A question about surround encoding

riredale wrote on 1/17/2005, 2:23 PM
I'm about to finish a surround project, and something just came to mind:

When I finish off the front and rear channels, both hit peaks very near 0db. Okay, so when the DVD player decodes the surround data, the front and rear sets will behave normally. But if the DVD is played back on a stereo setup, then the Dolby Digital decoder will mix the rears in with the fronts at a -6db level, which is what I specified in the encoding bitstream. But if the rears happen to peak close to 0db at the same time as the front channels, won't that mean that the modified stereo output will go beyond 0db and clip? Or is the Dolby decoder smart enough to back off the front channels a bit, knowing that it's mixing in the rears?

If it's not smart enough to realize that, then I guess it implies I need to make sure front peaks don't go higher than about -2db, so that there's a bit of headroom for the rear mix-in. Right?

Comments

JJKizak wrote on 1/17/2005, 3:00 PM
SPOT says don't go past -3db.

JJK
Spot|DSE wrote on 1/17/2005, 5:43 PM
As JJ mentions, don't go beyond -3dB, because of exactly what you're describing. The decoder does control dynamics, but it's still better playing safe. I just bought Troy, and was really surprised to hear the lack of control, and clipping that was audible in playback. At first I thought I had too much treble due to the clip, but turns out it was the encode being too hot. Something didn't get set right in their encode stage, but I've not torn it apart to find out what. The new Instant Surround from Jeffrey P. Fisher and VASST covers this reasonably well.