Dolby Cat 94 Pentoptical Format Module - Information Wanted

[Connor Munsch]

Hello all,

I've come across various references to the Cat. 94 Pentoptical Format Module for the CP100. I understand that it is required in order to use the various surround adapters with the CP100, but beyond that there seems to be very little information available about how it actually functions. Does anyone here recall using one of these back in the day, or have schematics or documentation that goes into more detail?

Looking over the SA3 and Cat. 116 diagrams, it appears that the external decoder derives the Left, Right, and mono Surround channels, while the Center channel is generated by the Pentoptical Module. However, the signal flow and the order in which this happens are still unclear. I am curious if the Center channel is derived before or after the surround decoder. Any insights would be greatly appreciated — thanks for your time.

-Connor

[Steve Guttag]

The channels are derived independently. The Pentoptical module and the Surround adapter both get to use the same Lt/Rt source to do their thing.

Another thing to understand is that "Dolby Stereo" as a format evolved over time. Its early incarnations didn't really have any "steering," as later systems that used the CAT150 did. The CAT150 would be housed in the SA4 (or SA5) such that the optical format module really just sent Lt/Rt up to the SA4 and then took the SA4's LCRS output and sent them to the outputs.

The CP100's format modules we a bit of a sound matrix card...routing signals as desired, for the format, as well as, possibly, performing some processing.

The CP100 treating the surround channel as an accessory was one of its achilles heel. Every later format had to contend with sending the surround change out and around to pick up possible processing, NR, and EQ.

So, on the early cards, like the CAT110 (CP50), CAT82 (CP100). The stereo effect was a bit diminished. It would sum for center and the steering amounting to a slight boost/cut based on where the sound was to go. We're talking something like 3 or 6dB...nothing approaching discrete or very convincing.

The Whole SA1-SA3 was trying to figure out what to do with the surround delay (do we do it on the card or in the chassis?)

But, once the CAT150 came out, there was honest-to-god steering where you could get each channel to play without having too much crosstalk out of the others.