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By LEO SIMPSON Dolby Surround Sound: How It Works You’ve been to the movies. You’ve seen the pictures. You’ve heard the sound. And now you want it all at home, in your living room. And while you may not be able to afford the latest widescreen TV or LCD video projection system, you can at least have the big sound of the movies. Read on, to find out how Dolby Surround Sound works. Many people, if they think about it at all, may assume that the latest Dolby Surround Sound used to such great effect in today’s action movies is somehow related to the “failed” surround sound systems of the early 1970s. These were variously referred to as surround, four channel and quadraphonic. There where three competing systems, all of which were incompatible with each other and all required two additional speakers in the rear corners of the listening room, to reproduce extra channels from specially encoded stereo LP records. Older readers will remember the names of those systems as “Sansui QS”, CBS “SQ” and JVC’s CD-4. Fig.1: this shows the arrangement of speakers used in a typical Dolby stereo cinema system. The film sound track provides two stereo channels & these are fed to the Dolby processor to provide four main channels, Left, Centre, Right & Surround, together with an optional Subwoofer channel for extra bass. LEFT SUB “SQ” and “QS” sound systems were “matrixed” systems in which the rear channel signals effec­ tively rode on top of the normal stereo signals and were recov­ered with relatively simple phase-splitting circuitry. The re­sulting rear channels did not have much separation from each other or from the front channels (about 7dB) but they did give an illusion of extra space or “ambience” to the sound quality. The CD-4 4-channel system, by contrast, was much more ele­gant and did provide quite good separation between front and rear channels (up to 20dB) but it was also much more expensive, push­ ing the existing LP recording technology to the absolute limits. Briefly, a CD-4 encoded record has the regular stereo sign­als plus a high frequency carrier at 30kHz. This was phase and frequency modulated with rear difference signals (up to 45kHz) CENTRE SCREEN POWER AMPLIFIERS OPTIONAL SUBWOOFER RIGHT Lt Rt DOLBY STEREO CINEMA PROCESSOR CENTRE LEFT SURROUND SURROUND 4  Silicon Chip RIGHT DOLBY A-TYPE NR OR DOLBY SR Lt Rt BASS EXTENSION 4-CHANNEL DECODER OPTICAL PREAMP ADAPTIVE MATRIX 7kHz LOW-PASS FILTER AUDIO DELAY MODIFIED DOLBY B-TYPE NR DECODER Fig.2: the block diagram of the Dolby stereo cinema processor. The key point to note is that the surround channel is fed via an audio delay of typically 20 milliseconds. This is partly to provide ambience or reverberation & partly to allow your ears get the cue for direction from the front speakers. which were subsequently recovered from the outputs of the RIAA preamplifier. It required a phono cartridge with a special elliptical (Shibata) stylus, very low compliance and extra special tracking capability. If you played a CD-4 disc with a conventional stereo cartridge, the high frequency carrier would quickly be ploughed out and the disc would no longer provide four channel sound. After a few years, consumers were so confused with the claims and counterclaims for the different systems that they avoided the issue entirely and four channel sound just died. Dolby Surround Sound Dolby Surround Sound is quite different from all three of the above ill-fated systems and is continually growing in its acceptance by movie-goers and consumers in the home. So let’s see how Dolby works. Dolby stereo surround sound based on optical sound tracks was introduced to movie theatres in the mid-seventies but its arrangement of the channels is quite different to that used in previous home surround sound systems. Fig.1 shows the arrangement of channels and speakers used in a typical Dolby stereo cinema system. In essence, the film sound track provides two stereo channels, Lt (left total) and Rt (right total) and these are fed to the Dolby processor or decoder to provide four main channels, Left, Centre, Right and Surround, together with an optional Subwoofer channel for extra bass. Notice that all the Surround speakers are fed with the same signal – there is no left rear and right rear, just surround and this signal happens to be delayed, by 20 milliseconds or more, with respect to the front speakers. Why three front speakers? The trouble with having just two stereo speakers in a cinema is that the screen is too wide. For people at the sides of the theatre, the nearest speaker predominates. This sounds and looks silly when the source of the sound is clearly at the centre or on the other side of the screen. This is where the centre channel comes into its own and provides much better sound localisation. The centre channel should not be thought of as merely a “fill-in” speaker but as quite separate from that provided by the two stereo channels. Indeed, when the centre channel is used in a typical movie, it is used only for dialogue and the stereo speakers are silent while it is in use. Once you become aware of it, you SUBWOOFER 3rd OCTAVE EQUALISER RIGHT 3rd OCTAVE EQUALISER CENTRE 3rd OCTAVE EQUALISER LEFT PARAMETRIC EQUALISER SURROUND will often notice the profound switch between stereo sound over to the centre channel and then back out again. In effect, Dolby provides three separate front channels: they are used separately for dialogue while the left and right are used for stereo music accompaniment. The surround speakers are used during action scenes, with the delay providing a great deal of ambience, reverberation, echo or whatever. While you may think that having all the surround speakers driven from the one source would make rear sounds some­what vaguely located, your ears get their cues from the front speakers so that the apparent localisation is quite strong. Dolby decoding Fig.2 shows the block diagram of the Dolby stereo cinema processor which is featured in the setup of Fig.1. This shows input preamplifiers for the projector’s optical pickups, followed by Dolby A noise reduction, and then the two stereo signals are fed to the 4-channel decoder. This is partly based on the 4-channel matrix systems of the early 1970s and the key to it is the box labelled “Adaptive Matrix”. The outputs of this matrix become the LEFT CENTRE Lt -3dB SURROUND -3dB BPF DOLBY NR ENCODER +90ø -90ø RIGHT Rt Fig.3: the basic Dolby surround encoding setup. The encoder accepts four separate input signals – left, centre, right & surround (L, C, R & S) – & creates two final outputs, left-total & right-total (Lt & Rt). October 1994  5 DIGITAL DECODER A systems both are four channel sys­tems, their method of operation is quite different. MONO Dolby encoding While you might expect the Dolby signal encoding process to be complicated, it LEFT is actually simpler in princiDIGITAL 2 CHANNEL DECODER ple than the decoding. Fig.3 STEREO B shows the basic encoding setRIGHT up. The encoder accepts four separate input signals, left, DOLBY SURROUND DIGITAL PROGRAM MATERIAL centre, right and surround (L, C, R & S), and creates two LEFT 4 CHANNEL ANALOG DIGITAL final outputs: left-total and CENTRE ANALOG PRO LOGIC DECODER DOLBY RIGHT right-total (Lt & Rt). DECODER C SURROUND SURROUND The L and R inputs go straight to the Lt and Rt outputs without modification. LEFT The C input is divided equalCENTRE ly to Lt and Rt but with a 3dB DOLBY 5 OR 5.1 CHANNEL DOLBY RIGHT SURROUND SURROUND DIGITAL level reduction (to maintain DIGITAL LEFT SURROUND (SUBWOOFER OPTIONAL) a constant acoustic power in DECODER RIGHT SURROUND the mix). SUBWOOFER The S input is also dividFig.4: this diagram shows the various decoding possibilities from Digital Dolby ed equally between Lt and Surround which is encoded on to optical tracks between the film sprocket holes. Rt but it goes through three Note that it provides left & right surround channels but this is not possible via Dolby more processing steps: (1) encoded video tapes. bandwidth limiting from 100Hz to 7kHz, (2) encoding four channels – left, centre, right and After the delay line, the surround with modified Dolby B noise reducsurround – but the surround signal signal is filtered to remove any noise tion, and (3) ±90° phase shifts which goes through a little more process- above 7kHz and then passed through are applied to produce a 180° phase ing before it is fed to the surround a modified Dolby B noise reduction difference between the signal compoamplifiers. decoder so that the net result is that nents added to Lt and Rt. First, it goes through an audio delay when no surround signal is supposed A number of points should be made line which is there for two reasons. to be present, the rear speakers are about the overall encod­ing process. First, it provides the echo or reverber- quiet. First, there is no loss of separation ation which results in the “big” sound Note that all channel signals are between the original left and right of cinemas. Second, by delaying the subjected to substantial equal­isation signals. Second, there is also no theo­ surround sound, your ears get the cue before being fed to their amplifiers retical loss of separation between the for direction from the front speakers and loudspeak­ers. It should be clear centre and surround sign­ als. This and this provides the localisation re- by now that while Dolby Surround follows because the surround signal ferred to above. and the ill-fated quadraphonic sound is recovered by taking the difference INPUTS Lt Rt LEFT INPUT BALANCE CONTROL RIGHT SURROUND MASTER VOLUME CONTROL L-R L/R BALANCE SURROUND TRIM SURROUND ANTIALIAS FILTER AUDIO TIME DELAY 7kHz LOW PASS FILTER MODIFIED DOLBY BTYPE NOISE REDUCTION UNIT Fig.5: this is the block diagram of a “passive” Dolby surround sound decoder which does not incorporate the directional enhance­ment feature of Pro-Logic decoders. 6  Silicon Chip OUTPUTS LEFT RIGHT SURROUND Lt VCA LEFT VCA RIGHT L+R VCA CENTRE L-R VCA SURROUND PASSIVE DECODER Rt Fig.6: directional enhancement could be provided if each decoded output had its own voltage controlled amplifier (VCA). However, this does not work well as dialogue in the centre channel could cause the music in the stereo tracks to be pumped up & down. CONTROL CIRCUIT between the Lt and Rt signals and any identical centre channel components will be cancelled in the surround output. Similarly, since the centre channel (when decod­ed) is derived from the sum of Lt and Rt, the equal and opposite surround sound components will be cancelled out. This concept of precise cancellation to maintain separation between the centre and surround channels presupposes that the two main transmission channels have virtually identical gain and phase characteristics. If they don’t, separation between the centre and surround channel signals will be poor. Digital Dolby In 1992, a new 35mm format called SR.D was introduced by Dolby Laboratories. Between the film sprocket holes on SR.D prints is a 6-channel digital sound track. The older Dolby stereo Surround information is also present so that cinemas without the digital sound equipment can still show them. Dolby Surround at home All the movies which have been subsequently released on videotape or broadcast from TV stations have the original Dolby surround encoding information still present in their stereo sound tracks. In 1982, Dolby introduced a surround decoder for the home. When driving the required number of amplifiers and loud­ speakers, this can provide a convincing reproduction of theatre sound in the home. Dolby Pro-Logic The most recent development in home surround sound equip­ ment is the Dolby Pro-Logic decoder, first introduced in 1987 and now more or less standard in deluxe home theatre systems. This gives a substantial enhancement to sound localisation compared with the so-called “passive” decoder depicted in Fig.5. Before we describe the Pro-Logic system we should discuss some other means of directional enhancement which have been tried. Directional enhancement refers to any technique that attempts to improve the separation between channels by modifying the outputs of the matrix decoder. The first of these is “gain riding” whereby each decoded output has its own voltage controlled amplifier (VCA), as shown in Fig.6. Consider the case where the dialogue is present in the centre channel (so that Lt = Rt). This means that the centre channel will have the dialogue but so will the left and right hand speakers. To enhance the centre channel, the decoder could increase the gain of the centre amplifier and reduce that for the stereo channels. The same procedure could be used to isolate the left channel when only left LEFT Lt INPUTS The new digital system provides five full range channels for left, centre and right speakers, plus separate left surround and right surround speaker arrays in a configuration known as stereo surround. A sixth, bass only channel for subwoofers gives rise to the de­ scription “5.1 channels”. Fig.4 shows the various decoding possi­bilities from Digital Dolby Surround. Dolby’s digital sound has been a major factor in the suc­cess of recent movies, particularly “Jurassic Park”. Fig.5 shows the block diagram of a Dolby surround sound decoder and this can be compared with the Dolby stereo cinema processor shown in Fig.2. Note that the centre channel is lacking. At present, the six channel digital sound encoding is not available via the video tape format but it may become available on future digital video discs. DOLBY PROLOGIC ADAPTIVE MATRIX INPUT BALANCE CONTROL Rt MASTER VOLUME CONTROL RIGHT CENTRE CENTRE TRIM SURROUND NOISE SEQUENCER ANTIALIAS FILTER SURROUND AUDIO TIME DELAY L/R BALANCE 7kHz LOW PASS FILTER MODIFIED DOLBY BTYPE NOISE REDUCTION UNIT SURROUND TRIM OUTPUTS LEFT RIGHT CENTRE SURROUND Fig.7: the block diagram of a Dolby Pro-Logic decoder. If you compare this with the passive decoder depicted in Fig.5, you will see that the main difference is in the Pro-Logic Adaptive Matrix & the addition of the centre channel. October 1994  7 INPUTS Lt Rt FULL-WAVE RECTIFIER BANDPASS FILTERS FULL-WAVE RECTIFIER LEFT/RIGHT DOMINANCE SENSE LOGDIFFERENCE AMPLIFIER DUAL TIME CONSTANT E POLARITY SPLITTER ER THRESHOLD SWITCHES L+R L-R FULL-WAVE RECTIFIER FULL-WAVE RECTIFIER LOGDIFFERENCE AMPLIFIER DUAL TIME CONSTANT Below: this photo shows our soon to be published Dolby Pro-Logic Sur­ round Decoder which is presently being assessed by Dolby Labora­tories in California, USA. 8  Silicon Chip EL E ER E CL EC E SL ES OUTPUTS LEFT COMBINING NETWORKS RIGHT CENTRE SURROUND E POLARITY SPLITTER FRONT/REAR DOMINANCE SENSE Fig.8: this diagram shows the Adaptive Matrix used in the Dolby Pro-Logic decoder. The four signals are fed to full wave rectifiers & then to logarithmic detection circuits to determine the dominant signal for subsequent directional enhancement. channel signal was present, by turn­ing down the gain of the other channels. Unfortunately, this system of gain riding with VCAs does not work with real film sound tracks. In particular, stereo music is usually present along with dialogue. If dialogue is the domi­ nant factor and is used to vary the gain of relevant channels, the volume of music will inevitably be pumped up and down. E To solve this problem, Dolby has come up with the concept of “signal dominance” – the sound that is most dominant in the sound mix at any instant in time. This can then be used to vary the gains of the relevant channels and thus give the desired direc­tional cues. Dolby also suggest that if a decoder is to detect and use signal dominance to set the channel gains, it needs two addition­al characteristics to work effectively. First, it must be fast enough to provide enhancement on an instantaneous basis when the signal peaks are prominent enough to be heard as separate events. Second, it must sense when the relative signal dominance ES 8VCAs falls below a threshold where it is no longer necessary to provide any substantial directional enhancement. For these reasons, the Pro-Logic decoder has sensing circuits which ignore the absolute signal levels but respond to the logarithmic difference in levels between signals and thereby determine direction of signal dominance. Thus it can provide the correct degree of directional enhancement or gain adjustment of the four channels. Fig.7 shows the block diagram of a Pro-Logic decoder. If you compare this with the so-called passive decoder depicted in Fig.5 you will see that the main difference is in the Pro-Logic Adaptive Matrix and the addition of the centre channel. The detail of the Adaptive matrix is shown in Fig.8 and it has two sections: Left/Right dominance sense and The construction of our proposed Dolby Pro-Logic Sur­round Decoder will be quite straightforward, with most of the parts mounted on a single large PC board. It is based on a Dolby Pro-Logic chip set from Mitsubishi. Front/Rear dominance sense. In practice, the decoded channels from a Dolby Pro-Logic decoder can have a separation of up to 30dB which is more than adequate to provide strong directional information. The proof is in the listening, of course, and any action movie gets a major benefit from the Dolby sound track. Loudspeakers & amplifiers For a typical home Dolby surround setup you need quite a lot of equipment, apart from either a hifi stereo VCR (if you wish to play back Dolby encoded tapes) or a stereo TV set. If you have a conven­tional stereo VCR (ie, without hifi sound) you’ll be wasting your money since tape hiss will be a problem. You will need a stereo amplifier and a pair of stereo speakers for the front channels and a stereo amplifier for the rear speakers. It is normal practice to have a stereo amplifier to drive the rear speakers so that you can easily set the balance between them. For the centre channel, you have two approaches. The first is to use the “phantom channel” option available on most Dolby decoders. This divides the centre channel sound bet­ween the front stereo speakers and can give a good result in most home living rooms. The second approach is to use a centre channel amplifier and centre channel speaker which sits on top of or below the TV screen. Centre channel speakers must have full magnetic screening otherwise they will seriously degrade the picture quality from your TV, to the point where it is completely unwatchable. To emphasise this point, if you place a conventional unshielded loudspeaker within 30cm of your TV’s screen, you will magnetise the shadow mask inside and thoroughly “screw up” the colour purity. This will cause horrible colour blotch­es all over the screen and the set will then require de-guassing to restore its picture quality. The big problem about using a centre channel speaker, even if it is shielded, is that it should have the same sound quality as the main ste- reo speakers, as far as the treble and midrange is concerned. If it is a lesser quality speaker, the transition from centre to stereo speakers will be very no­ticeable – the centre channel will “squawk”. Most Dolby decoders have integral amplifiers for the rear and centre channel speakers and of course, you can purchase a full-on stereo receiver with Dolby Pro-Logic decoding builtin. This will have five power amplifiers so that external power amplifiers will not be necessary. Rear speakers Since the frequency bandwidth of the surround signal is limited to between 100Hz and 7kHz, the requirements for the rear surround speakers are not demanding and virtually any small pair of speakers will do the job. Do-it-yourself Pro-Logic Dolby Pro-Logic decoders are licensed by Dolby Laboratories who maintain strict control over technical standards. As this article is being written, a fully licensed design by SILICON CHIP staff is currently under review by Dolby Laboratories. We hope to SC publish the design very soon. October 1994  9