Surrounded By Space Page 2
Like two of the quad systems, Dolby MP uses matrixing to encode directionality as phase differences between the left and right stereo signals. No phase difference places images at front and center, a 45 degrees difference places them at the left or right stereo speakers, and a 180 degrees difference (reverse-phase) places them in the rear surround channels. If you add side speakers, a 90 degrees difference places sounds at one or the other of those. For films, each sound image is panned (footnote 2) to its proper location so it appears there unambiguously in playback.
Conventional stereo recordings do not have rear-channel information specifically encoded into them, but they do contain some reverse-phase ambience information, because ambience has random phase and a certain amount of that randomness is reversed. A Dolby Surround decoder can separate out this ambient information and direct it to the rear channels. Subjectively, the effect is surprisingly like being in the hall where the recording was made. But Dolby MP differs from quad in one important respect: the surround information is monophonic. So even with two widely spaced rear speakers, the surround channel conveys no impression of space. What it does do, though, is draw the ambience from behind the front speakers into the listening room, enveloping the listener as it would in a hall. The result is usually a startling improvement in realism, mono ambience or not.
But Dolby Surround has never been used for audio-only recordings. In fact, Dolby Labs discourages such use of their system, for reasons they seem hard-pressed to explain. However, they cannot prohibit it, and considering the number of surround-equipped audio-video systems out there in consumerland, it seems surprising that no one is making ambience-encoded, or sound-in-the-round, audio recordings for them.
Actually, there's a much better system for ambience reproduction. It's called Ambisonics, and Bill Sommerwerck has described it often enough in these pages that I won't reiterate. But, like better Beta, Ambisonics has just not (as yet) garnered consumer interest. Purists dislike it in principle because there is so much active electronic circuitry involved, while the general public---even that sector now delighted with their Dolby Surround---either scorns Ambisonics because they see it as another quad, or is totally unaware of its existence. Only one record company, Nimbus, records Ambisonically, and their catalog is relatively small. It is unlikely that other firms will join them until there is more consumer demand, and there won't be more consumer demand until there are more Ambisonics recordings available. It's a classic case of Catch-22.
Is there no hope, then, for a perfectionist's surround-sound system? There are two possibilities: Compact Disc and R-DAT.
Few people realize that the CD can accommodate two more discrete signal channels, which could be used as a rear ambience pair. These would not require matrixing or dematrixing; all four signals would be as clean as today's stereo pair from CD, and the soundfield re-creation would be as good as it is possible to get. But no one is hurrying to release surround-sound on CD either, because: 1) it would halve the playing time; and 2) there are no recorders or players capable of handling four digital signals in CD format (footnote 3).
The R-DAT system also has discrete 4-channel capability, the standard allowing for the sampling rate to be reduced to 32kHz to cope with the extra data-storage requirements, but there are no players yet in US homes and it's anybody's guess when there will be. Copycode may be a dead issue, now that the NBS has reported so negatively on its effects, but the music industry has nevertheless resolved to continue its efforts to prevents R-DAT's US introduction. (See George Graves's "Industry Update" elsewhere in this issue.) Several hardware manufacturers have announced Spring '88 release dates for R-DAT machines, but so many release dates have come and gone uneventfully, it's hard to put any credence in the latest ones.
Where do we go from here? I hate to say it, but the best bet for domestic ambience reproduction as of now is the worst system for it: Dolby Surround. Good low-distortion decoders are available for it; there are lots of them in American homes; and there are lots of CD players hooked up to home-video systems. (Vinyl discs have never been entirely satisfactory for encoded surround sound because any mistracking causes horrible rear-channel distortion.) Dolby-encoded rear ambience on CDs would not affect CD playing time, would appear up-front when played on stereo systems, and could be separated out and reproduced from the rear by anyone equipped to do so. The initial encoding could be done passively, so those who don't choose to decode wouldn't have to worry about signal degradation due to active devices. The results wouldn't be as good as discrete four-channel sound, but they would be much more realistic than what we're accustomed to now from stereo. And that, after all, is what we're supposed to be seeking.
Footnote 2: In stereo, panning is the placement of a mono source at a particular location between the speakers, by adjustment of the relative level of the signals in the two channels. In the case of Dolby Surround, it also involves phase shifting.---JGH
Footnote 3: Conventional stereo CDs have the data streams representing the two audio channels multiplexed/interleaved on the disc; on replay, they are demultiplexed by the appropriate data being alternately read out of a RAM buffer memory. Four-channel replay, which I understand is not part of the CD Standard, would be no more complicated in principle, but would require dedicated hardware.---JA