Making a DCP from a Blu-Ray with only L-R sound

[Carsten Kurz]

The MidSide Decoder in DCP-o-matic has a flaw in that it inverts L and R in the output as part of the process. But it works nice enough to extract a center from both plain stereo as well as Prologic/Lt/Rt files.
Making a straight Mid/Side decoder for digital files otherwise is easy, and usually safe to use, even if your source track is not known to be Lt/Rt or matrix encoded. I just wanted to stress the difference between a matrix/prologic decoder and an upmixer. For matrix/Prologic decoders to give good results, you have to make sure the source IS actually matrix encoded.

DCP-o-matic has two different 5.1 upmixers in addition to the Mid/Side decoder, their way of working is documented, and they sometimes give decent results, but need to be checked on a proper surround audio system, as they may run havoc on some tracks. They have been assigned experimental status and need to be enabled in advanced prefs since quite some time.

[Steve Guttag]

For matrix/Prologic decoders to give good results, you have to make sure the source IS actually matrix encoded.

That's not really true at all. Sure, you'll only get what the person hear in the mixing suite if it was, in fact, Lt/Rt encoded but using a Matrix Decoder on any stereo source, generally has good (and desired) results. I'm not saying that you can't find the odd-ball case-in-point where it doesn't but that would definitely be the exception. By and large, getting the dialog in the center with a matrix decoder, including for sources that just put mono on both tracks is a HUGE improvement over Lo/Ro. Even "magic surrounds" (a term we used for sources that were not mixed in surround sound, like older music CDs, that might have out-of-phase information that shows up there) is often pleasing and rarely, if ever, distracting. Dolby even incorporated them from the CP55 onwards for Non-Sync as an option.

So, if putting a matrix decoder into DOM isn't a big thing...here is the request...please do it!

[Carsten Kurz]

When you apply phase calculations/extinction to an unkown multichannel signal, there is always a risk that parts of the signal get eliminated. I have seen (heard) that happen often enough. Sometimes, those parts are irrelevant, sometimes, it's the dialog. Send non-preencoded stuff to the surrounds, and very unpleasant things may appear. Not arguing against matrix decoders. Just against the suggestion that it improves all sound.

[Harold Hallikainen]

The JSD-60 offered a simple matrix that used L and R to generate center and surrounds. It is described starting at the bottom of http://ftp.uslinc.com/Products/JSD-6...06.pdf#page=56 .

[Marcel Birgelen]

The best upmixing results from Stereo to 7.1 I've achieved using the Waves DTS Neural Upmixing plugin. It comes as a VST plugin, so it can be used in your favorite DAW, but also for live mixing purposes in any live mixing environment supporting VSTs. If QSys would ever start accepting VSTs directly, you could even implement it as part of a standard flow.

I've never upmixed mono to stereo, but the upmix from 2.0 to 7.1 was way better than any "plain old matrix decoding" technique. It certainly isn't perfect, but nonetheless delivers pretty stellar results for most types of input I've used it with.

[Harold Hallikainen]

Speaking of upmixing, there's the Lexicon QLI-32 that upmixes 5.1 or 7.1 to 32 channels ( https://lexiconpro.com/en/products/qli-32 ). I think some other manufacturers may have had something similar. I watched several movies with it, and it sounded pretty good!

Harold

[Steve Guttag]

When you apply phase calculations/extinction to an unkown multichannel signal, there is always a risk that parts of the signal get eliminated. I have seen (heard) that happen often enough. Sometimes, those parts are irrelevant, sometimes, it's the dialog. Send non-preencoded stuff to the surrounds, and very unpleasant things may appear. Not arguing against matrix decoders. Just against the suggestion that it improves all sound.

Again, you are quoting the exception, not the rule. I'd be willing to wager that well over 99% of all 2-channel (or mono on 2-channels) going through a pro-logic decoder, in a cinema will sound significantly better than leaving it as 2-channel or dual-mono. And, again, I'm not saying that you can't find the rare example where this isn't the case, but it will be the rare example.

If DOM would offer a pro-logic decoder, it would get used (a fair amount).

[Carl Hetherington]

There are 2 decoders in DOM and one of them was done basically as I described and works reasonably well, but is only for front channels (everything mono goes to center, everything left only goes to left, everything right only goes to right).

I guess you are talking about the mid-side decoder... that calculates

centre_out = (left_in + right_in) / 2

then does

left_out = left_in - centre_out
right_out = right_in - centre_out

Are you suggesting also putting the left_out and right_out in the surrounds?

The other 5.1 mixer is a disaster and sounds terrible. I believe it was done by working off of frequencies or something, but it should be dumped.

This has been hidden from the main GUI for a while (behind an "advanced" option). Some people find it useful and others find it makes things much worse.

The MidSide Decoder in DCP-o-matic has a flaw in that it inverts L and R in the output as part of the process.

Can you expand on this remark a little?

If DOM would offer a pro-logic decoder, it would get used (a fair amount)

If there's any open source code that does something useful along these lines, and makes things sound better (or at least, not worse) most of the time, I'm more than happy to add it to DoM. Adding support for VST is also an option, but rather more complicated.

[Marcel Birgelen]

If there's any open source code that does something useful along these lines, and makes things sound better (or at least, not worse) most of the time, I'm more than happy to add it to DoM. Adding support for VST is also an option, but rather more complicated.

VST integration would be a complete game-changer for DCP-o-Matic, as it would open it up to the many free and commercial plugins out there, including quite a few mixing, downmixing and upmixing plugins.

[Mattias Mattsson]

If there's any open source code that does something useful along these lines, and makes things sound better (or at least, not worse) most of the time, I'm more than happy to add it to DoM.

ffmpeg has an audio filter for upmixing that I haven't tried https://ffmpeg.org/ffmpeg-filters.html#surround

Also suggested some years ago in the DoM bug tracker: https://dcpomatic.com/mantis/view.php?id=1080

[Carl Hetherington]

ffmpeg has an audio filter for upmixing that I haven't tried https://ffmpeg.org/ffmpeg-filters.html#surround

Also suggested some years ago in the DoM bug tracker: https://dcpomatic.com/mantis/view.php?id=1080

Thanks for the reminders. If anyone would like to try these filters and report back how they work, that would be great! A positive review would definitely increase the motivation to add them to DCP-o-matic.

[Marcel Birgelen]

I have, in the past, back in late 2017 when it was first added, I upmixed a few indie DCP productions, primarily to get a somewhat OK center channel and they're fine for that purpose, but not much better than the "hand-o-matic" option, where you sum left and right, boost the center a bit and work some delay magic on the surrounds, maybe put some cutoff filters on surrounds. This filter is easier because it "asks" the right questions in the form of some parameters. But it still requires a lot of experimentation, especially with the window filter types provided and the windows size, to get the most out of it. It's certainly not a "mostly hands-off" upmix options like Dolby Pro Logic and it also certainly doesn't beat the commercial DTS Neural upmixing.

[Jesse Crooks]

The problem is that the 2 channel audio is actually the LtRt tracks for matrix 4 channel. As such there is a hole in the middle of sound field which leaves dialogue floating depending on your seat

The vast majority of the 2.0 DCPs we get are mastered for a Blu-ray release and the distributor was too lazy to create a 5.1 DCP. If you decode them the audio gets pushed to the correct channels. The telltale examples are the mono rep titles that are actually mono but are shipped as 2.0. Decode them and they run through the center channel with the low frequencies on the sub. Decoding them PREVENTS the hole in the middle of the sound field.