Restoration tips: HD matrix surround™

Author: spoRv
Time: 23-Jun-2015 7:54 PM
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HD matrix surround™

What is HD matrix surround™?
It's a discrete multichannel track, originally matrixed in a stereo sound, and then hardware decoded (hence the HD) using the best, and rarer, matrix hardware decoder.

Why?
Because some vintage hardware contains certain dematrix methods different from the usual Dolby Surround, Pro Logic etc. found in every modern decoders; they had that analog sound that could be even better than newer methods.

How does it work?
The matrixed stereo track (usually encoded in Dolby Surround, but could also be DTS Stereo, UltraStereo, CHACE Surround Stereo etc.) is played in the best hardware player (laserdisc, VHS, VHD etc.) using the best cables; the stereo output go in a matrix surround hardware decoder (the older and high-end, the better) to extract at least four different channels (usually Left, Center, Right, Surround) to get a discrete 4.0 multichannel track; those four channels are then captured using two sound cards with a stereo input, one sound card with four discrete inputs, or capture the track twice using a single sound card with a stereo input.
It's possible to get a stereo surround using an hardware stereo synthesizer or a software solution; an LFE could be obtained using a low/high pass filter to route the basses to the LFE and leave the other channels bass-free, either using an hardware or a software solution.

But will it sound better than, for example, Dolby Pro Logic II?
Albeit DPL II is a great matrix surround decoder, some older technology *may* sounds better, or in a different way... at the end:

Quote:Analogue domain matrixing encoding: Dolby Surround (only one spec of its kind there, 4 channels to 2)
Analogue domain matrixing decoding: Dolby Surround
Analogue domain matrixing decoding with steering and individual channel level control through h/w logic (proprietary circuitry depending on manufacturer, with, if at its most basic can decode Dolby Surround as is, then permitted to use logo): Dolby Pro Logic
Analogue domain matrixing decoding with steering and individual channel level control through h/w logic with different emphasis on center and rear for reproducing conventional non-Dolby Surround encoded stereo music tracks: Dolby Pro Logic II

(taken here)

Any hint about which decoders are better?
I've made some researches, and the following are among the best, alternative active matrix surround decoder ever produced:

Shure HTS 5300 / 5200 / 5000 (Acra-Vector technology)
Fosgate Tate II 101A
Fosgate Model 3 / 3a / 4 / 5
Aphex ESP-7000 / Proton SD-1000
Harman Kardon Citation 7.0 / 5.0 (6-axis technology)
Lexicon LOGIC 7 (first models)
Involve Surround Master

avoid Circle Surround I / II and DTS-Neo 6, as a Dolby Pro Logic II seems to be always better than those.
Is it always possible to use more than one decoder: for example using one for the Left/Right and another for Center/Surround, or add a stereo synthesizer to obtain a pseudo-stereo surround where the decoder outputs just a mono surround.

Notes
Here we are talking about HD matrix surround™ - where HD stands for hardware decoded... of course, we could also have SD matrix surround™ - where SD stands for software decoded; there are some interesting plugins for popular software like Foobar2000, that decode somehow a matrixed track; it's difficult to say how good could be the final result, but it *seems* hardware decoders produce decoded tracks of better quality.

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Author: junh1024
Time: 4-Jul-2015 5:58 PM (Edited)
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What is HD matrix surround™?
It's a discrete multichannel track, originally matrixed in a stereo sound, and then hardware decoded (hence the HD) using the best, and rarer, matrix hardware decoder.

It is matrixed, and therefore not discrete.

Matrix encoding does not allow one to encode several channels in fewer channels without losing information: one cannot fit 5 channels into 2 (or even 3 into 2) without losing information, as this loses dimensions: the decoded signals are not independent. The idea is rather to encode something that will both be an acceptable approximation of the surround sound when decoded, and acceptable (or even superior) stereo.

via https://en.wikipedia.org/wiki/Matrix_decoder

But will it sound better than, for example, Dolby Pro Logic II?
Albeit DPL II is a great matrix surround decoder, some older technology *may* sounds better, or in a different way... at the end:

Reading the info at https://en.wikipedia.org/wiki/Dolby_Pro_Logic , I can conclude it will ONLY sound better (or more discrete) than DPL2 if :

(It was thouroughly tested and checked on a DPL1 system to make sure that the artefacts and intent sound acceptable OR no discrete surround mix exists) AND (Only during section when the steering/gating system is active)

I am assuming that the decoder is single-band. A multiband decoder will go somewhat towards DPL2

When the gating system is not active (ie, cannot decide btw M/S and L/R dominance), a DPL2 decoder would do better as it can steer effectively 128 to 1024 (or more) bands depending on the FFT size.

there are some interesting plugins for popular software like Foobar2000, that decode somehow a matrixed track;..., but it *seems* hardware decoders produce decoded tracks of better quality.

Software decoders are typically free, you can make one fairly quickly (based on consumer DPL1), but would not sound goot/discrete. Some Proprietary software decoders will sound better than the free ones, because they put in more effort to make it sound goot (FFT/active matrix, etc). FFT takes much more effort to figure out what is right, and how to do it, etc.

I'll quote DTS Neural upmix (used FFT) as one of the best surround upscalers. It is proprietary (&paid), but easily available. It works as well upscaling normal stereo as on matrixed stuff. I also typically put on additional DSP to process the back to move transients back to the front, to give more discreteness (QQ Mashtactic).

===

E3:

I beleive it is possible to make a DPL1 Pro decoder in software, but I have never come across documentation comprehensive enough to determine the gating thresholds/etc. OTOH, I can probably figure out the thresholds for DPL2 decoding because I know FFT & DTSnu well enough.