dbx 128 User manual

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HOW TO USE THIS MANUAL .
To achieve maximum benefit from your 128, we
recommend that you read the entire dbx 128 Operator’s
Manual and keep it handy for future reference. However, if
you are familiar with expansion and compression, and with
the use of dbx II tape noise reduction, the FUNCTIONAL
CALLOUTS and BRIEF OPERATING INSTRUCTIONS
should provide all the information you need to connect
and operate the 128. These sections also serve as quick
reference guides for occasional review.
The INTRODUCTION explains why and when to use
dbx {I tape noise reduction, expansion, compression, peak
limiting and peak unlimiting. Combination applications are
also covered. The remainder of the 128 Operator’s Manual
is a detailed set of connection diagrams, operating
instructions and explanations of the 128’s functions.
WARNING: TO PREVENT FIRE OR SHOCK
HAZARD, DO NOT EXPOSE THIS APPLIANCE
TO RAIN OR MOISTURE.
INDEX
FUNCTIONAL CALLOUTS 2
BRIEF OPERATING INSTRUCTIONS 4
INTRODUCTION TO THE dbx 128 6
WHEN TO USE COMBINATION APPLICATIONS 9
“DETAILED OPERATING INSTRUCTIONS —————~<C~s~“‘C=S:*«SC
“CONNECTIONS —————...—.
OPERATION
14
“HINTS TOGETTHEMOSTOUTOF ——S20~
dbx TAPE NOISE REDUCTION
“USING THE DYNAMIC RANGE ENHANCER SYSTEM 21
HOW TO SET UP COMBINATION APPLICATIONS 24
SYSTEM PERFORMANCE 29
TECHNICAL DETAILS OF dbx PROCESSING 30
“COMPRESSOR/LIMITER AND EXPANDER, ~—~—=«33
A SUMMARY
SIMPLE PRECAUTIONS WHEN USING ANY 35
PROGRAM EXPANDER
SPECIFICATIONS 7 36
SCHEMATIC 38
dbx PRODUCT WARRANTY 39
FACTORY SERVICE 39
dbx GLOSSARY I

FUNCTIONAL CALLOUTS
Fig. 1 — Front Panel Callouts
BELOW, ABOVE THRESHOLD (L.E.D.’s)
When the amber indicator is illuminated, the
input level to the 128 is below the threshold
level adjusted by the LEVEL control. When
the red indicator is illuminated, the input
level to the 128 is above the threshold level.
The L.E.D.’s do not measure output tevel.
DYNAMIC RANGE ENHANCER: PRE, POST
(switches} When the 128 is in TAPE mode and the
PRE switch is depressed, compression or expansion
takes place ahead of the tape recorder’s input (and
before tape noise reduction encoding if the 128
is in REC mode); when the POST switch is
depressed, compression or expansion takes place
after the tape machine's output (and after tape
noise reduction decoding if the 128 is in PLAY
mode). When the 128 is in DISC mode press the
POST switch.
PLAY LEVEL MATCH (control) Adjust
this control if the playback level from a
dbx I] encoded tape is considerably differ-
ent from the playback level of a non-
encoded tape played back in the bypass
PILOT (L.E.D.
indicator light)
Shows that the AC
power is ON
(illuminated), or
OFF (dark).
POWER (switch)
Controls the AC
power to the dbx
128. Push once
for ON, again
for OFF.
US
NY
Gi
¥
rd
DYNAMIC RANGE
ENHANCER
Lonoise reouction I
ABOVE THRESHOLD/LINEAR
(switch) In the LINEAR position
(switch in), the compression/
expansion section of the 128
operates in a normal fashion over
the entire program dynamic range
(see Figure 14 A & C). In the
ABOVE THRESHOLD position
(switch out), the compression/
expansion section acts only on
program levels that exceed the
threshold set by the LEVEL con-
trol (see Figure 14 B & D). This
changes the 128 into a peak limiter
(corresponding to COMPRESS
settings of the COMPRESS/EXPAND
control) or a peak unlimiter (cor-
responding to EXPAND settings of
the COMPRESS/EXPAND control).
Passages.
LEVEL (control) Sets the
“threshold’’ level above
which compression or ex-
pansion begins when the
ABOVE THRESHOLD/
LINEAR switch is in the
ABOVE THRESHOLD
position. When the ABOVE
THRESHOLD/LINEAR
switch is in the LINEAR
position, set the LEVEL
control so that ABOVE
THRESHOLD illuminates
during loud passages, and
BELOW THRESHOLD
illuminates during quiet
VANCE
COMPRESS/E XPAND (control)
Counterclockwise rotation of this
control past the 1.0 setting turns the
128 into a compressor with a ‘‘com-
Pression ratio” given by the numbers
on the surrounding scale.* Clockwise
rotation of the control past the 1.0
setting turns the 128 into an ex-
pander with an ‘expansion ratio”
given by the scale numbers.* When
the COMPRESS/EXPAND control is
set at the “1.0” setting (12 o'clock),
the 128 does not compress or
expand, and signals pass unchanged.
mode. The setting is not critical.
TAPE NOISE REDUCTION: REC, BYP, PLAY
(switches) When the REC switch is depressed, the
128's tape noise reduction circuitry is ready for
recording and encoding a tape with dbx {I tape
noise reduction. When the PLAY switch is de-
pressed, the 128's tape noise reduction circuitry
is ready for playback and decoding of an
encoded tape. When the BYP switch is depressed,
the 128's noise reduction circuitry is bypassed for
recording or playback of non-encoded tapes.
TAPE/dbx DISC (switch) Set this switch to
the DISC position, and press the PLAY
switch for playback of dbx II encoded discs.
The TAPE/dbx DISC switch should be in
the TAPE position for recording or play-
back of dbx II encoded tapes.
*The term “ratio,” whether associated with
“expansion” or “compression,” refers to the
amount of change in output /evel versus the input
level. A compression ratio of 2:1 (a COM-
PRESSION setting of 2.0) means that for a 2dB
change in input level there is a 1dB change in the
output level. An expansion ratio of 1:2 (an
EXPANSION setting of 2.0) means that for a
1dB change in input level there is a 2dB change
in the output level. A ratio of 1:1 (a setting of
7.0) suggests neither compression nor expansion,
so that for any change in input level there is a
corresponding change in the output level.

. RECORD LEVEL This screw-
Fig. 2 — Rear Panel Callouts driver control is factory
adjusted to match the record
levels encountered in most
home stereo equipment. Ad-
justment is not critical for
encode/decode performance; PLAY INPUTS
if you have too much or too Connect the cables
little level at the input to from your tape
your tape recorder, see page 14 : recorder “playback”
for adjustment instructions. (or ‘‘“monitor’’) out-
puts to these inputs.
q
4
WH
QUAD COUPLER For
compressing or expanding
four channel programs,
connect the QUAD
COUPLER jacks of two
dbx 128’s together. This
allows the compression/
expansion circuits in both
128’s to track” each
other so that the ‘‘quad
image” remains stable.
if
}
RECORD
HOSS
LEANING
i
ROM PREAMP TO TAPEBRECORDER FROM TAPE
TAPE OUTPUT AUXEBARY OR RECORDER
LINGINPUT OUTPUT
SERIAL NO.
A
f
py
WY
PLAY OUTPUTS Connect the cables
RECORD
INPUTS
For
connec-
RECORD
OUTPUTS
from
these
outputs
to
the
tape
(‘’play-
tion
of
the
cables
from
your
Connect
the
cables
back"’)
input,
preamp
input,
or
tape
preamp’s
record
output
or
tape
from
these
outputs
into
monitor
inputs
of
your
preamplifier
output.
If
there
is
no
tape
out-
the
“record”
inputs
of
amplifier
or
receiver.
Before
installing
put,
use
the
main
preamp
out-
your
tape
machine
the
dbx
128,
these
cables
came
directly
put.
Before
installing
the
dbx
(line
level
or
auxiliary
from
the
tape
recorder
“playback”
(or
128,
these
cables
were
plugged
inputs,
not
the
mic
“monitor’’)
outputs.
(Figure
3,
Page
7
directly
into
the
‘‘record’’
inputs
inputs).
illustrates
these
connections.)
“
of
your
tape
machine.
Micro-
7
phones
cannot
be
connected
NOTE:
Never
connect
any
other
directly
to
the
128;
a
preampli-
electronic
equipment
between
the
128
fier
or
mixer
must
be
used
to
and
the
tape
recorder.
See
page
9
provide
line-level
signals
to
for
connection
of
a
graphic
equalizer,
the
128.
reverb
unit,
etc.

BRIEF OPERATING INSTRUCTIONS
To Play a dbx-Encoded Phonograph Disc
1. Set your preamplifier’s INPUT SELECTOR Switch to
PHONO, and the TAPE MONITOR Switch to TAPE.
2. Press the 128’s DISC, PLAY and POST Switches,
and set the COMPRESS/EXPAND Control to the ‘’1.0’
Position.
3. Play the disc and adjust the preamplifier’s VOLUME
Control for the desired listening volume. If the volume is
considerably louder or quieter than normal, adjust the
128’s PLAY LEVEL MATCH Control.
To Make a dbx-Encoded Tape
1. Set your preamplifier’s INPUT SELECTOR Switch
to the appropriate source (i.e. PHONO, TUNER or AUX),
and the TAPE MONITOR Switch to TAPE.
2. Press the 128’s TAPE, REC and POST Switches, and
set the COMPRESS/E XPAND Control to the ‘1.0’
Position, Do not switch the 128 while recording.
3. With your tape machine in the Record Ready (Pause)
mode, set recording levels for peaks typically 3 to 6 VU
lower than the levels normally used. (With dbx processing,
significantly lower recording levels are often required to
avoid tape saturation, particularly in cassette recording.
Experiment during the initial setup to find the appropriate
levels.)
To Play a dbx-Encoded Tape
1. Set your preamplifier’s TAPE MONITOR Switch to
TAPE.
2. Press the 128’s TAPE, PLAY and PRE Switches,
and set the COMPRESS/EXPAND Control to the ‘1.0’
position.
3. Play the tape. If the volume is considerably louder
or quieter than normal, adjust the 128’s PLAY LEVEL
MATCHControl.
Fig. 3 — Basic Hook-Up Diagram

To Expand a Program
1. Set your preamplifier’s INPUT SELECTOR Switch
to the appropriate source (i.e. PHONO, TUNER or AUX),
and the TAPE MONITOR Switch to TAPE.
2. Press the 128’s DISC, BYP and POST Switches, and
set the ABOVE THRESHOLD/LINEAR Switch to the
LINEAR position (IN).
3. Set the COMPRESS/EXPAND Control to the desired
degree of expansion (usually between 1.2 and 1.4). Do not
over-expand, especially on classical music.
4. Set the LEVEL Control so that the ABOVE
THRESHOLD LED lights on loud passages and the
BELOW THRESHOLD LED lights on quiet passages.
To Peak Unlimit a Program
1, Set your preamplifier’s INPUT SELECTOR Switch
to the appropriate source (i.e. PHONO, TUNER or AUX),
and the TAPE MONITOR Switch to TAPE.
2. Press the 128’s DISC, BYP and POST Switches, and
set the ABOVE THRESHOLD/LINEAR Switch to the
ABOVE THRESHOLD position (OUT).
3. Set the COMPRESS/EXPAND Control to the desired
degree of peak expansion (usually between 1.2 and 1.6).
4. Set the LEVEL Control so that the ABOVE
THRESHOLD LED lights-only on musical peaks, or for
the desired effect.
To Compress a Program
1. Set your preamplifier’s INPUT SELECTOR Switch
to the appropriate source (i.e. PHONO, TUNER or AUX),
and the TAPE MONITOR Switch to TAPE.
2. Press the 128’s DISC, BYP and POST Switches, and
set the ABOVE THRESHOLD/LINEAR Switch to the
LINEAR position (IN).
3. Set the COMPRESS/EXPAND Control to the desired
degree of compression (usually between 1.2 and 3.0). Too
much compression can cause unwanted side effects.
4. Set the LEVEL Control so that the ABOVE
THRESHOLD LED lights on loud passages and the
BELOW THRESHOLD LED lights on quiet passages.
To Peak Limit a Program
1. Set your preamplifier’s INPUT SELECTOR Switch
to the appropriate source (i.e. PHONO, TUNER or AUX),
and the TAPE MONITOR Switch to TAPE.
2. Press the 128’s DISC, BYP and POST Switches, and
set the ABOVE THRESHOLD/LINEAR Switch to the
ABOVE THRESHOLD position (OUT).
3. Set the COMPRESS/EXPAND Control to the desied
degree of peak limiting (usually 10.0 or higher).
4. Set the LEVEL Control so that the ABOVE.
THRESHOLD LED lights only on musica! peaks, or for
the desired effect.
To Record with Peak Limiting and
dbx I! Tape Noise Reduction
1. Set your preamplifier’s INPUT SELECTOR Switch
to the appropriate source (i.e, PHONO, TUNER or BUN).
and the TAPE MONITOR Switch to TAPE.
2. Press the 128’s DISC (IN) BYP and POST Switches,
and set the ABOVE THRESHOLD/LINEAR Switch to the
ABOVE THRESHOLD position (OUT).
3.
Set
the
COMPRESS/EXPAND
Control
for
the
desired
degree
of
compression.

4. Set the LEVEL Control so that the ABOVE
THRESHOLD LED lights only on musical peaks, or for
the desired effect.
5. Now press the 128’s TAPE (OUT) REC and PRE
Switches.
6. With your tape machine in Record Ready (Pause)
mode, set its input level controls.
7. Record the tape.
8. To play back the tape just recorded, press the 128’s
TAPE (OUT), PLAY and PRE Switches, and set the
COMPRESS/EXPAND Contro! to the 1.0 position. Play
the tape.
To Play a dbx-Encoded Tape or Disc and Add Peak Limiting
1. Press the 128’s PLAY and POST Switches, and either
the DISC or TAPE Switch depending on the source.
2. Set the ABOVE THRESHOLD/LINEAR Switch to
the ABOVE THRESHOLD position (OUT).
3. Set the COMPRESS/EXPAND Control for the desired
degree of peak limiting (usually 10.0 or higher).
4. Set the LEVEL Control so that the ABOVE
THRESHOLD LED lights only on musical peaks, or for
the desired effect.
5. Play the tape or disc.
INTRODUCTION TO THE dbx 128
A live musical performance is an exciting experience,
the kind of experience that record manufacturers and radio
stations attempt to capture for reproduction in home
listening.
Unfortunately much of the excitement of the original
live performance is lost, even when using the best speakers,
amplifiers, tuners, turntables, and tape machines. The
primary reason for this lack of excitement and realism is
the dynamic range limitations in the recording and
broadcast mediums.
Dynamic Range and Why It Is Important
Dynamic range can be defined as the difference in
sound level, measured in dB, between the loudest and the
quietest passages in music. Acoustical loudness is measured
in decibels of sound pressure level, or dB SPL. The dB
expresses the smallest change in sound level that the average
ear can detect. 0 dB SPL is the threshold of human hearing,
the faintest sound most people can hear at a midrange fre-
quency (1,000 Hz). The loudest sound the average person
can tolerate without physica! pain is approximately 120 dB
SPL (some people can tolerate 130 dB, others leave the
room when the sound level reaches 110 dB). Thus, human
hearing has a maximum dynamic range of about 120 dB SPL.
Noise is the jower restriction of the dynamic range of
most recorded material. Typical sources of noise are phona-
graph disc surfaces, turntable rumble, tape hiss, and FM
broadcast hiss. When the original program’s quietest sounds
are too low in level, they are buried in the noise. ““Rumble”
and “scratch” filters can remove some of this noise, but
unfortunately, they also remove parts of the program. A
simple but effective way to remove or reduce noise
significantly without subtracting anything from the music
has long been sought. One method to avoid noise-imposed
restrictions in the recording and broadcast mediums would

be to raise the overall program level, allowing the louder
sounds to become even louder. However, when the pro-
gram’s loudest sounds are too high in level, they cause
distortion. Thus, the difference between the noise level
and the maximum undistorted level of a tape machine, disc,
or radio broadcast is not wide enough to accommodate the
full dynamic range of a live performance.
To get around the inherent dynamic range limitations in
a given medium, the original program is often ‘‘compressed.”’
This means that its dynamic range is restricted by making
the loud passages quieter and/or the quiet passages louder.
Compression thus avoids the dynamic range limitations of
the medium by reducing the dynamics of the program.
Compression is effective, but unfortunately it also removes
the program’s excitement and realism. Given the necessity
for compression, engineers long ago realized that if there
were some way to reverse the process, restoring the
dynamic range of the compressed program,the quality of
recorded music could be significantly improved. Until
recently, this ‘reverse compression” process, known as
“expansion,” had not been successfully applied in any
consumer product.
dbx has developed techniques that make a dramatic
improvement in the dynamic range of any recorded musical
material: (a) the patented dbx II Tape Noise Reduction
System for tape machines and phonograph discs, and
(b) the linear decibel compression/expansion system. The
dbx 128 incorporates both of these systems which may be
used independently or in combination with each other.
You can use the 128’s dynamic range enhancer to reduce
the noise of records or FM broadcasts, or you can capture
all of the widest range program material on your conven-
tional cassette or reel-to-reel tape recorder using dbx II
tape noise reduction.
dbx Il Tape Noise Reduction
dbx 11 tape noise reduction circuits ‘‘encode”’ a program
before it is recorded on a tape recorder and “‘decode” it
upon playback. This process reduces tape noise to inaudi-
bility and retains the original program’s dynamic range.
This means that a recorded program will not have any more
audible noise than the original source. Also, the dbx-
encoded program will retain the dynamics and excitement
of the original source without causing tape saturation.
When to Use the dbx I! Tape Noise Reduction System
Use the 128’s dbx tape noise reduction circuitry to
avoid
audible
tape
noise
and
increase
headroom
when
you
record a tape of a live performance, dub (copy) a con-
ventional disc onto a tape, or record any other source,
such as an FM broadcast.
The 128’s tape noise reduction circuits can also be used
to
decode
a
dbx-encoded
phonograph
disc.
dbx
encoded
discs have been recorded with dbx I! noise reduction
processing, and already have all the advantages of dbx
noise reduction.
dbx tape noise reduction processing cannot reduce noise
on an existing tape if it ha’ been recorded without dbx
encoding. Similarly, the dbx tape noise reduction circuits
cannot reduce noise on conventional phonograph records.
However, the dbx 128’s expander circuits can reduce
these types of noise, as explained in the next paragraph.
dbx Expansion and Compression
For the most part, noise in an FM broadcast or ona
phonograph disc is only audible when the program levels
are quiet. dbx linear decibel expansion increases the
program dynamics so the loud levels become fouder and
the quiet levels, inciuding the noise, become quieter.
Simultaneously, expansion restores much of the dynamic

range and, therefore, the excitement of the original per-
formance. The same circuits that do the expanding can be
used to compress a program for background music or for
making car stereo tapes, making the loud levels quieter,
and the quiet levels louder. Depending on front-panel
control settings, the linear decibel compression/expansion
circuitry can also “‘peak limit’ or “peak unlimit.” Peak
limiting is simply compression with the 128 set for “above
threshold” operation. This prevents program levels from
rising above an adjustable level (threshold), but has no
effect on levels below that point. Conversely, peak un-
limiting is simply expansion withthe 128 set for ‘‘above
threshold’”’ operation. This increases the amplitude
during the loud passages of the music, without affecting
the program below the set threshold level.
When to Use Expansion
Use the 128’s expansion capabilities to reduce the noise
present in the original source. For example, you can expand
a highly compressed and older recording and increase its
dynamic range to the point where it is comparable to your
best conventional phonograph discs and, at FM broadcast
time, significantly reduce the hiss. Or you can expand a
conventional disc and dramatically reduce surface noise
and turntable rumble at the same time you increase its
dynamics to recreate sound like a live performance.
When to Use Compression
Compression is used for recording a program which
must have a relatively constant level, as for example when
playing or recording background music. At the low listen-
ing levels of background music, quieter passages must be
kept loud enough to be heard, yet loud passages must not
be too loud. Similarly, if you are recording music for play-
back in your automobile or any other high noise area,
compression can keep the quiet sounds above the noise,
while it holds the loud sounds below distortion-causing
levels. Compression is also useful for a live recording of
several people with different voice levels. In this case, the
compressor acts like an automatic volume control and
saves you the task of “gain riding.”
When to Use Peak Limiting
If a tape must be recorded without the benefit of dbx Il
tape noise reduction (for later playback without dbx
decoding), then peak limiting is very useful because it can
prevent musical peaks from causing tape saturation. Peak
limiting can also be used to protect speaker systems from
extremely loud musical peaks, or from loud ticks or pops
on a damaged phonograph disc.
When to Use Peak Unlimiting
Many record manufacturers, and all radio stations use
some form of peak limiting in order to record or broadcast
high average program levels without having sudden musical
peaks saturate the tape or overmodulate the radio
transmitter.
“Unlimiting’’ (peak expanding) with the 128 can correct
the dynamic alteration created by peak limiting operations.
For example, suppose you have a particularly good record-
ing of a concert, one with excellent overall dynamics, how-
ever, some passages may lack the expected dynamic impact,
giving an unnaturally controlled feeling where instruments
should stand out. Such a recording probably had very little
original compression, but had peak limiting applied to high
level passages. The 128’s peak unlimiting action could make
this recording sound better.

WHEN TO USE COMBINATION APPLICATIONS
You can simultaneously use the 128’s tape noise reduc-
tion circuitry plus any one of the following: expansion,
peak unlimiting, compression or peak limiting.
When to Use dbx Ii Tape Noise Reduction Plus Expansion
By recording and playing back a program with dbx I|
tape noise reduction and then expanding it, you can end up
with a program that actually has lower noise and better
dynamic range than the original. This is especially effective
on noisy phonograph discs.
When
expanding
or
peak
unlimiting
a
program
that
has
been
recorded
with
dbx
II,
the
128’s
expansion
circuitry
should
come
after
the
dbx
I!
tape
noise
reduction
circuitry
(push
the
POST
button).
This
allows
the
dbx
tape
noise
reduction
circuits
to
properly
decode
the
program,
before
the
expansion.
It is not necessary to use the 128’s expansion circuitry
when playing a dbx-encoded disc, since the encoded disc
already has an extremely wide dynamic range. To avoid
over expansion, always press the PRE button and set the
COMPRESS/EXPAND control to the “1.0” (12 o’clock)
position when playing a dbx-encoded disc.
When to Use dbx II Tape Noise Reduction
Plus Compression
Normally, when making a recording with dbx Tape
Noise Reduction, additional compression is not necessary.
Compression might normally be used to restrict dynamic
range, and to “get the program on the tape,’’ but dbx
encoding alone overcomes the dynamic range limitations
of recording tapes. If you use compression plus dbx tape
noise reduction, the compression would primarily be for
special effects purposes.
When playing a tape which has been recorded with
dbx II tape noise reduction, or when playing a dbx-
encoded disc, you can use the 128 for peak limiting
(above threshold compression).
When adding compression or limiting while playing back
a dbx-encoded tape or a dbx-encoded disc, the 128’s com-
pression circuitry should come after the 128’s tape noise
reduction circuitry (press the POST switch).