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Look what I just snagged off eBay :)
Here it is:

A little background: I've been trying to find a good way to transform the stereo surround track on the Brazil Criterion Collection back into its original 4 channels, as is intended. Turns out it's a little more tricky than expected. 

For one, the "Dolby Stereo" matrix is always the same. In other words, the encoding is always the same. BUT, there is a vast variety of different decoders. 

The most "faithful" ones seem to be actual Cinema Processors, like the Dolby CN65. Way too expensive for me sadly. There is the CN45, but it has a bad reputation and supposedly doesn't come with the surround decoder by standard.

This unit here apparently is/was used for monitoring Dolby Stereo matrix mixes in actual studios. So, chances are, this unit is something like audio engineers use to monitor their Dolby Stereo/Surround mix. It has symmetrical (XLR) inputs and outputs, perfect for a studio environment where you want to minimize electrical noise and stuff like that. This is perfect for me because I have a nice RME Fireface audio interface that I connect this to and create a high quality decoded 4-track from any stereo mix.

It actually outputs 4 channels. L, C, R and S (Surround). Many decoders I found somehow split the surround channel up further, which is of course not really faithful to the original. The Pro Logic II decoder is an example of this. Also, original Dolby Stereo mixes only had a 7kHz limited surround channel, so it's kind of pointless to get any kind of high frequency content in the surround channel, as its guaranteed to be just bleeding from the L,R channels. This one should get me as close as possible to how it was originally "meant". 

Now, it's apparently still built on a Pro Logic integrated circuit, so it's still basically the consumer variant and basically a later "fancy" thing that introduced steering (detecting which channels are dominant and pronouncing them, vaguely said). That's not totally faithful I think, because steering wasn't used at the time when Brazil was produced afaik. So ... meh. It won't be perfectly faithful.

On the positive side, from what I read on Wikipedia, the standard consumer Dolby Pro Logic decoders lack the surround delay that is typical for cinema processors. Basically, this is used to improve the localization. Since bleeding of the Front channels into Surround cannot be completely avoided, a delay is introduced to make sure that the sound always appears first in the front, improving the localization of the sound source in the front. This decoder here does have that delay adjustment (which apparently can be changed between 10ms and 150ms, depending on the size of the room, not quite sure how this all works together yet).

So, to summarize:

- Has 4-channel Pro Logic decoding plus surround delay, like cinema processors. So, no unfaithful splitting of the rear channel through weird frequency modulating stuff. Also likely has the 7kHz cap on the surround channel, making it more faithful again.

- Sadly, it's Pro Logic based, so a certain form of artificial channel separation is introduced.

- Should be on par with very good early home theater surround decoders. So while it won't allow me to hear what you would have heard in cinema (that would require a much more expensive cinema processor), I will hear what good early surround consumer hardware would have produced. Plus the delay. Plus the symmetrical outputs for perfectly recording the output and creating surround tracks out of it.

- No fancy Dolby Pro Logic II type stuff with all the advanced steering etc. that was never really intended back then. Smile

- Afaik it's analogue. Smile

I just hope this thing works. If so, I'll be trying it with other movies too. Should work with any movie that was done in "Dolby Stereo". In theory it should work very fine on the optical Dolby Stereo tracks, for example in the captured Ghostbusters 35mm. It's not technically meant for decoding the optical tracks, but I think it's as close as possible without an actual Cinema Processor unit. Might work on some Laserdisc PCM tracks maybe? Don't really know anything about them, but I imagine they will have put the matrix-encoded audio in there too?

Either way, if it works, you're all welcome to send me some audio to run through it. I'll capture it at 48kHz, 24bit. I could theoretically capture up to 192 kHz 24 bit, but most tracks out there are only 48kHz anyway, so there seems to be little use for that.

Also, for my own releases, I might use the converted output further to create a 5.1 set; The L C R I can keep. The rear surround I can split up into two identical Ls and Rs channels, attenuated by -3 dB to create the "phantom center" in the back. LFE I can simply use a lowpass around 125 Hz on all the channels. Should be fair enough I think.
Thanks given by: Feallan
I'd like to be part of your clientele for my releases. For the purposes of making surround tracks for my projects, I'd like 48 kHz 24 bit for the standard AC3 5.1 tracks and, for futureproofing purposes, 192 kHz 24 bit for the uncompressed 4.0 and 5.1 tracks. If you can do a 7.1 set, I'd like that as well, in 96 kHz 24 bit. All for compliance with the Blu-ray spec, of course.
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@Koopa Luath: If possible, I'd like to stay at a maximum of 96 kHz unless the source really is digitized at 192 kHz. Technically the hardware can do more, but it kinda brings a few ... handling issues with it. I would say, if you actually have a 192 kHz audio, I'm willing to do it at that frequency, else I feel like it's a bit of a wasted effort. 96 kHz is absolutely fine though. 24 bit is standard anyway.

You can easily downconvert the uncompressed 96 kHz material digitally to 48kHz (it's a multiple by 2, so no real resampling issues) and then encode in AC3 or anything you like. As for the 5.1, I'll just write a little tutorial how to get that out of the 4.0 mix (the way I would personally do it, anyway, as there is no official standard for that, although I could read some cinema processor manuals to get inspiration), so I have less work and bandwidth spending. Smile Should work with 7.1 too, but I don't know if you really get more out of it. Basically (someone with more knowledge please correct me if wrong) you could just attenuate by -6 dB (is that the right number?) and split the surround to 4 channels (side and rear), which is similar to how cinemas did it back then. Though, for the 7.1 I would need to figure out the approach yet, software-combination wise.

A little update in general:

I just read the user's manual and it makes no mention of Pro Logic, just normal surround. Though it's possible they simply didn't use that name. It has an internal Cat. 150 Surround decoder, which some sources say dates back to 1979. Pro Logic is from 1987, apparently. So ... this might be the real deal. In any case, if this is a component from 1979, it should actually be more faithful than I assumed at first. Only thing I'm confused about is whether Pro Logic is a newer thing or not. Or whether Pro Logic is simply the old technology including steering that was later branded Pro Logic for consumers. Some more research is due, I guess.
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Dolby Surround/Pro-logic was the consumer name only, professionally it was always named Dolby Stereo (matrix or 70mm 6-track)
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That's what I read too. This unit uses the word "Surround" though, despite clearly not being targeted at normal consumers (at least a semi-professional/prosumer audience I'd say). The Cinema Processors use "Dolby Stereo" though, as expected. Maybe "Dolby Stereo" is simply the name of the printed optical track for cinemas?
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Nice find, Tom! Ok
Fundamental collection thread | Vimeo channel | My personal blog
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(2017-11-14, 10:26 PM)spoRv Wrote: Nice find, Tom! Ok

I hope so. This thing is the reason I'll be eating spaghetti for the rest of November. Big Grin
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