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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.
Follow Ups:
That's bad news for music videos with TrueHD.
Yes, he probably heard a difference depending on the title. I haven't done any serious comparisons yet, but I did notice that 300 's PCM and TrueHD tracks are really out of whack (I only watched the first few minutes of the movie the day I got it to see how it looked). I wouldn't put it past the morons at Warner Bros to have given the dialog normalization wheel a first rate spin and really crank up the crap factor.
It's really sad what Dolby has become re: audio quality. I used to respect the company for the audio tools they provided to consumers. From Dolby B, C and S noise reduction for tape decks to Dolby Surround and Pro-Logic Surround for home theaters to Dolby Surround and Spectral Recording for the cinema. They were truly a company you could count on to deliver a better audio experience for the end user.
Starting with AC-3/Dolby Digital, they began sucking donkey dicks. Given the time they've had to correct that mistake and have yet to do so, I believe they've moved on to elephants.
> > I did notice that 300's PCM and TrueHD tracks are really out of whack < <
I don't know if this applies to the player you used, but I do know that if you playback TrueHD on the Panasonic DMP-BD10 and BD10A players via the analog outs, the player overrides speaker size/distance settings, making them all "large" and equidistant. Also, if the number of speakers doesn't match what the soundtrack is configured for (i.e. it's a 7.1 and you only have 5.1), you'll lose information.
Why not stick to one then? I don't believe many discs have DD True HD at this juncture for BD though.
If there is "Dialog normalization" going on, TrueHD is messing up the audio tracks. I don't understand why Dolby needs to screw around with the master audio tape content during the encoding process.
I can understand the need for TrueHD track to save on space, but I fail to see why you would want both a TrueHD and an uncompressed PCM track on the same disc unless you like to "A/B" audio tracks. Or there is some subtlety with HDMI I haven't bothered to read.
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