SURROUND SOUND

How it works Build a passive matrix speaker network which decodes surround sound Build a wireless surround sound system for as little as $40.00

WARNING!

The circuits and wiring diagrams below have been tested on several hi-fi stereo systems with no damage to the systems. However, I cannot be responsible for some hair-brained engineer who may have used a radical design that might be damaged by an unorthodox speaker hookup. Use these circuits at your own risk.

History of Surround Sound

After reading many articles about surround sound I have come to the conclusion that most of the writers haven't done their homework. Some writers trace the origin of surround sound to the 1976 movie Star Wars. Others trace it to the abortive attempts at marketing quadraphonic (four-channel) stereo in the early 1970's. Where did surround sound originate; what was the first surround sound movie? So far, the earliest reference I have seen to what is now called surround sound was a documentary on the making of the 1940 Disney movie, Fantasia. The concept of encoding a third channel into a stereo signal is so simple that I'm sure it was thought of as soon as stereo sound was developed.

What about Star Wars? Star Wars was released in a six-channel stereo format which was already at least 15 years old in 1976. I suspect the truth is that, starting with Star Wars, surround sound became almost universally used for movie prints destined for theaters lacking the more expensive multichannel sound systems. Home theater surround sound is simply a by-product of these prints. There is no need to re-record the sound for home video. Once a movie is encoded with surround sound, it remains with the movie for the home video release.

How Surround Sound Works

With standard stereo, the separate signals are sent to separate speakers, one to the right of the listener and one to the left. The sounds produced by the speakers may seem to come from one speaker or the other, or may seem to come from empty space anywhere between the speakers.

Amplifier connections

Figure 1: speakers in unison, sounds appear to come from between the speakers
If the sound level in both the right and left speakers are equal, the sound will seem to come from a point in space half-way between the speakers.

Figure 2: If a sound is louder in one speaker than the other the sound will be heard closer to the louder speaker
If one speaker is louder than the other, the sound will seem to come from a point in space closer to the louder speaker.

When a stereo signal is recorded, microphones are placed so that the sounds in each speaker is at the correct level to make sounds seem to come from the directions they originated form. Because of this all of the instruments of a symphony orchestra will seem to come from their proper places between the speakers; violins, trumpets and timpani toward the left; violas, woodwinds and horns toward the center; cellos, basses and trombones toward the right.

Phasing

Stereo sound, as described above, works only if the speakers are in phase, meaning that the speakers are wired to work in unison. To be in phase, the speaker cones must move together; they must move toward the listener at the same time and must move away from the listener at the same time.

If you accidentally wire one (but only one) speaker backwards, the speakers will work out-of-phase; one speaker cone will be moving toward the listener while the other speaker cone is moving away from the listener and vice versa. This causes a total collapse of the stereo effect. All sounds will seem to come from everywhere between the speakers at once or may seem to come from both speakers at once. To be sure your speakers are in phase; tune to an AM radio station (or use another non-stereo signal); adjust the volume to be equal in both speakers. The sounds should seem to be coming from a single point in space between the speakers and nowhere else. If this is not the case, the speakers are out-of-phase. To fix the phasing, reverse the wires on one (and only one) speaker.

Figure 3: speakers out of phase, listeners can't tell where sound comes from

Using Phasing to Add a Third Channel to a Two Channel System

Encoding surround sound is very simple. The third channel is simply mixed into the left channel as if it were any other microphone channel. However, before being mixed with the right channel, the phase of the third channel is reversed. This has the same effect as if the wires on the right speaker were reversed, but only for sounds encoded for the third channel. Therefore, sounds intended for the left and right speakers will sound normal. Sounds encoded for the third channel will be hard to locate and may even be lost in the shuffle unless deliberately listened for.

Decoding the Surround Channel

In order for a speaker to work there has to be an electric current flowing through it. As the current through the speaker changes, the speaker cone moves back and forth. The more current flowing through the speaker, the farther the cone moves. The current actually reverses direction so that the speaker cone is alternately pushed out and pulled in, creating sound waves. [missing diagram of typical speaker]
Figure 4:
To decode surround sound is also very simple. All you have to do is add a third speaker. The secret is how the third speaker is hooked up to the system. Normally a speaker is hooked up to the red and black (or + and -) connectors on the stereo amplifier. The third speaker has both wires hooked to the red (or +) connectors on the amplifier. This is the simplest way to create a surround sound system.

Figure 5: Surround sound with sounds equal in the front channels and no sound in the rear channel
Notice in Figure 5 that current flows through the front speakers, in one side and out the other. Current doesn't flow in the third speaker because current is trying to flow in both sides at once.

Figure 6: Surround with sounds only for the rear channel
With sounds encoded for the third channel, the current flows out of phase for the front speakers. Notice in Figure 6 that when the current flows out of phase in the front speakers, it is presented to the rear speaker in such a way that it is able to flow through the speaker. Therefore, sounds encoded for the rear speaker are also heard in the front speakers, but are out of phase so they are difficult to locate. On the other hand, these sounds are heard solidly in the rear speaker so are only perceived to come from there. It actually works better if you use two speakers for the surround channel. When two speakers are used for the surround channel, the black (or -) wires are connected together as shown in Figure 7.

Figure 7: Decoding surround sound with two rear speakers
In the above example the regular stereo sounds are considered to be equal, thus heard halfway between the front speakers and totally unheard in the rear speaker. In reality, sounds are often unequal in the front speaker and thus heard off center. With these sounds there is a proportional leakage to the rear speaker. This has two effects: first, the off center sounds are "pulled" farther off center; second, it adds a pleasing ambiance to the sounds in the front speakers even when no third channel is encoded. This is exactly what Radio Shack's Quatravox four channel simulator did. If you happen to have a Quatravox it will decode surround sound.

Dolby Surround and Dolby Surround Pro Logic

So what about Dolby Surround® and Dolby Surround Pro Logic®? Dolby Surround® adds a 20 millisecond delay to the rear channel. This makes your room sound larger. Since the sounds from the rear channel arrive late, it seems as though they came from a greater distance. Pro Logic® add a sophisticated "gain riding" amplifier to the system. This amplifier attempts to determine where the loudest sound is and boosts the appropriate amplifiers to emphasize that sound. The reason for this is to help solve the problem of signal leakage from the front channels to the rear channels, as mentioned above. Mathematically, there is only a 3db separation between the front and rear channels. Theoretically, this separation would be barely detectable; you would have to listen hard to tell that there was any difference at all. However, in reality the human auditory system hears things differently.

First of all, any sound that is equal in the front channels (one which would be centered between the two front speakers) is completely rejected by the rear speaker (as mentioned above); it will not be heard in the surround channel. Any sound which is out of phase between the two front channels (as the surround channel will be) will be passed through and heard at full volume in the rear speaker. Since the surround channel is out of phase in the front speakers, the listener cannot distinguish where, in relation the front speakers, the sound is coming from. As a result, by the time the surround channel reaches your brain, it seems to come completely from the rear speaker.

The second problem Pro Logic® attacks is that sounds toward the edge of the TV screen tend to be pulled off the screen altogether; someone talking near the edge of the screen will seem to be heard off screen. This is once again caused by leakage of the front channel sound to the rear channel. Pro Logic® adds a fifth speaker between the two front speakers. This fifth speaker is fed with an equal mix of the front channel signals, mixed in phase; sort of the opposite of the rear channel. This pulls sounds back to the screen. Placing your front speakers close to our TV helps solve this problem also. (Do not place them too close as the magnets in the speakers will cause color shifts on the TV screen.)

Two Speaker Systems

There are some two-speaker surround sound systems on the market. These systems use sophisticated phasing and time-delay circuits to fool the auditory system into hearing sounds from points in space that are not between the speakers. I have yet to listen to one of these system myself. Reviews in magazines report that sounds that should come from behind actually come from the side. They also report that some sounds come from random directions. A four speaker system is cheaper and works better. (Lately, I've noticed that two-speaker surround sound is being pre-encoded into some TV commercials. In these commercials, with no surround sound decoding equipment at all, sounds seem to come from places well outside the speakers.)

Front-to-Rear balance

Now, what about controlling the balance between the front and rear channels? There are two basic approaches to this:

One approach would be to buy a second stereo amplifier for the rear channel. Simply wire the speakers to both + or red terminals (leaving no connection to the - or black terminals); A second method would be to wire in a stereo fader (available from Radio Shack). These faders are designed for automobile stereo systems and can handle only 25 Watts per channel. However, all you have to do is wire in the fader according to Figure 8. Figure 8 includes a switch to switch between surround mode and regular stereo mode.

Figure 8: Wiring a stereo fader into the matrix

Balancing the System

In order for the above surround sound systems to work, the stereo balance must be adjusted to null out the front channels in the rear speakers. To do this: tune in an AM radio station (as mentioned above for checking the phasing); disconnect the front speakers and adjust the speaker balance control until the lowest possible sound level is heard in the rear speakers; hook up the front speakers and you are ready to go. For the surround sound to work properly this balance must be maintained.

Building an Active Decoder

Commercially made surround sound systems use an electronic circuit called a differential amplifier to decode the surround sound. Since a differential amplifier uses active electronic components, such as transistors, it is considered "active".

The differential amplifier simply does, electronically, what the rear speaker does in the passive decoder described above. The differential amplifier rejects any signals that are identical in the front channels and pass any differences to the rear channel. The output of the differential amplifier is sent to the regular audio amplifier and then the rear speaker(s). One advantage of this type of decoder is that you don't have to worry about the null balance adjustment mentioned above. This adjustment is made in the differential amplifier once and does not need to be readjusted.

If you want to take this route you can make a differential amplifier with only four components, all available from Radio Shack (some Radio Shack catalog numbers are given preceded with RS). With a few extra support components a self-contained surround sound decoder can be built. The active decoder is so simple that it can be built on a "breadboard" PC board also available from Radio Shack. The full schematic is shown, including the pinout of the IC, in figure 9.

Figure 9: An active surround sound decoder
The circuit is powered by a 9V DC converter. The IC, however, is designed to operate from dual power supplies. To make it work with a single power supply an artificial ground is used. The artificial ground is generated by using the second amplifier in the LM1458 IC as a voltage follower. A voltage divider (two 100k resistors) established the ground point at 4.5V (half of the 9 Volt supply). In the decoder circuit, the artificial ground should be considered as zero Volts, the V+ as +4.5 Volts and the V- as -4.5 Volts. However, since the circuit is actually powered from a single power supply, the power input is labeled as +V (9V+) and -V (9V-) representing the + and - terminals of the DC converter. Bus connections, such as +V, -V and GND are shown by symbols only. This keeps the schematic from looking too confusing.

The input of the active decoder is to be connected to the "Tape Out" of your stereo amplifier. The output of the decoder will go to another amplifier to drive the rear speaker(s). The DPDT switch can remove the decoder from the system for regular stereo in both the front and rear speakers.

The null adjustment is made as described above for the passive decoder except instead of adjusting the speaker balance the 20k 15 turn trim-pot is adjusted.

Wireless Surround Sound System

One thing you can also do with the active decoder is attach it to a Ramsey Electronics FM-10x Stereo FM Transmitter. With this combination you can transmit the surround channel to any FM radio for a wireless surround sound system.

You can save some effort by wiring the active decoder right into the FM transmitter. Power is derived from the transmitter's power supply (a 9 Volt battery if the transmitter is not modified). The RCA connectors and the ground generator are eliminated, reducing the circuit to the four components actually needed for the differential amplifier. The stereo/surround switch could also be added to the transmitter.

Figure 10: Decoder to add to Ramsey FM-10 stereo transmitter
Since I am unfamiliar with the newer versions of the FM-10x transmitters I cannot give a wiring diagram. You will have to cut the circuit board traces where the signals enter the transmitter and wire the decoder in there. Pin 11 of the BA1404 IC acts as an artificial ground and is where the GND connection of the decoder goes.

One More Thing

One unadvertised thing a surround sound decoder will do is reduce the volume of the lead vocal in most popular music. Reduce the volume in the front speakers and you have an instant karaoki machine.