Dolby Laboratories SR 280 Operation manual

[](I DolbY SR
Cat. No. 280
Spectral Recording Module
User Information

USER INFORMATION
FOR
SPECTRAL RECORDING MODULE
CAT. NO. 280
Dolby Laboratories Incorporated
U.S.A. 100 Potrero Avenue, San Francisco, CA 94103
Tel: [phone]; Telex: 34409; Fax: [phone]
U.K. 346 Clapham Road, London SW9 9AP
Tel: [phone]; Telex: 919109; Fax: [phone]
WARRANTY INFORMATION: Equipment manufactured by Dolby Laboratories is warranted against
defects in materials and workmanship for one year from the date of purchase. There are no other express
01' implied warranties and no warranty of merchantability or fitness for a particular purpose. During the
warranty period Dolby Laboratories will repair or, at its option, replace components which prove to be
defective, provided the unit is shipped, prepaid, to the manufacturer directly or via an authorized
distributor. Defects caused by modifications, misuse or accidents, and any further damage caused by
inadequate packing for service return, are not covered by this warranty.
Dolby Laboratories' obligation is restricted to the repair or replacement of defective parts and under no
circumstances shall Dolby Laboratories be liable for any other damage, either direct or consequential.
All requests for repairs or information should include the unit serial number to ensure rapid service.
ill1986 Dolby Laboratories Inc.
S871741717493

This information is intended to serve as an introductory user's guide for the
Cat. No. 280 spectral recording module. It assumes previous knowledge of the
basic principles of audio recording, complementary signal processing, and some
familiarity with Dolby A-type noise reduction and interface frames.
Important Note
The Cat. No. 280 spectral recording module is
both pin and level compatible with the
Cat. No. 22 A-type module. In most cases, the
Dolby A-type user can simply replace the
Cat. No. 22 module with the Cat. No. 280
module and continue recording using
established studio procedures. (A switch on
the rear of the Cat. No. 280 must be set to the
appropriate frame type.)

Dolby® Cat. No. 280
Spectral Recording Module
The Cat. No. 280 module is the
first commercially available
embodiment of the new spectral
recording process developed by
Dolby Laboratories. The module
has been designed for ease of instal-
lation in existing 360-Series and
M-Series frames from Dolby
Laboratories.
Dolby spectral recording is a
new audio recording technique
which provides audible signal
integrity superior to that of any
other recording method in use. The
performance of Dolby SR with
contemporary analog professional
tape recorders stands up to ex-
tensive scrutiny. The Cat. No. 280
module has been designed to allow
easy operational use of the Dolby
SR process, using existing frames,
with no major studio equipment
changes or capital expenditure.
Auto
Co",pare
DOlbY ••
Noise Ref Ta pe
The Cat. No. 280 spectral
recording module utilizes function
logic similar to that of the Dolby
Cat. No. 22 A-type noise reduction
module; consequently the control
facilities of 360, 361 and M-Series
units operate normally, such as
IN/OUT, RECORD/PLAY and
input /output gain controls.
Dolby SR modules incorporate
a new recorder alignment and
verification tool called Auto
Compare . A built-in pink noise
generator in the module applies
bursts of noise (Dolby Noise) at a
calibrated level to the recorder.
The signal is used at the head of a
tape to identify the use of Dolby
SRI and to assist in recorder cali-
bration (however, the spectral
recording process is more tolerant
of input/output gain errors than
Dolby A-type noise reduction).
During playback, the Auto Com-
pare circuitry automatically alter-
nates the monitor output between
the Dolby Noise off-tape and the
DD DoibySR
internal noise generator. LED
indicators on the module front
indicate the current monitor con-
dition. Auto Compare assists in
recorder set-up alignment, but
additionally provides a rapid
method of checking a recorder
track by ear during a session.
Dolby Laboratories has de-
livered over 80,000 Cat. No. 22
noise reduction modules through-
out the world , and this widespread
availability of interfaces accepting
the Cat. No .280 modules will
facilitate adoption of the new
Dolby SR process.
Other Dolby SR configurations
are available, including
the Cat.
No.
431 , a module for Dolby SP-
and XP-Series units. In addition,
two testers are available to assist
process evaluation
and module
performance verification.
Full descriptive information
about
the Dolby spectral recording
process,
and product leaflets
describing other Dolby
SR modules
and testers are available on request
from Dolby Laboratories.

Dolby® Cat. No. 280
Specifications
INTERFACES:
The Cat. No. 280 module can be installed
in the following frames:
Models 360 and 365
All module functions available, including
simultaneous
record/play Auto Compare
if two Model 360s are used in dedicated
Record
and Play modes.
Model 361
Auto Compare functional durin g
tape-playback following recording,
but
not during simultaneous recording.
M-Series Units
Simultaneous record/play Auto Compare .
The Cat. 280 requires a special top cover
(Cat.
No . 359) to fit in Channels 2, 10, and
18 of M-Series interface.
CAT. NO. 280 MODULE
SPEGFICATIONS:
Layout:
A pair of printed circuit cards mounted
togeth er form the module. Signal input/
output and control functions compatible
with Cat. No. 22 noise reduction
modul e.
Input Circuit:
680 k ohms unbalanced, 300 mV rms for
reference leve
l.
Peak Encode Input Level:
3.0 V rms (20 dB above reference level).
Peak Decode Output Level:
3.0 V rms from Output 1,
5.0 V rms from Output 2.
Line Amplifier:
When mounted in interface, maximum
output + 22 dB into bridging load, + 21 dB
into 600 ohms
(0 dB = 0.775 V rms).
(A 6 dB higher level is possible using
+ 36 V line-amplifier supply voltage.)
Overall Frequency Response:
± 1 dB, 20 Hz-20 kHz (encode/decode).
Bandwidth Limitation: Control Inputs:
Internal filters: 10 Hz-50 kHz. External + 18 to + 30 V to actuate record
Overall Harmonic Distortion:
mode (provided in interface).
External single pole switch for process
2nd & 3rd harmonic each 0.3% at 3 dB
in/out (provided in interface).
below
peak level, 20 Hz-20 kHz. External single pole switch for
Dolby
Negligible higher order distortion com- Noise
mode (provided in interface).
ponents at any level. Internal three position switch to adjust
operating logic to
Models 360 and 365,
Overall Dynamic Range of
Model 361, or M-Series interfaces.
SR System (typical}:l
105 dB-clipping level to CCIR/ARM
Stability:
noise level. System
is highly stable-does not require
93
dB-clipping level to CCIR Rec. 468-2 routine alignment (no adjustable internal
weighted noise level. controls).
105 dB -clipping level to NAB
A- weighted noise leve
}2
Operating Temperature:
95 dB -clipping level to unweighted noise Up to 45 degrees
C.
level, 20 Hz-20 kHz.3 Construction:
Typical Obtainable Dynamic Fiberglass printed circuits, solid state
Range, 15 ips: devices
throughout; clear anodized
90-95 dB.
aluminum covers, black characters.
Matching Between Units: Size:
±1 dB at any level and any frequency, 192 x155 x 20
mm (7.6 x 6.1 x 0.8 inches).
20 Hz-20 kH z.
Weight:
Signal Delay: 500 gm
(18 oz.).
Approximately 7.0 J.l.s per channel,
14
J.l.S overall, encode/decode, plus delay Power Requirements:
of interfaces used.
SR circuit: +20 V to +28 V, 100-140 mA.
Line amplifier:
+18 V to +36 V, 13-17 mA.
Calibration Facilities:
Dolby Noise generator for establishing
correct levels
and frequency response, via D[] Dolby
built-in meter amplifier and interface
meter.
Output signal can also be fed to
external analysis facilities.
Automatic
audible Auto Compare function, allowing
Dolby Laboratories Inc., 100 Potrero Avenue ,
San Francisco, CA 94103-4813,
comparison of Dolby Noise from tape Tel
ep h one [phone], Telex 34409.
with internally generated reference
346 Clapham Road, London SW9 9Ap,
pink noise. Tel
ep h one (01) 720 -1111, Telex 919109.
"Dolby" and the double -D symbo l are trademarks of
Status Indicators:
Dolby Laboratories lic en sing Corporation.
5871741717493
Yellow LED on front of module indicates
Specifications subject to ch ange without notice.
Dolby Noise mode,
Auto Compare Reference/Tape function
status indicated
by red and green LEDs
on front of module.
(LED control signal available for remotely
situated LEDs
or lamps.)
1. Processor a lon e; in interfaces, may depend on line
l evels u se d.
2. Weighting filter supplemen t ed by 25 kHz 4- pole low -
pass filter to ensure that only audible nois e is measured.
3. Rms or average responding meter, 4-pole filters.

SECTION 1
II
TABLE OF CONTENTS
INTRODUCTION
1.1 Dolby Spectral Recording.
1.2 Dolby SR Packaging . .
1.3 Dolby
SR Compatibility .
1.4 Dolby SR Calibration. .
1.5
Cat. No. 280 Applications.
360 Series
M-Series .
SP/XP Series . . . . . . . .
Cinema Products (CP50, CP100, CP200)
Page
1-1
1-1
1-1
1-1
1-2
1-2
1-2
1-3
1-3
SECTION 2 CAT. NO. 280 INSTALLATION
2.1 General Installation . . . . .
2.2
Configuration Switch and Indicators
2.3 Interface Frame Switch Labeling
2.4 Optional External Connections
2-1
2-2
2-2
2-3
SECTION 3 CAT. NO. 280 CALIBRATION
3.1 Dolby Noise. 3-1
3.2
Auto Compare . 3-1
3.3
Auto Compare Control 3-2
3.4
Auto Compare Metering 3-2
3.5
Auto Compare Synchronization. 3-2
3.6 DolbyTone
and the Cat. No. 280 3-3
SECTION 4 SYSTEM ALIGNM(i:N'I'
4.1 General System Alignment . 4-1
4.2 Recording
Levels for Dolby SR 4-1
4.3
General Alignment Using VU Meters 4-2
4.4
Using VU Meters with High Level Reference Tapes. 4-3
4.5
Aligning with Peak Program Meters 4-3
SECTION 5 PRACTICAL OP~:RA'I'ION
APp(i~NnIX A
APP(i~NUIX B
5.1 Model 360 Operation . . . .
5.2
Auto Compare in the Model 360 .
5.3 Model 361
Operation. . . .
5.4
Auto Compare in the Model 361 .
5.5
M-Series Operation
5.6 Auto Compare in the M Series
Technical Aspects .
Optional Cat. No. 280 Connections.
5-1
5-1
5-2
5-3
5-5
5.5
A-I
B-1

1-1
1.1 Dolby Spectral Recording
SECTION 1
INTRODUCTION
Dolby spectral recording, or Dolby SR, has been developed to answer the needs of
modern professional recording. The Cat. No. 280 SR module is the first
implementation of this new recording process. It can be used in existing interface
frames manufactured by Dolby Laboratories and will, therefore, allow a
widespread introduction ofthis new high-quality recording format.
1.2 Dolby SR Packaging
To facilitate easy interchange, the Dolby SR circuit has been packaged in the
same card profile and pin out as the familiar Cat. No. 22 noise reduction module.
This will allow the process to be conveniently used in commonly available 360-
Series and M-Series interface frames. The Cat. No. 280 cannot be used in the SP
unit due to incompatibilities with the Cat. No. 230 carrier card. A specific Dolby
SR module, the Cat. No. 431, will be available for the existing SP- and XP-Series
multi-track frames.
While the Cat. No. 280 is generally compatible with the Cat. No. 22 module, there
is one major difference: the standard Cat. No. 280 module is physically thicker
(about .15 inch, 3 mm) than the Cat. No. 22, and therefore will not fit in some
existing Cat. No. 22 applications. To address situations where the Cat. No. 22
cards have been mounted close together, such as in the Model 362, a special
packaging adaptation of the Cat. No. 280 has been developed, the Cat. No. 280-L.
This long but thin adaptation extends the Dolby SR circuits forward of the front
panel by about 7 112 inches, or 190 mm, thereby allowing Dolby SR modules to be
used in close-pitch situations.
The increased thickness of the Cat. No. 280 will not allow standard modules to fit
in a few channels of the M-Series frame. To accommodate M-series channels 1,10
and 18, special module covers (Cat. No. 359) must be installed on the Cat. No. 280
modules used in these channels only.
1.3 Dolby SR Compatibility
Electrically, the Dolby SR process is very different from A-type noise reduction.
Both Dolby A-type NR and Dolby SR are complementary processes that, when
decoded properly, give excellent results. Dolby SR, however, goes far beyond
noise reduction systems in its ability to improve the overall quality of a recording
or transmission channel. Dolby A-type and Dolby SR are not considered cross-
compatible--Dolby SR encoded tapes should not be decoded with Dolby A-type
cards and vice versa.
1.4 Dolby SR Calibration
As with A-type noise reduction, Dolby SR requires the decode module to be
correctly set in level with respect to the encode module. With A-type, this is
accomplished using Dolby Tone; with the Dolby SR process, it is achieved using a
new, easily identifiable alignment signal: Dolby Noise. Dolby Noise, abbreviated
DN, is always used to establish correct decode level.

1-2
In addition to the Dolby spectral recording circuits and the Dolby Noise
generator, the Cat. No. 280 contains an important new feature--Auto Compare.
The Auto Compare circuit, using Dolby Noise as a signal, gives the user valuable
gain and frequency response information about the recording channel. The Auto
Compare mode allows an automatic aural comparison of the recorded Dolby Noise
signal and the internally generated pink noise. The user can quickly, by ear,
verify the performance of the recording system by comparing the reference pink
noise with the Dolby Noise from the tape. The Auto Compare feature is discussed
fully in Section 3 -- Calibration.
1.5 Cat. No. 280 Applications
The following is a summary of how the Cat. No. 280 can be used in specific Dolby
interface frames. A full discussion of Cat. No. 280 operation in these units is
covered in Section 5 -- Practical Operation.
360 Series
Model 360
The Model 360 is intended for single-channel, dedicated
encode or decode operation. Two interface frames and
two Cat. No. 280 modules are required for one channel of
simultaneous record and playback operation. The Auto
Compare feature can be fully operational during both
record and playback.
Model 361
The Model 361 is designed for changeover operation of a
single module for both record and playback. The
Cat. No. 280 module is switched into the record and
playback signal paths just as with the Cat. No. 22. The
Auto Compare sequence is available only when playing
back previously recorded Dolby Noise.
Model
362
The Model 362 is designed to hold two Cat. No. 22 A-type
modules, and functions as a dual-channel Model 360.
Due to height restrictions, two standard Cat. No. 280
modules cannot be installed. To incorporate Dolby SR in
the Model 362, a special adaptation, the Cat. No. 280-L,
must be used. The Cat. No. 280-L contains the two
circuit cards that make up the standard Cat. No. 280
module, with one mounted behind the other. This
results in a module that is of single thickness but double
length, therefore extending forward of the front panel by
7t inches (190 mm).
M-Series
The M-Series chassis was designed to hold 16 Cat. No. 22
A-type modules. Two chassis can be linked to
accommodate 24 or 32 channels. It is possible to use
Dolby SR in all channels, provided that special covers

are installed on the Cat. No. 280 modules used in
channels 1, 10 and, in the M-24 unit, channel 18.
Cat. No. 280 modules with standard covers can be used
in all other M-series channels. The special cover,
referred to as the Cat. No. 359, will be available through
Dolby product distributors.
SP/XP Series
The greater thickness of the Cat. No. 280 prohibits its
use in the SP Series. Additionally, there are some
electrical incompatibilities with the Cat. No. 230 carrier
card. The Cat. No. 431, due in late 1986, will fit both the
SP and XP multi-track units. This module will be a
plug-in replacement for the entire Cat. No. 230/22
combination in existing SP units and for the
Cat. No. 331 A-type module in the newer XP frames.
Cinema Products (CP50, CPI00, CP200)
The Cat. No. 280 requires minor modification for use
within cinema units. In a few special situations,
however, it may be necessary to use a Cat. No. 280 in a
CP50, CP100 or a CP200. In this case, contact Dolby
Laboratories for more information.

2.1 General Installation
2-1
SECTION 2
CAT. NO. 280 INSTALLATION
If the Dolby interface frame has been previously aligned for use with the
Cat. No. 22 A-type module, the system is basically ready for use. Installation is,
therefore, simply a matter of removing the existing Cat. No. 22 from the Dolby
interface frame, setting the configuration switch on the Cat. No. 280, and
plugging in the module.
For optimum performance of the Dolby SR circuits, however, calibration of the
recording system and interface frame input and output levels is recommended
and is covered in Section 4. If the interface frame (360 Series or M Series) is being
installed at the same time, refer to the appropriate interface frame manual for
information regarding connections, etc.
The Cat. No. 280 module consumes roughly the same amount of power as the
Cat. No. 22 A-type module and, therefore, has the same cooling requirements. It
is important that adequate free air be provided around the Dolby interface frame,
particularly the M-Series unit, which should always have a blank panel above
and below the chassis.
2.2 Configuration Switch and Indicators
Before installing the Cat. No. 280, the user should be familiar with the basic
function of the switches and indicators mounted on the module. There are three
indicators and one switch on the Cat. No. 280. The configuration switch should be
set for the appropriate usage condition.
• • • Set s~itCh to
360 M 361 usage mode
On 0 II-i
SR DOLBY SPECTRAL RECORDING Cat. No. 280
U 0 1.17 Dolby laboratories Inc. San francIsco and London
~~--~~~---,--~------~~--------~--------~---------~---~~-~~~-
D(] D lb ·
SR DOLBY SPECTRAL RECORDING
o ry Cat. No. 280 Module
GREEN

2-2
Cat. No. 280 Configuration Switch
Selects usage mode. (360,361, M Series)
This switch is found on the rear of the Cat. No. 280
module. It selects the appropriate internal logic for
correct operation of the Auto Compare circuits. In
general, it should be set to match the type of interface
frame containing the Cat. No. 280 module. An exception
exists, however, when Model 361 frames (or channels of
an M-Series unit) are used for dedicated encode or
decode operation, as if they were Model 360 units. In
this case, the configuration switch should be set to the
"360" position.
Since the Model 362 functions as a dual-channel Model
360, the configuration swith on the Cat. No. 280-L
should be set to the "360" position.
If the Cat. No. 280 module is plugged into a frame with
this switch incorrectly set, the Auto Compare circuit
may not operate correctly. No damage will occur, and
the Dolby SR circuits will continue to operate correctly.
The tape will always be properly encoded or decoded
regardless of the position ofthe configuration switch.
Cat. No. 280 Indicators
YELLOW indicator.
Dolby Noise ON
The yellow indicator is ON whenever the Dolby
Tone/Cal button on the interface frame is pressed. The
Dolby Tone/CAL button is used to control the generation
of Dolby Noise and to enable the Auto Compare mode.
RED and GREEN Indicators.
Reference (R~~n) ITape (GREEN)
These two LEDs indicate the source of Auto Compare
audio. RED ON indicates the internal reference pink
noise. GREEN ON indicates the Dolby Noise being
played off the recorder. Neither LED is ON if Auto
Compare is inactive.
2.3 Interface Frame Switch and Meter Labeling
The Cat. No. 280 card has been designed to be installed in interface frames
originally designed for the Cat. No. 22 A-type NR module; therefore, several
switches are labeled inappropriately. For example, with the Cat. No. 280
installed, the switch labeled Dolby Tone/CAL is used to select Dolby Noise, and
the switch labeled NR ON/OFF is used to switch ON or OFF the Dolby SR process.

2.4 Optional (i:xternal Connections
The main edge connector on the Cat. No. 280 has terminals which can be used for
remote display of the Auto Compare status indicators. The reference pink noise is
available at the edge connector for other uses. Additionally, a terminal has been
provided that will allow Auto Compare circuits to be synchronized. The user is
encouraged to use these optional connections to make the use of Dolby SR and
Auto Compare more convenient. Appendix B gives specific information on using
these options.

3-1
SECTION 3
CAT. NO. 280
CALIBRATION -- DOLBY NOISE
This section describes Dolby Noise, Auto Compare and their use during
calibration. Alignment and operation of the Cat. No. 280 in specific Dolby
interface frames is covered in Section 5 -- Practical Operation.
3.1 Dolby Noise
Dolby Noise is an alignment signal derived from internally generated pink noise.
The pink noise is generated at an accurate level with respect to the Cat. No. 280's
internal signal levels and then interrupted every two seconds by a 20 ms gap of
silence. These periodic interruptions (sometimes called "nicks") serve to
positively distinguish Dolby Noise from other pink noise. These interruptions
also provide the synchronization signal for the Auto Compare circuit used during
playback.
The Cat. No. 280 is switched into the Dolby Noise mode whenever the "Dolby
Tone"
or "Cal" button on the interface frame is pressed. The YELLOW LED on
the front of the Dolby SR module will be ON.
When making a recording using the Dolby SR process, the user should always
record a short section of Dolby Noise at the beginning of each reel (just as is done
with Dolby tone for A-type recordings). This Dolby Noise positively identifies the
recording as being Dolby SR encoded, and provides an accurate alignment
reference for the decoder. When Dolby Noise is recorded (the "Dolby Tone" or
"Cal" button on the interface frame is pressed), the Dolby Noise is sent to the
recorder at 15 dB below Dolby Level. This lower level is compensated by gain in
the calibration meter amplifier so that Dolby Noise will read on the Dolby Level
dot in the center of the calibration meter. In other words, the level of the Dolby
Noise
on the calibration meter will read very low during normal playback, but
will read Dolby Level--the center dot--when the Dolby Tone/Cal button is pressed.
3.2 Auto Compare
The Auto Compare feature provides a simple and convenient way of quickly
verifying the recording channel. During the Auto Compare mode, the internal
reference pink noise and the recorded Dolby Noise are alternately switched to the
monitor output on four-second intervals. This provides, in effect, a continuous
A/B comparison between the reference pink noise generator and the Dolby Noise
off the tape.
The Auto Compare feature in the Cat. No. 280 module allows the easy
identification of errors in level, high and low frequency response (including wrong
equalization settings), azimuth, or any other audible conditions.
The Dolby Noise recorded on the tape has interruptions every two seconds. The
internal reference noise, on the other hand, is continuous and does not have the
interruptions. This leads to an easily identifiable pattern of four seconds of
continuous reference noise and four seconds of the interrupted or "nicked" Dolby
Noise.
After several initial alignment sessions, the "nicked/continuous" pattern
is easily identified as a tape/reference pattern, even though the two noises may
sound identical-- which they will, if the recorder and Dolby SR levels are aligned
properly. Two LEDs on the front of the Cat. No. 280 indicate whether the monitor

3-2
is receiving the internal reference nOIse (RED) or the Dolby Noise from the
recorder (GREEN).
3.3 Auto Compare Control
Auto Compare is controlled by the Dolby Tone/Cal button on the interface frame
and the status of the tape recorder. An Auto Compare sequence will
automatically begin IF the recorder is playing Dolby Noise AND the Dolby
Tone/Cal button on the interface frame is pressed. In other words, when the
Dolby Tone/Cal button is pressed, the Cat. No. 280 automatically recognizes
Dolby Noise
at its input; the four- second alternating reference/tape sequence will
automatically begin.
When a Cat. No. 280 is installed, the Dolby Tone/Cal button is used only in
association with Dolby Noise and has no application with other tones or program
material. If the button is inadvertently left pressed while playing program
material, the tape will not be decoded and the Auto Compare circuit may
occasionally be triggered by transient program material, resulting in bursts of
reference noise being sent to the monitor. If the Dolby Tone/Cal button is
released, the situation returns to normal and the tape will be correctly decoded.
The exact operation of the Auto Compare feature depends on which Dolby
interface frame the Cat. No. 280 has been installed in and whether the interface
frames are used for dedicated encode/decode or changeover operation. The mode
of
operation is selected by the rear switch on the module. These specific
differences
are discussed in Section 5 -- Practical Operation.
3.4 Auto Compare Metering
The calibration meters in the interface frames (360 Series and M Series) were
designed to read correctly on sine waves at a single level--specifically Dolby
Level.
In order for Dolby Noise to produce an accurate and reliable meter
reading, the Dolby Noise is band limited to reduce meter bounce and reduce the
effects of recorder frequency response errors at extreme frequencies. The meter
signal is then amplified so that Dolby Noise reads Dolby Level--the dot--on the
calibration meter. When the Dolby Tone/Cal button is not pressed, the meter
functions normally, as with a Cat. No. 22 installed.
An important point to remember:
Although the Dolby Noise is always recorded at -15 dB with respect
to the established Dolby Level, it is metered at Dolby Level on the
calibration meter whenever the Dolby Tone/Cal button is pressed.
3.5 Auto Compare Synchronization
In currently available Dolby interface frames, the Auto Compare switching
circui ts are not synchronized between channels, unless the user has modified the
interface frame as suggested in Appendix 1. In the Dolby SR module for the SP
and XP Series, the Cat. No. 431, Ref/Tape switching will be synchronized between
channels. Regardless of whether synchronization is provided, it is important to
remember to listen to only one channel at a time for valid Auto Compare
information.

3.6 Dolby Tone and the Cat. No. 280
Dolby Tone is used to establish the decode level for Dolby A-type NR and has no
application in setting the decode level for Dolby SR. When the Dolby Tone/Cal
button is pressed with the Cat. No. 280 installed, the calibration meter on the
Dolby frame cannot be used for reading external tones because of the band
limiting and gain used. When the Dolby Tone/Cal is not pressed, the calibration
meter functions normally as if a Cat. No. 22 were installed, without the extra
gain and band limiting.
If the Dolby interface frame and the tape recorder have been properly aligned
using external tones, Dolby Noise will be correctly set for the proper decode level
during playback. (Dolby Noise will read Dolby Level on the calibration meter
when the Dolby Tone/Cal button is pressed.) If tapes are played that have been
recorded with the tape recorder set up for a different reference flux level, the
"Play Cal" on the recorder (or, sometimes more conveniently, the input
potentiometer on the interface frame) will have to be adjusted to obtain the Dolby
Level
reading while the Dolby Noise is being played AND the Dolby Tone/Cal
button is depressed.
Note:
As a safeguard, Dolby SR levels should always be
established using Dolby Noise, even though the recorder
and overall system may have been carefully aligned
using tones. System levels are set using tones and test
tapes in the conventional manner with the Dolby SR
processing switched off (NR OFF on the associated
interface frame). Following that, the Dolby SR level
alignment is checked using Dolby Noise with the Dolby
Tone/Cal button pressed. As Auto Compare provides the
user with an accurate reference for setting playback
frequency response and decode level, the user should
always attempt to get the Dolby Noise that has been
recording on the tape to audibly match the reference
nOise.

4-1
4.1 General System Alignment
SECTION 4
SYSTEM ALIGNMENT
This section covers general recording levels for optimum performance of Dolby
SR. As covered in Section 3, the signal levels through the Dolby interface frame
should be set using external tones and test tapes with the Dolby SR switched OFF
(NR OFF on the interface frame).
4.2 Recording Levels for Dolby SR
Dolby Level can be established anywhere in the 100 n Wblm through 500 n Wblm
region, and the Dolby SR system will give good results in all practical recording
situations. Because of the overload characteristics associated with modern
recording tapes, however, it is recommended that Dolby Level be established in
the 185 n Wblm or 200 n Wblm region in order to achieve the very highest
measured signal-to-noise ratio performance.
The Dolby SR process is capable of giving magnetic recording the capacity for
extraordinarily high dynamic range, in some cases as high as 100 dB for 15 IPS
1I4-inch tape. In order to measure such a high signal-to-noise ratio (under test
bench conditions), the noise and overload points of the tape and that of the
associated electronic circuits in the recording system (including the Dolby
interface frame and the Cat. No. 280 module) must be reasonably matched. In
most real recording situations with Dolby SR, the overall performance of the
recording system is limited by the microphone pre-amplifiers or the ambient
noise level found at the recording location, so such attention to precise matching
is unwarranted.
The basic Cat. No. 280 based Dolby SR system (encode-decode) has a clip-to-noise
window of approximately 107 dB. The internal reference level of the Dolby SR
electronics, or Dolby Level, is set to be about 20 dB below the internal clip point
and about 85 dB above the electronic noise floor of the system. Modern recording
tape has a saturation point around 15 dB above 185 nWb/m; therefore, if 185
n
Wblm or 200 n Wblm is the chosen reference, tape saturation will occur just
before the Cat. No. 280 clips, and the noise floor measured will be essentially that
of the tape -- the desired effect. Recording studios that use 185 n Wblm or 200
n Wblm as a reference can continue to record using already established
procedures, and obtain maximum benefits of Dolby SR.
Note:
While the clip point of the SR circuits themselves is
about 21 dB above Dolby level, the actual headroom
through the system may be limited by the line amplifier,
the interface frame, and how the output level has been
adjusted. For example, when the Cat. No. 280 is
installed in the Model 360 Series, the maximum output
level is +21 dBm into 600 ohms -- that is, 17 dB above
Dolby Level. If the output potentiometer has been set so
that Dolby Level is equal to + 8 dBm, (a common 0 VU
output level for U.S. broadcast applications), the
headroom will be limited to 13 dB above Dolby Level.

4-2
Listed below are the maximum output levels of Dolby
interface frames with the Cat. No. 280 module installed:
360 Series
M Series
+20 dEm into 200 ohms
+21 dEm into 600 ohms
+22 dEm into 10 K ohms (bridging)
+21 dEm in to 200 ohms
+22 dEm into 600 ohms
+23 dEm into 10 K ohms (bridging)
4.3 General Alignment Using VU Meters
The following is a general alignment procedure for use with external VU meters
and reference tapes with a 185 n Wblm or 200 n Wblm test tape:
1. Switch the Dolby SR process OFF (NR OFF on the associated Dolby
interface frame).
2. Play the reference level tape on the recorder.
3.
Adjust the Dolby frame "input" potentiometer so the calibration meter on
the interface frame reads the dot in the center -- Dolby Level.
4.
Adjust the "output" potentiometer to restore reference level at the Dolby
interface frame's output.
5. Send a tone at reference level from the console to align the record aspects of
the Dolby interface frame. (With the Model 361, this has been already
accomplished with the steps above.)
6.
Adjust the record sensitivity of the recorder to give a Dolby Level reading on
the Dolby frame calibration meter.
7. With the Dolby Tone/CAL button depressed, record a short section of Dolby
Noise.
Play this section back and verify that it reads Dolby Level on the
calibration meter.
Note:
The Dolby SR circuits in the Cat. No. 280 have been set
to have unity gain at Dolby Level within the 700 - 1 kHz
region. If the external tone is in this frequency band, it
will make no difference if the alignment is done with or
without the Dolby SR process switched in. Tape recorder
frequency response measurements and equalization
adjustments should always be done with Dolby SR
switched off, as there will be noticeable deviations
outside the 700 - 1 kHz region. As Dolby A-type must be
switched off for alignment, it is probably a good practice
to be consistent, and turn Dolby SR off as well.

4-3
4.4 Using VU Meters with High Level Reference Tapes
It has become common practice for many U.S. studios to use a reference flux level
of 250 nWblm, 261 n Wblm or 320 n Wblm, any of which can be established as a
reference level for Dolby SR with good results. Some European studios, however,
have established an alignment tone that is closely related to "peak recording
level" and can be as high as 1000 n Wblm, a few dB below tape saturation. If such
a high flux level is set to equal Dolby Level in the interface frame, much of the
headroom of Cat. No. 280 will go unused and the noise floor will be influenced by
that of the Dolby SR processing electronics; the full effect of the Dolby SR process
will
not be obtained. In order to get the maximum measured performance from
Dolby
SR when using such a high flux level reference tape, it is necessary to alter
the suggested alignment procedure. The following are two possible solutions,
either of which will give acceptable results:
a. Establish a secondary tone, lower than the reference flux level by a
fixed
amount, that will serve as the Dolby calibration tone. The
secondary tone will read lower on the recorder and console meters but
at Dolby Level--the center dot--on the interface frame. The reference
alignment tone will read high, or even off scale, on the Dolby
calibration meter.
b. Always align the Dolby interface frame with the high flux level
reference tone to the "DIN" dot on the upper part of the Dolby
calibration meter. The "DIN" indication is placed approximately 5 dB
above Dolby Level on the Dolby calibration meter. (Some older meters
may be labeled "32" or not have any upper mark at all.) Dolby Noise
will still be generated and metered at Dolby Level--the center dot.
Regardless of the specific procedure used to set reference level through the Dolby
interface frame, Dolby Noise must be set to read Dolby Level--the center dot--on
the Dolby calibration meter. It is important to remember that the level of Dolby
Noise
is pre-set to 15 dB below the established Dolby Level at the tape recorder.
The Dolby Noise is automatically restored to read Dolby Level when the Dolby
TonelCAL button is pressed. When playing back previously recorded Dolby
Noise,
the Dolby TonelCAL button should be pressed and, if necessary, the "Play
Cal" on the recorder adjusted so the Dolby calibration meter reads Dolby level.
4.5 Aligning with Peak Program Meters
Like VU meters, peak program meters can be used to set signal levels through the
Dolby interface frame. Peak program meters are widely used, especially in
Europe, because their faster rise times more accurately indicate the peak level of
complex program material, while VU meters read the average program level.
Studios using peak reading meters, therefore, need less headroom between their
100% reference level and the clipping or overload point on the tape. It is typical
when aligning Cat. No. 22 Dolby A-type modules to set Dolby Level about 5 dB
below
the 100%, or "peak reference" alignment tone, normally used by some
studios. In the case of Dolby SR, much of the available headroom in the
Cat. No. 280 will go unused and the measured noise floor will be influenced by the
Dolby SR processing electronics. In order to obtain the full benefits of the Dolby
SR process when aligning with peak reading meters, Dolby Level should be
established at roughly 10 - 15 dB below the peak indications on the meters. This
should, in most cases, result in a flux level of around 200 n Wblm being chosen for
Dolby Level.

4-4
The following is a general alignment procedure that should be followed when
aligning Dolby SR using external peak program meters:
1. Switch the Dolby SR process OFF (NR OFF on the associated Dolby
interface frame).
2. Send a tone from the console at the chosen Dolby Level (10 to 15 dB below
100%) or if adjusting the playback side, playa tape recorded at this level.
3. Adjust the Dolby interface frame "input" potentiometer so the calibration
meter reads the dot in the center -- Dolby Level.
4.
Adjust the "output" potentiometer to restore unity gain through the Dolby
interface frame.
5. Send a tone at reference level from the console to align the record aspects of
the Dolby interface frame. (With the Model 361, this has been already
accomplished with the steps above.)
6.
Adjust the record sensitivity of the recorder to give a Dolby Level reading on
the Dolby frame calibration meter.
7. With the Dolby Tone/CAL button depressed, record a short section of Dolby
Noise. When it is replayed, verify that it reads Dolby Level on the
calibration meter.
Note:
Various peak reading meters use different dynamic
characteristics and scales to indicate peak level. When
aligning a Dolby interface frame with external peak
reading meters, remember that reference level actually
corresponds to a numerical indication on the scale that
indicates the studio's standard operating level. For
optimum performance of the Dolby SR circuit, the level
which corresponds to the center dot on the Dolby
calibration meter should be about 10-15 dB below the
maximum program level indicated on the peak reading
meter.

5-1
SECTION 5
PRACTICAL OPERATION
There are differences in the detailed alignment procedure and the operation ofthe
Auto Compare circuit between the Model 360, Model 361 and M-Series units;
therefore, a separate procedure is provided for each. Please refer to the
appropriate section on the following pages.