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My 5.1 mix is finished, dial norm came out to -26. I'm ready to make an AC3 for the screening test.

The Question: How do I ensure the stereo fold down of the AC3 doesn't clip? Is there a some kind of built in limiting function?

If I just fold down the 5.1 mix it does clip at some points, so I was going to put a limiter on the stereo version, but I can't figure out how to make a dvd with both a 5.1 mix and stereo mix separate, so I'm settling for an AC3.

Any help much appreciated!

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What you call a fold down is called a downmix in ac3 standards.

The default settings for an ac3 decoder making a downmix of a 5.1 stream to a stereo output is the following (as mentioned here : http://www.dolby.com/us/en/technologies/dolby-metadata.html) :

  • Left and Right channels are sent to their respective L and R channels.
  • Center channel is sent to both L and R channels with a -3 dB gain.
  • Left Surround and Right Surround channels are sent to their respective L and R channels with a -3 dB gain.
  • Usually, LFE channel is ignored.

(Notice there are some differences between a Lt/Rt downmix and a Lo/Ro downmix). (Also notice that some of the downmix settings are metadata dependant, and you could for example prevent the surround channels to be used in the downmix by setting the appropriate metadata at the encoding stage).

So there's obviously an opportunity for clipping when (for example) summing

L + 0.707 * C + 0.707 * Ls

But the ac3 decoder will usually apply the Dynamic Range Control when downmixing, which is supposed to prevent such clipping. A licensed Dolby hardware decoder will usually apply this control. Software decoders might not implement this control.

Nevertheless, you should also be able to add a second soundtrack while authoring the DVD, you could use a handmade downmix encoded as 2.0 ac3. The end user can then chose which soundtracks he wants to hear trough standards dvd player interface. See your authoring software user manual to achieve this.

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AC3 is a data stream not an audio stream so its not really the same as doing a straight stereo foldown. You just tell the software which channel/file is which and if you want to include dialnorm and the number.

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