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The Stereophonic Zoom The stereophonic zoom The stereophonic zoom By Michael Williams A variable dual microphone system for stereophonic sound recording Introduction In the field of monophonic sound recording the It is unfortunately too often the case that the sound recording engineer has a large range of position of the microphone system is a different types of microphone from which to compromise between a good stereophonic sound choose his particular preference for recording image and the optimum ratio of direct to any specific sound source or sound environment. reverberant sound. In more general terms, it is All manner of frequency response curves can be evident that the different parameters that must be chosen either to modify the timbre of the sound taken into ac- count during the process of sound source, or, if desired, to reproduce it as faithfully recording are infinitely variable, each situation as possible. The directivity of the microphone and encountered being in itself unique. The conditions its position in relation to the source, can also be that are necessary to achieve optimum results in a chosen to obtain, fairly easily, the desired specific context with the present "tools of the relationship of direct to reverberant sound. trade" are but rarely encountered. Unfortunately we do not have the same flexibility Even in the face of this very limited choice, many in the choice of microphone systems for stereo- attempts have been made to establish which, of the phonic sound recording. The number of available different systems available, could be considered as microphone systems and the types of micro- having the best overall performance. The futility of phones used, is relatively limited, and, almost this type of approach becomes immediately evident without exception, those that are available have in the light of the fact that each specific system has fixed characteristics. It is undeniable that each of a unique set of characteristics and that the sound these systems may be optimum in one particular recording sources and environment are infinitely situation, however none of them can be variable. considered as meeting all our needs for stereophonic sound recording. On the contrary, rather than reduce the choice of systems, an effort must made to increase the In no way can we consider our present stereo- number of systems available. Each sound recording phonic sound recording microphone systems as engineer must have the largest possible se- lection enabling the listener to experience natural of systems to choose from, in order to solve the perception of the original sound source. The specific problems presented by a particular sound image as perceived by the listener is recording situation and, to express his own conditioned by the choices imposed by the sound personal interpretation, as freely as possible. recording engineer. Balance, sound perspective, localisation, etc are determined by the sound The ideal is of course an infinitely variable system engineer and his skill in transmitting these covering all situations. This document describes parameters to the listener will remain part of the just such a variable dual microphone system called art of sound recording for many years to come. « The Stereophonic Zoom » and explains 1 The stereophonic zoom the basic theory of dual microphone systems in The operational approach general and how this has been interpreted to develop this variable system. I. 1 Characteristics of the standard listening configuration The "Stereophonic Zoom" enables the sound The basic characteristics of a stereophonic sound recording engineer to get much nearer to the recording system are determined as a function of optimum result in the great majority of recording the relative position of the loudspeakers in relation environments. It uses any desired first order to the listener during reproduction. It is almost microphone directivity with it's associated universally accepted as a Standard Listening frequency response curve, and enables Configuration, that the listener must be placed at reasonably independent control of: the summit of an equilateral triangle, the loudspeakers being positioned at each extremity of • Stereophonic Recording Angle, the base of the triangle and directed towards the • Angular or Geometric Distortion, listener (Figure • Reverberation distribution, 1). • Early reflection localisation . The analysis of the physics and psychoacoustics involved in this type of system has already been published in papers given by the author to various Conventions of the Audio Engineering Society: 1984 - 75th AES Convention in London - Preprint 2072, "Stereophonic Zoom ; a practical approach to determining the characteristics of a spaced pair of microphones" 1987 - 82nd AES Convention in London - Figure 1 – Standard listening configuration Preprint 2466, "Unified Theory of Microphone Systems for It is essential to attenuate reflections from the Stereophonic Sound Recording" ceiling, floor and walls and in addition, symmetry should be maintained in relation to the shape of the 1990 - 88th AES Convention in Montreux - listening room, so that any remaining reflections Preprint 2931, affect equally the sound heard from the left and "Operational limits of the Variable M/S Stereo- right channels. Only then will the conditions be phonic Microphone System" adequate to hear with clarity and precision the stereophonic image generated by the specific 1991 - 91st AES Convention in New York - microphone sys- tem used during the recording Preprint 3155, session. "Early Reflections and Reverberant Field Distribution in Dual Microphone Stereophonic Further improvements can be made to the listening Sound Recording Systems" environment by following the IEC recommendations concerning the specification of a standard listening The basic operational simplicity of this system is room. Another interesting solution to the problems described in this document. This should enable in designing a listening room has been developed by the sound recording engineer quickly to use the Bob Walker of the BBC Research Dept., and St Zoom System for everyday sound recording published in Audio Engineering Society Preprints with a minimum of study. 3543, 3853 and 4645. 2 The stereophonic zoom I. 2 Localisation The intention in this document is to describe Without going into all the details described in a variable dual microphone system that will Chapter II, one can say that the localisation of a reproduce realistic stereophony, thereby sound source between the loudspeakers is creating a good "impression" of relative obtained : acoustic size for each sound source and maintaining the continuity of the sound • by varying the intensity ratio between the two environment. It is important again to loudspeakers, emphasise the word "impression" in this • or by creating a time difference between them, context, as we are indeed only concerned by • or by a combination of both intensity and time impressions. For instance, in recording an difference. orchestra, we must create the impression of depth and localisation of individual If the same sound is produced by each loud- instruments, as realistically as possible. The speaker at the same level and at the same instant, acoustic signals, received by our ears in the then one will get the impression that the sound listening room coming from the loud- image is situated in the centre between the two speakers, bear very little relation to the loud- speakers (0° in Figure 1). If on the other hand acoustic information actually received by the the sound produced by the right channel is louder listener in the concert hall. In fact, it could be than the left, then the localisation of the sound said that, any recording and reproduction image will be situated somewhere to the right sys- tem which is capable of giving this hand loud- speaker (between 0° and 30° in Figure "impression" is acceptable, no matter what 1). If on the other hand the sound intensity of both means are used to obtain it. channels is the same, but there exists a small time Intensity Difference (or Intensity Ratio) and difference (less than a millisecond) between each Time Difference information can be generated channel then a similar effect is obtained i.e. if the by two spaced directional microphones. The left channel is in advance of the right then the Intensity Difference information generated by sound is localised to the left and vice versa. the microphones, is a function of the position of the sound source and the angle between It is quite commonplace to create a variation of the axes of the directivity patterns of the the Intensity Ratio on a mixing desk, by means of a microphones. The Time Difference simple potentiometer, normally called a "pan pot". information, on the other hand, is a function of On the other hand it is quite exceptional to find a the position of the sound source and the variable delay line associated with a mixing desk to distance between the microphones. To obtain create Time Difference information between left Intensity Difference information only, the and right channels. Both of these techniques microphones must be "coincident", whilst produce localised sound sources between the Time Difference information only, will be loud- speakers, unfortunately without any obtained with spaced omnidirectional information necessary to produce natural microphones or parallel spaced directional acoustic size and stereophonic acoustic microphones. environment. This process is very aptly called in I. 3. 1 Operational characteris- French "Monophonie
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