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tragichero 16th November 2009 11:42 AM

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!

bombcode 16th November 2009 01:01 PM

1. Play DVD on different DVD player (and TV) to verify that it is not your system.

2. Turn Dialog Norm off.

3. Make sure that any form of compression is off.

minister 16th November 2009 01:37 PM

It's happening because whatever device you are using to play it back has DRC (Dynamic Range Compression) on. You need to turn it off on the playback device. In the computer, it is in DVD Player in the preferences. It doesn't happen with aiff because there is no Dolby DRC engaged on playback.

georgia 16th November 2009 04:11 PM

BTW.... this will happen often as eveyone's DVD player in a gazillion homes made by a bazillion companies are all different. I would venture to guess that 99% of the players are just taken out of the box, plugged in and left at the default. So if this is an issue with your mix, I'd drop the mix a bit to assure that it doesn't happen on the test DVD system you are using. Remember - Lowest common denominator.....You might try a Music Light Compression and see how it feels.



cheers
geo

Ben B 16th November 2009 04:14 PM

What are your mix levels prior to encoding as AC3?

-Ben B

pneyrinck 16th November 2009 05:36 PM

A DVD player can play back in one of two modes: Line Mode or RF Mode. It sounds like yours is playing back in RF Mode. RF Mode applies 11 dB of gain and will limit any peaks above -11 dbFS even if you set the compression profile to None. The only way to work around this is keep your peak levels below -11 dBFS. Anything above will be limited in a pretty bad way (fast attack with a slow linear release). I agree with the idea to try a different DVD player but if you are using the RF cable output, then it will always be RF Mode so don't use the RF cable out. If you are using the RCA plug line outputs, it should be a Line Mode decode. Look in the DVD player menu options to see if it is incorrectly set. But since you never know what kind of system your DVD will play back on, I recommend you always limit peaks to -11 dbFS for a stereo mix. For a 5.1 mix you have to limit even lower so that a downmix doesn't go above -11... trick stuff! By the way, we are working on a product that helps you navigate all this while mixing/mastering.

Paul Neyrinck
Neyrinck Audio

Ben B 16th November 2009 05:49 PM

Quote:

Originally Posted by pneyrinck (Post 4788519)
A DVD player can play back in one of two modes: Line Mode or RF Mode. It sounds like yours is playing back in RF Mode. RF Mode applies 11 dB of gain and will limit any peaks above -11 dbFS even if you set the compression profile to None. The only way to work around this is keep your peak levels below -11 dBFS. Anything above will be limited in a pretty bad way (fast attack with a slow linear release). I agree with the idea to try a different DVD player but if you are using the RF cable output, then it will always be RF Mode so don't use the RF cable out. If you are using the RCA plug line outputs, it should be a Line Mode decode. Look in the DVD player menu options to see if it is incorrectly set. But since you never know what kind of system your DVD will play back on, I recommend you always limit peaks to -11 dbFS for a stereo mix. For a 5.1 mix you have to limit even lower so that a downmix doesn't go above -11... trick stuff! By the way, we are working on a product that helps you navigate all this while mixing/mastering.

Paul Neyrinck
Neyrinck Audio

This is exactly why I asked the OP what his mix levels were prior to encoding!

Good info, Paul.

-Ben B

tragichero 16th November 2009 09:49 PM

Quote:

Originally Posted by minister (Post 4787871)
It's happening because whatever device you are using to play it back has DRC (Dynamic Range Compression) on. You need to turn it off on the playback device. In the computer, it is in DVD Player in the preferences. It doesn't happen with aiff because there is no Dolby DRC engaged on playback.

you nailed it! When i disabled this, the program material returned to its original level and all the transients during that loudest section play back beautifully. I had no idea this was part of the Dolby spec.

Quote:

This is exactly why I asked the OP what his mix levels were prior to encoding!
The majority of the material peaks between -18dbfs to -14dbfs. However, during the loudest section, i have peaks around -3dbfs. It seems i may need to add some 2bus compression to make the dynamic range smaller, in addition to bringing the entire level of the program material down so my peaks will be retained even if the DRC is engaged (and it very well could be as Geo's post points out).

Quote:

A DVD player can play back in one of two modes: Line Mode or RF Mode. It sounds like yours is playing back in RF Mode. RF Mode applies 11 dB of gain and will limit any peaks above -11 dbFS even if you set the compression profile to None. The only way to work around this is keep your peak levels below -11 dBFS. Anything above will be limited in a pretty bad way (fast attack with a slow linear release). I agree with the idea to try a different DVD player but if you are using the RF cable output, then it will always be RF Mode so don't use the RF cable out. If you are using the RCA plug line outputs, it should be a Line Mode decode. Look in the DVD player menu options to see if it is incorrectly set. But since you never know what kind of system your DVD will play back on, I recommend you always limit peaks to -11 dbFS for a stereo mix. For a 5.1 mix you have to limit even lower so that a downmix doesn't go above -11... trick stuff! By the way, we are working on a product that helps you navigate all this while mixing/mastering.

Paul Neyrinck
Again, this is fantastic information! Thank you!

If I follow Paul's advice and make sure that i do not have any peaks above -11dbFS, will my mixes play back most reliably in the majority of systems?

When mixing/encoding Live concerts for DVD, is there a standard level i need to follow or a general rule of thumb for my RMS and Peak levels?

I really appreciate everyone's expertise and guidance, you are brilliant resources!

Ben B 17th December 2009 05:33 PM

Quote:

Originally Posted by tragichero (Post 4789394)

If I follow Paul's advice and make sure that i do not have any peaks above -11dbFS, will my mixes play back most reliably in the majority of systems?

When mixing/encoding Live concerts for DVD, is there a standard level i need to follow or a general rule of thumb for my RMS and Peak levels?

I am trying to find out this exact information right now. It's starting to seem like no one really knows!

-Ben B