Dolby Prologic decoding support

[Daniel]

Good day everyone.

 

I’d like to request a very important feature that I think will resonate with the videogame preservation oriented folks in here.

 

Dolby Prologic support, you know, that ancient surround solution from back in the day that encoded multichannel information into a stereo track to be decoded by a fancy AV System.

 

SNES, PlayStation, N64, Gamecube, PlayStation 2, Wii and many more consoles took advantage of it even though sometimes it wasn’t even advertised.

TV shows and a LOT of VHS tapes, Laserdiscs and DVDs also have tracks encoded in this format.

 

The trouble is, more and more AV Systems are dropping Prologic decoding support altogether and because of space constraints people are moving to Soundbars and those will certainly not pay an extra fee for 20th century tech so most of them have their own upmixing algorithms that mess up the Prologic soundstage.

 

I noticed that the OSSC Pro has a quad stereo mode that, in theory, could easily be modded to support old school Prologic and it would be a game changer for gamers and physical media collectors out there, I mean, come on, we are sitting on real, developer intended surround sound out of SNES and PlayStation 2.

 

Pretty please?

 

 

[Zacabeb]

By-the-book decoding of Dolby Surround and Dolby Pro Logic II is a bit complicated as it relies on active steering logic. The existing Quad mode in the OSSC Pro just replicates the fronts in the surrounds from what I can tell.

Passive decoding (L-R) is probably simple to implement but produces a lot of “magic surround”, i.e. unintentional surround content.

On playback devices, Dolby Pro Logic II and Dolby Surround Upmixer decode Dolby Surround correctly and remain part of the Dolby Audio licenses as far as I know. Dolby has done a terrible job promoting Dolby Surround Upmixer and not explaining how it works, making it seem arbitrary even though it follows the same set of rules as its predecessors.

Edit: Even though passive decoding has its disadvantages, it could still be useful as a compromise.

Mono surround can be extracted by Ls = Rs = 0.707L – 0.707R and stereo surround by Ls = 0.866L – 0.500R and Rs = 0.500L – 0.866R. In my experience it’s best for encoded content if the two surrounds are extracted in phase with each other. Preferably the surrounds should be bandpass filtered to about 100 Hz – 7 kHz and delayed by 10-20 ms to reduce some of the audible leakage from the front channels.

• This reply was modified 3 months, 1 week ago by Zacabeb. Reason: Added thought

[marqs]

This would indeed be a nice feature, but unfortunately only one I2S data line (stereo/front) is connected from FPGA to HDMI transmitter while the 2 remaining lines are connected directly from HDMI RX. From analog sources it’s thus only possible to transmit 2-channel LPCM and optionally mirror it to side/back speakers.

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