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Old 16th November 2009
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AC3 distortion on DVD playback (dolby stereo)

Hey there everyone, hopefully the collective minds here can help shed some light on a confusing issue i'm having.

I'm currently having a problem with audio playback on a live concert DVD i'm working on. The issue seems to only be occurring when playing the compressed AC3 mix from the physical DVD-video disc. i am using DVD Studio Pro to author the disc.

My final mixes were printed to 48k/16bit AIFF files and encoded to Dolby Stereo using Apple Compressor. I left ample headroom on my mixes to ensure that the dynamic range of the material was left intact. Therefore, i set the dialnorm to -31db -- there was no need to attenuate further. Here are the parameters i used to encode to ac3:

Description: 2.0 Dolby Digital audio at 320kbps
File Extension: ac3
Estimated size: 137.33 MB/hour of source
Audio Encoder
Format: AC3
Sample Rate: 48.000kHz
Channels: 2
Bits Per Sample: 16
Target System: DVD Video
Data Rate: 320 kbps
Compression Preset: None
Audio Coding Mode: 2/0 (L, R)
BitStream Mode: Complete Main
Center Mix Level: -3dB
Surround Mix Level: -3dB
Dolby Surround Mode: None
LFE Exists: No
Dialog Normalization: -31 dbFS
Copyright Exists: Yes
Original Content: Yes
Audio Production Information Exists: No
RF Overmodulation Protection: Off
Channel Bandwidth Lowpass Filter: Off
DC Highpass Filter: Off
LFE Channel Lowpass Filter: On
3dB Attenuation: Off
phase 90: Off
Deemphasis: Off


Strangely, when I playback the DVD, the program level of the audio is being boosted significantly... so much so, that when the mix does reaches its loudest parts, the transients are getting squashed in a very unpleasant way.

When i play the AC3 file from my computer however, this level boost is not happening... it only happens when i play it from the authored DVD.

As a test, i authored a DVD using the uncompressed AIFF version of my mix. In this case it played back at the appropriate level -- no unexplained level boost, all dynamics retained. Unfortunately, multiple concerts are being put onto the disc and there is no room available to use raw uncompressed AIFF audio. I will need to use compressed audio, but why is this drastic level boost happening on playback?

Is this a problem caused by the authoring software - DVD Studio Pro? Or have i missed something with the Dolby Stereo compression settings to create a this issue in the AC3? I don't believe it is a problem with the AC3 since it plays back normally prior to authoring... but it may be possible.

Any ideas would be a great help! thanks so much!