Reproduced fromavailable on most modern stereo equipment.

Studio Sound, July 1986
One of Blumlein's many discoveries' was that increased

Stereo Shuffling:
New Approach – Old Techniquewidth could yield stereo images beyond the left andright speakers. This useful discovery would permit one
to pan sounds over a wider stage than normally used intoday's studio. There is no reason why panpots should

Michael Gerzon introduces an approach for

not be designed to cover such an increased stage widthexperimentation– yet I am unaware of a single mixer in which this isactually done.

Although many recording engineers and studios don't
realise it they already have the equipment to produce aThis is not to say that width control is without problems

marked improvement in the stereo quality of many of– which we shall discuss in more detail further on –their recordings. Not digital effects using reverberation,

however, these problems can often be solved by a more

delayed echoes or the like but a technique that has

sophisticated process called 'shuffling', also based on

been known but almost unused for over 30 years. This

Blumlein's work. Blumlein noted that one could notis the stereo 'shuffler'.

merely alter the gain of the difference signal S, but one
could alter this gain in a frequency-dependent way by

What is a shuffler, and how come you didn't know you

using an equaliser. By this means he showed how one

had one? Last question first – a shuffler can be

could improve the directional quality of particular stereo

produced by unconventional connections between the

microphone techniques (including one pseudo-dummy-

inputs and outputs of many mixers (my initialhead technique rediscovered at the BBC a few years

experiments were with an £80 mixer (!) but it shouldago). The process of equalising the difference and sum

work at any price level) along with a stereo graphic

signals differently before recovering left and right isequaliser. But to get the best out of this, and in thetermed 'shuffling'. In effect, shuffling is a frequency

absence of a dedicated commercial shuffler, it isdependent width control.

important to understand what you are doing.
The first systematic commercial use of shuffling was in
The basic idea of the shuffler goes back to Alan

EMI's Stereosonic system in the mid 1950s2, in which

Blumlein's invention of modern stereo in 19311. (Histhe bass width of recordings made with coincident

British Patent 394,325 repays detailed study as perhaps

crossed figure-of-eight microphone pairs was increased

still the best source text on how stereo works.) Blumlein

relative to the treble width. The reason why EMI usedconceived stereo not just as a left (L) and right (R)shuffling was that research had revealed that stereo

speaker signal but also in terms of a sum signal M (=images at bass frequencies reproduced more narrowly

L+R) and a difference signal S (=L–R). The letters M

than at treble frequencies for a given intensity ratio inand S stand for 'mid' and 'side' signals (as in the M-S

the two speakers, and the increased bass width

microphone technique): M is the signal containing

attempted to compensate for this. This didn't work

information about the middle of the stereo stage,adequately with the actual recording techniques EMI

whereas S only contains information about the sides –used at that time, so they dropped it.

since S=0 for a central signal.
From time to time, shuffling (or processes achieving
Given M and S, the original left and right signals can be

identical results to shuffling) has been revived – forrecovered by a second sum-and-difference operation,

example, various domestic hi-fi products under thevia 2L=M+S and 2R=M–S. By thinking in terms of theRealistic name have shuffler stereo 'enhancement'

sum and difference signals, Blumlein was not merely

circuits built in – but in my opinion these are poorly

able to devise the MS microphone technique (which was

implemented, giving an exaggerated bass-heavy

rediscovered and named by Laurisden in Denmark inquality. Various American authors have revived or

the 1950s) but was able to modify the stereo effect ofrediscovered shuffling in recent years, notably Richard

other recordings. In particular, Blumlein was able toKaufman3 and David Griesinger4.

modify the width of the stereo images of coincidentmicrophone recordings by increasing (or decreasing) the

Kaufman has proposed using the system illustrated ingain of the S signal relative to M before recovering theFig 2 for shuffling. This system requires the construction

left and right signals (Fig 1). An increase in the relativeof special sum-and-differencing circuits. Although suchgain of S increased width, whereas a decrease of S gain

circuits are quite simple, they have to be constructed bydecreased width. In view of the fact that width control

the user, since they are not available as standard

was known in 1931, it is strange that it is still not

products (except for special purpose MS microphoneprocessors). Nevertheless, Fig 2 is a useful way ofcoincident microphone techniques. My own experiments

understanding shuffling, and we shall use this for ourusing coincident and near-coincident (spacings of aboutbasic descriptions of what it does. Other methods of

5 to 7cm) microphones yield the same frequency and S

achieving the same result are often easier to implement

channel boost for best results in many cases, however,and use but their theory is harder to understand.

unlike Griesinger, I believe that this enhancement
should be tried on a case-by-case basis, rather than as

Essentially, in order to widen the stereo image at aa blanket processing technique. This is because thegiven frequency, one increases the gain at thattechnique is not uniformly effective with all locations

frequency in channel 2 of the graphic equaliser, possibly

and microphone techniques, and also there are someslightly decreasing the gain at that frequency in channel

unwanted side effects on stereo image quality whose

1 in order to retain a flat frequency balance in theseriousness varies considerably between recordings.

resulting overall sound. Similarly, to decrease the width
at a given frequency, one reduces that frequency's gain

In doing any shuffler processing, it is important toin channel 2, increasing it slightly in channel 1 tomonitor over loudspeaker systems optimised for good

maintain the overall frequency balance.

stereo imaging – this unfortunately excludes moststudio monitoring systems. Something like the classic

To change the overall width, one similarly increases orminiature BBC LS3/5A speaker is still hard to beat for

decreases the overall gain in channel 2 of the graphic

accurate monitoring of stereo images – both Griesinger

equaliser of Fig 2. Moreover, the processing system ofand myself seem to have settled on it as a reference forFig 2 is a powerful technique of reducing various

work in this area, although it is worth trying results withproblems with stereo. For example, reducing the width

other speaker types. If one doesn't have precision

at low frequencies makes vinyl records easier to cut,monitoring of stereo imaging (eg if one uses speakers

since low frequency S signals at a high level are hard to

optimised for spaciousness of reproduction rather thancut. Noise from FM stereo reception can be reduced by

precise images), then one doesn't know if the shuffling

cutting the width around 7kHz, since S channel noiseis producing an authentic improvement in a recording or

around this frequency contributes most to the perceived

just compensating for the anomalies of the specificnoise. Stereo mics picking up thumps from transmitted

monitoring system used. For location recording work, it

floor vibrations can often yield more thump-free

is advisable to make recordings without shuffler

recordings by selective bass filtering of the S channel.

processing, postponing shuffling to post-tape processing
in a familiar accurate monitoring environment.

Besides such problem-reducing roles, shuffling also has
uses in enhancing overall stereo quality. One use, withAn understanding of some of the problems that arise incoincident microphone recordings, is to use shuffling to

the use of width and shuffler systems will speed up therender imperfect frequency-dependent images, due toprocess of finding optimum shuffler and width settings

microphone imperfections, more sharp by compensating

in a given case.

for the width variations in the image. This can beparticularly useful in the extreme bass, where

Normal panpot stereo and also stereo from truly

conventional microphones such as cardioids tend tocoincident perfectly cardioid microphones, produce

become more omnidirectional at the very lowest

stereo images by placing individual sounds in the two

frequencies. An enhancement of extreme-bass width

speaker channels in identical phase but with differing(typically below 100Hz) can sometimes compensate for

amplitudes. If sounds are in opposite phase (ie with one

this. In a similar way, providing that the capsules arechannel phase-inverted relative to the other) on the two

sufficiently coincident, treble irregularities ofstereo channels with one louder than the other, thenmicrophone polar diagrams can be partiallythe sound image tends to have a more diffuse quality

compensated for. Also, as described further below,

but can often be located beyond that stereo loudspeakersome of the phase anomalies caused by small spacings

which is louder. While it is wonderful to have stereo

of a few cm between microphones can also be partiallyimages located beyond the loudspeakers, such images

compensated by suitable shuffling.

can cause problems.

It has been suggested by Griesinger4 that the sense ofFirstly, if the 'beyond the speaker' images are bassy

spaciousness of recordings can be improved bysounds, then they can cause vinyl records to be hard toincreasing the bass width below about 600Hz. He

cut at high levels due to the large S signal produced.

suggests that, if the S gain is increased relative to that

Secondly, because such beyond-the-speakers imagesof M at low frequencies by from 4 to 8dB, this ishave larger S than M signals, such sounds can beeffective in creating an impressive reconstruction of thedrastically reduced in level when reduced to mono,sense of space of the recording location when used withgiving an unbalanced mono mix. For cassette, compact

disc and video media, however, these problems may not

spacing of microphones, width control should be

be too important. Thirdly, the beyond-the-speakers

confined to those lower frequencies at which the sounds

images do have an unconvincing localisation quality.

from all directions arrive at both microphones more-or-This often results in unstable, fuzzy or confusing

less in phase. In practice, this would mean confining

images, although at other times the images can bewidth control to frequencies below about 5.4 d kHz wherequite dramatic in effect.

d is the distance between the microphones in cm, or
2.1

d kHz where d is the distance between the

If beyond-the-speaker images are produced by

microphones in inches. An exception to this rule is whenincreasing the width of an already-made recording or

the microphone spacing is so large (eg more than 2m)

submix, then additional problems can arise if thisthat the sound arrivals at the microphones arerecording does not consist of amplitude-difference

effectively incoherent, and the microphone signals canstereo images to start with. The four cases when otherbe treated as effectively independent signals.

types of stereo images with phase as well as amplitudedifferences can occur are:

It has to be said that on many recordings where width• recordings made with microphones slightly spaced

increase over a wide frequency band 'shouldn't' work, it(by a few centimetres) from one another;does seem to give an enhanced wider image. This• recordings made with coincident hypercardioid,seems to be unpredictable, so should be tested on afigure-of-eight or MS techniques, in which antiphasecase-by-case basis. The effect of other sources ofimages can occur;

interchannel phase differences, on width control, such

• when stereo studio effects involving phasing of

as tape azimuth errors, can be similar to the effects

channels are used, eg autopanning systems, somedescribed above with spaced microphones.

stereo reverb devices and some stereo synthesiser
outputs;

We have just seen a reason with particular kinds of• recordings with time differences in the two stereo

microphone techniques, and with interchannel tapechannels due to analogue tape azimuth error or digitalazimuth errors, why shuffling should be confined to bass

converter timing differences.

frequencies (eg below about 320Hz for ORTF
technique). Experimenting with width control andConsider by way of example, a recording made with

shuffling yields other good reasons why width control

ORTF technique, in which two cardioid microphones,

should be confined to the lower frequencies. Given thatangled about 110° apart, are spaced apart by ear

beyond-the-speaker images have anomalous

distance spacing – about 17cm – as illustrated in Fig 3.

localisation low frequencies do seem to localise quite

A sound arriving from due left will arrive at the left

reliably beyond the loudspeakers. So, if we increase themicrophone about 0.5ms before it arrives at the rightwidth only of lower frequencies (say below 600Hz or

microphone (since sound travels in air at about 340so), those low frequency signal components that dom/s) and will be picked up about 20dB down on thelocalise well are widened, while those at higher

right channel compared to the left (because of thefrequencies are retained in their usual easy-to-localise

cardioid directionality patterns of the microphones). At

positions. Here the different frequency components oflow frequencies, the phase difference of the sound at

sounds are no longer in precisely the same position.

the two channels is small but at 1kHz, the sound has totravel half a wavelength between left and right

We shall, for convenience, adopt David Griesinger’s

microphones. As a result, at low frequencies (and also

term 'spatial equalisation' to describe width increaseat 2kHz, 4kHz, 6kHz, etc) the sound arrives in phase at

below about 600Hz. Actually, the effect of spatial

the two microphones, and a width control increase will

equalisation isn't as simple as just described. First, as

indeed widen the stage, reducing crosstalk below 20dB

observed in the fifties by Clark, Dutton and Vanderlyn2,

for moderate width increases. However, at 1kHz (andand as rediscovered by Griesinger, many stereo

also at 3kHz, 5kHz, 7kHz, etc) the sound arrives at the

reproduction systems give narrower reproduction ofright microphone out of phase compared to the left

bass than treble, so that spatial equalisation might bemicrophone, so that a width control increase will

expected to give sharper images in those cases. This,actually reduce the crosstalk below 20dB at those

however, depends on the assumption that wideband

frequencies.

sound localisation is simply the sum of the separate
narrowband effects, something that is not confirmed by

As a result, any attempt to use width increase withall research (eg that of Bower5,6). This suggests that the

ORTF technique will indeed widen lower frequencies but

ears actually do some sort of cross-referencing of

it will have the effect of alternately narrowing andlocalisation data at different frequencies to produce thewidening higher frequencies, resulting in a possibly

perceived imaging effect.

confused and degraded stereo imaging. Thus, with any

Listening shows that often, with a single sound that has

On recording techniques capturing a good sense oflowish-frequency fundamentals and high frequency

space (eg spaced omnis and – in good acoustics –overtones, the subjective effect of increased bass width

Blumlein crossed figure-of-eights) the processing is

is often (but not always!) that the high frequency

often superfluous, and can sometimes even giveovertones are also pulled out to or near to the position

exaggerated spaciousness. If one uses crossedof the fundamental frequencies. In such cases, thecardioids, one cannot normally capture a good sense ofimages can remain sharp. Unfortunately, high frequency

space, especially as this technique seems to lend itself

sounds without low frequency components (eg cymbals)

best to relatively close placement to the musicians.

are not shifted. This can have strange effects on the

Spatial equalisation seems often to give excellent

stereo image – for example, on one recording with aresults with crossed cardioids, often giving a good sense

drum kit spread across the right half of the stereoof the acoustics of the recording venue.

stage, the kick drum is moved over from the left side ofthe drum kit to the right side by spatial equalisation!

With live recordings made with a crossed cardioid stereo

Similar anomalies can change the distribution ofpair, it is actually not true that precise coincidence of

instruments across the stage in an orchestral recording.

the two microphones gives best results (contrary to thecase for many other directional characteristics). A

One thing that spatial equalisation (bass width increase

degree of spacing can actually improve the stereo

below 600Hz) undoubtedly does is give all the sense ofimage quality. What one should certainly not do is space

increased spaciousness that normal width increase canthe microphones as in Fig 4 – this might liven up thegive but without the gross anomalies that the latter cansound but it degrades stereo imaging. Fig 5 indicatesgive at higher frequencies. It appears4 that the sense of

the optimum kind of spacing – this is similar to thespaciousness in stereo recordings largely depends on'crossed-over' ORTF technique, except that the optimum

the directional handling of bass frequencies. This is very

spacing for imaging is only about 5cm (2in). Thisevident on, say, stereo recordings of audience applausespacing, widely used in cheap Japanese stereo electrets,

in a live acoustic. The use of spatial equalisation can

was first commended to me by Tony Faulkner.

give a sense of being enveloped in the audience, eventhough there is no obvious change in the high-Remarkably, for normal stereo listening configurations,frequency imaging. The lower frequency components of

it turns out that the 5cm spacing produces roughly thethe clapping seem to be enough to create a sense ofsame phase/amplitude relationships between the twobeing almost there among the audience.

ears of a listener in the stereo seat as does a live soundfrom the same apparent direction up to about 2kHz –On many recordings, spatial equalisation does not only

and in this respect is better than true coincidence. Such

improve the sense of spaciousness but can also improve

5cm-spaced crossed-over cardioids, angled about 115°

the stereo imaging heard by listeners sitting away fromto 120° apart, seem to be an optimal cardioid technique

the stereo seat.

for stereo imaging accuracy. The use of bass-widening
up to 600Hz with this technique seems to give a much

In general when reprocessing recordings using spatial

better sense of space than the use of ORTF technique,equalisation, one should avoid aiming at a grossly

and without the latter's 'phasiness' anomalies.

spectacular effect as this will probably prove to beunnatural and tiring on casual or repeated listening.

There is another reason why spatial equalisation

One should listen carefully to what happens to thematches this cardioid recording technique particularly

positioning of different instruments to make sure thatwell – the existence of phase shifts between sum andthe altered positioning is acceptable – sometimes it

difference channels in the shuffler circuits described in

won't be. In preparing panpot recordings with thethis article. Although not mentioned so far for simplicity,

intention of using spatial equalisation, the original mix

such phase shifts occur because equalisers not only

should either be monitored (under appropriate

alter the amplitude of signals but also their phase. Thisconditions) through the spatial equalisation, or if not,generally degrades the localisation of amplitude stereo

instruments with lowish or mid-frequency fundamentals

and is usually a defect. It is possible (as realised byshould be panned rather more narrowly in the stereo

Vanderlyn2 as early as 1957) to 'phase compensate' theimage than is finally intended. One can, of course,

equalisers to match the phase in the M and the S

experiment with using bass widening only on parts ofchannels but this generally involves more complex

the mix – eg on stereo reverb and on sounds intended

circuitry, so will not be discussed further here.

to be placed beyond the speakers only.
Griesinger's results that bass widening sounds best if
Spatial equalisation seems most beneficial on stereo

concentrated below 600Hz (which my own tests

recordings on which the sense of space is inadequate.

confirm) might partly be a side-effect of the lack ofphase compensation – since a bass boost of S relative

exaggerating the artificialities of the pseudo-stereo

to M of 8dB produces a phase lag of about 25° in the S

technique. For example, I have found it to work wellsignal centred around 600Hz – at a frequency at whichapplied to the Orban pseudo-stereo of Robert Parker's

the ears are particularly sensitive to such 'phasiness'.

well-known reprocessing of old jazz recordings.

To make things even worse, the ears are more sensitive
to phase lags in the S channel than corresponding

Having indicated many of the possibilities andphase leads, as can be demonstrated from BBC

limitations of shuffling (and there is of course much

psychoacoustic data5,6.

more that could be said at a technical level), we
describe the practical implementation promised earlier

In the absence of proper phase compensation of the M

using readily available equipment and with easy

and S signals, one normally has to tolerate someadjustment. Ideally, for the implementation to bephasiness and blurring at mid frequencies if using

described, one should use a stereo graphic equaliser inshuffling or spatial equalisation. It may well be (Iwhich the two channels are ganged. Failing that, twohaven't tried it yet) that spatial equalisation might work

separately adjusted stereo channels can be used,

to frequencies significantly higher than 600Hz if proper

although this means one has to adjust more controls. (Iphase compensation is used. However, it is interesting

use a 7-band ganged graphic for this application.)

to note that small microphone spacings of the typeshown in Fig 5 produce phase leads in the S signal

The method to be described has the unusual feature of

relative to M at frequencies around the crucial 600Hz

automatically decreasing the sum channel gain as theregion. As a result, spatial equalisation without phasedifference channel gain is increased, so as to maintaincompensation can actually improve the 'phasiness' foran even overall frequency balance however the controls

crossed-over cardioid stereo, especially for sounds fairly

are set. Its description is so late in the article because it

close to the centre of the stereo stage and foris not as easy to understand how it works as Fig 2, andmicrophone spacings of 5 to 10cm. Simple spatial

it would have been confusing to introduce that

equalisation has defects that undo the defects of thesecomplication earlier. However, once set up, it is very

microphone techniques, giving a happy 'synergy'. The

easy to use.

converse is that the defects of the microphone
technique of Fig 4 are made even worse by spatial

The basic idea is to use a mixer having a phase-

equalisation without phase compensation!

inverting stereo signal path, which we shall term the'processing signal path'. In many mixers, this can be

One can, in other situations, reduce the phase errors inprovided by two of the input channels plus a suitable

S relative to M by arranging that the transition between

stereo output bus or headphone outputs. One needs

low and high frequency gains is as slow as possible,

gain controls somewhere in that signal path, preferably

rather than changing rather sharply around 600 Hz. I

ganged, eg a headphone output gain control, or ahave found that images do sound sharper and lessganged input gain on the two input channels. By way ofphasey if one sets the graphic equalisers so that theexample, on the cheap Realistic Cat No 32-1200B

transition takes place over a few adjacent bands, andmixer, the path between the auxiliary stereo input tothis is something that can be adjusted by ear for best

the headphone outputs is phase inverting and has in-effect.

path gain controls. One then feeds the processing path's
outputs back to the processing path's inputs, but

Spatial equalisation is particularly suitable forconnects the leads to interchange channels, so that the

reprocessing recordings for professional use made onleft output is fed to the right input and vice-versa (Figamateur equipment. This is because, as already noted,

6). Besides the processing path which is fed back as

many cheap stereo electrets have a spacing matched todescribed, the mixer also needs other stereo inputs

the properties of spatial equalisation, and also because

mixed into the processing path, and a main stereo

tape azimuth errors in cassettes or poorly maintained

output subject to its own gain control (Fig 6 – in theanalogue reel-to-reel machines are not worsened by the

Realistic mixer, any other input can be used, and theprocessing. Additionally, unlike simple width

main output is used for outputting the shuffled signal).

enhancement, spatial equalisation does not worsen
audible noise, since it leaves frequencies above about
1kHz unaltered.

One then mixes the stereo from other inputs, which is
to be processed, into the fed-back signal path (Fig 6).
The effect of the external feedback loop is to modify the

Another situation where spatial equalisation can provestereo. For signals identical in both input channels (ieeffective is with pseudo stereo derived by the Orban

central mono or M), due to the inverting property of thestereo synthesiser device from a mono original. Spatial

processing signal path, the feedback is negativeequalisation can enhance the spaciousness without

feedback, so that the M gain is reduced. For signals

opposite in phase in the two channels (ie the S signal)

right channel input's panpot to the point at which no

the feedback is positive since the fed back signal adds

output signal emerges from the left channel mainto the signal in the other channel rather than cancelling

output. Turning both input gains back up, the narrow

it – this is due to the signal in the other channel being

image produced by the panpots at the signal inputs isin opposite phase, so being in phase with the inverted

now adjusted to counteract the widening of the

fed back signal from the other channel. Thus the S gain

feedback path at its unity gain, so as to retain normal

is increased. As a result, as one turns up the gain in thestereo at this setting. (On the Realistic mixer, if theprocessing signal path (by stereo-ganged gain controls

headphone and auxiliary input gains are both set

at its input or output) the width is increased – up to thehalfway to 5, then panpots set to about 2.2 divisions

point where the positive feedback becomes unstable.

from centre counteract the feedback effect.)

When setting up the feedback (in the Realistic by

When one now re-inserts the graphic equaliser into thefeeding the headphone output into the auxiliary inputs

feedback path (Fig 7) its central unity gain settings willwith channels swapped) take care to keep input andagain give normal stereo, however, boosting any

output gains down to start with and turn them up slowly

frequency band on the equaliser (equally in bothto find out the point at which feedback howl occurs.

equaliser channels) widens the stereo image in that(Warning – keep speaker or headphone levels well downband, and cutting it narrows the stereo in that band.

when doing this!) Below this point, the circuit acts as a

Thus one has achieved an effective stereo shuffler bystereo widening control for stereo signals mixed in withthe circuit of Fig 7, and adjustment of the stereo-

the processing signal path, allowing adjustment all theganged equaliser bands simultaneously modify S and M

way up to infinite width at the point of feedback howl.

gains so as to preserve frequency balance. If the

The simultaneous reduction of M gain as S gain

equaliser has a bypass switch, one can use it to directly

increases give subjectively almost constant gain ascompare the processed and unprocessed stereo.

width is varied in many mixes, although central images

Because of the ganging of the stereo bands, this system

become quieter and edge images louder as the width isrequires fewer control adjustments when being altered

turned up. Interestingly, the configuration of Fig 6 turns

than the system of Fig 2, and so is easy to use.

the mixer into one whose panpots cover a wider stereo

Moreover, one also has overall width control available

stage than usual (depending on the setting of the width-

by adjusting the overall gain within the feedback path.

control gain) – so there is no reason why most studios
cannot start using their mixer as a wide-stage mixer.

Rather than using an external graphic equaliser, it isalso possible to use equalisers built into the mixer

If one now inserts a stereo graphic equaliser into thechannels used in the processing signal path instead.

feedback path (assuming the equaliser is not phase

This has the advantage of requiring no external mixer

inverting – if the equaliser phase inverts, then a non-circuitry other than connector leads, however, the two

inverting processing signal path should be used) one

equaliser channels have to be adjusted separately, not

has the possibility of varying the amount of feedback(usually) being gangable. The other disadvantage herewith frequency (Fig 7). Unfortunately, the equaliser willis that the equalisers built into the mixer are nothave some gain at all frequencies (even at maximum(usually) graphics, being designed for creative alterationcut), so will tend to widen the image at all frequencies –of tonal quality rather than shuffling. This makes it

this is not usually wanted or desirable.

more difficult to visualise instantly the kind of shufflingproduced by settings of the equalisers.

To counteract this, one first sets up the system without
the equaliser (or with the equaliser bypassed) at

For regular use as an overall width processor andreasonable feedback settings, and feeds the stereo

shuffler, it may well be worth obtaining a modest mixer

signals to be processed into the mixer via panpots. The

such as the Realistic, with the minimum of unnecessaryidea is to narrow the inputted stereo image with thefacilities, just for use as a width and shuffler control, in

panpots to counteract the widening at unit gain in the

conjunction with a stereo-ganged graphic equaliser of

feedback path. One pans the two channels in towards

the type encountered in some domestic hi-fi equipment.

the centre. If one turns down the right channel input

The alternatives are building equipment specificallylevel on the main stereo input, while feeding a signal

designed for this processing, or using more highly

into the left channel, then turn the left channel panpot

specified professional equipment that is less convenientto that position at which no output emerges from theto adjust.

right channel main output (so that the panpot
counteracts the widening of the feedback path).

Inevitably, this article has only scratched the surface of

Similarly, one then turns down the left channel input

stereo image reprocessing. More sophisticatedgain, feeding in a right channel signal, and adjusts thetechniques are possible. These include improvements inshuffling equaliser design (eg using phase-compensated859-871 (November 1985).

2

equalisers ) through methods of modifying image
sharpness, to dynamic signal dependent modificationsof the stereo such as have been used in variable matrix
decoders. Beyond that, there are the additional
enhancements of Ambisonic reproduction technology7,8,either applied to decoding stereo signals or to'transcoding' them into an Ambisonic format8,9.

8) Michael Gerzon and Geoffrey Barton, ‘Ambisonic
Surround-Sound Mixing For Multitrack Studios’, Preprint
C1009, 2nd Audio Engineering Society International
Conference 'The Art And Technology Of Recording', May
11-14, 1984.
9) Richard Elen, ‘Ambisonic Mixing – An Introduction’,Studio Sound, vol 25, no 9, pp 40-42, 44, 46

Although I don't agree with everything in them, I(September 1983).

recommend references 3 and 4 for additional ideas on
possible uses of shuffling. I hope this article has
provided you with a useful introduction to stereo

Width settings and crosstalkenhancement techniques and useful tools in day-to-day

It is often useful to know what the gain of S (relative torecording and reprocessing work. You are certainlythat of M) is, so that one knows how much width

encouraged to experiment using different shuffler

increase has been applied. This can most easily be done

settings with different kinds of recordings andby measuring the crosstalk of a left only signal on to theprocessing techniques. There is no telling what kinds ofright channel (or vice-versa) – something that can beeffects and improvements that you might come up with.

done on the mixer's meters. The crosstalk is the same
whether the S/M gain is reduced or increased by a given

Acknowledgements
1 would like to thank David Griesinger, Tony Faulkner
and Ivan Vernon McKinney, Jr, for drawing my attention
to various aspects of microphone and shuffling

number of dB! If S gain is increased, this crosstalk isout of phase but if the S-gain is reduced, the crosstalkis in phase.
S/M gain

CrosstalkS/M gain

Crosstalk

dB

dB

dB

dB

±0.0

–∞

±7.0

–8.3

±1.0

–24.8

±8.0

–7.3

1) A D Blumlein, British Patent 394,325 (14 June 1933),

±2.0

–18.8

±10.0

–5.7

reprinted in Journal of the AES, vol 6, pp 91-98, 130

±3.0

–15.3

±12.0

–4.5

(April 1958).

±4.0

–12.9

±15.0

–3.1

±5.0

–11.1

±20.0

–1.7

±6.0

–9.6

techniques used in this article.
References

2) H A M Clark, G F Dutton and P B Vanderlyn, ‘The
Stereosonic Recording and Reproducing System’ IRE
transcript. Audio, vol AU-5, pp 96-111 (July-Aug 1957),
reprinted with additions in Journal of the AES, vol 6, pp
102-117 (April 1958).
3) Richard J Kaufman, ‘Frequency Contouring For Image

The figures in this table are accurate only for the casewhen S has no phase shift relative to M, and so aremost easily applied to frequency-independent width
control.

Enhancement’, Audio, February 1985, pp 34-39.
4) David Griesinger, ‘Spaciousness and Localization in
Listening Rooms – How To Make Coincident Recordings
Sound As Spacious As Spaced Microphone Arrays’,
Preprint 2294, 79th Audio Engineering Society
Convention, New York, October 1985.
5) J S Bower, ‘The Subjective Effect Of Interchannel
Phase-Shifts On The Stereophonic Image Localisation Of
Wideband Audio Signals’, BBC Research Department
Report BBC RD 1975/27 (Sept 1975).
6) J S Bower, ‘The Subjective Effect Of Interchannel
Phase-Shifts On The Stereophonic Image Localisation Of
Narrowband Audio Signals’, BBC Research Department
Report BBC RD 1975/28 (Sept 1975).
7) M A Gerzon, ‘Ambisonics In Multichannel

The Gerzon Archive

Broadcasting And Video’, Journal of the AES, vol 33, pp

www.audiosignal.co.uk

Fig 1: Blumlein’s sum-and-difference width controlBlah

Fig 2: Sum-and-difference shuffling system

Fig 3: ORTF spaced crossed-over cardioid techniqueFig 4: Non-recommended spacing of cardioid microphones for stereo recordingFig 5: Crossed-over spaced cardioids for improved stereo imaging

Fig 6: Stereo widening control implemented by feedback around a mixer.
The phase inversion and feedback can be anywhere in the feedback or processing signal pathFig 7: Complete shuffling system based on Fig 6 plus stereo graphic equaliser (preferablywith ganged controls). Panpots in the main signal inputs compensate forthe basic widening effect of the feedback loop