Mixing 5.1 Audio with LD PCM

[alleycat]

So I've got a couple of projects in mind where I will need to insert some additional footage from an extended version (which is 5.1 only) into the main movie for which I'm using 2.0 Laserdisc PCM (Dolby Pro Logic I). The first thing I need to do is convert the 5.1 to 2.0, in a manner that it gets unpacked the same way the Laserdisc PCM does i.e. DPL I. 

Chewtobacca previously detailed an avisynth script to convert to DPL1 and suggested that BeHappy could be used - it's an avisynth audio GUI.

So the first thing I did was convert the DTSHD 5.1 track to wav using eac3to. I then loaded that into BeHappy, selected Downmix DPL I and that gave me a 2.0 file. Interestingly there were several input methods, bassaudio, FFAudioSource and LwLibavAudioSource all produced identical sources, but WavSource produced a very different final third of the file so I used bassaudio. 

Anyway I know eac3to also has a DPL downmix option but it's DPL II not DPL I. So in order to compare I produced another file through BeHappy but this time DPL II and also produced one in eac3to using -downDPL.

What I did not expect was the results to be so different. So below, top is eac3to and bottom is BeHappy. Just to reiterate both of these are DPL II downmixes not DPL I. 



Anyone got any idea why these are so drastically different? Is eac3to maybe applying some kind of boost or normalisation? I just expected the two programs to produce similar results, if they are both using an industry standard method of downmixing i.e. DPL II?

I appreciate once I've decided on which option I'm using (I'm leaning towards BeHappy) I may well need to play around with the volume levels once I'm joining so it matches the Laserdisc PCM.

[spoRv]

It seems second is "only" leveled down... try to amplify it: find in Audacity what is the max level, and boost the second to the same; they probably have similar (if not equal) waveforms.

[alleycat]

Thanks spoRV, boosted to the same peak level it looks like this, now they look very similar. So think I'll stick with BeHappy as that has the option for DPL I. Cheers.

[maksnew]

Thanks spoRV, boosted to the same peak level it looks like this, now they look very similar. So think I'll stick with BeHappy as that has the option for DPL I. Cheers.

Using such diagrams it is impossible to determine the best sound quality, adding volume is not a problem, I usually use the best sound quality, even if it is stereo, and then you can work on processing it.

[NeonBible]

Hi @alleycat . I need to do something similar and downmix 5.1 to 2.0 in order to patch into LD audio.

Do you have a step by step or could point me to chewtobacca's script? I'm a bit confused what needs to be done.

[alleycat]

Hi @NeonBible,

Chewtobacca's post is hidden in another thread so it's not easy to find but this is it:

Yeah, the question is how multi-track mixes were matrixed for LD (and whether the process varied from disc to disc). Given that DPL2 was introduced in 2000, my preference has always been to use DPL1 (mono surround, like the old four-track mixes) because the results have always struck me as being the best match for matrixed tracks on LD.

Code:

Code:
#AviSynth
a=WhateverSource("6-track_file")

flr = GetChannel(a, 1, 2)
fcc = GetChannel(a, 3, 3)
lrc = MixAudio(flr, fcc, 0.3205, 0.2265)
bl  = GetChannel(a, 5)
br  = GetChannel(a, 6)
sl  = MixAudio(bl, br, 0.2265, 0.2265)
sr  = MixAudio(bl, br, -0.2265, -0.2265)
blr = MergeChannels(sl, sr)
mix = MixAudio(lrc, blr, 1.0, 1.0)

return mix#.ConvertAudioTo16bit()

Use BeHappy instead if you like. Uncomment ConvertAudioTo16bit() if you want, but you might prefer to adjust the volume (or EQ or whatever) first.

I don't know whether other downmixing methods (e.g. DPL2) will affect playback via your AVR. Maybe experiment and report back.

[alleycat]

BeHappy is pretty straight forward, open the GUI, drop your 5.1 file into it, select BassAudio from the drop down (or whichever one you prefer) and then tick downmix DPL1.

[NeonBible]

Thanks alleycat, I’ll give it a go.