dbx 3-BX Owners manual

INDEX

BRIEF OPERATING INSTRUCTIONS INTRODUCTION

Why Use An Expander

Dynamic Range

Restricting Dynamic Range Compression and Limiting dbx Tape Noise Reduction

HOW THE 3BX AND OTHER EXPANDERS WORK

CONNECTIONS OPERATION

EXPANSION To Expand an FM Broadcast or Conventional Phonograph Disc

To Expand a Tape During Playback

How to Expand and Tape Copy a Program Combining 3BX Expansion with dbx Tape Noise Reduction

SIMPLE PRECAUTIONS WHEN USING ANY PROGRAM EXPANDER

SPECIFICATIONS dbx PRODUCT WAR RANTY

FACTORY SERVICE SCHEMATIC GLOSSARY

WARNING: TO PREVENT FIRE OR SHOCK HAZARD, DO NOT EXPOSE THIS APPLIANCE

TO RAIN OR MOISTURE.

2 4

4 4 4 5

14 14 14

15 15

17 18 19

BRIEF OPERATING INSTRUCTIONS

Fig. 1 - 3BX Front Panel

REMOTE INDICATOR EXPANSION Adjust this slide con- This L.E.D., above the trol for the desired degree of expan- R EMOTE switch, glows sian. A "1.4" setting means that an when the REMOTE CON- input signal with a 40dB dynamic TROL UN IT is activated range will be expanded to 56dB, or by the R EMOTE switch. that an input signal with a 50dB

PI.LOT This. L.E ..O. 70dB. A 1.2 setting would result in (lIght emitting diode), a 20% increase of dynamics; a 1.4

above the POWER setting would result in a 40% sWitch, glows when increase of dynamics etc

the 3BX is ON. ' .

. \ dynamic range will be expanded to

EXPANSION

POWER Push this switch once (IN) to turn the 3BX ON; push again (OUT)

for OFF.

REMOTE Push this switch once (IN) to activate the REMOTE CONTROL UN IT, and deactivate

the front panel controls; push again (OUT) to deactivate the REMOTE CONTROL UNIT, and reactivate the front panel con- trols. Without the REMOTE

CONTROL UNIT, this switch can be used as an expander in/ out (bypass) switch.

NOTE: If you do not have the optional remote control unit,

the REMOTE switch MUST be in the OUT position in order for the front panel cpntrols to operate.

GAIN CHANGE l.E.D.'s These L.E.O.'s indicate the amount of gain change in each of the 3BX's three frequency bands. The red L.E.O.'s indicate upward expan- sion (volume increase), the yellow L.E.O.'s indicate downward expan- sion (volume decrease).

HF •••••••••• HF

MF •••••••••• MF

LF •••••••••• LF

-20dB "'12d8

GAIN CHANGE

TRANSITION LEVEL When an incom- ing signal is above the level set by this

control, the 3BX expands upward; when an incoming signal is below the level set by this control, the 3BX expands down- ward. Set the control so that red GAl N

CHANGE L.E.O.'s glow during loud portions of the music, and the yellow GAIN CHANGE L.E.O.'s glow during the quiet portions of the music. This adjustment is non·critical. It primarily controls the activity of the L.E.D. display.

SOURCE Push thiS switch and the PR E switch to expand programs coming from your phono- graph, FM tuner or other auxiliary equipment.

TAPE Push this switch and the POST switch to expand programs coming from your tape recorder. If you do not hiJve a tape recorder, push the

SOURCE switch.

PRE Push this switch to expand a program

before recording it..,. POST Push this

switch and the TAPE

RECORDER

switch to expand a program played back from your tape recorder.

*Expansion will, in most cases,increase the dynamic range of a program beyond the capa- bilities of your tape recorder. To expand a program and then record it, we recommend u.<ingany dbx tape noise reduction system. This allows you to reduce the noise and expand the dynamic range of the original program, and preserve this increased dynamic range on tape. If you do not have a dbx tape noise reduction system, we recommend expanding a program upon playback from your tape recorder (press the TAPE and the POST switch).

Summary: The PR E switch expands the signal before the tape recorder input (PR E places the expander before the tape recorder input); the POST switch expands the signal from the tape recorder's output (POST places the expander after the tape recorder output). Pushing both PRE and POST switches simultaneously bypasses the expander functions.

RECORDER OUTPUTS*

Connect the cables from these

outputs to the Auxiliary or Line

inputs 01 your tape recorder. If you do not have a tape recorder, do not connect any cables to these outputs.

RECORDER INPUTS* Connect the cables from your tape re-

corder Line or Monitor outputs to these inputs. If you do not have a tape recorder, do not connect any cables to these inputs.

REMOTE CONNECTOR For connection of the optional

REMOTE CONTROL UNIT which allows control of threshold, and expansion ratio independently

for each of the 3BX's three fre- quency bands. II you do not have the optional REMOTE CONTROL UNIT, do not connect any cables

to this connector.

dbx

SERIAL NO.

INPUTS

1 2

10 TAP~ IHCOHO£R FRO""' TAPl

INPUTS Connect the cables from the Tape Outputs of your preamp- lifier, integrated amplifier/pre-

ampl ifier or receiver to these inputs. If your preamplifier, integrated amplifier/preamplifier or receiver has no Tape (Tape Monitor Loop), use the Preamp Outputs and Power

0utputs

Amplifier Inputs.

RECORDER

OUTPUTS

1 2

@/@

AUXILIARY OR Rf,CORDER

t '!\Il IM'VI OUTPUT

OUTPUTS Connect the cables from these outputs to the Tape Inputs, or Monitor Inputs of your preamplifier, integrated amplifier/preamplifier or receiver. If your preamplifier, integrated amplifier/preamplifier or receiver has no

Tape Inputs or Monitor Inputs (Tape

Monitor Loop), use the Main Amplifier Inputs on your power amplifier, inte- grated amplifier/preamplifier or receiver.

O~~I~~~ ~~'~'ltJ1

*/f you have a dbx tape noise reduction system {such as any of our 120 or 150 series} seepage 16 for connection diagrams.

II~11,.1(11 ~

C{)J<j~I(;TOH

MODEL 3BX

117V·

50·60Hz

30 WATTS

AC POWER CABLE Connect this cable to a 117V AC, 50 or 60Hz AC power

source only. Models for use with other power sources outside the United States are available. Contact the dbx factory for information. The Model 3BX requires a maximum of 30 watts AC power.

INTRODUCTION

Why Use An Expander

If you're a music lover or an audiophile (or both), you have pwbably noticed that much of the excitement of a live performance is missing in a recorded or broadcast per- formance. The primary reason for this loss of excitement is that the dynamic range of the recorded m bwaclcast per- formance has been purposely restricted to fit the dynamic range limitations of the recording or broadcast mediums.

The 3BX is a sophisticated, three band expander that can restore the dynamic range, and excitement, to a record- ing or radio broadcast, adding considerably to your listening enjoyment. By expanding dynamic range, the 3BX lowers the characteristic noise levels of a tape, phonograph record, or FM broadcast. It restores the "punch" of loud passages, and the whisper of quiet ones. It can add new life to an old record collection, and make FM broadcasts worth listening to. The use of a 3BX with a dbx tape noise reduction system (such as our 120 or 150 series), lets you make tape copies of recmds, FM broadcasts or other tapes that actually sound better than the original. With these capa- bilities, the 3BX will become one of the most valued com· ponents in your home music system.

Dynamic Range

Dynamic range is the difference in level between the loudest and the quietest portions of a program, expressed in dB*. Since the quietest pal'ts of a recorded program are usually restricted by noise, the dynamic range of a record- ing is usually defined as the difference in level (in dB) between the loudest parts of the program and the noise level.

Restricting Dynamic Range

The loudest sounds in a live performance may reach 120

dB SPL. The quietest sounds, however will not be heard if they are much quieter than the ambient wom noise (people coughing, air conditioning or other noises). The ambient room noise in a very quiet auditorium is somewhat over 30dB SPL. The useable dynamic range of a live perfor- mance is thel'efol'e derived by subtracting the wom noise (30dB SPL) from our tolerance of extremely loud sounds (120dB SPL), giving a maximum of about 90dB. Recording studios have less wom noise and a dynamic range of over

100dB can be realized.

The dynamic range of a recorded program is purposely

restricted to far less than 100dB in order to fit within the dynamic range limitations of the recording or broadcast mediums. For example, the dynamic range of a studio

quality tape recorder' is about 60dB. Tape noise restl'icts the quietest sounds that can be recorded, and tape satura- tion (distortion) restricts the loudest sounds that can be recorded. Home tape recorders, especially cassette and cartridge I'ecmders, have an even more restl'icted dynamic range ... often only 45dB. (dbx tape noise reduction systems can neal'ly double the dynamic range capabilities of any tape I'ecorder.)

'The "dB" or "decibel" is a unit of expression for sound level or intensity of sound. One decibel is usuallv described as the smallest detectable change in sound level. The threshold of human hearing

(the faintest sound vou can perceive at a midrange frequencV of

100Hz) is approximatelv "OdB SPL" (Sound Pressure Level) and

the threshold of pain (the point at which vou instinctivelv put vour

hands over vour ears) is about 120dB SPL. Some people can

tolerate 130dB SPL, others leave the room when the sound level

reaches 110dB. The difference between the "threshold of human hearing" and the "threshold of pain" is the dvnamic range of

human hearing (120dB).

The maximum dynamic range of only the very best phonograph discs is about 65dB' (and this is seldom achieved). The quietest sounds on a disc are restricted by the "grain" of the vinyl, and other surface irregularities that create noise; the loudest sounds are restricted by the maximum excursion of the groove. Loud levels are also restricted by the ability of the phonograph needle to "track" the record. To allow mOI'e playing time per side,

the dynamic range of many records is often restricted to less than 50dB,

~BV using dbx /I noise reduction during the manufacture of

phonograph discs, the dynamic range can be extended to 100dB.

Surface noise is reduced to inaudibilitv and the full dynamics of a

performance can be captured. dbx-encoded discs are commerciallv

available, and can be decoded with any of the 120 or 140 series noise reduction systems.

100dB

input

signal

The dynamic range of a radio program is about 60dB for

FM broadcasts, or 50dB for AM broadcasts. The quietest sounds are restricted by broadcast interference and noise, like FM hiss; the loudest sounds are limited by the maxi- mum allowable modulation of the transmitter (100%). Above 100% modulation, the transmitted signal would be distorted, and the station would interfere with adjacent radio stations, near the same I'adio frequency.

Compression and Limiting

Compression and limiting are the electronic techniques used to restrict the dynamic range of a Iive program to fit within the restrictions of the recording or broadcast

mediums, A COMPRESSOR is a device that increases the level of quiet passages, and decreases the level of loud

passages. The COMPRESSION RA

no is the ratio in dB of

SOdB

usable

dynamic

range

"window"

signal/noise

''':;:::';0"

Jracorder

Fig, 3 - Dynamic Range Limitations of the Tape Recording Process NOTE: The waveform is not a sinewave signal; it is the "envelope" describing the program's volume changes.

the compressor's input dynamic range to its output dynamic range. For example, if the compression ratio is

°1

1QdB

2: 1, the output level will only change 1dB for every 2dB change at the input (thus restricting), or "compl'essing" the dynamic range). The THRESHOLD is the level at which the compressor decides to increase or decrease levels. The

-51015 50

..•

Ihr61lhold

"5' 1

m-+-1l'''

~.!.

7!,,11J

compressor decreases the level of input signals that are above the threshold level, and increases the level of input

<lho"., th.nshold

comp"'S5,,,,1

..•

"t 40

signals that are below the threshold level. Some com- pressors act only on levels that are above the threshold level, and let any signal that is below the threshold level pass unchanged. A LIMITER is a compressor that acts only on signals above the threshold level, and that has a com- pression ratio of 10:1 or higher. The threshold of a limiter is usually adjusted so that it acts only on musical peaks, preventing them from exceeding the threshold level by more than a very small margin. dbx limiters have an infinity: 1 compression ratio setting. At this setting, peaks cannot exceed the threshold level by even a small margin.

Fig. 5 - Above Threshold Compression Above threshold com- pression has no effect on low level signals. When signal level reaches the adjustable threshold, the dynamic content of the music is decreased but only above that threshold. Higher compression ratios may be used in above threshold compression than in linear com- pression; however, all ratios are available. This is known as limiting if the compression ratio is 10: 1 or higher.

The restriction of dynamic range created by compression

and/ol' limiting is undesirable because it removes much of

55dB

,: ~:, I

:: : l'

'~"T

the excitement from a recorded performance. However, without this restriction, the quiestest parts of the program could be lost in noise, and the loudest parts of the pmgram could be severely distorted. Fortunately, there are ways to overcome this dynamic range restriction, by restoring "lost" program dynamics.

dbx Tape Noise Reduction

dbx tape noise reduction systems allow a program of up

...L

program

Fig. 4 - Linear Compression Linear compression reduces the entire dynamic range of the music irrespective of frequency

(20-20,OOOHz).

to 1OOdB dynamic range to be I'ecorded on tape (or on an encoded phonograph disc) without losing the quiet passages in the noise or distorting the loud passages. Professional recording studios throughout the world are using dbx professional tape noise reduction systems, and dbx has

become the new established leader in the field. dbx II noise reduction systems, such as our 120 series, are available for

use by the audiophile and home recordist. Like the original dbx tape noise reduction system, dbx II noise reduction systems make it possible to tape record up to 1OOdB of dynamic range, and in addition, dbx II noise reduction systems also facilitate playback of special dbx encoded discs (see footnote, page 5).

dbx tape noise reduction devices do not remove noise present in the original program; instead, they prevent additional noise (or distortion) from being added to the program by the tape recording process. This noise reduction is accomplished by a "compander," a circuit which compresses the program before recording, and

J. +20

+ '5 tape saturation + 15

-----------------

expands the program during playback. In addition, dbx tape noise reduction systems apply pre-emphasis of high frequencies before recording, and de-emphasis upon play- back to further reduce high frequency noise (hiss) and tape modulation noise.

Other noise reduction systems, such as Dolby and

ANRS also reduce noise levels by a similar process, although they are not nearly as effective as dbx tape noise

reduction systems. dbx recommends the use of its tape noise reduction systems whenever possible. However, dbx tape noise reduction cannot reduce the noise levels or expand the dynamic range of existing recorded program material. (Neither can Dol by, AN RS or other compander type systems). dbx developed the 3BX to increase dynamics

+10

-------

'"-r

-20

lOOd8 input

dynamic range

-40

-60

, - BO

Fig.6 - dbx Tape Noise Reduction Loud passages are decreased in level allowing them to be recorded below the level of tape saturation;

very quiet signals are placed on tape significantly above the tape noise level.

-20

-40

-60

l00dB output

dynamIC range

and reduce noise on phonograph records, tapes and radio broadcasts.

The 3BX is a "single-ended" device. This means that it can reduce the noise levels (and restore the program dynamics) of a recorded or broadcast program. Thus, the 3BX revitalizes your entire collection of records and tapes. Of course, 3SX expansion can be combined with a dbx tape noise reduction system to make tape copies that sound better than the original program.

HOW THE 3BX AND OTHER EXPANDERS WORK

An EXPANDER is a device that decreases the level of quiet musical passages, and increases the level of loud

musical passages. It is the opposite of compression. The EXPANSION RA T10 is the ratio of the expander's output dynamic range to its input dynamic range. An expander with a 1:1.4 expansion ratio will have an output level change of l.4dS for an input level change of 1.0dS. Given

·5

·28 77dB

e.panded

proq.amal14

Fig. 7 - Linear Expansion Linear expansion operates over the entire dynamic spectrum of music irrespective of frequency (20-20,OOOHz),

making loud passages louder and quiet passagesquieter, reducing audible noise. Expansion ratios are adjustable. For example: 1.0 = 0% increase in dynamic range; 1.2

= 20%; 1.5 = 50%; 2.0 = 100%.

an expander with an expansion ratio of 1: 1.4, and an input program that has a dynamic range of GOdB, the output

dynamic range will be (GO x 1.4 = 84) or 84dB. The

TRANSITION LEVEL (threshold) is the level at which

the expander decides whether to increase or decrease

program levels. When an input signal is above the threshold level, the expander increases its level; when an input signal is below the threshold level, the expander decreases its level.

All expanders have a level detection circuit. This

detection circuit is used to sense the input signal level and to determine whether it is above or below the threshold.

However, the method used to detect input signal level is different on various expanders. The detection technique is vital, as detailed below.

Peak Detection

Some expanders sense musical peaks in the input signal to determine whether the input signal level is above or below the threshold. The effect of this peak detection is that the expander' acts somewhat erratically, and may expand the program when it detects a noise spike, or brief musical transient that isn't really representative of the program level.

Average Detection

Some expanders sense the average level of the incoming program to determine whether the signal is above or below the threshold. Average detection circuits will not over- react on musical peaks, but may respond too slowly to accurately expand a program. The expander may respond too late to a rapid increase in program level after the actual input signal has begun to decrease again, causing an un- natural or swishing sound.

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