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question about encoding ac3

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Joined: 22 Sep 2006
Posts: 35

PostPosted: Fri 29 Sep 2006, 2:58    Post subject: question about encoding ac3 Reply with quote

I'm using VEGASto render a stero audio track to ac3 format. in the encoding custom seeting, there's a dialog nomarlization. the default setting is -27db. I'm not sure what it means. just thought it's too low, and I set it to -3db. then use the encoded ac3 to author a dvd, but the playback volume is very very low. I have searched the net for info, but still not quite sure about its meaning or function. can anyone explain it clearly?
thanks in advance.

It seems that the encoder does not change the gain of the file, only add a flag to the file to let the decoder know the file's average gain. is it right?
then since I changed setting to -3db, the decoder thinks it's too loud, and then lower the volume when playback. and here the default setting -27db is only a relative value, or a relative reference value, it has nothing to do with a file's real value. is it right?
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PostPosted: Sat 30 Sep 2006, 16:52    Post subject: Reply with quote

Neil Wilkes can probably give you more details, if he's reading this, but as I understand it, the "dialogue normalisation" value refers to to the relative volume of speech in the source sound file, compared to the highest possible value.

In other words, if you use a value of -27 dB, the decoder will assume that audio that is 27 dB below the maximum should be played at the "normal" volume for speech. If you use a lower value (ex., -3 dB), then it will decerease the playback volume so that audio 3 dB below the maximum sounds "normal".

In other words, to make the volume higher, you need to input a lower value (note that -27 is lower than -3).

Besides being used by the decoder, I suspect this value is also used by the encoder for psychoaural calculations (i.e., determining which sounds are more important to people, and compressing those less).

Also, note that, during decoding / playback, volume is usually not scaled linearly (it's "compressed" at the top, to account for the difference in range between home audio systems and the original theatre sound systems that the films were mixed for). In other words, an explosion that is 4x louder than speech in a movie theatre is likely to be just 2x or 1.5x louder than speech when the DVD is played back in a home threatre system. If the home player tried to keep the original ratio, it would either distort the loud parts (because home speakers can't deliver the same volume), or would make speech sound too low.

Since speech is generally considered the most important element of the audio, Dolby Digital lets you select the "dialogue normalisadion" value, to ensure that, no matter how the playback system scales the volume, the speech will remain more or less at the "right" level. Of course, different decoders and amplifiers have their own ideas about what the "right" level should be, so this is never 100% accurate, but assuming people have their sound systems adjusted for their personal taste, all DVDs with the normalisation level set correctly should sound close enough.

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