2017-11-14, 03:22 PM
Here it is: https://www.ebay.de/itm/Dolby-Surround-D...2968402357
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.
- Afaik it's analogue.
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.
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.
- Afaik it's analogue.
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.
