PCM vs. Dolby TrueHD

[Brain Sturgeon]

I posted this about the PCM v. THD thing in a similar thread:

Bit for bit, PCM and TrueHD, assuming both are from the same master, theoretically should be the same. But, unless the audio encoder set the audio attenuation on the encoder to -31dB (instead of its default: -27dB), then Dialogue Normalization (set at -4dB at default) will have been applied to the soundtrack during the TrueHD encoding process and the soundtrack is no longer identical to the studio PCM master. Now, whether DialNorm is a bad thing or not is a controversial subject, but assuming that the original studio master is the best representation of the desired audio for the movie, then any modification of that, even if it is as simple as a -4dB attenuation of the center dialogue channel, would seem to be a detriment. And it does not appear to be as simple as a volume issue.

A quote by DaViD Boulet from an AVS thread:

"the problem with DN isn't that it lowers the dynamic range (volume) by 4 (or more) db... the problem is HOW it does it.

It does it by digitally recalculating the audio data to digitally down-scale the waveform.

That means that you will NEVER get bit-for-bit accuracy from a TrueHD stream that's been flagged with dialog-normalization.

And it can't be bypassed because Dolby won't allow any in-spec consumer gear to let the user avoid it! It doesn't matter that it's just a meta-data instruction flag: if Dolby REQUIRED that it's processing instructions be followed by your decoder, then the fact that the original PCM lossless data was represented prior to DN processing is moot."

So unless DialNorm is defeated during the TrueHD encoding, then it is not the same as the studio master.

I'd take the PCM over TrueHD unless the dialnorm flag has been defeated. If it has, then the two tracks should be theoretically identical. I have not, as of yet, had a chance to compare a PCM track to a TrueHD track where DN was defeated, so I can't give my opinion as to whether this bears out on "real world" listening.

If this does bear out and both sound identically good, then it makes sense to use TrueHD without DN over PCM, as TrueHD would allow anywhere from a 2:1 to 4:1 compression of data without loss of quality, leaving us more room for higher bitrate video (assuming bandwidth is available), extras, etc...

FWIW

Cheers!