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®
NOV
30 1 997
Designed to lower the audio blasting that goes with TV-commercials, the unit also serves as a studio sound manipulator!
Stereo Compressor
BY JULES RYCKEBUSCH
An Audio Compressor is an essential tool for audio processing and it is commonly found in recording studios. Most recording studios have a selection of compressors, each chosen for some specialized feature or nuance. Your home may not be a professional recording studio, but you do need our Stereo Compressor, so read on. As the device's name implies, the
POPtronix Stereo Compressor
Stereo Compressor compresses or limits the dynamic range of an audio signal. (Make sure you read the sidebar How Compressors Work.) In a recording studio, compressed audio prevents tape-recorder overload during loud passages without losing quiet passages in the noise floor (hiss). Before compressors were invented, the recording engineer would ride the gain by manually adjusting the signal level.
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AUDIO IN VCA
AUDIO OUT
+V THRESHOLD RECTIFIER LOG CONVERTER
SIDECHAIN RETURN
Fig. 1. FUNCTIONAL BLOCK DIAGRAM of the Stereo Compressor. Sidechain portions of the diagram may be omitted from the unit's construction.
RATIO
SIDECHAIN SEND
Compressor, you don't need to constantly ride the gain. It will automatically do that for you! When you patch the Stereo Compressor between your hi-fi VCR and your stereo sound svstem, you can enjoy listening to movies late at night in full fidelity, at low volume, without being blasted out of your chair when Rambo raids your living room. You can also use the unit to prevent tape (recording) overload while making home audio recordings.
Compressors are commonly used to record vocalists and acoustic instruments, especially those that have large dynamic ranges. Compressed audio can also be used to create artificial effects while recording, such as increasing the apparent sustain of an instrument or completely squashing the level of a signal. We will learn how to accomplish these audio effects later. Compressors can also be abused. Have you ever wondered why TV commercials sound louder than regular TV programming? Well, the producer of the offending TV commercial has used a large amount of compression on the audio tracks and then boosts the gain tojust below distortion level. The end result is an apparent Increase in loudness to the listener. This works because modern TVs and home theaters have a large dynamic range for the audio signal. Film and TV producers take advantage of the dynamic range available to them by recording normal conversation and background sounds 6 to 12 dB below the maximum signal level. Then for dramatic effect, music and sound effects such as explosions can be that much louder than the dialogue track. This is very noticeable when watching a movie on a high-fidelity video tape recorder. In fact, for late-night viewing, when you are trying to be quiet, you have to turn the TV up just to hear the dialogue and then quickly turn it down when the car chase scene comes crashing into your living room. But, with the Stereo
How it is done
The block diagram of the Stereo Compressor Is shown in Fig 1. The input signal is fed to a voltage-controlled amplifier (VCA) that has a nominal gain of unity. Some of the output signal is fed to a precision rectifier followed by a logarithmic converter circuit. The output of this block is a DC voltage proportional to the log of the average level of the input signal. By sending some of this DC control voltage to the VCA we automatically reduce the gain of the VCA when the input signal exceeds a user-determined threshold level. It is important to note that we determine the signal level after the VCA and not before. This allows the output level to increase and sound normal, but not increase as much as the input signal does. By varying the amount of feedback we adjust the compression ratio, which in conjunction with the THRESH control, determines the operating characteristics of the compressor. The optional sidechain jacks permit external processing of the audio signal or substituting a completely different audio signal as the control signal. This add-on circuitry lets the user experiment and achieve some useful audio effects.
How it works
Figure 2 is the schematic diagram of the Stereo Compressor. The device has two independent channels of compression,
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POPtronix
Stereo Compressor
+V C12 47µF +V R52 10K L THRESH. D R16 1.8K R17 1.8K B R47 1K CW R14 2.2K V R7 330K C SG G C11 47µF
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1 D8 + C14 470µF C13 470µF R13 1.5K D9 R12 2.2K D2 R ACTIVE POWER S1 NC
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PWR1 12VAC 400mA
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+ 2 O IC7 7912 C I 1
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R48 1K R51 10K R THRESH.
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D3 R ACTIVE R55 100K R OUTPUT
R39 1.0K C5 10µF +V R45 2.2K R22 1.5 MEG V D6 K R53 100K R RATIO L R19 22K R24 200 + R41 68K 1 2 3 4 5
THR1 2 LGA1 COUT1 SOUT1 +VI1 GND 22 R8 330K +V 21 VI1 SOUT2 +V12 IC3 SSM2120 18 20 19 R28 200 NC R29 200 R27 1.5K 6 5 + R6 68K 3 +
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7 IC2-b 1/2 5532 R38 10K
R40 10K 6 I 5 + IC1-b 1/2 5532 J1 RIGHT IN R9 150K +V J R31 10K C8 2000pF
R44 10K 7 R43 39K + 8 R18 47 9 10 V 11
+ R37 IC5-b 1.0K 1/2 5532
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P OPTIONAL
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17 +V M D4 D5 13 12 R4 22K CW R54 100K L RATIO S J6 L SIDECHAIN IN R R3 2.2K R49 1.0K J5 L SIDECHAIN OUT
C4 10µF
SIN2 16 SIN1 RIN1 IREF V RIN2 15 COUT2 LGA2 THR2 + 14
J2 LEFT IN
R32 10K
R2 39K
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R26 68K
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1 C1 IC1-a 1/2 5532 2000pF
R1 47
R36 10K
R30 10K
R5 1.0K
J7 R SIDECHAIN OUT
U J8 R SIDECHAIN IN
Fig. 2. SCHEMATIC DIAGRAM of the Stereo Compressor. When the sidechain jacks are included, the unit is designated as the Stereo version by PAiA. The Home Theatre version eliminates the jacks.
POPtronix
Stereo Compressor
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right and left. The heart of the circuit is the SSM2120 dual dynamic range processor integrated circuit IC3. The SSM2120 features two complete dynamic range processors. Each one consists of a voltagecontrol amplifier, a logarithmic converter, and a precision rectifier. The chip also features a dynamic range of 100 dB at only .O1-percent total harmonic distortion at +10 dB input. The remainder of the Stereo Compressor circuit consists of input and output buffers, the control circuitry and the comparator. Since the electronics for both channels is identical, we will look at the right channel in detail. The input audio signal from J1 goes to an inverting buffer circuit consisting of R40 and R44 and IC1-b. Its output signal (IC1-b, pin 7) is coupled to the signal input on IC3, pin 8, via C4, which blocks any DC component of the input signal. The SSM2120 chip is actually looking for a current input source, and R43 provides the proper amount of current to the chip. The RC filter formed by C8 and R18 eliminates any stray interference. The output signal from the internal VCA of IC3 (pin 4) is actually a current signal. It is restored to a voltage signal by current-to-voltage converter IC1-a and its feedback resistor R46. The signal from IC1a, pin 1 is routed to output stage amplifier IC5-a via R34. The output signal from IC5a, pin 1, is coupled via R35 and C9 to jack J3. OUTPUT potentiometer R55 lets you adjust the unit's gain. The output signal is also sent to the rectifier input (IC3, pin 9) via R36 and C7 either directly, or via the optional sidechain jack circuitry including R50. The side-chain jacks consist of a standard 1/4inch open-circuit OUT phone jack (J7) and a 1/4-inch IN phonejack (J8) with a normally-closed switch built into them. These jacks form a normalized patch point for additional audio processing. With nothing plugged into J8, the signal path is uninterrupted. By inserting a phone plug into the side-chain IN jack (J8), the normal signal path is broken and either the processed original signal is sent to the rectifier input, or a completely different
(new) signal is sent to the rectifier input. We will later see how to use this feature for some really powerful audio processing. But first, the control side of the house. Resistor R22 provides a reference current to the log-averaging circuit within IC3 via pin 2. It also forms an RC timing circuit with C5. This RC circuit determines the response time for the compressor. The time constant is set so that the compressor will respond rapidly without distorting. Potentiometer R51 along with other resistors, develops the threshold level signal. The voltage from the wiper of R51 is sent to the threshold input of IC3 (pin l) via R42. Another resistor R41 across the threshold pin 1 and the control output pin 3 establishes the internal gain of the control stage. The control output signal from pin 3 goes to a voltage-divider network centered around potentiometer R53 then to the inverting VCA input of IC3 (pin 7). A positive voltage on this input reduces the VCA gain, which is what we need to make the compressor work. Note that both the inverting and non-inverting control inputs (pins 5 and 7) are tied to ground via R5 and R7. The control inputs must remain close to ground potential for proper operation. A 6-millivolt change in voltage at these control pins causes a 1 dB change in VCA output. Diodes D6 and D7 ensure a unipolar control voltage. Potentiometer R53 is the compression RATIO control. It gives an adjustable compression ratio of 2 to 1, all the way to about 25 to 1. The last portion of the circuit is the comparator formed by IC4-b, two input resistors R10 and R20, a bicolor lightemitting diode LED3, and current limiting R11 for the light-emitting diode D1. Notice that op-amp IC4-b is a comparator. Normally it is unwise to use an op-amp for this purpose because the output stage saturates, which will slow down the comparator's response time. (Note: D1 through D3 are light-emitting diodes or LEDs. This symbol identification code was used in this story in order to agree with PAiAs kit symbolism.) In this instance, we need to get an output that changes from
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POPtronix
Stereo Compressor
D8 G D9 A + + C14 IC7 IC1 IC6 + + C12 R14 R16 R15 R17 R13 D H B E R12 IC4 C11 C8 R18 L R10 J R9 R11 R7 C13 C9 + R46 R45 R44 R43 C4 + F R20 R19 D7 K R42 R41 D6 R24 R23 R21 R22 R8 R6 C IC3 R26 J R25 C3 + R39 + C5 SG J4 P C10 + J3 O R40 J J2 J I + C6 R29 R28 R27 D5 J1 + C7 R30
R37 R36 R35 T W R34 R31 V S R33 R322 D4 N R4 + C2 R3
*R50 Y IC5 X *R49
U
R38
R
J M IC2
J
R5
R2 C1 R1
D1
D2
D3 J= WIRE JUMPER
Fig. 3. PARTS PLACEMENT on the PAiA board. The silk-screen markings on the board provide an additional assist for the assembler. Resistors R49 and R50 are shown in place for the Home Theatre version. The studio version of the Stereo Compressor has these resistors mounted on jack terminals. BOTTOM VIEW of the PC board, shown same size, details the foil pattern homebrew experimenters must assemble.
one supply rail to the other. This makes dual-LED interfacing very simple. Along with being an excellent audio op-amp, the NE5532 functions well as a comparator in this application. The power supply circuit consists of an externally connected Wall-Wart 12-volt AC transformer PWR1 and associated diodes and capacitors. (See top-right corner of Fig. 2.) One side of the 12-volt AC line is tied to ground and the hot side goes to halfwave rectifiers D8 and D9. These diodes deliver bipolar, unfiltered DC, and each
POPtronix Stereo Compressor
supply is filtered by electrolytic capacitors C13 and C14. Even though PWR1 is rated at 12-volts AC, the filter capacitors charge closer to the peak value of the 12-volt AC, and just about 15-volts DC is delivered to the +12-volt DC regulator IC6 and 12-volt DC regulator IC7. The output of each voltage regulator section is filtered for decoupling purposes by electrolytic capacitors C11 and C12. Two ground systems are used in the Stereo Compressor, one for power return and the other for signal return. This design
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CORD TO PWR1
IC1
AUDIO OUT
SIDECHAIN JACKS
AUDIO IN
PC BOARD
BIRD'S EYE VIEW of the Stereo Compressor before the chassis cover is secured in place. Note the neat interconnection between the PC board and front and rear panels.
and make your own board. The SSM2120 chip (IC3) is available from several sources including PAiA, Newark and Allied. The other parts are common garden variety types available from local and mail-order parts suppliers. If you do breadboard the circuit, make certain you use good grounding techniques. Return all signal and power grounds to one common point to eliminate any ground loops. Should you IC4 IC3 IC2 IC5 elect to use the PAiA circuit board, the circled letters in practice reduces the possibilities of ground Fig. 2 are termination-point identifiers for loops that introduce unwanted AC hum to hookup wires that run from the front and the audio signals. rear panel-mounted parts to the PC board. These interconnections are shown in Figs. Construction 3 and 4. Assembly of the Stereo Compressor is The three light-emitting diodes D1 relatively straightforward. PAiA through D3 require lead extensions made Electronics has a complete kit of parts from #22 insulated hookup wire. Mark available for the Home Theatre version (no the leads so that you can readily identify sidechain jacks) and the Studio version the anode and cathode terminals. Twist (includes sidechain jacks). If you want to the leads of the LEDs and cut their length roll your own, the circuit can be built on a so that the LEDs will fit into the three holes Radio Shack experimenter's PC board or provided for them on the front panel. you could copy the same-size drawing of These holes are between the POWER switch the circuit board provided in these pages S1 and the THRESH potentiometer R52 (See
HOW COMPRESSORS WORK
0
dB
a
b
c
The gray band is the usable dynamic range of a quality audio tape. The light gray band below the gray band is the inherent tape noise (hiss) common to all tapes. The range (height) of these bands varies with tape quality. The audio signal in (a) shows a loud passage that is too loud for the listener, so the volume was turned down (b) to lower the loud passage in the tape's dynamic range. However, the quiet passage was
lowered into the noise band common to all tapes. The quiet passage is either lost in the hiss or will sound appalling when played back. A compressed audio signal of the same passage (c) permits the quiet passages to be recorded above the hiss in the tape's usable dynamic range while the loud passage is reducedi for the listener's pleasure.
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POPtronix
Stereo Compressor
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PWR1
L IN
J2
Fig. 4. INTERCONNECTING THE PC BOARD with the front and rear panel parts is simplified by PC board circed markings. The long cable runs from the board's output stage to two front-panel controls, which requires using thin, flexible, audio, coaxial cables such as the RG-174/U type. The unit's step-down power transformer PWR1 is contained in a wall-plug casing that plugs into a power outlet.
POPtronix Stereo Compressor www.poptronix.com
S1 1 2 3 R IN J1 R52 R47 D2 IC4 IC1 R48 R51 K F R54 IC3 JUMPER C N SHIELD R53 M R56 R SHIELD R55 W V S R49 IC5 X Y SHIELD J I T U D3 D H D1 B J4 E P L O J3 J2 J6 J1 J5 R50 9601 IN J8 RIGHT SIDECHAIN R50 OUT J7 G SG J4 R OUT J3 IN LEFT SIDECHAIN R49 OUT A D9 L OUT
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Fig. 4). If you choose to omit the sidechain option (A big mistake--Editor), eliminate jacks J5 through J8. Resistors R49 and R50 normally mount on the terminals ofjacks J5 and J7. When the sidechain jacks are omitted, these 1000-ohm resistors are connected to terminals R and S, and S and T, respectively, on the PC board. The light-emitting diodes (D2 and D3) combined ground path must return straight to the power supply common. The LED's comparatively high current that is switching on and off in this ground return could cause popping sounds in the audio output when connected to signal or even power grounds.
counterclockwise rotation there might be some distortion. This is to be expected.
Using the compressor
The main use for a compressor is to keep levels from getting out of control while recording vocals and acoustic instruments. This takes a little experimentation. For a vocalist I usually start out around a 4-to-1 compression ratio with the THRESH set so the LED indicates red when the singers reach their nominal level. This way if they hit a note 20 dB higher than nominal (which would definitely clip most tape decks) the signal out of the compressor only increases by about 5 dB. The Stereo Compressor can increase the apparent sustain of a sound. By using a large amount of compression and restoring the level with the OUTPUT control, the compressor will initially reduce the output signal by a large amount. As the input signal level decreases, the amount of compression will decrease and the output level will remain relatively constant. The Beatles used this on the final piano chord in A Day In The Life to make the sound linger on. To use the compressor as a limiter while recording, set the RATIO to about 20 to 1 and set the THRESH control so that the LEDs momentarily change from green to red on peak signals. This will preserve as much dynamic range as possible. When the Stereo Compressor is connected to a hi-fi VCR for late night viewing, set the RATIO control as high as possible without distorting and set the THRESH control to change the LED's color during quiet spoken passages. This will allow you to preserve the fidelity and stereo spread of the movie sound track, hear all the dialog, but not get blown out of your seat when the F-114 does a strafing run in your living room. The above settings are intended to be starting points, so feel free to experiment.
Testing
After you have wired the Stereo Compressor, check for solder bridges, coldsolderjoints, incorrect component polarities and all the other nasty errors that prevent a circuit from functioning normally and sometimes result in selfdestruction. Do this before applying power! The best way to test the Stereo Compressor is to connect it between a CD or cassette deck and your hi-fi audio system. Set the RATIO control (R53) fully counter clockwise and the THRESH control fully clockwise. Set the OUTPUT control to about ten o'clock. These are the minimum settings. You should hear undistorted, noise-free audio. Both LEDs (D2 and D3) should be green. If there is distortion or noise, you need to go back and check your wiring and component polarities. If everything sounds good and you have green LEDs, then everything should be working fine. Note: with no input signal the LEDs may drift slightly and not track the THRESH control. Once everything is working, slowly rotate the THRESH control. At some point around mid-rotation, the LEDs should start to indicate red. This means compression is starting to occur. Rotate the RATIO control clockwise. You should hear a decrease in volume as the compressor squashes the signal. At full
Using the sidechain jacks
Along with regaining control of your hi-fi VCR, there are all sorts of useful
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POPtronix
Stereo Compressor
POWER SWITCH S1
THRESHOLD CONTROLS R51, R52
AUDIO OUTPUT CONTROLS R55, R56
functions available via the sidechain jacks. By patching an audio processor in the sidechain jacks, all sorts of cool things are possible. One of the most useful is creating a de-esser. This is a device used to remove sibilance from vocals. Sibilance is that nasty RATIO CONTROLS LIGHT-EMITTING DIODES R53, R54 Shhhh sound that occurs when S D1D3 words are spoken or sung, because of the way S sounds are formed in signal is compressed normally. The end result is overall increase in the percussive the human vocal track. quality of the processed sound. When we form an S sound, air passes The Stereo Compressor can also be between the teeth and tongue forming a burst ofwhite noise and a short blast of air. used as a ducker. A ducker is a device that reduces one signal's level based on a If the speaker or vocalist is close to the different signal. This effect gets used a lot microphone, this is picked up as a brief on radio commercials. In this case, a overload and noise burst. This burst of different audio signal is fed into the sound mostly contains high frequencies. sidechain such as an announcer's voice. By setting an equalizer to boost high When the announcer speaks, the output of frequencies and patching it into the side the compressor is reduced. This is useful chain, the compressor will drastically for keeping background music at compress the signal when the high maximum volume, but letting the frequencies are present, but act normally announcer's voice cut through the when they are not. background music by reducing the music You cannot eliminate sibilance, but it level when the announcer speaks. Listen can be minimized with a de-esser. Any equalizer will work. The best way to figure closely to any radio commercial and you out what frequencies to boost is to listen to will notice this effect. Duckers are also great for DJs or a presentation with the audio through the equalizer. Start background music. boosting until you have noticeably This ducker effect is also used in the increased the sibilance. Anything above 3 studio to allow one instrument to cut kHz usually works; you may have to through on a mix. If you want a particular experiment. A similar problem, although instrument to be more noticeable, such as at the other end of the audio spectrum, a snare drum, send the snare signal into relates to P and B thump sounds. These the sidechain and have the rest of the mix can be minimized the same way by feeding the compressor normally. When boosting the offending bass frequencies ever the snare drum plays, it will reduce (less than 300 Hz) via the sidechain. the level of the main mix, increasing the Another abuse of a compressor is to presence of the snare drum without totally squash an individual instrument increasing its level. Just by adding a signal, then restore its level. This is done with vocals, snare drums, kick drums, etc. fewjacks, we have increased the power of an already useful tool. U2 does this on a lot of their recordings. The Stereo Compressor is a state-ofBy squashing instruments that have a the-art audio processor. It can be used to percussive quality (such as drums or slap upgrade your home recordings, as an bass) the amount of percussive attack is addition to a professional recording studio, increased. This occurs because the or it can just allow you to enjoy late-night compressor does not respond instantly. movies without riding the gain. Enjoy! The initial attack transient portion of the signal gets through the compressor unaffected while the remainder of the
POPtronix Stereo Compressor
FRONT PANEL view of the Stereo Compressor. Since the unit can be used in the recording studio, channel reference is made to 1 and 2 instead of LEFT and RIGHT, respectively.
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PARTS LIST FOR THE STEREO COMPRESSOR
RESISTORS
All fixed resistors are 1/4-watt, 5%. R1, R8--47-ohms R2, R33, R43, R46--39,000-ohms R3, R11, R12, R14, R15, R45--2200-ohms R4, R9--150,000-ohms R5, R35, R37, R39, R47-R50--1000-ohms R6, R26, R41, R42--68,000-ohms R7, R8, R10, R2Q--330,000-ohms R9--150,000-ohms R13, R21, R27--1500-ohms R16, R17--1800-ohms R22, R25--1.5-Megohm R23, R24, R28, R29--200-ohms R30-R34, R36, R38, R40, R44--10,000-ohms R51, R52--10,000-ohm panel-mount potentiometer R53-R56--1 00,000-ohm, panel-mount potentiometer The following are available from PAiA Electronics 3200 Teakwood Lane Edmond, OK 73013 Tel: 405-340-6300 FAX: 405-340- 6378 Email: http://www.paia.com Complete kit of electronic components including circuit board, knobs, wall-plug transformer, phone jacks, etc., but less case-- US$74.75 #9601K Home Theatre desk-top case, punched, anodized, legended, with wood end caps and hardware-- US$19.25 #9601DTC Studio rack-mount case, standard 19-in. wide, 1-3/4-in. high, punched, anodized, legended front panel. Includes 1/4-in. jacks and all hardware--US$29.50 #9601RMC Please add US$7.00 for and handling for kit orders in the USA. PC board only--$22.50 #9601PC (shipping prepaid)
CAPACITORS
C1, C8--2000-pF, ceramic disk C2-C7, C9, C10--10-µF, 16-volt, electrolytic C11, C12--47-µF, l6-volt, electrolytic C13, C14---470-µF, 25-volt, electrolytic
SEMICONDUCTORS
IC1, IC2, IC4, IC5--5532 dual low-noise op-amp IC3--SSM2120 dynamic range processor IC6--7812 +12-volt regulator IC7--7912 12-volt regulator D1--Light-emitting diode, red D2, D3--Light-emitting diode, bicolor, red and green D4-D7--1N4148 silicon signal diode D8, D9--1N4001 silicon power diode
MISCELLANEOUS
J1-J4--Jack, RCA-phono, PC-mount (Home Theatre version only) J1-J5, J7-- Jack, 1/4-in., mono-phone, panelmount (Studio version only) J6, J8--Jack, 1/4-in., mono-phone, closedcircuit, panel-mount (Studio version only) PWR1--Wall-Wart 12-volt AC transformer PC board, wire, audio coaxial cable (see text), knobs, hardware, case, solder, etc.
Copyright © 1997, POPtronix Inc.
All rights reserved.
As a service to readers, POPtronix, Inc. publishes available plans or information relating to newsworthy products, techniques and scientific and technological developments. Because of possible variances in the quality and condition of materials and workmanship used by readers, we disclaim any responsibility for the safe and proper functioning of reader-built projects based upon or from plans or information published online. Since some of the equipment and circuitry described in POPtronix, Inc. Project Articles may relate to or be covered by U.S. patents, we disclaim any liability for the infringement of such patents by the making, using, or selling of any such equipment or circuitry, and suggest that anyone interested in such projects consult a patent attorney.
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POPtronix
Stereo Compressor
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