User Guide
20/20 AD
20 Bit A/D Converter
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SAFETY SUMMARY
The following general safety precautions must be observed during all phases of operation, service and
repair of this instrument. Failure to comply with these precautions, or with specific warnings elsewhere
intheseinstructionsviolatessafetystandardsofdesignmanufactureandintendeduseoftheinstrument.
Lexicon assumes no liability for the customer's failure to comply with these requirements.
General definitions of safety symbols used on equipment or
in manuals.
GROUND THE INSTRUMENT
To minimize shock hazard the instrument chassis and cabi-
net must be connected to an electrical ground. The instru-
ment is equipped with a three-conductor AC power cable.
The power cable must either be plugged into an approved
three-contact electrical outlet or used with a three-contact to
two-contact adapter with the grounding wire (green) firmly
connected to an electrical ground (safety ground) at the
power outlet. The power jack and mating plug of the power
cable meet International Electrotechnical Commission (IEC)
safety standards.
Instruction manual symbol: the product
will be marked with this symbol when it is
necessary for the user to refer to the
instruction manual in order to protect
against damage to the instrument.
Indicates dangerous voltage. (Terminals
fed from the interior by voltage exceed-
ing 1000 volts must be so marked.)
DO NOT OPERATE IN AN EXPLOSIVE
ATMOSPHERE
Do not operate the instrument in the presence of flammable
gases or fumes. Operation of any electrical instrument in
such an environment constitutes a definite safety hazard.
The WARNING sign denotes a hazard. It
calls attention to a procedure, practice,
condition or the like which, if not correctly
performed or adhered to, could result in
injury or death to personnel.
WARNING
CAUTION
KEEP AWAY FROM LIVE CIRCUITS
The CAUTION sign denotes a hazard. It
callsattentiontoanoperatingprocedure,
practice, condition or the like which, if not
correctly performed or adhered to, could
result in damage to or destruction of part
or all of the product.
Operating personnel must not remove instrument covers.
Component replacement and internal adjustments must be
made by qualified maintenance personnel. Do not replace
components with power cable connected. Under certain
conditions, dangerous voltages may exist even with the
power cable removed. To avoid injuries, always disconnect
power and discharge circuits before touching them.
The NOTE sign denotes important infor-
mation. It calls attention to procedure,
practice, condition or the like which is
essential to highlight.
DO NOT SERVICE OR ADJUST ALONE
Do not attempt internal service or adjustment unless another
person, capable of rendering first aid and resuscitation, is
present.
NOTE:
DO NOT SUBSTITUTE PARTS OR MODIFY
INSTRUMENT
Because of the danger of introducing additional hazards, do
not install substitute parts or perform any unauthorized
modification to the instrument.
CAUTION
Electrostatic Discharge (ESD) Precautions
The following practices minimize possible damage to ICs
resulting from electrostatic discharge or improper inser-
tion.
DANGEROUS PROCEDURE WARNINGS
Warnings, such as the example below, precede potentially
dangerous procedures throughout this manual. Instructions
contained in the warnings must be followed.
•
•
Keep parts in original containers until ready for use.
Avoid having plastic, vinyl or styrofoam in the work
area.
WARNING
•. Wear an anti-static wrist-strap.
Dangerousvoltages, capableofcausingdeath, arepres-
ent in this instrument. Use extreme caution when han-
dling, testing and adjusting.
•
•
•
Discharge personal static before handling devices.
Remove and insert boards with care.
Whenremovingboards,handleonlybynon-conductive
surfaces and never touch open-edge connectors ex-
cept at a static-free workstation.*
SAFETY SYMBOLS
•
Minimize handling of ICs.
•. Handle each IC by its body.
•
•
Do not slide ICs or boards over any surface.
Insert ICs with the proper orientation, and watch for
bent pins on ICs.
•
Use anti-static containers for handling and transport.
*To make a plastic-laminated workbench anti-static, wash with a solution of Lux liquid
detergent, and allow to dry without rinsing.
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Contents
Introduction
Controls and Connectors ...................................................................... 1
Unpacking • Power • Mounting .......................................................... 1
Front Panel Controls.......................................................................... 2
The Converter • The Level Meters • The Compressor
Synchronization and Output Formatting • Alternate Modes
Rear Panel Connectors ..................................................................... 5
Audio Connections ............................................................................ 6
Connectors • Cables
Using the 20/20 AD................................................................................. 9
Analog Inputs..................................................................................... 9
Termination and Grounding via Internal Jumpers ............................. 9
Balanced and Unbalanced Input Signals......................................... 12
Front Panel Adjustments ................................................................. 13
Synchronization and Sample Rate .................................................. 14
AES Sync • Word Clock • Internal and External Modes
About Dither..................................................................................... 16
Psychoacoustically Optimized Noise Shaping (PONS)
Automatic Offset Removal ...............................................................18
Compressor Modes ......................................................................... 18
Dynamic Range Compression • Non-Linear Transfer
Functions • Overload Protection • Digitizing Analog Tape
Link • Display
Digital Audio Outputs ....................................................................... 23
Realigning Two DAT Recordings .................................................... 24
Digital Sinewave Generator ............................................................. 24
Operating the 20/20 AD in the Digitial Mode ................................... 25
Specifications.......................................................................................27
Definition of Terms .............................................................................. 29
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20/20 AD User Guide
Introduction
The 20/20 AD is an analog-to-digital converter which can be configured for
two20-bitchannels, orfour18-bitchannels. Twinon-boardsignalprocessors
provide 20-bit to 16-bit compression in the digital domain, as well as DC
removal and a choice of four dither types.
The compressor allows leeway in setting levels and provides insurance
against clipping in the digital domain. In addition, it provides 195 soft
compression curves and 13 time constants to allow for more creative
functions, such as emulation of tape saturation or analog compression.
This manual provides some basic information on conversion, but assumes
the user will be familiar with the principles of analog-to-digital conversion, as
well as with the digital audio interface formats output by the 20/20 AD.
Information on specific data which may be required by any target device
should be obtained from the manufacturer.
Lexicon has attempted to provide correct information and functional capability
corresponding to digital audio standards known and available to us at the time of
publication of this document and manufacture of this instrument. As standards
evolve, Lexicon may make updates to documentation and/or system software
available for purchase.
Lexicon's warranty on this product excludes consequential damages resulting from
the use of this product. Local jurisdiction may extend the user additional rights.
AES/EBU Interface (Professional)
The AES/EBU interface conforms to both the AES3-1992 (ANSI S4.40-1992) specification and the EBU
document, Tech 3250-E. Both inputs and outputs are balanced, transformer-coupled designs, with a female
XLR input and a male XLR output, conforming to the standard convention of IEC 268, Part 12. Input and output
levels comply with CCITT V.11. On this device, the professional form of the digital audio and auxiliary data has
been labeled "AES" in accordance with general usage.
S/PDIF – Sony/Phillips Digital Interface Format (Consumer)
The S/PDIF interface conforms to the Channel Status Type II format, as specified in the EIAJ CP-340 Digtial
Audio Interface Standard, dated September, 1987, and to IEC 958, First Edition 1989-03. As of November 20,
1991, amendments to IEC 958, which include specifications for the implementation of auxiliary data for the
consumer format, were under review. Until new specifications are published, this device must conform to
currently approved specifications. On this device, the consumer form of the digital audio and auxiliary data has
been labeled "SPDIF" in accordance with general usage.
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Controlsand
Connectors
20/20 AD User Guide
After unpacking the 20/20 AD, save all packing materials in case you ever need
toshiptheunit. Thoroughlyinspectthe20/20ADandpackingmaterialsforsigns
of damage. Report any shipment damage to the carrier at once. The following
accessories are included with the 20/20 AD:
Unpacking
1. Power Cable
2. User Guide
3. Quick Reference Guide
The 20/20 AD is equipped with a 3-pin IEC connector and detachable line cord.
Connect the cable end of the 20/20 AD line cord to the 20/20 AD power
connector. Then plug the line cord into an AC wall socket providing voltage
corresponding to the data plate on the unit.
Power
The 20/20 AD measures 19"W x 1.75"H x 13.9"D (483 x 45 x 353 mm). Make
sure that the 20/20 AD is securely screwed into the rack, and that support is
provided for the rear of the chassis during transport to avoid possible damage
from severe mechanical shock.
Mounting
Themaximumambientoperatingtemperatureis95°F(35°C). Provideadequate
ventilation if the 20/20 AD is mounted in a closed rack with heat-producing
equipment.
1
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Controls and Connectors
Lexicon
20/20 AD
Front Panel
Controls
The 20/20 AD front panel controls are divided into three functional groups for
conversion, compression, and synchronization/output formatting.
Synchronization and
Output Formatting
Converter
Compressor
UNBAL THRESH
UNBAL THRESH
UNBAL THRESH
UNBAL THRESH
OVL
0
OVL
0
POWER
DIGITAL COMP/LIMITER
6
1dB SCALE
ALT
DISPLAY
LINK
DITHER
WIDE
HF
20BIT
PL DN
44.1
100
mS
RMT
VID
3
3
-18
-14
4
CHAN
SAMPLE RATE
EXT SYNC
6
6
4
0
8
12
18
24
30
36
42
60
12
18
24
30
36
42
60
-22
-10
48
~
PK HOLD
ATTEN
FORMAT
20/20 AD
EMPH
2
10
10
1S
AES
WC
SPDIF
AES
20 BIT A/D CONVERTER
-26
-6
12
M
C
P
+
•
0
∞
ON
+
–
M
-36
0
P
C
•
AES / WC:
PRES
LOCK
MUTE
–
1
2
3
4
GAIN
THRESHOLD
RELEASE
The Converter
Gain Controls: 1, 2, 3 and 4
Provide continuously vari-
able gain control over a 30dB
range.
THRESH
UNBAL
When the compressor is on,
indicates that the input level
has risen above the selected
compression threshold.
Lights when there is a mis-
match between the input sig-
nal and the setting of the
rear-panel Balance switch.*
Note: In 2-channel mode,
only 1 and 2 are active.
4 CHAN
Push on-push off selection of
2-channel or 4-channel
mode; lit when in 4-channel
mode.
UNBAL THRESH
UNBAL THRESH
UNBAL THRESH
UNBAL THRESH
OVL
0
OVL
0
1dB SCALE
ALT
3
3
4
CHAN
6
6
12
18
24
30
36
42
60
12
18
24
30
36
42
60
~
PK HOLD
EMPH
M
C
P
+
•
ATTEN
MUTE
EMPH
–
1
2
3
4
Turns pre-emphasis on and
off.
PK HOLD
ATTEN
1dB SCALE
Press to cause the level Press to attenuate balanced
meter LEDs to hold at the input signals by 6dB. Press
highest level received. Press again to turn attenuation off.
Press to select 1dB Scale
display on level meters.
Also used to select alternate
modes.
again to deactivate.
Do not use this control on
unbalanced signals.
* Non-linearity of the potentiometer can cause the signal to become unbalanced when the level knob is
set to 11:30. To correct this, turn the knob slightly in either direction until the UNBAL light goes out.
2
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20/20 AD User Guide
Controls and Connectors
The Level Meters
Whenthecompressoris off, thebehaviorofthelevelmetersisdeterminedbythesettings
of the 1dB SCALE, and PK HOLD switches.
When the compressor is on, pressing DISPLAY will cause gain reduction (in dB) to be
displayed on the two right hand meters (2-channel mode) or on all four meters (4-channel
mode).
The following charts show all of the possible states of the level meter and the value in
decibels for each state.
GAIN
REDUCTION
Switches ON
Meters 1 & 2
Meters 3 & 4
NORMAL
1dB SCALE
OVL
0
- red -
- red -
OVL
0
OVL
0
1
2
none
Normal
1 dB Scale
Normal
1 dB Scale
4-Channel
Mode
1 dB SCALE
DISPLAY
3
- yellow -
- yellow -
- green -
- green -
- green -
- green -
- green -
- green -
- green -
-3
-1
-2
-3
-4
-5
-6
-7
-8
-9
3
Gain Reduction
Gain Reduction
6
-6
4
none
Normal
Normal
Normal
(Blank)
12
18
24
30
36
42
60
-12
-18
-24
-30
-36
-42
-60
5
2-Channel
Mode
6
1 dB SCALE
DISPLAY
1 dB Scale
7
Gain Reduction
8
9
10
11
The Compressor
DISPLAY
Press to cause gain reduc-
tion to be displayed on the
level meters — In 2-channel
mode, only the right two level
meters are activated; in 4-
channel mode, all four level
meters display compression.
Press again to deactivate.
DIGITAL COMP/LIMITER
DISPLAY
100
mS
RMT
VID
6
C
-14
-18
4
0
8
-22
-26
-10
-6
LINK
ON
2
10
1S
10
12
0
∞
M
P
-36
0
+
–
•
GAIN
THRESHOLD
RELEASE
THRESHOLD
GAIN
RELEASE
LINK
Adjusts the gain of low level
signals (below the compres-
Sets the level at which com-
pression starts.
This control allows adjust-
ment of the release time of
the compressor. Release
times from 0mS to ∞ are
available. The attack time of
the compressor is automati-
cally set as a function of the
signal rise time.
In 2-channel mode, links the
leftandrightcompressors;in sion threshold) between the
4-channel mode, where 1,2 converter and the compres-
and 3,4 are already linked, sor.
this control links the two
pairs.
ON
Turns the compressor on
and off.
3
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Controls and Connectors
Lexicon
Synchronization and
Output Formatting
DITHER
SAMPLE RATE
Selects the type of dither:
wide bandwidth, 20 bit, or
High Frequency .
Selects the sample rate of
the A/D conversion. This
switch should be set to the
same sample rate as your
sync source. When Word
Clockisselectedasanexter-
nal sync source, the AES
ChannelStatusSampleRate
bit will be set to the sample
rate value you have selected
here.
DITHER
WIDE
HF
20 BIT
PL DN
44.1
SAMPLE RATE
EXT SYNC
48
FORMAT
AES
WC
SPDIF
AES
FORMAT
Selects the format to be
available at the output con-
nectors, AES or S/PDIF.
AES / WC:
PRES
LOCK
Note: The format selected
here will be sent to all out-
puts.
AES/WC
In internal sync mode, PRES LOCK lights in external sync
indicates that either WC or mode to indicate the unit has
AES is available at the inputs successfully locked to the in-
(The PL DN setting of this
switch is reserved for future
enhancements.)
as a sync source.
coming signal (WC or AES).
EXT SYNC
Inexternalsyncmode,PRES
indicates that the selected
sync source is present at the
input connectors.
This switch allows selection
of: Internal mode (unlit),
Word Clock source, or AES
source. This is for sync
source only. See ALT-EXT
SYNC below for digital audio
input.
Alternate Modes
ALT - ATTEN
ALT - ON
Reserved for future en-
hancements.
To activate
Mutes the outputs. Note that
the level meters still work. To
unmute, hold ALT and press
ATTEN again.
the alternate modes...
press and hold
...then press one of these.
ALT - DITHER
Activates psychoacousti-
cally optimized noise shap-
ing. When activated, WIDE,
20BIT and HF will be lighted.
ALT - 4 CHAN
Activates output of a digital
sinewave at the AES ports.
1dB SCALE
DISPLAY
LINK
DITHER
ATTEN
ALT
MUTE
RMT
EXT SYNC
4 CHAN
~
Note that this type of dither
can only be used in 2-chan-
nel mode; activating will dis-
able 4-channel mode and
compressor functions.
ALT - DISPLAY
ON
VID
Activates remote mode/front
panel lockout. When acti-
vated, the 1dB SCALE and
DISPLAY LEDs flash and
front panel controls are
locked into their current set-
tings. To unlock, hold ALT
and press DISPLAY again.
Repeat to exit the alternate mode.
ALT - EXT SYNC
Enables the digital audio in-
put. See Operating the 20/20
AD in the Digital Mode.
ALT - LINK
Only active in 4-channel
mode. Activates a short tone
burst. See Realigning Two
DAT Recordings.
4
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20/20 AD User Guide
Controls and Connectors
20/20 AD
Rear Panel
Connectors
BAL/UNBAL
Push on/push off switch. Set
button to correspond to input
signal type. A mismatch be-
tween the input signal and
the setting of this switch, will
be indicated on the front
panel by the BAL LED.
Analog Input
AC Power
Standard 3-pin IEC power
connector.
3-pin XLR connectors. In 2-
channel mode, only inputs 1
and 2 are active.
3
+
DO
4
1
+
DO
2
4
IN
3
2
IN
1
WORD CLOCK
IN OUT
PUSH
PUSH
PUSH
PUSH
PUSH
UNBAL
BAL
UNBAL
BAL
AES
SYNC
IN
V ~
Sync/Digital Audio Input
3-pinXLRconnector,professionalformatAES/EBUbalanced
digital input.
Digital Output
Two 3-pin XLR connectors, professional format AES bal-
anced digital outputs.
75Ω, unbalanced BNC connectors for word clock in/out.
Two consumer format optical (fiber-optic) outputs.
Two consumer format unbalanced coaxial RCA outputs.
Note: In 2-channel mode 3 & 4 and 1 & 2 are identical.
The input and output connectors conform to published stan-
dards for professional and consumer formats. The 20/20 AD,
however,providestheuser-selectedformatatallconnectors,
e.g. the AES format, if selected, will be available at the
consumer-type outputs, as well as at the XLR outputs.
5
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Controls and Connectors
Lexicon
Audio Connections
Connectors
Signal
Mating Connector
Description
AES/EBU
XLR A3M
Balanced RS-422
Digital Input
AES/EBU
Digital Output
XLR A3F
RCA
Balanced RS-422
S/PDIF
Consumer Digital
Output
Unbalanced 75Ω
S/PDIF
Consumer Digital
Consumer Digital
Audio Optical
Output
Word Clock
Analog Input
BNC
XLR
Unbalanced 75Ω
pin 2 high
XLR Pinout
2 = high
Male
3 = low
1 = ground
Female
1 = ground
3 = low
2 = high
XLR Pinout — pin 2 high by convention
6
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20/20 AD User Guide
Controls and Connectors
Cables
This interface requires balanced connections using high-quality, low-capaci- AES/EBU
tance, controlled-impedance, data communication, twisted-shielded pair cable. Digital Output
Microphone cable may introduce a significant amount of jitter into the
signal, causing distortion.
S/PDIF
Use commercially-available, consumer audio optical cable assemblies.
Consumer Digital Audio
Output
Word Clock I/O
This interface is unbalanced but, because it carries digital signals, it requires the
use of 75Ω RG-59 coaxial cable.
Below are recommended cable and cable assemblies. Similar cables from
sources other than those listed here may be appropriate.
AES/EBU
Belden 9271 (foil shielded twisted pair, 124Ω, 12.2 pF/ft)
Gotham Audio GAC-2 (shielded twisted pair)
S/PDIF Consumer Digital Audio
Belden 9259 (RG-59/U: 22 AWG conductor, .242 O.D., 75Ω, 17.3 pF/ft)
Belden 8218 (27 AWG conductor, .150 O.D., 75Ω, 20.5 pF/ft)
Maximum recommended length: 32 ft. (10M)
S/PDIF Consumer Digital Audio Optical
Toshiba TOCP174y
Sony POC-15
Maximum recommended length: 16 ft. (5M)
Word Clock
Belden 9259 (RG-59/U: 22 AWG conductor, .242 O.D., 75Ω, 17.3 pF/ft)
Belden 8218 (27 AWG conductor, .150 O.D., 75Ω, 20.5 pF/ft)
Maximum recommended length: 32 ft. (10M)
7
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20/20 AD User Guide
Using the 20-/20 AD
The 20/20 AD has four analog inputs: CH1, CH2, CH3 and CH4. In two-channel
mode, only CH1 and CH2 are used, and the dynamic range of the unit is
extended. If you have two stereo pairs, one of which is used more often than the
other, the most often used pair should be assigned to CH1 and CH2, and the
other pair to CH3 and CH4. This allows you to take advantage of the extended
dynamic range of two-channel mode when using only a single stereo pair.
Analog Inputs
When digitizing stereo pairs, the left channel of each pair should be sent to CH1
and CH3; the right to CH2 and CH4.
The 20/20 AD accepts balanced or unbalanced signals with the following peak
input levels:
Signal Type
Attenuator
dBu
dBV
balanced
balanced
unbalanced
OFF
ON
OFF*
+2 to +22
+8 to +28
-4 to +16
0 to +20
+6 to +26
-6 to +14
* Unbalanced signals cannot be correctly attenuated with the front panel ATTEN button. Input levels
of unbalanced signals must be lowered before entering the 20/20 AD.
The analog XLR connectors should be wired as follows:
Signal Type
Pin 1
Pin 2
Pin3
balanced
screen
(shield)
positive
polarity
return
unbalanced
screen and
return
positive
polarity
open or
return
In a typical studio environment, each piece of equipment is connected to a “star
ground” through the safety ground on the AC connector, or through a ground
strap tied to the chassis. In these cases, connecting the shield of the audio
cable(s)topin1atbothendsofthecablewillcausethe20/20ADtobegrounded
to the audio source through two paths: the shield of the audio cable and the
safety ground or ground strap. This will result in a “ground loop” which manifests
itself as hum at the power line frequency.
Termination and
Grounding via
Internal Jumpers
A common solution to this problem is to connect the cable shields only at the
source end, and to leave pin 1 of the 20/20 AD end floating. If this is not practical,
you can open the ground jumpers (W1, W9, W11 and W13) on the 20/20 AD’s
analog inputs.
9
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Using the 20/20 AD
Lexicon
CAUTION
These instructions are for use by qualified personnel only.
Before opening the 20/20 AD, REMOVE THE POWER CORD. You may be
exposed to hazardous voltages even if power to the unit is turned off.
WARNING
Turn off power and REMOVE THE POWER CORD
before opening the unit.
Take a moment to study the figure below. This diagram shows the terminating
and grounding of input signals for the unit, and explains the function of most of
the jumpers on the 20/20 AD Main pc board. The configuration shown illustrates
the jumper settings when the 20/20 AD is shipped from the factory.
CHASSIS GROUND
XLR
CONNECTOR
ANALOG IN
W1
W9
W11
W13
1
2
3
+
-
XLR
CONNECTOR
W2
600 OHM
W10
W12
W14
AES OUT
1
2
3
W26
W27
SHIELDED
TWISTED PAIR
AES SYNC IN
SHIELDED
TWISTED PAIR
W18
1
2
3
RCA
CONNECTOR
W17 110 OHM
S/PDIF OUT
XLR
CONNECTOR
75 OHM
COAXIAL
BNC
CONNECTOR
WORD CLOCK IN
J13
75 OHM
COAXIAL
W16
75 OHM
CHASSIS CONNECTED TO
EARTH GROUND
THROUGH POWER CABLE
OR GROUND STRAP
Jumper settings
(as set at the factory)
10
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20/20 AD User Guide
Using the 20/20 AD
Thefollowingfigureindicatesthelocationsandfactorysettingsofthesejumpers
on the board. In their closed positions, the jumpers have their right two (or lower
two) pins shunted together. To open a jumper, move the shunting block to the
left(orupper)twopins.(Notethatthejumpersin J13haveonlytwopins;toopen,
simply remove the shunting block.)
W1
W2
W9
W10
W11
W12 W13
W14
Analog Jumpers
W17
W27
U30 U31
W18
W16
W26
U48
U42
U49
J13
Digital Jumpers
Jumper locations
(shown with factory settings)
Note that the 20/20 AD’s analog inputs are not transformer isolated. It is,
therefore, necessary to have one ground connection between the 20/20 AD and
the audio source.
In addition to the ground lift jumpers, each analog input has a 600Ω termination
jumper. If the analog source expects to see a 600Ω load, these jumpers should
be closed.
11
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Using the 20/20 AD
Lexicon
Make sure the BALANCE switches located on the rear panel are set to match
the type of input. (OUT=balanced , IN=unbalanced). The red UNBAL lights on
the front panel will indicate an error if there is a mismatch between the switch
setting and the signal type.
Balanced and
Unbalanced
Input Signals
Note that when an UNBAL light goes on, there is a good chance that the input
signal has been audibly clipped, even if the level meter does not indicate an
overload.
If it is necessary to mismatch the switch settings to the signals (for example,
when switching quickly between balanced and unbalanced inputs) you can
avoid clipping by setting the peak input levels about 3dB below full scale.
Undersomeconditions,theUNBALlightmayremainon,eventhoughthesource
and the 20/20 AD are both set for unbalanced operation.
1. The source has a pin-3-high unbalanced signal. Try swapping pins 2 and 3.
2. The source is floating (transformer-isolated). Tie pin 3 to pin 1 at either end
of the cable.
3. Non-linearity of the potentiometer can cause the signal to become unbal-
ancedwhenthelevelknobissetto11:30.Tocorrectthis,turntheknobslightly
in either direction until the UNBAL light goes out.
The UNBAL light may also flash intermittently if the source is balanced and
transformer-isolated, and the 20/20 AD shares the source with another con-
verter or an amplifier. The other box may be unequally loading the positive and
negative source signals, causing them to become unbalanced. Some solutions
to this problem are:
1. Drive the 20/20 AD by itself. This will ensure a balanced signal and minimize
common mode noise.
2. If the source's isolation transformer has a center-tapped output, ground the
center tap.
3. Reduce the signal level into the 20/20 AD and make it up with the front panel
gain control(s).
4. Remove the power cord, open the 20/20 AD, and short out R4, R28, R54 and
R95. Theseare10Kresistors(brown, black, black, red)locatedneartheinput
relays. Note that this change may degrade the 20/20 AD's CMRR.
5. Equalize the load resistances of the other box with the following procedure:
a) With the 20/20 AD and the other box connected, send a high level tone
through the source so that the 20/20 AD's UNBAL light goes on.
b) Connect a 1-Meg potentiometer (set at maximum resistance) between the
pin 2 signal and ground.
c) Gradually reduce the resistance until the UNBAL light goes out. (If the
UNBAL light does not go out, reconnect the pot between pin 3 and ground,
and repeat.)
d) Once the UNBAL light goes out, continue reducing the resistance until the
lightcomesonagain. Setthepotsothatitisinthemiddleoftherangewhere
the UNBAL light is out.
e) Remove the pot and measure the resistance. Solder a resistor of that value
between the correct pin and ground.
12
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20/20 AD User Guide
Using the 20/20 AD
Adjust the front panel Gain knobs until the peak input levels are balanced and
come within a few dB of clipping. For now, make sure that the DISPLAY switch
intheDIGITAL COMP/LIMITERsectionissettoOFF. Innormaloperation, each
level meter will indicate the level at the output of the converter in dB referenced
to digital full scale (dBfs). A lighted green or yellow LED indicates that the signal
has exceeded the level printed next to it. The red 0dB LED lights when the signal
exceeds -0.5dBfs. The red OVL LED indicates audible clipping, (clipping which
lasts for several samples).
Front panel
Adjustments
Normal Level Meter settings
(1 dB SCALE and DISPLAY set to OFF)
- red -
- red -
OVL
0
OVL
0dB
I dB SCALE
Turning on the 1dB SCALE switch increases the resolution of the top end of the
scale shown on the level meters. In this mode, each LED corresponds to 1dB.
- yellow -
- yellow -
- green -
- green -
- green -
- green -
- green -
- green -
- green -
3
-3dB
6
-6dB
12
18
24
30
36
42
60
-12dB
-18dB
-24dB
-30dB
-36dB
-42dB
-60dB
Meter Ballistics
Each meter consists of a peak meter (indicated by an isolated LED) and an
average meter (indicated by a growing and shrinking bar). The peak meter's
attack time constant is instantaneous, and its decay time constant is roughly 1
second. The average meter's time constants approximate those of a VU meter,
with attack and delay time constants of 10 mS.
PK HOLD
1 dB Level Meter settings
(1 dB SCALE set to ON,
DISPLAY set to OFF)
When you press PK HOLD, the meters will indicate the highest level received
after PK HOLD was pressed. To reset the peak, turn PK HOLD off and then on
again. Remember that the level meters will display either the normal scale or the
1dB scale, dependent on the setting of the1 dB SCALE switch.
- red -
- red -
OVL
0
OVL
0dB
- yellow -
- yellow -
- green -
- green -
- green -
- green -
- green -
- green -
- green -
3
-1dB
-2dB
-3dB
-4dB
-5dB
-6dB
-7dB
-8dB
-9dB
ATTEN
6
12
18
24
30
36
42
60
The ATTEN button should only be used on balanced input signals.
If a Gain knob is turned completely counter clockwise and the level meter still
indicates an overload, you must either press the ATTEN button (which reduces
the input level by 6dB), or reduce the signal level at its source. Note that ATTEN
should not be used unnecessarily as it causes a slight reduction in the Common
Mode Rejection Ratio (CMRR) and thereby increases the potential for hum.
4-CHAN
Whenthe4CHANswitchison, the20/20ADconvertsfourchannelsfromanalog
to digital, with a dynamic range of better than 16 bits. When the 4 CHAN switch
is off, the 20/20 AD only converts CH1 and CH2, buts adds 12dB (2 bits) to the
dynamic range. Your selection of mode depends primarily on how many
channels you need to digitize, and on whether or not you need the extra dynamic
range. If you plan to record to 16-bit media without using the on-board digital
compressor, 2-channel mode will give you duplicate digital outputs.
EMPH
The EMPH switch adds CD-standard 15/50µsec pre-emphasis to each input
signal before digitization.
13
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Using the 20/20 AD
Lexicon
The 20/20 AD can synchronize to its internally derived sample clock at 44.1kHz
or 48kHz, or to an external AES or Word Clock signal, in the range of 41-51kHz.
The EXT SYNC switch allows you to select external Word Clock (WC), external
AES (AES), or internal source (WC and AES off).
Synchronization
and Sample Rate
Before switching between Internal and External modes, be sure to reduce
monitor volume as some D/A converters do not mute when they lose lock.
AES Sync
The AES SYNC IN signal can be any AES3-compatible signal running at the
correct sample rate — It does not require a Digital Audio Reference Signal
(DARS). The 20/20 AD is shipped with a 110 ohm termination resistor across
pins 2 and 3 of the XLR connector, as required by the AES3 specification. If you
intendtodaisychainseveral20/20ADsfromthesameAESsource,youwillhave
to open jumper W17 on all units except the last one in the chain.
Jumper W18 connects the cable shield to system ground. The 20/20 AD is
shipped with W18 open, assuming that ground is supplied from the generator of
the sync signal. Note that, unlike the analog inputs, the AES SYNC input is
transformer isolated; it is not necessary to have a ground connection between
the 20/20 AD and the sync source.
All AES connections require twisted -shielded pair cable with a controlled
impedance between 100 and 125Ω, and less than 15 pF/ft of capacitance.
Microphone cable will not work reliably. Refer to Page 7 for recommended
cables.
Word Clock
The WORD CLOCK IN signal should be a logic-level (logic high=3V-5V) square
wave. When shipped, the 20/20 AD terminates the signal with a 75Ω resistor
through the leftmost jumper in J13. If the clock source is not capable of driving
high through 75Ω (most TTL and CMOS drivers are not), it may be necessary
to open this jumper. In this case, it is important that the cable be kept as short
as possible. As with AES SYNC, if you intend to daisy chain several 20/20 ADs
from the same Word Clock, you should open the leftmost jumper on J13 on all
unitsexceptthelastoneinthechain.Ifanalternativegroundpathexistsbetween
the word clock generator and the 20/20 AD,W16 can be opened to prevent a
ground loop.
Note that, unlike the analog and AES SYNC inputs, the Word Clock input is not
transformer isolated, and requires a ground connection between the 20/20 AD
and the signal source.
The Word Clock input requires a 75Ω RG-59/U coaxial cable. Use of ordinary
audio-quality cable may distort the edges of the Word Clock. This will add jitter
to the sample rate, which will, in turn, add noise and distortion to the analog/
digital converter. Refer to Page 7 for recommended cables.
The WORD CLOCK OUT signal is a logic-level square wave at the sample
frequency.Itiscapableofdrivinga75Ω load,andisalwaysavailable,evenwhen
an internal sync source is selected. This output may be used to synchronize one
ormoreslave20/20ADstoamaster. AswiththeWordClockinput, useonly75Ω
RG-59/U or similar cable.
14
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20/20 AD User Guide
Using the 20/20 AD
Internal and
External modes
The 20/20 AD is shipped configured for Internal mode. To operate in External
mode, press EXT SYNC until the desired sync source (WC or AES) lights. If the
selected sync source light flashes and the PRES light is not on, it indicates that
there is no signal present as a sync source. If the selected sync source and
PRESlightsareon,buttheLOCKlightisoff,theselectedsyncsourceispresent,
but is not within the 41-51kHz lock range.
External Mode
EXT SYNC
WC
AES
AES / WC:
PRES LOCK
With WC selected as the external sync source, the front panel SAMPLE RATE
switch determines the setting of the AES and S/PDIF Channel Status sample
rate bits, not the actual sample rate, which tracks the external sync source. It is
important to make sure that the sample rate selected at the front panel agrees
with the external sync source, as some digital tape recorders determine sample
rate from the Channel Status bits rather than from the actual sample rate.
External Mode Error
States
EXT SYNC
WC
AES
AES / WC:
AES / WC:
PRES
LOCK
EXT SYNC
WC
AES
LOCK
PRES
To operate in Internal mode, press EXT SYNC until the WC and AES lights are
both off. Then press SAMPLE RATE until the desired sample rate lights. In
Internal mode, the PRES light indicates that either WC or AES is available as an
external sync source.
Internal Mode
EXT SYNC
WC
AES
AES / WC:
PRES
LOCK
15
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Using the 20/20 AD
Lexicon
Dither is a low-level pseudo-random signal which is added to digital audio to
reduce quantization noise, in effect, by replacing it with a“nicer sounding” noise.
About Dither
If the recorder or workstation which follows the 20/20 AD is capable of recording
or processing 20 bits, the DITHER switch should be set to 20 BIT. If, however,
the equipment can only record or process a 16-bit signal, either WIDE or HF
dither should be selected. This is true even if the compressor is on; the dither
generator always sends the appropriate signal for a 16-bit recorder, regardless
of the compressor settings.
All three of these basic dither types have the same quantization noise reducing
effect, but differ in the spectral distribution of the “replacement” noise.
WIDE dither produces white noise.
HF (high frequency) dither produces noise whose amplitude is lower than WIDE
at low frequencies, but higher at high frequencies. Theoretically, this is less
audible than WIDE dither, but just as effective.
20 BIT dither has the same spectrum as HF dither, but with a lower amplitude.
This setting should be used when recording to 20-bit media.
It is possible to filter the quantization noise in such a way that almost all of the
noise within the frequency range where human hearing is most acute is shifted
tohigherfrequencies.Thisiscalledpsychoacousticallyoptimizednoiseshaping
(PONS) because it takes advantage of the variations in the ear's sensitivity to
noise within the 20 Hz - 20 kHz range.
Psychoacoustically
Optimized Noise
Shaping (PONS)
To activate this dither, press and hold the ALT button, then press DITHER. The
three dither lights will go on (WIDE, 20BIT and HF) to indicate that you are in
PONS mode. Note that, due to limitations in processing power, the compressor,
4-channel mode and the ALT-MUTE function are disabled while PONS mode is
active. To deactivate PONS, simply press DITHER.
16
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20/20 AD User Guide
Using the 20/20 AD
-60.00
-70.00
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-90.00
-100.0
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16.0k
18.0k
20.0k
20/20 AD in 2-channel mode, truncated to 16 bits without
dither. Note harmonic distortion (spikes at multiples of 1kHz)
and elevated noise floor.
Plot of noise + distortion vs. frequency for the 20/20 AD in 2-
channel mode with a -60dBfs sinewave at 1kHz.
-60.00
-70.00
-80.00
-90.00
-100.0
-110.0
-120.0
-130.0
-140.0
-150.0
-160.0
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18.0k
20.0k
HF dither is added before truncation. Noise floor in audio
rangeisloweredby3dB—attheexpenseofincreasednoise
above 20kHz.
WIDE dither is added before truncation. Noise floor is ele-
vated further, but harmonic distortion is removed.
-60.00
-70.00
-80.00
-90.00
-100.0
-110.0
-120.0
-130.0
-140.0
-150.0
-160.0
-60.00
-70.00
-80.00
-90.00
-100.0
-110.0
-120.0
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-140.0
-150.0
-160.0
0.0
2.00k
4.00k
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14.0k
16.0k
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22.0k
24.0k
0.0
2.00k
4.00k
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8.00k
10.0k
12.0k
14.0k
16.0k
18.0k
20.0k
PONS dither. Noise floor is down almost to the untruncated
level in the critical band (2-4kHz), but significantly increased
at high frequencies.
20BIT dither is added before truncation. This is similar to HF
dither, but at a lower level. The dither reduces spurious tones
(low-level spikes visible in the first plot) at the expense of a
slightly elevated noise floor.
17
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Using the 20/20 AD
Lexicon
DC offsets are created by input amplifiers and A/D converters for a number of
reasons. In the 20/20 AD, these typically include temperature changes,
changes between 2-channel and 4-channel mode, change of sample rate or
sync source, or large adjustments of input level controls. The DSP chips in the
20/20 AD automatically remove these DC offsets by passing data from the
converters through a digital high pass filter with a corner frequency of less than
0.1 Hz.
Automatic Offset
Removal
This high pass filter will reduce output offsets to less than -80dB within one
minute of a change in input offsets. In order to minimize output offsets, wait at
least one minute after any of the above changes before recording.
DC offsets created outside the 20/20 AD are removed by blocking capacitors
locatedbetweentheanaloginputconnectorsandthefirstamplifierstage. These
capacitors have a corner frequency of less than 1 Hz, which guarantees less
than 0.1dB of loss at 10 Hz (referenced to 1 kHz).
It is possible to extend the low frequency response down to less than 1 Hz by
shorting these blocking capacitors. This should only be done if the offset of the
input signal is relatively constant, and less than 1% of full scale (approximately
150 mV with a 22 dBu input). Transformer-isolated sources are generally safe,
but non-isolated sources should be checked with a voltmeter. Offsets greater
than 1% may take several minutes for the digital high pass filter to remove.
To short out the blocking capacitors, remove C5, C6, C30, C31, C51, C52, C80
and C81. These are upright tubular capacitors located near the analog input
connectors. Solder short lengths of wire in place of each. Alternatively, you can
leave the capacitors installed, and solder short lengths of wire across the pins
of each on the solder side of the board.
Until you become comfortable with the operation of the rest of the 20/20 AD, you
should keep the compressor turned off. Make sure that the DISPLAY, LINK and
ON buttons in the compressor block are all off (unlighted) and the compressor
gain knob is set to 0dB.
Compressor Modes
Although the function of the digital compressor is similar to that of a standard
analogcompressor, itsintendedusesaresomewhatdifferent.Threeofthemore
common uses (Dynamic Range Compression, Non-Linear Transfer Functions
and Overload Protection) are described below.
Dynamic Range
Compression
This mode is primarily useful when recording program material with a wide
dynamic range onto 16-bit media in 2-channel mode. (Although the compressor
will work in 4-channel mode, the dynamic range of 4-channel mode is so close
to the 16-bit level that any further compression will only raise the noise floor.)
This use of the compressor requires a bit of artistry, because all settings must
be optimized to the program material.
With the compressor on, set compressor GAIN to 6dB. This will increase the
gain of low level signals (below the THRESHOLD setting) by 6dB, effectively
squeezing the dynamic range of the converter in 2-channel mode from 110dB
to 104dB. The DITHER switch automatically adds just enough dither to squeeze
the remaining dynamic range into the 96dB dynamic range of 16-bit media.
18
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20/20 AD User Guide
Using the 20/20 AD
Note that the 6dB GAIN setting is given only as an example. Feel free to
experiment with any level in this mode. (GAIN settings above 12dB are not
available because at that point practically all of the dynamic range of the
converter will fit onto 16-bit media. )
Set THRESHOLD to -24dB. Now, when the signal level exceeds -24dB, the gain
isgraduallyreduceduntil,at0dBsignallevel,thecompressiongainisalsoat0dB
(unity).
The graph below shows a plot of gain vs. input level with compressor GAIN at
6dB and THRESHOLD set to -24dB. Note that, unlike some compressors, the
gain curve is not linear, but changes smoothly. This reduces high-frequency
artifacts which can result from aggressive compression.
Threshold
(-24dB)
"0" Level
Unity Gain
-104dB
-110dB
"0" Level
Input
The settings of the GAIN and THRESHOLD knobs determine the compression
ratio, according the formula:
___| threshold |___
ratio =
| threshold| – gain
In our example, the compression ratio is 24÷(24 – 6) = 1.33:1. This is not a very
intense compression by most standards but, remember, the point is to pack as
much music onto the recording media as possible without introducing audible
artifacts. As the following table of compression ratios shows, most of the ratios
are below 2:1. In order to minimize the audibility of compression artifacts, use
ratios from the unshaded portion of the table. Setting the threshold at any level
higher than a 4:1 ratio will force the ratio to 4:1.
19
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Using the 20/20 AD
Lexicon
GAIN
0dB
-3dB
1
2
3
4
5
6
7
8
9
10
11
12
1.50:1
1.20:1
1.14:1
1.11:1
1.09:1
1.07:1
1.06:1
1.06:1
1.05:1
1.05:1
1.04:1
1.04:1
1.03:1
1.02:1
3.00:1
1.50:1
1.33:1
1.25:1
1.20:1
1.16:1
1.14:1
1.12:1
1.11:1
1.10:1
1.09:1
1.08:1
1.07:1
1.06:1
4.00:1
2.00:1
1.66:1
1.42:1
1.33:1
1.27:1
1.23:1
1.20:1
1.17:1
1.16:1
1.14:1
1.13:1
1.11:1
1.09:1
4.00:1
3.00:1
2.00:1
1.66:1
1.50:1
1.40:1
1.33:1
1.28:1
1.25:1
1.22:1
1.20:1
1.18:1
1.15:1
1.12:1
4.00:1
4.00:1
2.66:1
2.00:1
1.71:1
1.55:1
1.45:1
1.38:1
1.33:1
1.29:1
1.26:1
1.23:1
1.20:1
1.16:1
4.00:1
4.00:1
4.00:1
2.50:1
2.00:1
1.75:1
1.60:1
1.50:1
1.42:1
1.37:1
1.33:1
1.30:1
1.25:1
1.20:1
4.00:1
4.00:1
4.00:1
3.33:1
2.40:1
2.00:1
1.77:1
1.63:1
1.53:1
1.46:1
1.41:1
1.36:1
1.30:1
1.24:1
4.00:1
4.00:1
4.00:1
4.00:1
3.00:1
2.33:1
2.00:1
1.80:1
1.66:1
1.57:1
1.50:1
1.44:1
1.36:1
1.28:1
4.00:1
4.00:1
4.00:1
4.00:1
4.00:1
2.81:1
2.28:1
2.00:1
1.81:1
1.69:1
1.60:1
1.52:1
1.42:1
1.33:1
4.00:1
4.00:1
4.00:1
4.00:1
4.00:1
3.50:1
2.66:1
2.25:1
2.00:1
1.83:1
1.71:1
1.62:1
1.50:1
1.38:1
4.00:1
4.00:1
4.00:1
4.00:1
4.00:1
4.00:1
3.20:1
2.57:1
2.22:1
2.00:1
1.84:1
1.73:1
1.57:1
1.44:1
4.00:1
4.00:1
4.00:1
4.00:1
4.00:1
4.00:1
4.00:1
3.00:1
2.50:1
2.20:1
2.00:1
1.85:1
1.66:1
1.50:1
-6dB
-8dB
-10dB
-12dB
-14dB
-16dB
-18dB
-20dB
-22dB
-24dB
-26dB
-30dB
-36dB
As both the formula and the table show, raising the threshold increases the ratio
— making the compression more audible above the THRESHOLD setting.
However, because signals below the threshold are boosted linearly through the
compressor, raising the threshold may decrease the amount of time the
compressor is reducing gain. This may decrease the audible artifacts.
Conversely, if you lower the threshold all the way to -36dB, you will pretty much
guarantee that all but the quietest signals will experience some amount of gain
reduction, but the ratio will be so small (1.2:1) that the total effect may be less
noticeable than a higher threshold setting.
Start with RELEASE time set to about 180 mS, then shorten the time for
percussive program material and lengthen it for material such as voice.
RELEASE times of 0mS to ∞ are available. Attack time of the compression is
automatically set as a function of the signal rise time.
Compressor Knob Values
100
180
6
-16
5
7
56
-18
-14
-20
-22
-24
-26
-30
-12
-10
-8
-6
32
320
560
4
8
18
10
5
3
9
2
10
11
1S
1.5
1
0
0
12
-3
∞
-36
0
GAIN
(in dB)
THRESHOLD
(in dB)
RELEASE
(in mS and S)
Note that changing the settings of GAIN and THRESHOLD while recording may
produce audible clicks. If, for artistic reasons, you wish to turn the compressor
off during a recording, do so by increasing the release time to ∞. (Using the
compressor on/off switch may produce a click.)
20
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20/20 AD User Guide
Using the 20/20 AD
When RELEASE is set to 0, the compressor acts like an amplifier with a non- Non-linear
linear transfer function. For example, setting GAIN to 6dB and THRESHOLD to Transfer Functions
-14dB produces a close approximation of a generic tape saturation curve. This
mode is also useful for overload protection of some types of percussive
instruments.
Note: This function is not available in 4-channel mode.
When recording live music onto 16-bit media, the recording engineer tries to Overload Protection
maximize dynamic range by keeping the signal as close as possible to full scale
without actually clipping. Peak levels are typically set a few dB below full scale,
because losing a few dB of dynamic range is far preferable to clipping.
The digital compressor eliminates this tradeoff. To use it, first set the input levels
to give yourself a comfortable margin between peak level and clipping. For
example, to give yourself 4dB of margin, push 1db SCALE and PK HOLD and
adjust the input level until the peak reads -4dB (the LED labeled 18).
Now, turn on the compressor . Set GAIN to 4dB. This increases the signal level
by 4dB (restoring 0dB peak level ). Set THRESHOLD to 0dB to automatically
select the maximum practical ratio.
Whenaninputsignalexceedsthepeaklevelby4dBorless,thecompressorgain
isautomaticallyreducedjustenoughtoassurethatthecompressoroutputnever
exceeds 0dB. After the overload has passed, the compressor gradually brings
the gain back up to 4dB. The RELEASE knob determines the speed of this
recovery.
Note: This automatic gain reduction can add considerable distortion to the
leading edge of the overload signal. With THRESHOLD set at 0dB (or at any
value less than or equal to the GAIN setting), gain reduction will not start until
the signal is about to clip — too late to prevent leading edge distortion. If this
distortion is unacceptable, try lowering the THRESHOLD setting so that
compression starts before the signal clips. If, for example, THRESHOLD is
lowered to -8dB, compressor gain reduction will be gradual, instead of being
reduced all at once at -4dB.
It is important to remember that the lower you set THRESHOLD, the sooner
compression will start, and the more likely you are to color signals which were
not in any danger of clipping.
21
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Using the 20/20 AD
Lexicon
To summarize:
1. Set the input levels so that the peak level is a few dB below clipping.
2. Turn on the compressor.
3. Set compressor GAIN to the input level safety margin, and THRESHOLD to
0dB.
4. Adjust RELEASE to suit the program material. The faster the expected
clipping source, the faster the release time should be.
5. Adjust THRESHOLD to suit the program material. If the expected source
material is percussive, the chance of clipping is relatively slim, and you don’t
want to color the rest of the music, leave THRESHOLD at 0dB. Otherwise,
itmaybenecessarytolowertheTHRESHOLDsettinguntiltheclippedsignal
sounds acceptable.
Note: The compressor can only prevent clipping if the analog/digital converter
doesnotoverload. Anysignalwhichoverloadstheconverterwillalsooverload
the compressor.
Although the compressor works in both 4-channel and 2-channel mode, the
available dynamic range in 4-channel mode is only a few dB greater than the
capacity of a 16-bit recorder, so nothing is gained by setting your margin greater
than a few dB — unless, of course, there is a specific audio effect you are trying
to achieve.
Digitizing Analog Tape Aslightlyunorthodoxuseofthecompressorallowsyoutoautomaticallyfinetune
the level so that the highest peak will be set to exactly 0dBfs when transferring
analog tape (or other repeatable source) to the digital domain.
To do this:
1. Do a dry run of the loudest section(s) of the tape through the 20/20 AD to
check the level. Using 1dB SCALE and PK HOLD, adjust the input level until
the peak comes within 6dB of clipping.
2. Turn on the compressor. Set GAIN to 6dB, THRESHOLD to 0dB, and
RELEASE to infinity. Do another dry run of the loudest section(s) of the tape.
If you turn on DISPLAY, you will see the gain reduction meter grow down
during the peaks and stay down.
3. Rewind the tape, turn on the digital recorder, and play the tape all the way
through. The compressor will add just enough fixed gain to boost the peak
to 0dBfs. Because the gain doesn't change, there are no compression
artifacts.
If you are in 4-channel mode, remember that you have less dynamic range
available, so you should try harder to set the input levels as close to 0dBfs as
practical, only relying on the 20/20 AD for the last 2 or 3dB.
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20/20 AD User Guide
Using the 20/20 AD
In 2-channel mode, when the LINK switch is off, each of the two channels is LINK
compressed independently. When the LINK switch is on, the louder of the two
channels sets the amount of compression. This serves the same function as
“stereolink” inanalogcompressors.Whenyouarerecordingastereosignaland
want to maintain the stereo image, LINK should be on. However, if you have the
compressor set up to protect against short overloads, or if you have the release
time set to 0, keeping LINK off will probably sound better. When recording two
mono signals, LINK should be off.
In 4-channel mode, when the LINK switch is off, channels 1 and 2 and channels
3 and 4 are linked in stereo pairs. When the link switch is on, all four channels
are linked. (There is no way to compress four mono signals.)
When the DISPLAY switch is on, the level meters indicate the amount of DISPLAY
compressor gain reduction in each channel. This shows the amount that gain (in
dB) which is being reduced as the input level exceeds the threshold.
The 20/20 AD provides two sets of digital audio output connectors with three
Digital Audio Outputs
connectorsineachset.Infour-channelmode,thefirstsetcontainsdigitizedCH1
and CH2, the second set contains CH3 and CH4. In two-channel mode, both
sets contain identical copies of digitized CH1 and CH2. The unused set may be
used to drive a backup recorder or to synchronize another 20/20 AD.
Each set of digital outputs contains three connectors: an XLR connector which
transmits AES3 digital audio, an RCA connector which transmits an S/PDIF-
compatible digital audio signal, and a TOSLINK-compatible optical transmitter.
All outputs are always active. The front panel FORMAT switch selects
between AES and S/PDIF-compatible Channel Status bits, but it doesn’t enable
or disable the actual transmitters. Make sure that the FORMAT setting is set for
AES when using the XLR connector and S/PDIF when using the RCA or optical
connectors.
Pin 1 of the AES outputs is connected to system ground through jumpers W26
and W27. If the shield is connected to ground at the receiver end, these jumpers
should be opened. Note that, because of the transformer or optical isolation, it
isnotnecessarytoconnectthegroundbetweenthe20/20ADandtheequipment
receiving the digital audio data, regardless of which output type you choose.
Asmentionedabove,onlyappropriatehigh-frequencycablesshouldbeusedfor
digital audio. Refer to Page 7 for recommended cables.
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Using the 20/20 AD
Lexicon
When recording in 4-channel mode to two DAT recorders, it may be useful to
record a "clapper" signal to both DATs to facilitate realignment during post-
production. When you press ALT-LINK, the 20/20 AD will generate 8mS of
silence, followed by an 8-cycle 1 kHz square wave burst at -12dBfs This signal
is time-aligned on all four output channels. You can align the two DAT tapes on
a disk audio production system by visually lining up the first edge of the burst,
or you can align them audibly by subtracting channel 3 from channel 1 and
adjusting the offset until the click disppears.
Realigning Two DAT
Recordings
The 20/20 AD includes a digital sinewave generator which is accurate to 20 bits,
with a dynamic range of more than 120dB. This generator is useful in testing the
digital outputs. To use it:
Digital Sinewave
Generator
1. Use the SAMPLE RATE, FORMAT and 4-CHAN buttons to select the output
mode. (These controls are locked out in sinewave generator mode.)
2. Press and hold down 1dB SCALE (ALT)and press the 4-CHAN button. 1dB
SCALEand4-CHANwillflashtoindicatethattheunitwillnowgeneratedigital
sinewaves. The compressor GAIN knob adjusts the frequency of the sine-
wave and the THRESHOLD knob adjusts the amplitude, as shown below.
3000
2062.5
1500
1125
937.5
750
5062.5
6937.5
9000
-16
-18
-20
-22
-24
-26
-30
-36
-14
-12
-10
-8
11062.5
14062.5
17062.5
20062.5
-6
562.5
375
-3
187.5
0
-60
(GAIN)
Frequency in Hz
(THRESHOLD)
Level in dBfs
48 kHz Sample Rate
2755
1895
1378
1033.5
861.5
689
4651
6374
8269
10163.5
12920
15676
18432.5
517
344.5
172.5
(GAIN)
Frequency in Hz
44.1 kHz Sample Rate
3. To turn off the sinewave generator, press and hold down 1dB SCALE (ALT)
and press 4-CHAN. The 20/20 AD will resume normal operation.
Note that in sinewave generator mode, only the compressor GAIN and
THRESHOLDknobshaveanyeffect.Pressinganydual-keycombinationsother
than ALT—4-CHAN may invoke undocumented diagnostics routines. If you
accidentally get into diagnostics mode, the best way to exit is to turn the power
off, then on again.
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20/20 AD User Guide
Using the 20/20 AD
The 20/20 AD's Digital Input mode can be used to make use of the digital
compressor, or to noise shape the dither spectra before recording 20-bit source
material to a 16-bit recording device*.
Operating the 20/20 AD
in the Digital Mode
In Digital Input Mode, format in = format out. No format conversion is
possible, and the front panel format indicator and selector are disabled.
This means that you must carefully consider the compatibility of your
source format and the requirements of any connected digital recording
device.
1. First, turn down your audio monitors. As the 20/20 AD will be connected in
the digital audio path, interconnects in the digital domain may disrupt any D/
As downstream.
2. Connect your digital audio source to the 20/20 AD rear panel AES/SYNC
input. The front panel PRES indicator should light.
3. Connect the 20/20 AD digital output to your recording device.
4. Pressandholddown1dBSCALE(ALT)andpresstheEXTSYNCbutton.The
20/20 AD will mute analog audio at the input, disable word clock on the BNC
connector, and lock onto the incoming AES signal. Once the 20/20 AD locks,
all four digital mode indicators will light.
Digital Input Mode
EXT SYNC
WC
AES
AES / WC:
PRES LOCK
Oncethe20/20ADisconfiguredfordigitalaudio, theanaloginputsaredisabled.
In conformance with AES standards, only two channels of digital audio can be
processed through the digital input. All six digital outputs are active.
The left front panel meters show digital audio input. The right meters show 1dB
scale (if selected), or gain reduction when the compression function is selected.
To return to analog input operation, turn down your monitors and press EXT
SYNC while holding down the 1dB SCALE button. This will reinitialize the unit
with the analog inputs active. Recalibration of the converters will take approxi-
mately one minute.
Note that powering off will also return the unit to analog input mode, which is the
default state of the 20/20 AD. If you want to operate in the digital input mode, you
must reconfigure the unit each time the unit is powered on.
*This feature is standard as of Serial # 1215, and is available as an update for
earlier units.
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Specifications
Digital Audio
Inputs and Outputs
Frequency Response
(relative to 1kHz)
Signal Delay
+0.1, -0.15dB, 10Hz to -20 kHz
112 samples
(2.54 ms at 44.1kHz
2.33 ms at 48kHz)
AES, S/PDIF: XLR, 3-wire balanced
RCA, 2-wire
unbalanced coaxial line MAX Input Level
Optical, optical fiber
Balanced:
Word Clock: BNC, 75Ω, unbalanced
Attenuator Off: +0 to +22 dBu
Attenuator On: +8 to +28 dBu
Unbalanced:
Attenuator Off: -6 to +16 dBu
(-8 to +14 dBV)
RFI Shielding
Complies with FCC Rules Part 15
Class A requirements for computer
equipment
Front Panel Controls
Converter Gain (4)
1dB SCALE/ALT
PK HOLD
ATTEN/MUTE
4 CHAN/~
EMPH
DISPLAY/RMT
LINK/ tone burst
ON/VID
Compressor Gain
Compressor Threshold
Compressor Release
DITHER (3 types, and OFF)
FORMAT
Power Requirements
100/120/220/240 VAC (+5%, -10%)
50-60 Hz 30W
Dynamic Range
4-channel mode:
100dB typ,
98dB min @ 1kHz
2-channel mode:
112dB typ,
Dimensions
19.0"W x 1.75"H x 13.9"D
(483 x 45 x 353mm)
Conforms to 19" rack mount
standard, 1U high
110dB min @ 1kHz
THD+N (at -2dBfs)
1kHz: <-94dB (0.002%)
20 Hz-20 kHz: <-90dB (0.0032%) 11 lbs 2 oz (5 kg)
Weight
SAMPLE RATE (2)
EXT SYNC (2 types, and none)
POWER
Shipping materials meet or exceed Proj-
CMRR
Attenuator Off:
ect1AoftheNationalSafeTransitAsso-
ciation (NSTA) packaged-product
specifications: vibration test, drop test,
static compression test.
>60dB below 1kHz,
>40dB above 1kHz
Rear Panel Controls
BAL/UNBAL switch
Attenuator On: >40dB
Environment
Front Panel Indicators
UNBAL
THRESH
Operating Temperature: 32° to
95°F (0° to 35° C)
Crosstalk
-80dB typ, 0-20kHz
Storage Temperature: -22° to 167°F
(-30° to 75°C)
Input level
Sample Rates
44.1 kHz, 48 kHz
Dither selection
Format selection
Sample Rate selection
External source selection,
presence and lock
Button lights indicate:
1dB SCALE on
PK HOLD on
Humidity: 95% maximum without
condensation
Sync Sources
Internal
Safety Approvals
UL, TUV
External AES (Digital Input*)
External Word Clock
(TTL level square wave)
Video (with optional daughter-
board)
ATTEN on
4-CHAN on
EMPH on
DISPLAY on
LINK on
Lock Range
+5%
*Units with Serial #'s <1215 do not have
thisoptioninstalled.Upgradesareavail-
able from Lexicon.
COMP/LIMITER on
Specifications subject to change without notice.
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Definition of Terms
AES : Audio Engineering Society.
frequency : the number of vibrations per unit of time, ex-
pressed in cycles per second (Hz).
aliasing : a form of distortion which occurs when an A/D
converter’s sample rate is less than twice the highest fre- harmonic distortion : the appearance of harmonics of the
quency component of a signal. input signal at the output of a device.
amplitude : the maximum departure of the value of a wave- link : the tying of two channels of the 20/20 AD compressor to
form from its average value. a shared gain curve.
attenuation : a reduction in the amplitude of an audio signal. lock : two or more audio signals which have the same
frequency and phase.
balanced signals : two equal and opposite signals run
through a twisted pair. The advantage of balanced signals is mS : millisecond: 1/1000 of a second.
that common mode noise tends to be added equally to both
portions of the pair; it can be removed by subtracting the
negative from the positive signal. (Common mode noise
rejection cannot be performed on unbalanced signals, which
oversampling : a method of sampling an audio signal at
many times the audio bandwidth (typically, 64-256 times) in
order to reduce the complexity of the anti-aliasing filter. The
oversampled data is then digitally filtered to reduce the
consist of an audio signal and a ground return.)
sample rate.
bandwidth : the arithmetic difference between the upper and
lower cut-off frequencies of an audio system.
period : the interval of time required for a waveform to
complete one cycle and begin to repeat itself. Period = 1/
bit : binary digit: the smallest unit of digital information, frequency.
equivalent to the result of a choice between two alternatives
(yes or no, on or off , 0 or 1). A series of bits make up a binary
number called a sample word.
phase : the angular relationship between two signals, meas-
ured in degrees, where 360° = the period of the waveform.
When two identical signals are always of the same polarity,
common mode noise :any signal which is common to both they are said to be in phase, or to have a phase of 0°. When
the negative and positive polarity of a balanced signal. In a they are always of opposite polarity, they are said to be out of
typical audio signal, common mode noise is introduced over phase, or to have a phase of 180°.
the length of the cable by outside interference. A typical
example is line hum.
polarity : the positive or negative direction of an electrical
force.
CMRR : Common Mode Rejection Ratio: measures the ability
quantization : the process of converting an infinitely-varying
of a system to reject common mode noise. It is expressed as
waveformintoafiniteseriesofdiscretelevels. Theinaccuracy
the ratio of common mode noise entering the system to the
introduced by this process is called quantization error.
common mode noise leaving it.
rms amplitude : root mean squared amplitude: a way of
assigning a DC power equivalent to an AC waveform. For a
dBfs : decibel full scale: this is the unit used to describe the
amplitude of a digital audio signal where 0 equals digital full
sine wave, this is 0.707 times the peak level.
scale.
sample rate : the periodic interval at which an analog audio
signal is sampled, e.g. 44.1 or 48 kHz.
digital headroom : does not exist, see dBfs.
distortion : an unwanted change in a waveform as it passes
signal-to-noise ratio : the ratio between the maximum peak
through an electronic component or from one medium to
signal and the wideband noise in the absence of signal. This
another.
number becomes more meaningful when distortion is added.
dither : a low level signal which is added to digital audio to It is then expressed as S/(N+D).
reduce quantization noise, in effect, by replacing it with noise
which is perceived as less offensive.
S/PDIF : Sony/Phillips Digital Interface Format.
synchronize : to arrange events to occur simultaneously. In
audio, the events are both the period and the phase of a
signal.
dynamic range : when used to describe digital-to-analog or
analog-to-digital converters, dynamic range is the ratio of the
full scale signal to the broadband noise (0-20 kHz) measured
THD+N : the ratio of the rms amplitude of the distortion and
with a -60dBfs signal. In other words, it is the S/(N+D)
noise that is added to a signal to the rms amplitude of the
measured with a -60 dBfs input signal and a 20 kHz low-pass
signal itself. This can be expressed in percent or decibels. S/
filter.
(N+D) = 1/(THD+N).
EBU : European Broadcast Union.
time constant : the amount of time it takes for a signal to rise
or fall 63% of its full scale. This only pertains to signals which
emphasis : a form of equalization which increases high
frequency signals in order to maximize the signal-to-noise
rise and fall exponentially, such as the 20/20 AD meter time
ratio. De-emphasis is the inverse of emphasis.
constants or compression gain curves.
equalization : the process of increasing or decreasing the
amplitude of some frequency ranges of a signal with respect
to others.
word clock : a square wave used as a synchronization
source. Its frequency equals the sample rate.
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