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Only he strutted around, proud as a peacock, when Dolby filed a suit against him for infringing on SR technology. I guess being 'recognized' in this manner provided its own approval, by bestowing the status of 'legitimate contender'!

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Then the CP50 came out. The "standard" CP50 only decoded the 3 screen channels. It had two (single channel) Cat.22 NR cards, an "QS" matrix card and three Cat.64 eq cards. Surround decoding was an extra cost option. Later, there was an automation interface to operate it remotely. The sub-woofer option was added later, all for extra cost.

Actually, the concept and equipment to create SVA was developed in (I think) the late 1930's by Western Electric. Being part of the phone company, WE was interested in all kinds of sound-type technology, but didn't seem to pursue the SVA concept and dropped it. So, years later, Dolby bought a few of those old optical sound recorders, and modified them with modern electronics. The WE patents had expired, so Dolby did'nt have to pay any roylties/rights.

So, now Dolby had a noise reduction process ("A"); a device to get two channels of optical sound on film (WE SVA), now they needed to get 4 channels. They wanted 4 channels, because they knew there were still many Cinemascope (4 channel magnetic striped film) installations around. So, they made their complete system as a replacement to the mag tracks, and to match the "3 speakers behind the screen and mono surround) setup. If you had a Cinemascope theater (and there were many of them, not being used) you could rather cheaply get back the 4 channel sound, with out the problems of magnetic striped film (EQ'ing, demagnitizing head, heads wearing out-they only lasted 5-6 features.)

At that time we were in the middle of the 4 channel home stereo "Quadraphonic" era. The idea of this home system was the same, (2 channels matrixed to 4) except the speakers were to be placed in the 4 corners of the room- you sat in the center- unlike the theaters. "Quadraphonic" was only for listening to records. There were video tape recorders, but no one released movies (in fact, then, the studios hated the idea of VCR's, thinking it would lead to piracy) so there was no thought of "Quadraphonic" being used as way to watch films. Several companies made a simlar 2 to 4 decoders for this home market, but Dolby liked Sanyo's "QS" decoder the best and licenced it from them to complete their theater package.

You have two sound channels on the film: left and right. Adding left and right gives you the center channel. Subtracting right from left (L-R) gives you the mono surrounds. This only works with sound material encoded in this way. It wouldn't work with a normal CD, for example. The circuitry for doing what I described is very simple and could be built entirely with Radio Shack parts. When someone advertises a "passive surround decoder", it's based on the principle above. This simple solution has problems with signal separation from the different channels, so Dolby added some of its own improvements. This is compatible with two-channel stereo. The center channel information is split evenly between the two stereo speakers, and the surround is out of phase and results in a sound which appears not to come from the center. If you've ever hooked up a stereo receiver at home out of phase, you'll know what I mean. If you were to mix that down to mono, the surround would cancel out and not be heard at all.

As to how many channels, yes it all started with just Dolby "A" noise reduction for mono tracks, but soon there after the three channel version came out when the Dolby CP-100 was still the only processor. To be sure, the concept went way back, but the problem was the noise level. Dividing the track into two doubled the background noise which was already not too good. By applying Dolby "A" NR the noise level could be reduced enough to allow for the two separate tracks. It seems to me that Ioan Allen (the daddy of the whole project) once told me that the resultant noise is about 1 dB below that of a well made old style mono track. In the meantime, both negative and positive film stocks have improved and when today's SR is added to the mix, we have some pretty damn fine optical tracks.

Back to the CP-100, but the decoder card in the original allowed for only the L,C,and R, which is why most of them eventually sprouted the "SA" (Surround Adapter) series of outboard chassis which housed the early surround cards which were separate from the front channel decoders. I still remember installing the very first hand wired Cat. No. 150 into the Century 21 Theatre in Denver, which must have been sometime in late '78 of '79. (I remember it well because of the truly professional projectionist who could not even run a sound-only reel without full lights down, curtain open. A real showman.)

To expand a bit on the how-it-works description above, after the two tracks (Lt and Rt, for "Left total" and "Right total") are read and been through the appropriate noise reduction, they go into the decoder where the L and the R go straight through almost to the end where they are presented to VCA's, or "Voltage Controlled Amplifiers". At the sametime, through a side channel they are summed and presented to the Center VCA. Also at the same time the two signals go to the steering circuit which analyzes them and if they are alike in both phase and amplitude commands are sent to the VCA's to raise the level for the C channel and reduce the level for the L and R channels. If the Lt and Rt are different, the reverse happens so that they VCA's steer the signals to the L and R channels as a stereo signal. When the steering circuit sees the two incoming signals as the same in level but with one of them leading 90 degrees in phase and the other lagging 90 degrees, these are identified as Surround, passed through a Dolby "B" NR circuit and passed on through the Sur VCA to the amplifiers. Thanks to today's technology they are now able to achieve a whole lot better separation than the 6 dB they started out with. The last I heard, it is now somewhere around 25 dB. No, I don't think that you are going to do this with Radio Shack parts.

In particular, the CP-100 is the original Dolby Stereo processor.

The CP-50 was the "economy" processor designed to be retrofitted into existing 4-track magnetic theatres. It stripped away the "flexibility" the CP-100 had (the CP-100 had 3 format slots so one could "program" it by installing the appropiate format into one of the three slots. The CP-50 on the other hand just did the basics...that it it just handled optical sound formats (magnetic was a pass through since the theatre was expected to already have it's 4-track magnetic system in place).

The first Cat. 150 should have come out around 1978-79 or thereabouts. The CP-200 came out in May of 1980 with the release of EMPIRE and the Cat. 150 was standard for CP-200.

Merry Christmas to all!

Steve

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