(idm) dolby?

From Alex Reynolds
Sent Thu, Sep 3rd 1998, 22:13
What's considered "true" quad sound? What vile beast has four ears? <g>

The easiest, no-brainer approach: four channels in via two needles on two
grooves, four channels out via RCA (or whatever you use).

However, I think that as far as the movies go, you can encode multiple
channels in "stereo" (two) channels, which is already available from a
record -- technically, surround sound (FL, C, FR, RL, RR) is accomplished
by phase differences put in the two channels. Front and rear signals are
separated by having a "front" wave cancel a "rear" wave via superposition,
and vice versa. (This, according to the Dolby decoder manuals I read whilst
running a film house back in school.)

Apparently, the way that the sound is optically encoded on the film doesn't
affect the poorer theatre's ability to serve regular stereo, or even
vanilla mono.

So with records as they are, you'd go with an encoder chip before pressing
the master and a decoder chip on each turntable to get four channels out.
Or you could go with the nasty, two-armed monster.

:: Alex

P.S.: With 35mm film, you only have so much space to put a soundtrack
before you start squeezing the picture frame, so Dolby's older solution was
good in that you only needed enough room for the two channels. In fact,
SDDS and Dolby Digital use the space between the cog holes (!) to store the
optical soundtrack, and DTS -- I think -- uses a special laser disc player
which carries the soundtrack for the entire movie.

>Couldn't "true" quad sound be done this way, then?
>
>On Thu, 3 Sep 1998, Alex Reynolds wrote:
>
>> That sounds almost like you're describing some sort of error correction on
>> a piece of vinyl...
>>
>> Such a two-tracked record could be played on a deck with a two-needle arm,
>> the distance between needle tips equalling the distance between groove
>> valleys. The advanced deck would be less susceptible to bumps *and* able to
>> take the two signals and subtract noise from either, like a pair of
>> chromosomes. Something like that could be "complete without surface
>> noise"...

__________________________________________________________________________
Alex Reynolds                               Distributed Support Specialist
Department of Biology                  School of Arts & Sciences Computing
University of Pennsylvania                                Philadelphia, PA
email:xxxxxxxx@xxx.xxxxx.xxx                            phone:215.573.2818