Broadcast levels of Prologic Downmix from 5.1

[Tim R.]

Say you are mixing a dense music performance in 5.1 (with program filling all 3 front speakers-Vocal center, band spread), and meeting guidelines for levels in surround (for example averaging -10 to -8 with peaks at -6). How do you handle (or do you need to worry about) the fact that the Dolby downmix (Prologic etc) can potentially peak above the -6 ceiling your supposed to be adhearing to?

[Postman]

Are you providing the downmix? If so you have to deal with it to hit the specification. Either manual level rides of the downmix, or put a limiter on the downmix.

I have not seen or heard of a spec that requires peak control of the surround mix so that a downmix happening somewhere else (like the set top box) maintains guaranteed peak levels, but frankly it wouldn't surprise me too much if such a spec were to be imposed.

[Tim R.]

We are only delivering the 5.1 mix (Dolby E). I'm just at a loss as to how the higher level in the downmix is handled. Especially since we're trying to avoid relying on metadata adjustments since they often are ignored.

[Postman]

Based on what I've see it is not dealt with very gracefully. Either there will be a limiter somewhere or more likely the signal will simply clip. If the consumer's set does the downmix, which is the usual situation for a surround broadcast watched on a typical receiver, there is little you can do beyond trying to prevent the downmix from exceeding 0dbfs. Since your surround mix is being constrained to -6dbfs, the downmix probably will be okay. There may be some limiting built into the Dolby chip in the consumer's device, I'm not sure if having one is standard practice or not, but you certainly cannot rely on it.

If the downmix is being handled by the broadcaster, to feed standard def/stereo outlets, you can be sure there is a big honkin' limiter in line that will take care of those peaks and most of your other dynamics too.

There are things you can do during the mix to lessen the possiblility of problems. For one thing, you can resist a temptation to spread loud sounds, like bass or singers or voice over, across many surround channels at once. Like, put the kick drum into L+R, or Center, but not all three.

[pneyrinck]

Originally Posted by Tim R.

We are only delivering the 5.1 mix (Dolby E). I'm just at a loss as to how the higher level in the downmix is handled. Especially since we're trying to avoid relying on metadata adjustments since they often are ignored.

The answer depends on where the downmix happens but I can explain a very common scenario that happens. The most common downmix will take place in a DVD player or cable TV set-top box. The DVD or cable TV box will be playing/receiving Dolby Digital (AC3) 5.1. When the AC3 was encoded for the DVD or the broadcast, the Dolby Digital encoder software/box inserts a metadata stream called "dynrng." The dynrng metadata stream tells the decoder to apply a gain change to the audio. The "dynrng" metadata will at minimum apply limiting to the 5.1 audio to make sure a stereo downmix does not clip. I hope that makes things more clear.

Paul Neyrinck

[soundthinker]

The problem is that metadata is still not reliably passed through the broadcast chain...