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Ambisonics

Full of Audio Promise

Modern developments in audio reproduction


Standard music listening systems in the home over the last twenty years or so have been two channel and thus two speaker 'stereo'. Stereo, with two transmission channels, is a great improvement on mono as the listener is given a degree of the spaciousness which exists in real live music. Obviously all this stereo from disc, FM radio and cassette has to be mono compatible.

Halfway during the stereo era, attempts were made by various protagonists to improve on the realism possible - quadraphony. But competing systems and the ill advised use of just 'double stereo' microphone techniques resulted in the failure of quadrophony in the most important area - the market place.

With this background any attempt to cover the same ground, even in a much more technically viable way, has an uphill struggle especially in the current economic climate.

Ambisonics, however, is here and its virtues and techniques should be of interest to musicians/composers and listeners alike.

If you accept that one of the aims of Audio reproduction is to do more than just reproduce instrumental sounds spread between a pair of loudspeakers (and all too often just in the loudspeakers) but to recreate the ambient environment in which the instruments were played, then it follows that ordinary 'stereo' has some way to go. Firstly, in the main, it only caters for reproduction of one horizontal quadrant of the original environment. One could go a stage further and cover the whole horizontal 360° of the original environment - horizontal Surround Sound. Or go even further and recreate the full sphere - Periphony. The latter is not commercially with us at the moment but the former is, horizontal Surround Sound, as one of the facilities of Ambisonics.

Quadrophony



Everyone thinks of quadrophony as a system with four loudspeakers and in its simplest form of arriving at the listener, this also applies to Ambisonics. However, that is where most of the similarity ends.

Quadrophony, in order to get into the home, was available in three competing systems. Two of the systems, SQ and QS, were 2 channel 'matrix' types, encoding the 'rear' information on the two channels of a disc, cassette or radio broadcast. Separately there was a more technically refined system, applicable to disc only, called CD4. This used a supersonic carrier to convey the 'rear' information. Obviously, there were commercial uncertainties between the three competing systems.

All of these systems, however, had one big failing - the microphone techniques were haphazard adaptions of those which had more or less worked in stereo. At the listening end it was sometimes said that, with two of the systems, things were 'better' when the wrong system was used to decode the matrix information!

There were also varying degrees of stereo compatibility in the systems, an important parameter as the greater number of listeners would still be listening in stereo. Worst still, one had a serious mono compatibility problem.

Ambisonic Miking



The development of Ambisonics could be said to have started at the microphone and with what it should do to convert the soundfield into a set of electronic signals. These electronic signals are called B Format signals and a basic understanding of these is at the heart of further thinking about Ambisonics. There are a set of four B format signals. Straightaway it must be stressed that these four signals are not directly related to the use of four loudspeakers.

The soundfield, at a point in space, can be described by three directional signals and one omnidirectional signal. The three directional signals are that which would come from three figure of eight microphones; one facing horizontally front to back (X signal), one facing side to side (Y signal) and one facing vertically up and down (Z signal). The final information needed to describe the soundfield at any point in space, is provided by an omnidirectional signal (called W).

Calrec Soundfield microphone.

Currently, there is only one microphone which can be used to produce these signals - the aptly named Soundfield Microphone (SFM). These are produced under licence by the Yorkshire firm, Calrec.

Using just one microphone in a recording or broadcast has its limitations, but is is possible to introduce the outputs of other mics into the XWYZ set of B format signals, by electronic mixing from panpots so that directional information is available. Many recordings exist on disc and cassette using a single SFM, but little has yet taken place with additional mics or solely with simulated B format signals.

What follows the B Format?



It is the B Format signals which can be used to create Mono, Stereo, Horizontal Surround Sound or the Periphony hinted at earlier.

Consider the stereo (or for that matter mono) usage. By manipulation and mixing in amplitude and phase, the B format signals can be used to produce a microphone that can be steered - controlling the direction and tilt, gives apparent movement (forward/back or up/down) - apart from the choice of polar pattern (omni, cardioid, hypercardioid or figure of eight) and 'capsule' angle! Further more, if the B format signals are recorded, then these facilities are available for modification afterwards.

However, we are digressing, the case in point is the Ambisonic use of B format signals. The X, W and Y signals are used for a quality of horizontal Surround Sound which far exceeds the performance of the ill-fated quadrophony with its unsuitable mic techniques. And you've guessed it - the X, W, Y and Z signals can be used to create the further extension of surround sound, that with height, Periphony.

In the home?



At the present time, this has to be with a 2 channel Matrix System, not dissimilar to the basic ideas behind the earlier matrix systems. With the undoubted improvement after many years of research into the use of matrix encoding techniques things are altogether more satisfactory. The system that has evolved is the UHJ system. It is difficult to try and relate the historical (and political) aspects of the meaning of the letters, but for the record -

U - Universal, in the sense of the amalgam of two systems.
H - The BBC Matrix H system of a few years ago.
J - The Ambisonic Research Teams 45° J system.

Internal view of the Soundfield microphone.

It is, in the writers view, a system of mono and stereo compatibility and I would even say that stereo is enhanced, this being confirmed by others, giving less listening fatigue and almost having a 3D effect, albeit from a normal stereo loudspeaker presentation. It is perhaps interesting, that when I give musicians a choice of crossed pair stereo from a SFM or of a UHJ stereo encoded version, they choose the latter!

To get from the B format signals to the 2 channel form suitable for Disc, Cassette or FM Radio, the X, W and Y signals are passed through a UHJ Encoder. At the playback or listening, end the encoded signal is passed through a UHJ Decoder and four 'loudspeaker' signals are derived. Of course, each needs its own amplifier to feed a loudspeaker. Incidentally, decoding at the listening end is not limited to just four loudspeakers (one might be financially!) but six can be used from the same four amps. However, the effects from four loudspeakers in a horizontal surround sound Ambisonic system are good.

Ambisonics vs Quadrophony



The writer dabbled in quadrophonics, years ago, recording four mics on four channels and simply playing back via four loudspeakers. I wasn't all that impressed. Now with the SFM, and the optimised feeding to four loudspeakers, one has a remarkable improvement. Consider the stage/ambience presentation which forms the basis of most Ambisonic UHJ material - the performers are correctly displayed, as in good stereophony, but with more depth, the ambience surrounding and enveloping the listener. There is not the crude, front and rear type of presentation of quadrophony. Also the speakers are not the apparent sound sources allowing one to move in the soundfield without the effect collapsing.

Practical ambisonics



Ideally one should have four identical loudspeakers, preferably with the units in a vertical array. But even if you haven't got, initially, four identical loudspeakers, it is still worth a try, the 'best' pair placed at the front. Again it goes without saying that low colouration speakers with little 'boom' and a 'peaky' top response are going to be best. If the loudspeakers are identical then the control of their levels is going to be much easier.

Ideally, four identical amplifiers should be used, again reproduction levels will be more easily set and any chance of frequency, or especially phase irregularity, reduced.

The most cost effective way of getting the UHJ Ambisonic Decode circuitry is to use the Minim AD2 module. A little DIY around this and one has produced an excellent source of four signals to feed the above mentioned four amplifiers and loudspeakers.

The author runs an Ambisonic system with ILP HY60 amplifier modules and four Coles 8" plus Motorola Piezo tweeters in very rigid reflex boxes. As 'tone controls' are not believed in, the whole affair is simple, straightforward and remarkably low cost considering the excellent performance.

The Minim AD2 Ambisonic Decoder.


The Minim AD2



The PCB is 100mm square with short ribbon cables to a potentiometer and a rotary switch. There is a unity gain between the UHJ input and the outputs. Levels ranging from 50mV to 2V are intended. The input impedance is 20K and the source impedance of the outputs is 6.7K. Three push buttons provide the following facilities:

UHJ/Stereo - allows playing of ordinary stereo sources with a simulated surround sound. The rotary potentiometer gives soundfield width control, from mono to super-stereo. This 'decoding' of stereo sources is a very useful, unsung feature of Ambisonics which performs very well. No need to feel that one's stereo sources are redundant!

Layout - compensates for loudspeaker layouts other than square. Used for 'oblong' layouts in either direction.

B Format - in case there should be B format signals available (the compact digital disc?) there are X, W and Y inputs.

For setting up this latter feature is very useful. The W (remember - the omni component?) input will feed equally to all outputs and will allow level setting. A suitable subjective signal is noise from an off station FM tuner.

The Minim AD2 module is well built with close tolerance components, many tests being carried out after construction. These are logged for possible future reference and each board has its own serial number.

Ambisonic recorded material



A number of record companies have quite full catalogues of UHJ material recorded with a single soundfield microphone. These include Nimbus, Unicorn, Music from York and the loudspeaker manufacturer IMF. Real Time duplicated cassettes on several labels are available from Whitetower Records at the address below.

The Future?



The NRDCs parent body, the British Technology Group, are currently of the view that the time is right for expansion of the marketing of Ambisonics.

It is recognised that it will only be when the multi-mic/multitrack techniques of the current stereo era, are readily incorporated into Ambisonic products from studios all over the world, will there ever be 'public acceptance'. It is of course this public acceptance and desire for Ambisonics that will make it commercially viable.

A program of design and construction of mixdown equipment to take multitrack material and convert it to the B Format form is underway and hence UHJ issues from such sources are likely soon.

Having more popular recorded material - the software - should increase interest among the playback equipment manufacturers - the producers of the hardware. However, with the advent of the Compact Digital Disc, has the boat been missed to incorporate in this medium, the actual B Format signals? This for many would be the imaginative step forward.

Whitetower Records are making available the MINIM AD2 Ambisonic decoder atthe special price of £45 including P&P. Orders should be sent with cheque or PO to DEPT E&MM: (Contact Details). A list of Real Time duplicated UHJ encoded cassettes is also available on receipt of a sae.



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Casiotone CT 7000

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Electro-Music Engineer


Electronics & Music Maker - Copyright: Music Maker Publications (UK), Future Publishing.

 

Electronics & Music Maker - Apr 1983

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