Lexicon 20/20 AD User manual

20/20 AD
20 Bit A/D Converter
User Guide

Lexicon
Copyright © 1992 Lexicon Inc.
3 Oak Park
Bedford, MA 01730
Tel: [phone]
Fax: [phone]
All Rights Reserved
Thispublicationisprotectedbycopyrightandallrightsarereserved.Nopartofit maybereproducedortransmitted
by any means or in any form, without express prior consent in writing from Lexicon.
Precautions
The 20/20 AD is a rugged device with extensive electronic protection. However, you should observe the same
reasonable precautions that apply to any piece of audio equipment:
• Always use the correct line voltage.
• Do not install the 20/20 AD in a closed, unventilated rack, or directly above heat-producing equipment such
as power amplifiers.
• Never attach audio power amplifier outputs (speaker outputs) directly to any of the 20/20 AD's connectors.
• To prevent fire or shock hazard, do not expose the 20/20 AD to rain or moisture.
Thisequipmentgeneratesandusesradiofrequencyenergyandifnotinstalledandusedproperly,thatis,instrict
accordance with the manufacturer's instructions, may cause interference to radio and television reception. It has
been type tested and found to comply with the limits for a Class A computing device in accordance with the
specificationsinSubpartJofPart15ofFCCRules,whicharedesignatedtoprovidereasonableprotectionagainst
such interference in a residential installation. However, there is no guarantee that interference will not occur in a
particular installation. If this equipment does cause interference to radio or television reception, which can be
determinedbyturningtheequipmentOFFandON,theuserisencouragedtotrytocorrecttheinterferencebyone
or more of the following measures:
Reorient the receiving antenna
Relocate the computer with respect to the receiver
Move the computer away from the receiver
Plug the computer into a different outlet so that the computer and receiver are on different
branch circuits.
If necessary, the user should consult the dealer or an experienced radio/television technician for additional
suggestions. The user may find the following booklet prepared by the Federal Communications Commission
helpful: "How to identify and Resolve Radio/TV Interference Problems."
This booklet is available from the U.S. Government Printing Office, Washington, DC 20402, Stock No. 004-000-
00345-4.
Notice
This triangle, which appears on
your component, alerts you to
the presence of uninsulated,
dangerous voltage inside the
enclosure...voltage that may be
sufficient to constitute a risk of
shock.
CAUTION Thistriangle,whichappearson
your component, alerts you to
important operating and main-
tenance instructions in this ac-
companying literature.
RISK OF ELECTRIC SHOCK
DO NOT OPEN
Printed in the United States of America
Lexicon Part # 070-09090

20/20 AD User Guide 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.
Instruction manual symbol: the product
willbe markedwiththissymbolwhenitis
necessary for the user to refer to the
instruction manual in order to protect
against damage to the instrument.
Indicatesdangerousvoltage.(Terminals
fed from the interior by voltage exceed-
ing 1000 volts must be so marked.)
TheWARNINGsigndenotesahazard.It
calls attention to a procedure, practice,
conditionorthelikewhich,ifnotcorrectly
performed or adhered to, could result in
injury or death to personnel.
The CAUTION sign denotes a hazard. It
callsattentiontoanoperatingprocedure,
practice,conditionorthelikewhich,ifnot
correctly performed or adhered to, could
result in damage to or destruction of part
or all of the product.
The NOTE sign denotes important infor-
mation. It calls attention to procedure,
practice, condition or the like which is
essential to highlight.
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
cablemeetInternationalElectrotechnicalCommission(IEC)
safety standards.
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.
KEEP AWAY FROM LIVE CIRCUITS
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.
DO NOT SERVICE OR ADJUST ALONE
Donotattemptinternal serviceoradjustmentunlessanother
person, capable of rendering first aid and resuscitation, is
present.
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.
DANGEROUS PROCEDURE WARNINGS
Warnings, such as the example below, precede potentially
dangerous procedures throughout this manual. Instructions
contained in the warnings must be followed.
Dangerousvoltages,capableofcausingdeath,arepres-
ent in this instrument. Use extreme caution when han-
dling, testing and adjusting.
SAFETY SYMBOLS
WARNING
WARNING
NOTE:
CAUTION
Electrostatic Discharge (ESD) Precautions
The following practices minimize possible damage to ICs
resulting from electrostatic discharge or improper inser-
tion.
• Keep parts in original containers until ready for use.
• Avoid having plastic, vinyl or styrofoam in the work
area.
•. Wear an anti-static wrist-strap.
• 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.*
• 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.
CAUTION

20/20 AD User Guide
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
Specifications.......................................................................................25
Definition of Terms ..............................................................................27

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'swarrantyonthisproductexcludesconsequentialdamagesresultingfrom
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
XLRinputandamaleXLRoutput,conformingtothestandardconventionofIEC268, Part12.Inputandoutput
levelscomplywithCCITT V.11.Onthisdevice,the professionalformofthe digitalaudioandauxiliarydata 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
AudioInterface Standard,datedSeptember, 1987,and toIEC 958,FirstEdition 1989-03.As ofNovember 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
currentlyapprovedspecifications.Onthisdevice,theconsumerformofthedigitalaudioandauxiliarydatahas
been labeled "SPDIF" in accordance with general usage.

1
Controls and Connectors
20/20 AD User Guide
Afterunpackingthe20/20AD,saveallpackingmaterialsincaseyoueverneed
toshiptheunit.Thoroughlyinspectthe20/20ADandpackingmaterialsforsigns
of damage. Report any shipment damage to the carrier at once. The following
accessories are included with the 20/20 AD:
1. Power Cable
2. User Guide
3. Quick Reference Guide
The20/20 AD is equippedwith a 3-pin IECconnector and detachable linecord.
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.
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.
Themaximumambientoperatingtemperatureis95°F(35°C).Provideadequate
ventilation if the 20/20 AD is mounted in a closed rack with heat-producing
equipment.
Mounting
Unpacking
Power
Controls and
Connectors

Lexicon
2
Controls and Connectors
1dB SCALE
ALT
PK HOLD
ATTEN
1
THRESHUNBAL THRESHUNBAL
2 3
THRESHUNBAL THRESHUNBAL
4
4 CHAN
~EMPH
OVL
0
3
6
12
18
24
30
36
42
60
OVL
0
3
6
12
18
24
30
36
42
60
MUTE
+
•
–
M
C
P
20/20 AD
Front Panel
Controls
The Converter
4 CHAN
Pushon-pushoffselectionof
2-channel or 4-channel
mode; lit when in 4-channel
mode.
EMPH
Turns pre-emphasis on and
off.
Gain Controls: 1, 2, 3 and 4
Provide continuously vari-
ablegaincontrolovera30dB
range.
Note: In 2-channel mode,
only 1 and 2 are active.
UNBAL
Lights when there is a mis-
matchbetweenthe inputsig-
nal and the setting of the
rear-panel Balance switch.
1dB SCALE
Press to select 1dB Scale
display on level meters.
Also used to select alternate
modes.
PK HOLD
Press to cause the level
meter LEDs to hold at the
highestlevelreceived. Press
again to deactivate.
ATTEN
Press to attenuate balanced
input signals by 6dB. Press
again to turn attenuation off.
Do not use this control on
unbalanced signals.
The 20/20 AD front panel controls are divided into three functional groups for
conversion, compression, and synchronization/output formatting.
THRESH
When the compressor is on,
indicates that the input level
hasrisenabove theselected
compression threshold.
20/20 AD
POWER
20 BIT A/D CONVERTER
1dB SCALE
ALT
PK HOLD
ATTEN
1
THRESHUNBAL THRESHUNBAL
2 3
THRESHUNBAL THRESHUNBAL
4
4 CHAN
~EMPH
OVL
0
3
6
12
18
24
30
36
42
60
OVL
0
3
6
12
18
24
30
36
42
60
DISPLAY
RMT
LINK
ON
6
GAIN
8
10
12
4
2
0
-14
-10
0-36
-18
-22
-26
100
mS
THRESHOLD RELEASE
DITHER
FORMAT
AES / WC:
SAMPLE RATE
EXT SYNC
DIGITAL COMP/LIMITER
-6
                                    
Converter Compressor Synchronization and
Output Formatting
MCP
1S10
0+•–
MUTE
∞
VID
+
•
–
M
C
P
WIDE
HF
48
AES
WC
PRES
20BIT
PL DN
44.1
SPDIF
AES
LOCK

3
Controls and Connectors
20/20 AD User Guide
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
displayedonthetworighthandmeters(2-channelmode)oronallfourmeters(4-channel
mode).
The following charts show all of the possible states of the level meter and the value in
decibels for each state.
The Compressor
DISPLAY
Press to cause gain reduc-
tion to be displayed on the
level meters — In 2-channel
mode,onlytherighttwolevel
meters are activated; in 4-
channel mode, all four level
metersdisplay compression.
Press again to deactivate.
GAIN
Adjusts the gain of low level
signals (below the compres-
sion threshold) between the
converter and the compres-
sor.
THRESHOLD
Sets the level at which com-
pression starts.
none
1 dB SCALE
DISPLAY
Normal
1 dB Scale
Gain Reduction
Normal
1 dB Scale
Gain Reduction
none
1 dB SCALE
DISPLAY
Normal
Normal
Normal
(Blank)
1 dB Scale
Gain Reduction
Switches ON Meters 1 & 2 Meters 3 & 4
4-Channel
Mode
2-Channel
Mode
1dB SCALE
GAIN
REDUCTION
NORMAL
OVL
0
-1
-2
-3
-4
-5
-6
-7
-8
-9
1
2
3
4
5
6
7
8
9
10
11
OVL
0
-3
-6
-12
-18
-24
-30
-36
-42
-60
- red -
- red -
- yellow -
- yellow -
- green -
- green -
- green -
- green -
- green -
- green -
- green -
OVL
0
3
6
12
18
24
30
36
42
60
DISPLAY
RMT
LINK
ON
6
GAIN
8
10
12
4
2
0
-14
-10
0-36
-18
-22
-26
100
mS
THRESHOLD RELEASE
DIGITAL COMP/LIMITER
-6
CPM
•–+
0
10
VID ∞
1S
LINK
In 2-channel mode, links the
leftandrightcompressors;in
4-channel mode, where 1,2
and 3,4 are already linked,
this control links the two
pairs.
ON
Turns the compressor on
and off.
RELEASE
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.

Lexicon
4
Controls and Connectors
AES/WC
Ininternalsync mode,PRES
indicates that either WC or
AESisavailableattheinputs
as a sync source.
Inexternalsyncmode,PRES
indicates that the
selected
sync source is present at the
input connectors.
Synchronization
and
Output Formatting
SAMPLE RATE
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.
(The PL DN setting of this
switch is reserved for future
enhancements.)
EXT SYNC
This switch allows selection
of: Internal mode (unlit),
Word Clock source, or AES
source.
DITHER
Selects the type of dither:
wide bandwidth, 20 bit, or
High Frequency .
FORMAT
Selects the format to be
available at the output con-
nectors, AES or S/PDIF.
Note: The format selected
here will be sent to
all out-
puts.
DITHER
FORMAT
AES / WC:
SAMPLE RATE
EXT SYNC
WIDE
HF
48
AES
WC
PRES
20 BIT
PL DN
44.1
SPDIF
AES
LOCK
Alternate Modes
ALT - ATTEN
Mutes the outputs. Note that
thelevelmetersstillwork.To
unmute, hold ALT and press
ATTEN again.
ALT - 4 CHAN
Activates output of a digital
sinewave at the AES ports.
ALT - DISPLAY
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.
ALT - LINK
Only active in 4-channel
mode. Activates a short tone
burst. See Realigning Two
DAT Recordings.
ALT - ON
Reserved for future en-
hancements.
ALT - DITHER
Activates psychoacousti-
cally optimized noise shap-
ing. When activated, WIDE,
20BIT and HF will be lighted.
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.
1dB SCALE
ALT
4 CHAN
~
DISPLAY
RMT
LINK
               
press and hold ...then press one of these.
To activate
the alternate modes...
ATTEN
MUTE
ON
VID
Repeat to exit the alternate mode.
DITHER
LOCK lights in external sync
mode to indicate the unit has
successfully locked to the in-
coming signal (WC or AES).

5
Controls and Connectors
20/20 AD User Guide
PUSH
DO WORD CLOCK
IN
DO
3 + 4 1 + 2 OUT
AES
SYNC
IN
V ~
PUSH 4IN3
UNBAL
BAL
PUSH PUSH 2IN1
UNBAL
BAL
PUSH
Digital Output
Two 3-pin XLR connectors, professional format AES bal-
anced digital outputs.
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.
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
3-pin XLR connectors. In 2-
channel mode, only inputs 1
and 2 are active.
AC Power
Standard 3-pin IEC power
connector.
Sync Input
3-pinXLRconnector,professionalformatAES/EBUbalanced
digital input.
75Ω, unbalanced BNC connectors for word clock in/out.
The input and output connectors conform to published stan-
dardsforprofessionalandconsumerformats.The 20/20AD,
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.
20/20 AD
Rear Panel
Connectors

Lexicon
6
Controls and Connectors
Audio Connections
XLR Pinout
Signal Mating Connector Description
AES/EBU XLR A3M Balanced RS-422
Digital Input
AES/EBU XLR A3F Balanced RS-422
Digital Output
S/PDIF RCA Unbalanced 75Ω
Consumer Digital
Output
S/PDIF Consumer Digital
Consumer Digital
Audio Optical
Output
Word Clock BNC Unbalanced 75Ω
Analog Input XLR pin 2 high
Connectors
2 = high
3 = low
1 = ground
1 = ground
3 = low
2 = high
Female
Male
XLR Pinout —pin 2 high by convention

7
Controls and Connectors
20/20 AD User Guide
AES/EBU
Digital Output
This interface requires balanced connections using high-quality, low-capaci-
tance,controlled-impedance,datacommunication,twisted-shieldedpaircable.
Microphone cable may introduce a significant amount of jitter into the
signal, causing distortion.
Use commercially-available, consumer audio optical cable assemblies.
Thisinterfaceisunbalancedbut,becauseitcarriesdigitalsignals,itrequiresthe
use of 75ΩRG-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)
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)
Cables
S/PDIF
Consumer Digital Audio
Output
Word Clock I/O

9
Using the 20/20 AD
20/20 AD User Guide
Analog Inputs
The20/20ADhas fouranaloginputs:CH1,CH2,CH3andCH4.Intwo-channel
mode, only CH1 and CH2 are used, and the dynamic range of the unit is
extended.Ifyouhavetwostereopairs,oneofwhich isusedmoreoftenthanthe
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.
Whendigitizingstereopairs,the leftchannelofeachpairshould besenttoCH1
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 OFF +2 to +22 0 to +20
balanced ON +8 to +28 +6 to +26
unbalanced OFF* -4 to +16 -6 to +14
*UnbalancedsignalscannotbecorrectlyattenuatedwiththefrontpanelATTENbutton.Inputlevels
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 positive return
(shield) polarity
unbalanced screen and positive open or
return polarity return
Inatypicalstudio environment, each piece of equipment isconnectedtoa “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
safetygroundorgroundstrap.Thiswillresultina“groundloop”whichmanifests
itself as hum at the power line frequency.
A common solution to this problem is to connect the cable shields only at the
sourceend,andtoleavepin1ofthe20/20ADendfloating.Ifthisisnotpractical,
you can open the ground jumpers (W1, W9, W11 and W13) on the 20/20 AD’s
analog inputs.
Using the 20-/20 AD
Termination and
Grounding via
Internal Jumpers

Lexicon
10
Using the 20/20 AD
Jumper settings
(as set at the factory)
W18
CHASSIS GROUND
ANALOG IN
AES SYNC IN
WORD CLOCK IN
AES OUT
S/PDIF OUT
XLR
CONNECTOR
SHIELDED
TWISTED PAIR
BNC
CONNECTOR
75 OHM
COAXIAL
600 OHM
W1
W9
W11
W13
W2
W10
W12
W14
W17 110 OHM
W16
J13
75 OHM
SYSTEM GROUND
RCA
CONNECTOR
W26
W27
CHASSIS CONNECTED TO
EARTH GROUND
THROUGH POWER CABLE
OR GROUND STRAP
3
2
1
+
-
75 OHM
COAXIAL
SHIELDED
TWISTED PAIR
3
2
1
3
2
1
XLR
CONNECTOR
XLR
CONNECTOR
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
thejumpersonthe20/20ADMainpcboard.Theconfigurationshownillustrates
the jumper settings when the 20/20 AD is shipped from the factory.

11
Using the 20/20 AD
20/20 AD User Guide
Thefollowingfigureindicatesthelocationsandfactorysettingsofthesejumpers
ontheboard.Intheir closed positions,thejumpershave theirrighttwo(orlower
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.)
U30 U31 W18
W17
U42 U48
U49
J13
W27
W26
W16
Analog Jumpers
Digital Jumpers
W2 W9 W10 W11 W12 W13 W14W1
Jumper locations
(shown with factory settings)
Note that the 20/20 AD’s analog inputs are not transformer isolated. It is,
therefore,necessarytohaveonegroundconnectionbetweenthe20/20ADand
the audio source.
Inaddition to the ground lift jumpers, eachanalog input has a 600Ωtermination
jumper. If the analog source expects to see a 600Ωload, these jumpers should
be closed.

Lexicon
12
Using the 20/20 AD
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.
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.
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
negativesource 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. Removethepowercord, openthe20/20AD,andshort outR4,R28,R54and
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
UNBALlight does not go out,reconnectthe pot between pin 3andground,
and repeat.)
d) Once the UNBAL light goes out, continue reducing the resistance until the
lightcomesonagain.Setthepotsothatitisinthemiddleoftherangewhere
the UNBAL light is out.
e) Removethepotandmeasuretheresistance.Solderaresistorofthatvalue
between the correct pin and ground.
Balanced and
Unbalanced
Input Signals

13
Using the 20/20 AD
20/20 AD User Guide
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
todigitalfullscale(dBfs).AlightedgreenoryellowLEDindicatesthat the signal
hasexceededthelevelprintednexttoit.Thered0dBLEDlightswhenthesignal
exceeds -0.5dBfs. The red OVL LED indicates audible clipping, (clipping which
lasts for several samples).
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.
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
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.Rememberthatthelevelmeterswilldisplayeitherthenormalscaleorthe
1dB scale, dependent on the setting of the1 dB SCALE switch.
ATTEN
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
theinputlevelby6dB),orreduce the signallevelatitssource.NotethatATTEN
shouldnotbeused unnecessarilyasitcausesaslight reductionintheCommon
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
channelsyouneedtodigitize,andonwhetherornotyouneedtheextradynamic
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.
Normal Level Meter settings
(1 dB SCALE and DISPLAY set to OFF)
OVL
0dB
-3dB
-6dB
-12dB
-18dB
-24dB
-30dB
-36dB
-42dB
-60dB
- red -
- red -
- yellow -
- yellow -
- green -
- green -
- green -
- green -
- green -
- green -
- green -
OVL
0
3
6
12
18
24
30
36
42
60
1 dB Level Meter settings
(1 dB SCALE set to ON,
DISPLAY set to OFF)
- red -
- red -
- yellow -
- yellow -
- green -
- green -
- green -
- green -
- green -
- green -
- green -
OVL
0
3
6
12
18
24
30
36
42
60
OVL
0dB
-1dB
-2dB
-3dB
-4dB
-5dB
-6dB
-7dB
-8dB
-9dB
Front panel
Adjustments

Lexicon
14
Using the 20/20 AD
The20/20ADcansynchronize to its internally derived sample clock at 44.1kHz
or48kHz,ortoanexternal AESorWordClocksignal,intherange of 41-51kHz.
TheEXTSYNCswitch allows you to select external Word Clock (WC),external
AES (AES), or internal source (WC and AES off).
BeforeswitchingbetweenInternaland Externalmodes,besure toreduce
monitorvolumeassome D/Aconvertersdonot mutewhentheylose lock.
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
pins2and3oftheXLRconnector,asrequiredbythe 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
shippedwithW18 open,assumingthatground issuppliedfromthe generatorof
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.
TheWORDCLOCK INsignalshouldbe alogic-level(logichigh=3V-5V) square
wave. When shipped, the 20/20 AD terminates the signal with a 75Ωresistor
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 75ΩRG-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.Itiscapableofdrivinga75Ωload,andisalwaysavailable,evenwhen
aninternalsyncsourceisselected.Thisoutputmaybeusedtosynchronizeone
ormoreslave20/20ADstoamaster.AswiththeWordClockinput,useonly75Ω
RG-59/U or similar cable.
Synchronization
and Sample Rate
AES Sync
Word Clock

15
Using the 20/20 AD
20/20 AD User Guide
The 20/20 AD is shipped configured for Internal mode. To operate in External
mode,pressEXTSYNCuntilthedesiredsyncsource(WCorAES)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.
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
withtheexternalsync source,assomedigitaltaperecordersdeterminesample
rate from the Channel Status bits rather than from the actual sample rate.
Internal and
External modes
External Mode
AES / WC:
EXT SYNC
WC AES
PRES LOCK
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
Internalmode,thePRESlightindicatesthat eitherWCor AESisavailableasan
external sync source.
Internal Mode
AES / WC:
EXT SYNC
PRES
WC AES
LOCK
External Mode Error
States
AES / WC:
EXT SYNC
AES / WC:
EXT SYNC
WC
PRES LOCK
AES
WC LOCK AES
PRES

Lexicon
16
Using the 20/20 AD
About Dither Dither is a low-level pseudo-random signal which is added to digital audio to
reducequantizationnoise,ineffect,byreplacingitwitha“nicersounding”noise.
Iftherecorderorworkstationwhichfollowsthe20/20ADiscapableofrecording
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(highfrequency)ditherproducesnoisewhoseamplitudeislowerthanWIDE
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
noisewithin the frequency rangewhere human hearing ismost acute is shifted
tohigherfrequencies.Thisiscalled psychoacousticallyoptimizednoiseshaping
(PONS) because it takes advantage of the variations in the ear's sensitivity to
noise within the 20 Hz - 20 kHz range.
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.Notethat,duetolimitationsinprocessingpower,thecompressor,
4-channelmodeandtheALT-MUTEfunction are disabled whilePONSmodeis
active. To deactivate PONS, simply press DITHER.
Psychoacoustically
Optimized Noise
Shaping (PONS)