Surround Sound Decoder Mk1

Originally published by Paul Stenning in ETI, July 1992

Many owners of satellite receivers, Nicam Stereo equipment and stereo video recorders are taking advantage of the vast improvement in realism that can be obtained on stereo film soundtracks, by feeding the sound through the Hi-Fi, and positioning the TV centrally between the speakers.

However, by comparison to the cinema, there is still something missing! Apart from the from the large screen and the kids throwing popcorn around, the main thing missing at home is the Dolby Stereo surround-sound system. Many films are now made with Dolby Stereo sound, and if it says "Dolby Stereo in Selected Cinemas" at the end of the credits, the surround sound information will still be encoded into the stereo soundtrack whether the film is transmitted by satellite or in Nicam stereo, or released on video.

There are some excellent Dolby licensed decoders available for home use, however these tend to be rather expensive. The unit presented here is a "sound-alike" circuit which, whilst not preforming as well as the real thing, still gives superb results for a more affordable price. The only other requirements are a small amplifier and a pair of cheap speakers. Some constructors may wish to incorporate a power amplifier circuit in the same case as this decoder, suggestions for this are given later in this article.

The project is quite straightforward to assemble and requires no setting up, so should therefore be well within the capabilities of most constructors. To this end, detailed assembly instructions will be given (more experienced constructors please bear with us!).

How it Works - General
Constructors requiring more information on the "Dolby Stereo" system in general are recommended to read the excellent article on the subject by Robert Ball in "Electronics - The Maplin Magazine", Issue 37.

When the film soundtrack is recorded, the surround sound channel is encoded by first filtering it and attenuating it by 3dB, and then passing it to two phase shifting circuits. One of these shifts the signal by +90 degrees and sums it to the left channel, the other shifts the signal by -90 degrees and sums it to the right signal. Thus the signal is 180 degrees out of phase between the two channels.

To extract the signal, basically all that is required is a fixed left minus right network. However, for better realism the extracted signal requires additional processing in the form of a short delay, typically between 12mS and 25mS, in this case 16mS. The purpose of the delay is to make any front channel sounds that stray to the rear channel, less noticeable. This is achieved by confusing the ear; since any stray sounds will come from the front channels a few milliseconds before the rear, they will appear to originate from the front. The delay also gives the sound more depth. A similar delay is also used in licensed Dolby decoders, and is allowed for by the film makers.

Before the delay circuit is a low pass filter, to limit the bandwidth of the signal to just below 4kHz, half the delay sampling frequency (8kHz). The delay is followed by a similar circuit, to remove the sampling frequency from the signal. In addition, this filter also gives a degree of bass boost (+6dB below about 300Hz).

Limiting the bandwidth in this way does not significantly detract from the performance of the unit. The bandwidth of the surround signal in a proper Dolby system is only 100Hz to 7kHz, and the upper end of this can only be achieved by using Dolby licensed noise reduction circuitry to remove stray signals. The surround channel exists for effect, not for listening to in its own right. So as long as the Left and Right channels are of reasonable quality the overall result will be fine!

How it Works - Detail
The circuit diagram is shown in Fig 1. The input signals should be within the range 0.5 to 1.0 volts RMS, this is the usual level from the audio output (NOT Speaker) connectors on equipment. IC1:A is the left minus right amplifier circuit, with a gain in differential mode of about five. This gain will overcome the loss caused by the balance control, and give adequate signal to drive subsequent stages. The Balance control, RV1, is used to offset any difference in the level of the two channels, this is will require fairly careful setting up in use, to reduce the amount of left and right channel signal from the rear channel.

R5 and R6 set the bias for IC1:A and, since the circuit is directly coupled, for the delay and remaining op-amps.

The extracted signal then passes through a low pass filter, built around IC1:B, with a roll-off of about 18dB per octave, and the -3dB point a little below 4kHz. The signal then passed through an analogue bucket brigade delay line, with a delay of 16mS.

IC2 (MN3101) is the clock for the delay line, and produces two anti-phase square-wave signals, with a frequency of 16kHz, which gives a delay sampling frequency of 8kHz. The IC also generates a reference voltage for the delay line, which is decoupled by C7. IC3 (MN3004) is the delay line itself, and the two outputs from it are summed by R15 and R16.

This summed signal then passes through another filter built around IC1:C. This is similar to before, but with the addition of R20 and C11, which reduce the signal by about 6dB above 300Hz, effectively giving a little bass boost. The signal is then buffered by IC1:D, and fed to the volume control via C12. The output from the volume control will then be connected to your power amplifier as required.

Please note that the frequency responses of the filter circuits have been calculated using analysis (Spice) software, based on the op-amp specified. Using a different device will give incorrect results, and may result in instability.

The mains enters via SW1, and passes to the transformer, T1. The output from the transformer is rectified by D2 and D3, and smoothed by C13, to give about 23V. The power supply voltage required is 15V, which is regulated by IC4.

PCB Construction
The component overlay for the PCB is shown in Fig 2. Double check the position, value and orientation of each component, against the Overlay and Parts List, before soldering it into place. Take care when soldering, to ensure reliable joints. In particular, take care where tracks pass close between pads, to avoid solder bridges.

Start by fitting the resistors, these components can be fitted either way round. The colour codes are given in the parts list. If the resistors used have a tolerance better than 5%, the Gold band will be Brown (1%) or Red (2%).

Next fit the diodes. These must be fitted the correct way round, as shown on the Overlay.

The IC sockets should then be fitted. Again orientation is important, the notch in the centre of one end should be positioned as shown on the Overlay. Do not plug the IC's into the sockets yet.

The terminal pins (if used) should be inserted now, in the positions marked P1 to P9. These will be a fairly tight fit and should be pressed firmly but carefully into place with the soldering iron. Support the board whilst doing this, and check the pins are properly in place and straight before finally soldering.

The ceramic capacitors can be fitted next, these can be fitted either way round. The values of these may be shown in a variety of ways, the most likely markings are shown in the parts list.

The electrolytic capacitors can be fitted now. Polarity is important, the positive connection is shown on the overlay with a +. On most electrolytic capacitors, the negative connection is marked with a -, and the positive lead is the longer one. Fit with the negative marking away from the + on the overlay.

The voltage regulator, IC4, is fitted next. Orientate as shown on the Overlay, and gently push down until the base of the body is about 5mm above the PCB before soldering.

Finally fit the potentiometers. Before fitting, trim the shafts to suit the knobs being used. Grip the end of the shaft (not the body of the pot) in a vice, and cut to the length required with a junior hacksaw, then remove any burrs with a fine file. The two pots are different types, so check the markings before fitting into the PCB. Push the pots well down and make sure they are correctly aligned before soldering.

Now give the board a thorough visual examination, checking in particular for incorrectly placed components, badly soldered joints and solder bridges.

The prototype was constructed in a plastic case, approximately 170 x 70 x 190mm, however a somewhat smaller case could be used if required. Since the unit contains dangerous (mains) voltages the case must be held together with screws, a clip together case is not suitable. If you intend to include a power amplifier circuit (see later), use a metal case and make sure it is large enough to house all the components comfortably.

Since the PCB is so small and light, it will be perfectly secure mounted by the potentiometers, unless the unit is to receive some particularly rough treatment! Two holes are required in the front panel for the pots, 2 inches apart and the right diameter for the bushes on the pots being used. If the small PCB mounting pots from Maplin are being used, the holes need to be 7mm (9/32") in diameter. If the pots have any small lugs to locate into the panel, these can be carefully cut or broken off.

A third hole will also be required on the front panel, to suit the power switch being used. The switch used on the prototype was supplied by Maplin as a Miniature Rocker Switch (YX65V), and required one 18mm diameter fixing hole, and a small notch to prevent rotation. It may be easier to use this type of switch, rather than one which requires a rectangular cutout.

The front panel can then be marked with rub-down transfers and sprayed with lacquer if desired, before the components are fitted. When tightening the pot nuts, support the body of the pot from behind, and do not over-tighten. The washer should be placed against the front panel, behind the nut. The suggested switch has a plastic nut which should only be tightened a little more than finger tight. The knobs can now be fitted, ensuring they do not scrape on the panel.

The rear panel can now be prepared, with a hole for the mains cable grommet or clamp, and appropriate holes for the audio connectors being used. The prototype has 5 pin DIN sockets, however phono sockets would probably be more suitable in most setups. You may prefer to use two sockets each for Left Input and Right Input, connected in parallel, to save using Y splitters or making custom cables (see the Installation Diagram, Fig 4). The same could apply for the Output if using a stereo amplifier, so the signal can be taken to both channels.

The transformer should be mounted in the base of the case. Mount it away from the circuit board and audio connectors, but where the leads will still comfortably reach the board and switch. Fit a solder tag under one of the mounting nuts for the Earth connection, having scraped away any lacquer from around the hole in the transformer first. The prototype used a 100mA wire ended transformer, which was more than adequate for the job.

Wiring Up
The Interwiring Diagram is shown in Fig 3. Start with the audio connections, using suitable screened cable. If you wish to sleeve the screens, use a piece of the outer insulation removed from the wire. If two phono sockets are being used for the Inputs and/or Output, as suggested earlier, they can be linked together with lengths of tinned copper wire, providing the sockets are no more than about 40mm apart.

Connect the "Left Input" socket to P4 (core) and P5 (screen) on the PCB, the "Right Input" socket to P6 (core) and P7 (screen), and the "Output" socket to P8 (core) and P9 (screen).

The mains wiring should now be carried out, with great care! Mistakes here can be costly and dangerous.

The mains cable is standard 3 core mains flex, rated at 3 Amps or greater. A grommet or cable clamp must be fitted in the hole where the mains cable enters. The cable must be secured in some way. As a last resort you could tie a knot in the cable, but try to do something better.

Remove sufficient outer insulation for the earth (Green/Yellow) wire to reach from the switch to the tag on the transformer mounting. Trim the Live (Brown) and Neutral (Blue) wires to about 30mm, strip the ends, and solder to the switch as shown in the Interwiring Diagram, ensuring that there are no loose strands. These joints should be sleeved for safety.

The thick Brown and Blue wires from the transformer primary should now be soldered to the switch, as shown in the Interwiring Diagram, and sleeved as before. To save possible damage to the transformer, leave these wires the full length as supplied.

The Earth (Green/Yellow) wire in the mains flex should now be soldered to the tag under the transformer mounting nut, together with a length of wire sufficient to reach the PCB. If the case has a metal front panel, a wire should be connected from this to the earth tag (a metal rear panel will probably be earthed by the connectors mounted on it, if not connect it to the tag also). It may be easier to remove the tag whilst soldering. Recheck the tightness of the transformer mounting screws after soldering these joints.

The wires from the secondary of the transformer will be two of one colour (possibly Green), and one of another colour (probably Black). Connect the single wire, together with the wire from the transformer earth tag, to terminal P3 on the PCB. Connect the two same colour wires to terminals P1 and P2 on the PCB, either way round. As before, it is best to leave the transformer wires full length.

Connect a 13 Amp plug, fitted with a 3 Amp fuse, to the end of the mains flex.

Now thoroughly recheck the wiring, particularly the mains connections. Remember that any mistakes in the mains wiring are potentially lethal.

Before plugging the unit in for the first time, carry out the following tests. Make sure the power switch is off. Set your test meter to it's highest resistance range, and measure the resistance between Live & Neutral, between Live & Earth, and between Neutral & Earth on the mains plug pins. In all cases the meter should read open circuit.

Now set the power switch to ON and carry out the above tests again. The resistance between Live & Neutral should now be between 1K and 2K (transformer primary), the other two tests should still read open circuit.

Set your test meter to the lowest resistance range available, and measure the resistance between the Earth pin on the mains plug and the following points: both transformer mounting screws, the screens of all audio connectors, and any exposed metal parts of the case. In all cases the meter should read less than one ohm.

If any of the above results are incorrect, recheck your wiring thoroughly and find out why! It is fairly easy to incorrectly wire a mains switch so that it shorts across the mains when switched on. Do not plug the unit into the mains until the results of all the above tests are correct.

Ensure that the plug-in IC's (IC1, IC2 and IC3) are not fitted. Plug the unit into the mains and switch on. Set your test meter to 30 Volts DC or greater, and connect it's negative probe to the earth tag on the transformer. Connect the positive probe to the cathode (bar) end of D2 or D3, the meter should read between 20V and 27V. Now connect the positive probe to pin 3 of IC1 socket, the meter must now read between 14.25V and 15.75V.

If all is well so far, switch off and wait five minutes, for the power supply smoothing capacitors to discharge. Now plug in the IC's, and switch on. Measure the voltages at the following points: IC1 pin 1, IC1 pin 7, IC3 pin 13, IC3 pin 14, IC1 pin 8, and IC1 pin 14. In all cases the voltage should be between 5 and 7 volts (it may be necessary to switch your meter down to it's 10V DC range for this). Switch off and assemble the case.

Now install the decoder and test out, as detailed below.

Installation and Use
The ideal viewing room setup is shown in Fig 5, although some compromises may well have to be made in practice. Two rear speakers are really required, if only one speaker is used it's position will be heard. These speakers should be behind the viewer, towards the rear corners of the room. The effect is a little better if the rear speakers are further away from the viewer than the front speakers. Remember that the rear speakers do not have to be of particularly good quality, since the rear channel bandwidth is only 100Hz to 4kHz. The author used a pair of small wooden cased speakers which originated from a stereo music centre, and a low power stereo amplifier. His supply of Dolby Stereo films arrived via an Amstrad SRD400 satellite receiver.

Connect the unit to your home entertainment equipment, as shown in Fig 4. Note that the Audio Out from the satellite receiver or VCR needs to connect to both the decoder and your main amplifier, this is the reason for the suggestion of paired sockets. Set RV1 central and RV2 to minimum. Leave the main amplifier off for now, and switch on the surround amplifier, this unit and the satellite receiver or video recorder.

Choose a mono program (if you have satellite try Sky News), and turn up the volume control (RV2). If the balance control (RV1) is adjusted towards either end the sound should be heard from the rear speakers. When the balance control is adjusted towards the centre it should be possible to find a point where there is virtually no sound from the rear speakers.

Leave the balance control at this position, and select a stereo film channel (Sky Movies or The Movie Channel). If the film is in Dolby Stereo, you should get a significant amount of sound (but not speech) from the rear channel. If you can't get this to work, check your satellite receiver is set to the main stereo channel (AU1 on Amstrad receivers), and check the film really is in Dolby Stereo (the blockbusters shown on Sky in the evenings often are).

Switch on the main amplifier and set the volume to your usual listening level. Now adjust the level of the rear channel to a level where the rear sounds contribute to the overall sound, without intruding. Dialogue should still appear to come from the front, whilst crowds, music and dramatic sound effects should fill the room.

A certain amount of practice will be needed to get the best results from the unit, it's really a matter of trial and error, but it's worth the effort!

The results from this simple unit can be quite stunning, it really does add a whole new dimension to home entertainment. The effects on some modern films are dramatic! Sky Television are obviously well aware that people use this sort of equipment, since even the trailers on their film channels make very good use of the rear channel.

If your main amplifier has an output available, after the volume control, try connecting the decoder to this, that way the volume of the rear channel will follow adjustments on the main amplifier.

This decoder will only work correctly with Dolby Stereo encoded film soundtracks. The effect of the delay will cause awful results if the unit is used with normal stereo programmes or music.

A Silly Idea!
Here's a further idea that a few of you might like to play around with. If the decoder is used without the front amplifier, on a normal stereo music source, the balance control can be adjusted to give the music in mono with little or no vocals. Do-it-yourself Karioke for those who like that sort of thing! Of course the bandwidth is very limited and the bass beat may be cancelled out as well, but it will probably still sound better than the singing!!

Adding a Power Amplifier
The information in this section is believed to be correct but has not been fully tested. It is therefore offered to more experienced constructors, for further experiment.

Some constructors may wish to incorporate a power amplifier into the same case as this decoder.

As only a few watts of power are generally needed for the rear channel, a suitable amplifier might be the Maplin 8 Watt Power Amplifier kit (LW36P, Price #7.45, Page 292 of the 1992 catalogue). This requires a power supply of about 21V, and a suitable circuit is shown in the catalogue, this could be constructed on a piece of tag strip or Veroboard.

It would be sensible to use the amplifier's power supply to power the decoder circuit. On the Surround Sound Decoder PCB, remove D1 and D2, and replace D1 with a link. The 21V form the amplifier power supply can now be connected to P1 (positive) and P3 (negative). If a different amplifier is used, a supply between 18V and 35V can be used to power the decoder this way.

The audio output from P8 and P9 can be connected directly to the amplifier input, using suitable screened cable. If you get a hum loop, try disconnecting the screen at the amplifier end. It may be necessary to reduce the gain of the power amplifier, by adjusting the values of the appropriate resistors on the amplifier PCB. A suitable connector or terminals for the loudspeaker can be fitted on the rear of the case.

Unless continuous operation at high volume is likely, a metal case should be adequate for heatsinking the amplifier and power supply voltage regulator.


Zip Files

The MN3004 and MN3101 ICs are believed to be obsolete.

I have been advised that they are currently available from "DigiKey" - see www.digikey.com for more information. It appears that you have to buy in lots of 25 devices, and the price is not quoted...

Parts List

RESISTORS (All 1/4 watt 5% or better)

R1,2,3,4,8,10,11,17,19,20 22K
R5 220K
R6 180K
R7,13,14 100K
R9,18 1K0
R12 68K
R15,16 4K7

POTENTIOMETERS (Miniature, PCB Mounting)

RV1 100K LIN


C1,2 220n
C3,4,8,9 4n7
C5,10 2n2
C6 220p
C11 10n
C14,15 100n


C7 10u 25V
C12 1u0 25V
C13 470u 35V
C16 100u 25V


IC1 LM324
IC2 MN3101
IC3 MN3004
IC4 78L15


D1 1N4148
D2,3 1N4001


P1-9 Terminal Pin
SW1 DPDT Rocker Switch
SK1,2,3 Phono Socket (or as required)
T1 240 to 15-0-15 100mA

IC Sockets (1x8 way and 2x14 way), Case, Knobs for RV1 & RV2, Mains Flex, Screened Cable, Sleeving, Fixings for T1, Solder Tag, Grommet or Cable Clamp, 13A Plug with 3A Fuse, Audio Connection Leads as Required.

Dolby, Dolby Stereo, Dolby Surround and the Double-D Symbol are trademarks of Dolby Laboratories Licensing Corporation. This project is based on generally available information and to the best of the authors knowledge does not infringe any of Dolby's patents.

All other trademarks acknowledged.

Some aspects of this project are based on information from an article by Robert Ball in Electronics - The Maplin Magazine, Issue Number 37.

This project, including all text, images and diagrams, is copyright 1991 - 2003 Paul Stenning. No part of this article may be reproduced in any form without prior written permission from Paul Stenning and WallyWare, inc. All details are believed to be accurate, but no liability can be accepted for any errors.

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