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 with a
basic matrix (BMX) that is unprecedented in the effectiveness of its use of only two channels of multidirectional information.
This effectiveness carries the implication that artificial one-at-a-time enhancement of acoustic imaging, ordinarily disruptive of that highly-prized psychoacoustic dimension of ambience, need rarely be considered. This effectiveness is traceable, in part, to the fact that the sole encoding parameter is audio phase angle, and that this parameter is used as the direct analogue of panorama bearing angle. This particular use of phase, not withstanding the fact that it is the only signaling parameter capable of representing the 360 repetitive character of bearing angle, is unique to the UMX system.

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.

As a companion to the disc system, a broadcast system called QFMX has been developed, It is one of five systems under study by the National Quadraphonic Radio Committee (NQRC) in the USA. The committee was organized by the Electronic Industries Association (EIA) for reporting to the Federal Communications Commission (FCC), The system is mono/stereo compatible with existing FM broadcasting, and the stereo signal is susceptible to RMX decode, The 38 kHz sub carrier, moreover, besides the modulation of its normal phase, bears the T signal as a modulation of its quadrature phase. The broadcaster may elect to stop there (TFMX option), to avoid using any further frequency bands than in current practice, while providing a quality of service virtually indistinguishable from the ultimate, QFMX, which ultimate requires the use of a 76 kHz subcarrier, Further systems. both for studio use and for use with the audio cassette, are under development as worthy companions to, and supporting systems for, the UD-4 disc.

Because of its use of the first truly rational matrix system capable of providing unsurpassed imaging quality commensurate with the number of disc channels in use, and because of its restrained carefully planned use of carrier channels, avoiding the principal hazards of such use to provide a sound quality the equal of contemporary stereo discs, we, together with many friends and advocates in the audio engineering community the world over, believe that a responsible commercial introduction of the UD-4 system may now be undertaken.

Announcements of commercial intention were made in September 1973 by Nippon Columbia Co., Ltd., a manufacturer in consumer electronics and disc records, and by its parent company Hitachi, Ltd., and the marketing of UD-4 discs and playback equipment in Japan has begum. Also, in November of that year, marketing for England was started through Johnsons of Hendon Ltd. in London. This is to be followed by marketing on the Continent and in the USA. Our proponents will be able to join us in the very near future in technical contracts and patent agreements based on ordinary kinds of terms.

The UD-4 discs and equipment manufactured by Nippon Columbia Co. bear the brand name Denon, a name chosen for the most select high-grade audio and acoustical products of its manufacture. In addition, the UD4 mark is in use.

REFERENCES
1. D. H. Cooper and T. Shiga: Discrete-Matrix Mullichannel Stereo. J. Audio Engrg. Soc., vol. 20, pp. 346-360 (June 1972).
2. O. Kohsaka, E. Saloh. and N. Nakayama: Sound Image Localization in Multichannel Matrix Reproduction, J. Audio Engrg. Soc., vol. 20. pp, 542-548 (Sept. 1972).
3. D, H, Coopur, T. Shiga, and T. Takakgi: QMX Carrier-Channel Disc. J. Audio Engrg. Soc., vol. 21. pp. 614-625 (Oct. 1973).
4. D. H. Coop"r: QFMX-Quadruplex FM Trans- mission Using the 4-4.4 QMX Matr,x System,
J. Audio Engrg. Soc" vol. 22, pp. 82-87 (March 1974).

 

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