Restoration tips: HD matrix surround™

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.

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:

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.
EDIT 2021-08-24:
Even if the original Circle Surround decoder should still be avoided, Circle Surround II seems to be a very improved version – it extracts up to 6.1 channels (L C R LFE Sl Sr Sc)

About DTS Neo 6, there are various versions available: difference lies in particular to the number of bands it splits the signal, but the big problem is that it’s difficult (if not impossible) to understand which version a particular decoder uses. According to Disclord, it made a good job upmixing DS encoded tracks.

My last thought: even if DPL I is IMHO the best encoder to get the 4.0 track AS CLOSER as possible to the one heard in the theaters, the others may (or may not) do a good job too; take in mind, though, that the results between different decoders could be slight or huge, hence comparisons should be made. /END EDIT

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.

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.