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here's where they go their separate ways

Posted by Joe Murphy Jr (A) on August 30, 2007 at 21:41:39

In Reply to: RE: PCM versus DDTrue HD posted by oscar on August 30, 2007 at 15:00:16:

Dolby insisted on having Dialog Normalization as part of the TrueHD "features": it's not needed or wanted and can be detrimental to sound quality.

Dialog Normalization, something these idiots used for their DVD audio tools as well, is basically digital volume manipulation. To lower the volume or make it seem equal in level to other sountracks/audio encodes, the encoder/decoder recalculates the digital level. The encoder defaults to -27dB ("ON", which is essentially any setting -30dB and lower) in the metadata section of the codec, instead of -31dB (the "OFF" setting). This means that if the sound engineer knows absolutely nothing about the "feature", it's set to digitally manipulate the volume from get-go. If the sound engineer knows about the "feature" and leaves it "ON" or, worse yet, lowers the level (which increases the manipulation), the audio could be degraded even further. For movies, if you switch back and forth between an uncompressed PCM track and a Dolby TrueHD soundtrack, the volume level should not change because the advanced audio track encode, after it's decoded to PCM, is supposed to be bit-for-bit identical (aka losslessly compressed) to the original PCM source. If it does change, chances are that dialog normalization is being used on the TrueHD track. In case you're wondering, any product that "turns OFF" dialog normalization is considered out-of-spec by Dolby and the manufacturer will not be granted a license (or use of the Dolby logo) for that product. Unfortunately, I've read that DTS has added dialog normalization to their audio tools as well. But unlike Dolby, I'm willing to bet that DTS defaults to "OFF", rather than "ON", to preserve the audio quality of the source that it's supposed to match.

So far Sony is the only studio that has said their Dolby TrueHD soundtracks will have the encoder set to -31dB/OFF.

Is dialog normalization ever useful? Yes, for digital television (ie, the control of those over-compressed, cranked-to-the-max commercials). Dialog normalization "normalizes" the volume level of other sources to match the level of the program that you're watching on TV. And that's about the only place it actually makes sense to use it. Digital volume manipulation is not needed for Blu-ray, HD DVD or DVD and it was not needed for AC-3 (Dolby Digital) encoded laserdiscs. You have a volume control on your receiver/surround processor that, in almost 100% of the products in use, changes the volume in the analog domain (ie, no bits are thrown away or manipulated). It's not like you're "flipping" through movies like you "flip" through channels on television.

I'm sure there are people who will point out that movie soundtracks are not the purist form of audio, so manipulation of the volume to match it to some unknown source is no big deal. I'll agree with the first part; however, I'd like to point out that music is part of many soundtracks and I don't think that even the least bit audiophilish person wants their music altered. As to the second part, it is a big deal. Why? Because music artists are eventually going to embrace a Hi-Def format. As long as TrueHD defaults to "ON", we're almost guaranteed to get manipulated audio for audio encodes made with Dolby TrueHD -- despite the fact that they claim it's a bit-for-bit decode to the original PCM source, which it can't be because the digital volume level has been altered.

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