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Old 12-01-2007, 04:18 AM   #5
crackinhedz crackinhedz is offline
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Feb 2007

Originally Posted by ProvenFlipper View Post

It kills dynamic range... it's bad for movies

Dialnorm Myth.

Myth #1: Dialnorm reduces dynamic range.

The main criticism that you are likely to encounter is that Dialnorm affects dynamic range, specifically that it reduces it. Indeed, from Fig. 3, it looks like we’ve chopped off the bottom third of the commercial’s soundtrack. In truth we haven’t: We’ve only turned the volume down. Lowering the volume knob on your system would have the exact same effect. If you really want that car commercial to blast you in the face, turn up your volume (though I doubt you'd want to), and you will still have all 105 dB dynamic range. Consider again that most films will use the same -27 setting, and that other values are appropriate for other material. In other words, if you've set your playback level to one that you are comfortable watching a movie at, Dialnorm is going to maintain that comfort level for you as the program changes, but there is nothing stopping you from further adjusting your volume one way or the other. "Controlled" values of Dialnorm may someday be imposed in such areas as broadcast TV (Dialnorm was a major point of attraction when Dolby Digital was chosen as the audio format for HDTV), but you always have the final say with your volume knob.

Myth #2: Dialnorm reduces everything by 4 dB, altering reference level playback of a movie.

A common criticism is that Dialogue Normalization "normally" reduces the level of the soundtrack by about 4 dB. Reduces it as compared to what? You have to compare it to something else first, and then the question becomes: is the Dolby Digital soundtrack 4 dB too low, or is the other material 4 dB too high? Follow me on this one.

A lot of home theater enthusiasts are concerned with what is called "reference level playback". In a nutshell, you use test-tones (as may be found on such DVDs as AVIA) to set the volume to the same standard levels used in cinemas. The reason to do this is to hear the soundtrack at the level the movie makers intended. A concern naturally arises that if volume is being altered by Dialnorm, the sound engineer's vision is compromised. Reference level playback is in practice very very loud in the relatively small acoustic spaces of home, and we must caution you against it at this point. Not only do most find it uncomfortably loud, but as we noted in our article explaining the LFE channel, it can quickly bring a subwoofer to its knees. But for the record, let's press on.

The default power-on setting for Dialnorm on Dolby's professional AC-3 encoder, the DP569, is -27 because as we noted, that value is a perfect fit for movie soundtracks. True, this value calls for your decoder to attenuate its output by 4 dB. Fact is, the two most common reference DVDs, Video Essentials and AVIA, were encoded with the same -27 Dialnorm value, so their test noises are also being attenuated by 4 dB, making them a perfect reference for Dolby Digital movies. If you've set-up a system with either of these tools, then any movie you play will not be "reduced" by 4 dB as compared to the reference.

DTS soundtracks, unlike Dolby Digital, are not attenuated by 4 dB by your decoder. This means that if you've set up your system using AVIA or Video Essentials, the DTS soundtrack is actually going to play 4 dB too high. Yes, that's right. You read it right: On a system calibrated for reference level playback with Video Essentials or AVIA, DTS soundtracks play 4 dB too loud. Conversely (and to be fair), if you set up a system using DTS test noise, the Dolby Digital soundtrack will be 4 dB too low. Yet what is important here, and what I really want you to take away from this, is that regardless of what actual level you watch a movie at, relative to one another, there exists this 4 dB difference between DTS and Dolby Digital movie soundtracks played over consumer equipment. If at any time you are comparing soundtracks, you must turn your volume down when listening to the DTS track and/or raise it when listening to the Dolby Digital track (as the case may be) in order to hear the same level from both.

We should note that most THX-certified receivers and processors address this by attenuating DTS material by 4dB after the decode stage, effectively putting everything on level ground.

Myth #3: Dialnorm adversely affects S/N (signal-to-noise) ratio.

Another concern that comes up is the notion that if the volume is being adjusted by the decoder (for any reason) in the digital domain, there is a reduction in quality (S/N ratio) from bit-width reduction. Audiophiles should actually appreciate that, when performed in the digital domain, the Dialnorm adjustment is extremely accurate. Dolby Digital is capable of 24 bit resolution. Thus, a volume reduction of 4 dB would mean a bit reduction of less than 1 bit. IF (big if) the D/A converters were silent to -144 dB, you might be able to measure this. In the real world where the D/A's performance is less than that, these sort of level changes at the decoder stage will not have an effect on S/N ratio for a given volume level. The same holds true for dynamic range.
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