HI-FI NEWS & RECORD REVIEW - MARCH 1975
THE UD-4 SYSTEM
Duane H. Cooper (University of Illinois)
Toshihiko Takagi (Nippon Columbia Co. Ltd.)
|THE UD-4 System is a phonograph disc
system for the recording and reproduction of four channels of
multidirectional audio signals. Its name derives from its use of UMX
encoding of directional information and from its use of a discrete
set of four information-bearing channels. Two of these appear as
ordinary phonograph-groovewall modulations, and the two further
channels appear as FM (frequency modulation) carriers superposed on
the same groove walls.
Although it exhibits similarities to the CD-4 System) in being a mono-stereo compatible, discrete, carrier-channel disc system, the UD-4 is the only discrete system that simultaneously offers a low-cost matrix playback option (UMX), and a 'discrete' playback option (QMX) in which many of the signal-distorting hazards previously characteristic of carrier-channel discs have been avoided. Thus, accurate sound reproduction, previously characteristic only of discrete discs, are combined into one discrete system. While there is necessarily some limit in localization accuracy in the matrix playback option, exceptional accuracy in both sound reproduction and imaging is maintained in the UD-4 discrete playback option.
The UMX encoding system, first described in
1971, has been almost universally recognized as the optimal means of
using a limited number of audio channels for the portrayal of a
continuous azimuthal panorama of sound images. The system starts
The far reaching implications and elegance of the UMX concept won early favour among colleagues at Nippon Columbia Co., Ltd., where work on matrix problems dated from 1970. The step-by-step development of successive members of the UMX family, even including experiments with an eight-loudspeaker, five channel system (PMX), began at once with BMX.
The basic matrix (BMX) makes use of a true omni directional mono channel and a stereo-difference channel that is also omni directional, except for a frequency- independent phase lag that, in comparison to the mono channel, is made equal to the panorama bearing angle for each of the acoustic images to be portrayed. The left and right stereo signals are the usual sum-and-difference combinations of these mono and stereo-difference channels and correspond to center left and center right decode points in the panorama. As with all diametrically opposite points in the BMX decode, full signal separation obtains between them. Consequently the stereo playback mode offers the full range of stereo imaging as a front-back fold of the encoded panorama, while the HMX playback mod feeds four loudspeakers to present the full panorama.
The symmetry of the loudspeaker-feed patterns provides further insight to BMX accuracy. The patterns are shown in fig. I for eight different source positions. The intercept of the curve with the radial lines indicates, by radial distance thus encompassed, the amplitude of the signal at the output terminal that is supplied to the loudspeaker in question. Terminals for four loudspeakers are shown. The patterns exhibit axial symmetry that remains invariant as the pattern rotates to follow the source. The decode phrasing pattern, not shown, constituting a half degree of phase per whole degree of azimuth, is locked to the amplitude pattern. The QMX pattern, shown in figure 2, may be similarly described.
The two further UMX channels on the UD-4 disc are a conjugate stereo-difference channel in which leading, in contrast to lagging, phases are used and a double-phase-angle channel, called T and Q channels, respectively. The decode of this pair, combined with the BMX decode, provides a full independence (separation) among speaker signals at 90 intervals in bearing angle; This technical discreteness provides a psychoacoustic crispness to the panoramic image quality that is undiminished even when the technical discreteness is maintained only up to a few kilohertz of audio frequency. This is a consequence of the acuracy of BMX imaging that continues to the top of the audio band, and of the reliance in human hearing primarily upon frequencies in the rage below only a few kilohertz for the sense of localization crispness.
Psychoacoustic trials have verified that reliance upon only moderate frequencies for the sense of localization crispness. Careful statistical analysis of the data obtained also showed that QMX localization exhibited much more stability for phantom imaging that any previously proposed means of using four discrete channels, and that a three channel system (TMX) came very near to the QMX system in localization crispness. These results persisted as the frequency range available for establishing localization crispness was progressively reduced.
The reliance upon a limited frequency range
greatly diminishes the frequency-band requirements for the FM
channels to those which may be more readily accommodated in the
present disc art - - without prejudice to future advances in the
art-diminishing at the same time one of the major sources of
distortion and noise to which such channels are otherwise prone. A
second source of distortion, arising from modulating the carriers in
a difference mode, has been virtually eliminated through choosing a
constant-phase-deviation equalization scheme for that mode.
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