Reproduced from
Studio Sound, June 1990

superior quality from such systems. This work has
used much of the psychoacoustic theory of directionalsound localisation, originally developed in connection

Three Channels:
The Future of Stereo?
Michael Gerzon takes a fresh look at 3channel stereo in the light of recent highdefinition TV technology
The development of widescreen and HDTV systems for
television has renewed interest in 3-speaker/3-channel
stereo, supplementing the traditional left and rightspeakers with a central one in the middle of thescreen. Such 3-channel stereo systems are far fromnew – they were first investigated at Bell TelephoneLabs in the USA in the early '30s, and have been usedby the film industry since Walt Disney's Fantasia in
1939.
The prime motivation for using an additional channel
for a centre speaker is to 'lock' central sounds, such as
on-screen dialogue, into a stable physical relationshipwith the screen. With 2-speaker stereo, the nominallycentral image is liable to wander around according to
the location of the listener/viewer.
Domestic 3-speaker stereo operating from threechannels is potentially capable of far more than merely'locking' central dialogue into place. The extra speakerand channel can be used to improve the quality ofother non-central image locations to be markedly
better than from 2-speaker stereo. Via such improved
phantom imaging at other locations, 3-channel stereo
has the potential for a marked improvement even inaudio-only applications.
Yet there is a strange paradox here. Despite the factthat 2-speaker stereo (deriving from Alan Blumlein's
famous 1931 patent) and 3-channel/3-speaker stereo(deriving from the work of Snow, Fletcher andSteinberg at Bell Telephone Labs in 1933) are about
equal in age, no one to my knowledge has seriously
tried to design 3-channel stereo to optimise phantom
images in the way that Blumlein and others have donewith 2-speaker stereo. Snow and others did design 3channel panpots in 1934 to create phantom 3-speaker
images. Such panpots are still widely used in the film
industry but their work was on a much more empiricalbasis than Blumlein's and no systematic
psychoacoustic optimisation analogous to Blumlein's
work with two channels was done.
Here we outline recent work done by the writer on the
optimisation of 3-channel/3-speaker stereo systems torealise the potential for improved phantom images andwith Ambisonics but optimised here for frontal stage
sound. This work has involved not merely
understanding and optimising 3-speaker results, butalso the development of simple practical technology
and system design for the use of this know-how in thesound-mixing studio environment.
In previous work, the theoretical potential for 3speaker stereo to improve on the phantom-image
illusion of 2-channel stereo has not been realised inpractical systems. In fact, some 3-channel panpots
give inferior images near the central position
compared to 2-speaker stereo for a central listener.
This is very important when on-screen visual action is
to be matched in sound position. This inferior imaging
was not too important in large screen film presentation
in auditoria, as creating localisation illusions in large
auditoria is more hit-and-miss than in the home, due
to the longer paths and larger time delays from theloudspeakers to the listener.
It is often forgotten that the Bell work on 3-channel
spaced-microphone stereo was specifically aimed at
reproduction in auditoria, not in the home. As a result,enthusiasts for Blumlein-type stereo have often
unfairly criticised the Bell work as 'not properly
understanding stereo' but they forget that Bell were
not trying to solve the same problem (reproduction ofphantom images in smallish rooms with reasonably
centrally-placed listeners) as Blumlein.
While the Bell approach may be appropriate for large
auditoria, including widescreen film presentation, it isundoubtedly not the optimum for domestic 3speaker/3-channel reproduction. Such domestic
systems can only realise the potential improvement ofphantom image performance by careful design work
extending Blumlein and others' work on 2-speaker 2channel stereo.
Domestic uses
It now seems likely that 3-channel stereo will be used
for the reproduction of frontal stage sounds inconnection with domestic widescreen and HDTV
systems. Extra channels for this are available in thevarious D-MAC and other broadcast systems acrossthe world – and 'sub-band' systems can be used to
add additional data-compressed channels compatibly
to almost any digital sound broadcasting system,
conveying the additional information via the leastsignificant bits. The obvious advantage is that central
dialogue is locked to the middle of the screen
irrespective of listener/viewer position.

At first sight, one requires little more from a 3-channel

screen but to distribute them across a narrow stage

TV audio system than a conventional 2-speaker stereo

around the centre. But to do this well requires goodfrom the outer speakers for ambiance and soundquality phantom imaging near the centre of theeffects, plus a central 'dialogue' channel. This isscreen, not just at the centre.

because of the various typical problems encounteredin matching sound to picture.

A typical example is the broadcasting of a TV quiz
show. It is generally better if each contestant's voice isOver the decades, we have learned to accept

in a slightly different position from the others, even if

conventions of visual presentation that involve

this does not match the visual image. This way, if two

constantly changing angles and perspectives on acontestants speak at the same time, it is easier for thescene unlike anything encountered in real life. If one

viewer/listener to follow what both are saying. A

attempts to change the stereoism of the sound tosimilar consideration applies to interviews, broadcasts

match the picture, the ears are much less tolerant ofof drama, and of musical groups. The ability to form

sudden changes of position than the eye and inconvincing phantom images greatly helps thegeneral, it is better to present a fixed stereo soundintelligibility of stereo sound.

image even if the picture is constantly cutting between
different viewpoints. (An exception to this rule is that

A second reason for good phantom imaging is that,

it is sometimes desirable to fade up the level of sounds

especially with HDTV (or even with widescreenin the mix when the object producing the sound isconventional-resolution TV with displayed-image

subject to a camera close-up.)enhancement), the need for rapid cutting of camera
angles may well diminish in some kinds of

The result of this need to keep the sound image fixed

programmes. A fixed-angle presentation usually loses

is that there is bound to be a mismatch between thea lot of important fine image detail, as compared tosound and the visual positions of objects. Researchwhat one can see in real life. Close-ups often make up

has suggested that mismatch is audible over 4°,

for this loss of resolution. For example, an overall

although up to about 11° need not be too

camera shot of an orchestra shows little detail where aobjectionable. Thus such mismatches can often be

close-up may show the strings of the violins.

kept to within the 'acceptable' region by piling up all
the on-screen sounds at the centre position.

With HDTV, and its higher (albeit far from perfect)
resolution, there is less need for close-ups, and theyAt first sight (and hearing?) all this makes true

can use less drastic changes of camera perspective

phantom imaging between the three loudspeakers

and angles. While, on artistic or conventional grounds,

pretty much irrelevant for TV use. But this is not so formany programmes will retain current approaches toa number of reasons we have so far not considered.

changing camera angle (and for several years tocome, producers will still have to take into accountMatching the position of sound and vision is not theviewers of low-definition TV), there will be an

only reason for having stereo. In audio-only use, theincreasing area of programming using relatively stable

main advantage of stereo (and probably the reason

camera angles. For such programmes, being able towhy it has become the standard domestic audio

match sound image positions to the picture is asystem) is not the direct ability to localise sounds atrealistic and useful option.

particular positions. Rather, if sounds are localised at
different positions, it becomes much easier for theGood phantom imaging causes a higher audio fidelity

ears and brain to separate out the different sounds inand quality such as is required for music programmes

a mix. The intelligibility improves, listener fatigue

and also ones requiring a very natural ambience or

reduces and the subjective distortion is substantiallysubtly creative sound effects (including adverts). In

reduced. The better the illusory quality of phantom

many such cases, matching the visual image is either

images, the more these other advantages of the ears'

totally unimportant, eg when a creative sound effect isability to 'listen through' the sounds are obtained.

used, or sound fidelity takes artistic precedence overimage matching. In all such cases, being able to use

In conventional 2-speaker stereo, one of the simplest

the three channels to improve phantom images istricks to improve the subjective quality of a mix is

important – it is certainly not adequate merely toinstead of piling up several sounds into precisely theconvey 2-channel stereo via the outer loudspeakers,

same stereo position, giving each sound a slightly

especially if these are widely spaced apart.

different position. The same is true for stereo TV
production and there is a strong argument not to pile

If the technical means for improving phantom imagery

up all on-screen sounds precisely at the centre of thevia three channels are implemented, one might be inthe strange situation where TV audio will be capable ofbeing of both the audio and video industries. It seems

better overall fidelity than conventional audio-onlywise for audio professionals, studios and theprogrammes based on two channels. If this should

manufacturers of studio equipment to start facing up

happen, there would be strong pressure to incorporate

to the issue of 3-channel stereo at this stage, and not

the improvements from three channels into audio-only

to leave all the important decisions to backroom

media.

industry committees who might get things wrong fromthe end-user point of view if they do not take on board

Any technology for improved 3-channel stereo isthe user's needs. It is also important to ensure that,

therefore also potentially important for audio-only

as soon as is practicable, any programmes produced

media in the future. There are various possible

will be maximally compatible with future 3-channel

technical means of adding a third channel compatiblyuse, so as to prevent premature technicalto conventional audio media, ie with digital media theobsolescence of programmes.

third channel can be smuggled in as a datacompressed signal using up the two least significant

Psychoacoustics

bits of the two existing stereo channels, with special

All the above is contingent on getting good phantom

precautions taken to psychoacoustically mask thisimaging from 3-speaker stereo. Yet it is a fact that, as

altered information for existing listeners. There iscurrently implemented, 3-channel stereo does not giveroom for a third channel in FM radio broadcasting (theparticularly good phantom images even for central

quadrature modulation of the 38kHz subcarrier) andlisteners. Far from being an improvement, the quality

cassette also has the possibility of a third channel

for central listeners of phantom images away from thewithin the space currently occupied by two. All thesecentre of the stereo stage can actually be worse via

cases can be done in a manner compatible withthree speakers and three channels.

existing mono and 2-channel stereo uses.
To understand this and try to remedy it, we have to
The only media where it could be difficult adding a

examine the psychoacoustics of image localisation.

third channel are the vinyl records, which are unlikely

This is a complex topic but the theoretical methods

to survive long into the 3-channel era in any casewere presented in relatively simple form in Wirelessbecause of CD, and AM stereo broadcasting, which is a

World1 some time ago. Using these methods of

low-quality medium in any case.

psychoacoustic analysis, plus additional empiricalknow-how, one can optimise the speaker feeds for

One of the main uses of 3-channel stereo is much lessthree speakers to give as good a localisation quality as

obvious. It turns out to be an extraordinarily useful

possible. One certainly can't get everything right (andproduction medium for mastering for mono and 2-

surround-sound Ambisonics can get certain things

channel release. If one mixes down to three channels

right that a 3-speaker system can't even in the frontal

as an intermediate stage, one has many options forstage sector) but the results can be markedly

improving the quality of the results obtained in the

improved and rendered subjectively much morelater final remix to 2-channel stereo. For example, oneconvincing.

can derive 2-channel stereo with a wider image widthor with better phantom-image psychoacoustics than

There are two basic theories used in analysing

with conventional 2-channel panpots by going throughlocalisation of sounds, although there are otherthe intermediate 3-channel stage.

significant methods used by the ears and brain. One
theory applies to frequencies below about 700Hz, andAlso, a 3-channel mix can be remixed to other formats

the other between about 700Hz and 6kHz. There are(mono, stereo, 3-channel Ambisonic surround-sound,

two different theories because the ears and brain use

psychoacoustically-improved stereo and even binaural)different methods of localisation below and above

and allows special AM airplay, video mixes orabout 700Hz, which is the frequency at which therebalancing to be done simply without having to go

wavelength of sound becomes comparable to the sizeback to the original multitrack. Thus, providing 3-of the head. In practice, the transition between thechannel technology can be got right, there is a strong

low and high frequency theories is not sudden but

reason to start mixing down to a 3-channel mastering

there is a rather fuzzy band of frequencies over which

format even when current release formats are still 2-

both theories have some application.

channel.
Two-speaker stereo is capable of quite good low
Whatever means are used to convey a third channel,

frequency localisation – the theory of which was

we see that the optimisation of 3-channel stereo is anunderstood by Alan Blumlein in 1931. However, thisimportant issue, affecting the whole future and well-

localisation is somewhat unstable when the listenermoves or rotates his/her head. As the speakerin 1968, and by Kohsaka and others at Nippon

separation is widened beyond the usual 60° angle

Columbia for discrete quadraphonic systems. However,subtended at the listener, this poor stability of

the HF detent effect at the central speaker of the 3-

phantom images markedly worsens, leaving thechannel panpot law of Fig 1, shown in Fig 3, is very

famous 'hole in the middle' and also in images that

extreme – even sounds supposedly panned

can be perceived as being elevated as discovered bysignificantly far from the centre are pulled right to thede Boer at Philips in the 1940s.

middle.

This instability of low frequency phantom images

What this means is that the LF panpot law – ideal forcannot be cured even by improved 2-channel stereo

LF localisation – is just about the worst possible atpanning methods (such as the transaural stereo ofhigh frequencies. The localisation of the Bell panpot

Duane Cooper and Jerry Bauck) or by special

law of Fig 2 is shown in Fig 4. While this has muchloudspeaker types (eg those recently designed bybetter HF localisation, the localisation stability under

Canon) designed to improve image centring. This islistener movement is quite poor – including for central

precisely the area where 3-channel stereo can give

images. Also, the apparent HF localisation is aboutmarked improvements, as can Ambisonics.

1½x as wide at high frequencies as at low frequencies.
(There is a somewhat similar discrepancy between LF

An improved image stability at low frequencies can be

and HF localisation for conventional 2-speaker stereo.)achieved by using a 3-channel panpot law as shown in
Fig 1. This shows, for each intended panned image

The question thus arises of finding an optimised HF

position, the amplitude gain of the sound in the three

panpot law and of devising a reproduction method thatchannels. It will be noted that in this panpot law, theconforms to the optimum LF law at low frequencies,

gain becomes negative, ie with a polarity inversion inand to an optimum HF law at high frequencies.

one extreme speaker when the sound position ispanned between the other two. The optimisation ofThis question is complicated by several practicalthis LF panpot law requires careful theoreticaloperational constraints. A frequency-dependent panpot

mathematics but its general form shown in Fig 1 iscircuit would be quite complex to implement, and theenough for general descriptive purposes. It is possible

resulting 3-channel stereo would not be compatible forto design simple analogue panpot circuits and simple

mixdown to conventional 2-channel stereo or mono –digital algorithms to implement such a panpot law wellboth of which will remain important for TV and otherwithout any great complexity.

use. Moreover, ideally, one wishes to use the samemix both for auditorium film and home TV use, since

By way of contrast, Fig 2 shows the panpot law

remixing can be extremely expensive. However, the LF

devised by Snow and others at Bell Telephone Labs inpanning law is inappropriate for auditorium

1934. This obviously very different law has poor LF

reproduction, where the large time delays from thelocalisation properties but it was designed for use withloudspeakers make optimal LF localisation academic.

reproduction in large auditoria, in which the fullRather, for auditorium reproduction, one wishes torequirements of LF localisation cannot be met anyway.

optimise according to HF localisation laws even atlower audio frequencies. It can be shown that such HF

Where the Bell panpot law wins over the law of Fig 1 islaw optimisation gives the best obtainable LF phantom

in its high frequency phantom image localisation. Fig 3

images under auditorium conditions.

shows the computed HF theory localisation of the
panpot law of Fig 1 via three speakers. This displays aThis need for two different optimisations of 3-speakerpronounced and severe 'detent' effect at the centre

feeds for auditorium and home use means that it isspeaker, whereby sounds panned (by the panpot law

wise to use a single basic frequency-independent 3-

of Fig 1 optimised for low frequencies) fairly near tochannel panpot law, as shown in Fig 1, but that forthe centre are pulled right into the centre speaker atplayback or monitoring, the resulting 3-channel signal

high frequencies. There is also a (much lessshould be subjected to an additional 3-input 3-output

pronounced) detent effect near the two outer

processing (termed 'decoding') to produce the three

speakers.

speaker feeds with optimum psychoacoustics for agiven environment. For film release prints, carefully

Now such detent effects are familiar with all directional

designed crosstalk can optimise phantom image

sound reproduction systems with a 'discrete' panning

localisation in large auditoria, whereas for home use,law (ie one which positions some directions atsuch crosstalk will only be implemented at highish

individual loudspeakers without any crosstalk) and was

frequencies in the consumer decoder. The home

identified in conventional 2-speaker stereo by Harwood

decoder should be in the home and not at therecording or transmitter end, since one requires thatgiving an improved phantom image illusion for half-leftthe recorded or transmitted signal be compatible also

and half-right positions, Ambisonic 3-channel

with mono or 2-speaker stereo reproduction, or evensurround-sound for reproduction via a surround-soundwith reproduction via alternative decoders via 4-

speaker array, binaural reproduction for headphone

speaker stereo systems or Ambisonic surround-soundlistening, and transaural reproduction, ie a form ofsystems.

stereo reproduction via two loudspeakers aiming torecreate via loudspeakers binaural signals at the ears

Should the home user place the highest priority on

after the sound has mixed in the air.

locking central images to the middle of the screen, theraw left-centre-right signals from the panpot can be

For example, for conventional stereo, the left, centre

fed direct to the three loudspeakers. Those requiringand right channels are conventionally panned (withmore subtle phantom images will use a simple decoder

constant power gains) or a mixing circuit having the

network to achieve this. Fig 5 shows the HF

same effect is employed. This results in a conventional

localisation that can be achieved from the panpot law

2-channel stereo mix in which intermediate positionsof Fig 1 via a simple decoding network. Comparing thisare panned conventionally with an almost perfectlywith Fig 3 shows that the detent effect has been

constant power gain (within about 0.2dB).

almost completely eliminated and that the low andhigh frequencies now match in position to within about

A mono mix having substantially constant power gain

4° over a 90°-wide reproduction stage. (Similar

at all positions in the original 3-channel stereo stage

matching of low and high frequencies can be obtained

can be obtained simply by summing the three

for other reproduced stereo stage widths.) The

channels at equal level.

particular decoder involved still gives excellent stabilityof central images – far better than conventional 2-

In Ambisonic surround-sound mixdown, the centre

speaker stereo or the original Bell 3-channel panpot.

channel is panned to front centre, and the left andright channels to a given angle θ (up to say 80°) to the

System considerations

left and right of centre at the same level. This results

The many possible reproduction modes of a 3-channel

in an Ambisonic mixdown covering a stage width 2θ

stereo signal require careful system design to ensure

wide, which can extend up to 160° wide whilethat all modes work well. We have already seen that itconforming to Ambisonic encoding specifications evenis possible to use the LF panpot law of Fig 1 with afor intermediate phantom positions.

decoder to provide improved phantom image
localisation quality via three speakers, while giving a

A wide 2-channel stereo mix, with stereo images

more rudimentary effect over three speakers without abeyond the left and right speakers, can be obtained bydecoder.

mixing down to 2-channel stereo with the left andright channels panned to those beyond-the-speakerWe have already mentioned the practical importancepositions, and the centre channel panned to centreof having a compatible mix in mono and 2-speaker

position, all at equal levels. This results in a 'wide' mix

stereo and that the use of a frequency-independent

with all phantom positions at almost the same level.

panpot law, such as that of Fig 1, is vital for this if

This is unlike widened conventional 2-channel stereo,

frequency-dependent mono and 2-channel results arewhich has excessive sound levels at the edges of theto be avoided. However, 3-channel stereo mixdown

stereo stage.

proves to be a powerful production tool for a wide
range of other important uses than mono, 2-speaker

Further refinements of 2-channel stereo are possible

stereo and 3-speaker stereo.

to give improved subjective results. Two examples ofthis are as follows. It has long been proposed toFig 6 provides an overview of how many of the'shuffle' conventional stereo, with wider width at low

different uses of the 3-channel material can be derived

frequencies than at high frequencies, but suchby subsequent signal processing. The basic idea isshuffling conventionally introduces an undesirable

that, whatever reproduction mode is chosen, the left,

position-dependent frequency response. If one uses a

centre and right channels are panned to theirseparate low and high frequency panning matrix fromrespective associated positions by panpots satisfying

three channels, via a (phase compensated?) crossoverthe appropriate law for that reproduction mode.

network, with the low frequency stereo mix being
wide, then the resulting shuffled 2-channel stereo will

Other possible reproduction modes include 'wide 2-

have a substantially flat frequency response for all

speaker stereo' conveying image positions beyond theimage positions. Such optimally shuffled stereo is ideal

two loudspeakers, 'psychoacoustic 2-speaker stereo'

for widening reproduction from two closely-spaced

speakers, such as those to the sides of the screen incan be faded up (or down) by increasing (oran all-in-one TV set or in an all-in-one portabledecreasing) the centre-channel gain before'ghetto-blaster' unit.

reprocessing into the final reproduction format. This
allows listeners/viewers of 3-channel stereo

A second psychoacoustic 2-channel mixdown frombroadcasts or recordings to alter balance to taste andthree channels uses the extra degree of freedom in thesolves the problems of those with non-standard

3-channel law to optimise the phantom images

hearing who find it difficult to hear dialogue in the

halfway between centre and left or right, improving on

presence of background music or sound effects. Such

conventional amplitude panning. Such psychoacoustic

listeners can fade up the centre-channel feed relative

2-channel stereo mixdown can achieve sharperto the outer channels, whether they are actuallyphantom images away from the central panned

listening in mono, 2-speaker stereo or 3-speaker

position without a marked deviation from flatness ofstereo modes.

frequency response in either mono or stereo. Thus, byinitially mixing to three channels and then going

Another use of such level-balance alterations is withthrough a psychoacoustic mixdown processor, thesound effects libraries or with library music. If

phantom image results from ordinary 2-speaker stereo

recorded in 3-channel stereo format, the level-balancecan be significantly improved without the complexityof the mix can be changed as it is mixed into the finalof mixdown (or the mono incompatibility) of transaural

programme in order to meet that particularstereo.

programme's requirements. Other uses for preparing
AM airplay mixes or dance remixes are also evident.

Three-channel stereo material can be converted into abinaural or transaural mix by separately encoding andOperational aspects

mixing the left, centre and right channels via binaural

Fortunately, there are various simple technologies foror transaural panpots. This gives good imagery of

implementing optimised 3-channel panpot laws

those three positions, although some intermediate

without great complexity. The simplest 3-channel

positions will not be encoded correctly at higher

analogue panpot can be realised by a minor

frequencies. It is possible to modify the mixdown frommodification of existing 2-channel panpot designs,

three channels into binaural or transaural formats to

involving two modifications to existing 2-channel

spread these errors more uniformly across the soundstereo mixers: the addition of a few extra components

stage, and thereby reduce their magnitude.

in each mixer channel strip and the addition of another

Conversion from 3-channel stereo to binaural or

mixing bus (or the reallocation of one of the post-fadetransaural formats, however, can never be perfect but

mixing buses); the addition of a moderate amount of

is a much better compromise than is possible fromextra signal processing after the three mixing buses.

ordinary 2-channel stereo.

This will mean that most existing 2-channel stereo
mixers can easily be redesigned for 3-channel use andFinally, the raw 3-channel signals can be reprocessed

most existing designs should be retrofittable for 3-

to give sharper phantom image reproduction via threechannel use at low cost.

(or more!) loudspeakers via suitable decoding
networks. The optimum decoding network depends on

A 3-channel mixing desk can still be used forthe circumstances. A normal domestic environment

conventional mono and 2-channel stereo use byrequires a different decoding circuit to a large

incorporating the conversion circuits for mono andauditorium environment (eg film, SR or A/V

stereo into the mixer – indeed the mono, stereo andapplications). A properly-designed psychoacoustic 3-3-channel mixes can be achieved simultaneously forspeaker decoder can give quite well-defined phantom

different release formats. Thus, at relatively little extra

image positions without excessive image movement

cost, studio and PA desks can be provided with thewith listener position. It is also possible to design

facilities for mono, 2-channel stereo and 3-channel

decoders that are to be used with four or more

stereo (perhaps also incorporating a 3-speakerloudspeakers.

decoder for monitoring). Thus, for example, a PA desk
normally used for mono or stereo would be '3-channel

Besides handling all these different modes ofready' for those venues where a central speakerreproduction, it is worth noting an important

cluster is practical, and could give a 3-channel output

subsidiary advantage of 3-channel stereo mixdown.

for recording purposes even when the actual live

This is in applications where it is necessary to alter thesound is reproduced in mono or 2-speaker stereo.

level-balance of the mix. At the expense of somenarrowing (or widening) of the stereo image near

Importantly, studio or SR desks of the 3-channel kindcentral image positions, the centre-channel material

described will be operationally identical to present-day

2-channel stereo desks, apart from the user having toloss in the original channels of these stolen bits can beselect the output mode to be used. Thus operationally,

compensated. For example, the loss of the two least

there is no relearning involved in using a 3-channel

significant bits in CD can still give a psychoacoustically

mixing desk.

weighted S/N ratio for existing 2-channel listeners ofaround 94dB – about 3dB better than currently

Further refinements of the panpot law can be achievedachieved with the full 16 bits. This subjective

by designing desks around purpose-designed

improvement is due to the use of optimally noise-

optimised 3-channel panpots. There are very simple

shaped subtractive dither, based on work by Peter

designs of such optimised panpots using all three

Craven and myself on optimal dither and noise

major panpot technologies: digital mixing, VCA

shaping.

technology and using ganged pairs of linear
potentiometers. Such optimised 3-channel panpots

In digital satellite broadcasting, a compatible digital

differ from the 'modified 2-channel panpots' described'sub-band' method of using the least significant audio

above in having slightly better psychoacoustics andbits for additional channels has been proposed bynoise performance – but in most situations, thePhilips. Such systems can be fully compatible withdifferences are marginal. The optimised 3-channel'existing' listeners, ie those listening to just the basic

panpots are, however, the preferred option when new

channels, since the altered least significant bits can bemixer designs are developed mainly oriented towards

well masked by the basic audio signal by a judicious

the 3-channel market.

use of noise-shaping and dither. To minimise loss ofquality, this must be done with great care, using

As noted earlier, 3-channel mixers will be ofproven results. Decoded 3-channel results can beadvantage even to those users only requiring 2-

further improved by using a carefully-designed

channel stereo, since the psychoacoustics of thesubjectively compatible companding system, and I

resulting 2-channel stereo can be improved overhave devised appropriate algorithms for this.

standard amplitude-panned 2-channel stereo by usinga psychoacoustic 3-to-2-channel conversion network.

Intercompatibility

Also, by mastering in 3-channel format, the master

There is an issue we have not yet really dealt with –can be re-released in future audio formats, eg 3-

the intercompatibility of different multichannel audio

speaker stereo, Ambisonic, without remixing from theformats. For example, supposing that we have a 2-

original multitrack, thereby protecting the investment

channel stereo programme, eg sound effects, libraryin mixdown time.

music, commercial music recordings, how can this best
be conveyed via a 3-channel stereo medium? Again,

There is still the problem of the lack of a standard 3-

suppose we have a 3-channel Ambisonic surround-channel tape format. The only 'standard' 3-channelsound mix. How can this best be reproduced over 3-

tape format is the 1950s ½ inch 3-channel analogue

speaker stereo? With a variety of different signal

format, which still has much to commend it, especiallyformats co-existing, all conversion options need to be

at 30in/s. One of the available 4-channel digitalconsidered if we are to avoid chaos. The fact is thatrecording formats could also be used, although it

several audio formats do exist and one will often needshould be possible to modify existing digital 2-channel

to use material from one format in another. This

reel-to-reel formats to handle three channels. Users ofproblem is not a new one in that binaural and 2-

digital 48-track machines can lay down a 3-channel

channel stereo have never really been compatible withmaster mix on three spare channels, and users of

one another – both binaural over speakers and stereo

videotape formats having three or four audio channels

over headphones sound wrong. So far, we haveshould also have no problems in 3-channel mastering.

merely lived with this problem without solving it but
the addition of further formats make it important toThis being said, the industry needs to look carefully atthink out solutions before the problems get worse.

mastering media for multichannel stereo.
The problem of reproducing 2-channel stereo overThree-channel media

three loudspeakers turns out to have some reasonable

There is also the question of how we get three (orsolutions – although the results are obviously not asmore) channels to the consumer. Digital media offer

good as full 3-channel stereo. For narrow stage

an 'easy' way to incorporate additional channels.

widths, 2-channel material can simply have its left andEssentially, the least significant bits of the existingright channels panned (using 3-channel panpots) tostereo channels can be 'stolen' and re-allocated tothe desired positions but this solution works poorly for

additional data-compressed channels. By appropriate

wide stage widths, especially when one wishes to fill

dithering and noise-shaping, most of the subjective

the whole stereo stage.

We have discovered some remarkably effective linear

the transmitted mode so decoding can be optimised

2-channel 3-speaker decoding matrices capable offor that mode.

improved image stability for non-central listeners andimproved image sharpness for central listeners, asThis complication arises because the optimum

compared to ordinary 2-speaker reproduction, and to

decoding for each reception mode is frequency-

prior proposals, eg the Bell/Klipsch 'bridged centre

dependent due to the frequency-dependence of humanchannel' method, for 3-speaker reproduction of two

directional hearing, and the frequency-dependent

channels. Obviously, such 3-speaker decoding of two

speaker feeds for optimum reproduction are notchannels cannot be as good as true 3-channel

compatible with a frequency-independent mono anddecoding but it is quite effective on a wide range ofstereo fold-down for mono and stereo listeners.

material. We hope to be able to publish both thetheory and methods of such 3-speaker decoding of two

Also, the transmission of a mode flag is importantchannels in the near future but it does seem to offer abecause future technological developments may reveal

good second-best option to true 3-channel stereo.

future improved decoders, and set designers shouldhave the option of incorporating these improvements

We are rather sceptical about the use of 'logic', 'gain

into receivers, which should detect the mode being

riding' or 'variable matrix' 3-speaker decoders for 2-

received. However, system standards should be suchchannel stereo, due to their signal-dependentthat a basic 3-channel stereo receiver without mode

'pumping' side-effects, which can cause both dynamic

switching will receive an acceptable, if notwandering and instability of subsidiary images andpsychoacoustically ideal, result.

increased listening fatigue. This negative comment is
not based only on experience of commercially

Besides the modes described above, TV transmitting

available designs but also on detailed development

systems may also wish to include other modes, suchwork on advanced experimental multiband logic

as 4-speaker stereo, different varieties of horizontal

decoders based on more sophisticated psychoacoustic

surround sound, and even full-sphere surround-sound.

design than those on the market. Most commercial

Any system design for HDTV sound should find a way

logic decoders have paid very little attention to theof making all these modes as intercompatible asproper localisation of image directions between thepossible. Such a system design is feasible but requires

loudspeakers.

very careful thought. Certainly, the one approach to
avoid is one that assumes a once-and-for-all rigid

A 3-channel receiver needs to know which reception

loudspeaker layout since this will limit any futuremode is in operation to reproduce each mode

improvements in the art.

optimally over the chosen loudspeaker layout. I
suggest that the reception modes involve at least theProgramme origination

following options: mono, 2-channel stereo, 3-channel

Besides 3-channel panpots, one also needs means ofstereo with the panpot law of Fig 1, 3-channel 3-

producing 3-channel stereo from non-monophonicspeaker feed-signal mode and 3-channel Ambisonicsources, including live soundfields. For live

surround-sound mode. For digital broadcasting,soundfields, one possibility is to use spaced mono

suitable flags in the data stream could indicate themicrophones (as did the experiments at Bell in 1933)

mode being transmitted.

panned to positions across the 3-channel stereo stage,eg four microphones might be panned to nominal

For each of these transmitted modes, a different

azimuths at ±45° and ±15° for the panpot law of Fig

optimum psychoacoustic matrix is required to feed the

1. An ideal 3-channel [signal] cannot easily be derived

three loudspeakers of a 3-speaker stereo receiver. For

from available coincident microphone arrays but aexample, 2-channel stereo requires the use of anquite reasonable non-ideal 3-speaker feed signal can

optimised 2×3 decoding matrix as described above, abe obtained via a suitable 3×3 matrix circuit from amono signal will be fed to just the centre speaker, asoundfield microphone. Where such a very

'figure 1 law' 3-channel signal will be fed to via a 3-

approximate feed is not adequate, a soundfieldchannel decoder, speaker-feed 3-channel signals will

microphone can be matrixed to give an accurate

be fed straight to the loudspeakers, and Ambisonicmatch to the panpot law of Fig 1 for sounds arriving

signals will be fed to the speakers via another matrix.

from a frontal stage but at the expense of anexcessive pick-up of sounds from the rear. If it is

These transmitted signals can be designed to minimise

possible to place a large acoustic absorber behind thethe need for mode switching if only a basic 3-channelsoundfield microphone, eg below or above the field ofstereo effect is required but a receiver wishing to getview of an HDTV camera, or disguised behind scenery,

optimum results from each mode will require to know

then such an accurate matrix might be workableotherwise rear sound pickup is a serious problem.

performance away from the stereo seat.

Two-channel stereo material, such as from sound

There is no obvious optimum subtended total angle ofeffects recordings, library music, stereo microphones

the loudspeaker layout at the ideally-positioned

or commercial music recordings, can be mixed into alistener – figures between 60° and 180° have been

3-channel programme either by restricting it (bysuggested. Our panpot law of Fig 1 is optimised formeans of 3-channel panpots) to a small part of thegood results for any subtended angle up to 160°

stereo stage, or by using suitable 2×3 matrix decoders

although image stability degrades as the subtended

as described earlier. The latter option does tend toangle increases, being poor beyond a 120° angle.

give less good image quality than true 3-channelmaterial, and has the problem that it must be madeFor audio applications, there is no need to adopt a

compatible with 3-channel decoding from the 3-

rigid standardisation of angle as long as thechannel mixed programme.

reproduction method is designed to give good
phantom images for the angle used by the listener. For

An alternative microphone technique can use spaced

TV applications, however, it is important that sounds

stereo-pairs of microphones each, panned across afrom on-screen images should substantially match therelatively small part of the 3-channel stereo stage.

position of the visual image. However, for a given

This gives more convincing phantom images than doloudspeaker layout, it will be possible to incorporate aspaced mono microphones. The use of two or more

3-channel 'width control' adjustment that will allow

stereo pairs placed at different locations and mixed

audible and visible image positions to be matched.

into the 3-channel stage might often prove to be apractical means of live stereo pickup for HDTV

While decoding to three loudspeakers is the nominallyapplications, with 3-channel panpots being used tocorrect way of reproducing the 3-channel sound, thecontrol the imaging from each pair within the 3-

use of three loudspeakers may not always be practicalchannel stage.

or desirable. This is because a middle speaker will beeither in the middle of the TV screen, or in the middle

Given the fact that new microphone techniques areof a control room window.

still being developed for 2-channel stereo almost 60
years after its initial development, we expect much

Ways round this are either to accept a speaker below

innovation to occur with 3-channel microphoneor above a picture, with an associated height error, or

technique in the future – varying from theto use four (or more) loudspeakers. These speakers

development of proper all-in-one 3-channel stereo

can involve either a narrow or 'inner' stereo pair and a

microphones to quite sophisticated 'matrix' techniques

wide 'outer' one, or can split the central loudspeaker

developing the 2-channel MS technique. However, it isinto a 'below-picture' and 'above-picture' pair. In

not to be expected that the empirical rules-of-thumbeither case, the speakers must be provided withfor 2-channel stereo mic technique will always work

psychoacoustically optimised feeds adapted to thewith 3-channel stereo.

specific layout in use in order to get an optimised
image illusion. Naively-chosen speaker feeds will not

The artificial reverberation of 3-channel stereo ideallywork well. A number of possible 4-speaker decoders

requires the use of a reverberation unit with three or

have been devised for use with 3-channel stereo

more appropriately related independent outputs

signals.

panned across the 3-channel stereo stage. There may
be suitable units on the market, eg the Yamaha DSP

Although there are several different options forprocessor series, and other surround reverb units.

monitoring a 3-channel signal, different monitoringTwo-channel output reverb units can be used if theyarrangements do sound slightly different, so thought

are fed into the three channels by an appropriate 2x3

needs to be given either to devising a standardised

decoding matrix as described earlier but this will inmonitoring arrangement or to understanding thegeneral not give as good results.

differences between different arrangements, so theireffects can be allowed for.

Monitoring and domestic playback
The ideal loudspeaker layout for 3-speaker stereo is of

The basic design theory for decoding 3-channel stereo

the general form shown in Fig 7, with all threeassumes that all speakers are at the same distanceloudspeakers lying on a circle centred at the nominalfrom a central listener. If this is not the case, eg if theideal stereo seat. The equal distance of all speakers

loudspeakers all lie in a straight line, then thefrom the ideal stereo seat gives maximum phasespeakers closer to the listener can be fed via a

coherence for phantom imaging, and helps optimise

compensating time delay (and also a slight gain

reduction) to restore the correct phase coherence ofa superior sound and a more social listening

the sounds reaching the stereo seat. Such delay

experience for several listeners in a room for audio-

compensation is difficult in analogue systems but isonly applications.

quite easy to implement in systems with digitalrecording, transmission and signal processing.

It is essential that the industry makes the rightdecisions both about the systems aspects of 3-channel

In order to design different monitoring and decoding

stereo (including mono and 2-channel compatibility)

arrangements to meet the widest range of needs, it isand about the right production technology (notably

important that there be a basic reference method of

mixer design and monitoring methods, but also 3-

panning sounds into three channels, such as that ofchannel mastering formats) to fully realise theFig 1. This acts as a reference for evaluating thepotential gains. This would include the potential

quality of imaging of different designs. One expects

benefits of using 3-channel mastering as a production

future innovations to discover improved or refinedformat even for 2-channel releases, and the adoption

decoders for different speaker layouts but theof the same formats for audio-only and TV

optimisation of such decoders requires knowing what

applications.

is to be decoded.
I am preparing a detailed technical report aimed at
Surround soundprofessional equipment manufacturers and major

This article has been primarily about stereo over ausers that will flesh out the above with much detail,frontal stage since the instability of phantom images ofboth in general theory and detailed designs and2-speaker stereo is an important defect of existing

methods, particularly as regards mixers and decoders.

technology, especially with an associated visual image.

However, anyone seriously interested in keeping up

However, the extension to the 360° of horizontal

with the future of stereo would do well to familiarise

surround-sound, to height portrayal, and even to thethemselves with the literature covering work already

4π steradians of full-sphere surround-sound, is also an

done over the decades. The history and basic theory ofimportant issue, which we cannot fully deal with here.

2- and 3-channel stereo is well covered in a useful

The most reliable existing surround-sound technology

compendium of important technical papers3. Anis that of Ambisonics, which requires the use of three

informed knowledge of the technical foundations oftransmission channels for horizontal surround soundstereophony among audio professionals will help themand four transmission channels for full-sphere

contribute to the important technical decisions that will

surround-sound2.

determine the future of stereo technology.

Such surround-sound is capable of reproducing sounds

The author is preparing a detailed technical report on

from every direction while satisfying a variety of

3-channel stereo, which will be made available topsychoacoustic requirements for directional

professional audio equipment manufacturers in thelocalisation. There is empirical evidence thataudio and video industries.

supplementing such systems with an additional frontcentre channel and loudspeaker for large-screen andReferences

auditorium applications can be a useful enhancement.

1) Gerzon M, 'Surround Sound Psychoacoustics',

Such an enhancement can be done in a way

Wireless World, December 1974

compatible with 3-channel stereo for the frontal sector

2) Gerzon M, 'Ambisonics in Multichannel Broadcastingof directions.

and Video', JAES, November 1985
3) Stereophonic Techniques, Audio Engineering

Conclusions

SocietyThree-channel stereo is not simply two sets of stereo
pairs (left/centre and centre/right) but properly
designed technology using all three speakers andchannels together and capable of subjectively
enhanced realism as well as the improved stability ofcentral images.
To the writer, one of the big hidden gains of 3-speaker
stereo is its lower listening fatigue and artificiality ascompared with 2-speaker systems. If properlydesigned studio technology is used, the results will not

The Gerzon Archive

only provide a better match to widescreen TV but offer

www.audiosignal.co.uk

Fig 1: Typical 3-channel panpot law for optimum LF localisation

Fig 2: Three-channel panpot law used by Bell Telephone Labs in 1934

Fig 3: Apparent HF localisation angle θE plotted against LF localisation angle θV for the
panpot law of Fig 1, via a 3-speaker layout subtending 90O at the listening position

Fig 4: LF (θV) and HF (θE) localisation azimuths for 90O speaker layout
for the 1934 Bell 3-channel panpot law of Fig 2

Fig 5: Apparent HF localisation angle θE plotted against LF localisation azimuth θVfor one design of improved 3-channel decoding network

Fig 6: Different uses of basic 3-channel signals satisfying
the LF frequency panning law of Fig 1