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  #11  
Old 03-12-2007, 10:23 PM
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minister minister is offline
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Default Re: AC3, is it Sample Accurate?

thanks for the discussion, guys.

i do not doubt that the Neyrinck plugs are as accurate as anything. so, i beleiuve you SLeadley. i actually now own both the Dolby and the DTS versions.

yet i don't know what the AC3 library coding issues might be...i just got to thinking about perceptual encoding and red flags went up for sync as well as delay...for anything even smaller than a frame.

but, for THIS project, the editor was assembling the DVD from my PCM files. I was not encoding the AC3 or DVD. and that is why i asked the question...because i knew there was no way for her to check very well...and i knew she would be using Compressor (with settings i gave her.) and authoring in DVD Studio Pro.

Sonsey, i had a feeling that they would experience the problems that you did, especially because final chords of piences of music would happen RIGHT ON THE HEAD of a new reel...and it would suck to have even HALF a frame of issue.
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  #12  
Old 03-12-2007, 11:03 PM
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Default Re: AC3, is it Sample Accurate?

Your splice might or might not work well depending on some factors. The main fact you need to keep in mind is that an AC3 stream is divided into "frames" that each produce 1536 samples of audio when decoded. Thus, the AC3 frames are not aligned with the video frames most of the time. However, if you do a little math, you can see that 125 AC3 frames (in a DVD which is always 48 kHz) will produce exactly 4.0 seconds of audio, so if the video and AC3 stream of a roll start at the same timecode value, one frame of video and one frame of AC3 will be aligned every 4.0 seconds thereafter.

So, if each roll has a length that is a multiple of 4.0 seconds, the splices will work fine.

If they are not a multiple of 4.0 seconds, you must be very careful how you author the DVD. You say that the AC3 files will be butt spliced. That sounds dangerous because the start of the next AC3 file might not be aligned with the first frame of the next video file. As a result, the audio could go out of sync by somewhere between plus or minus 1535 samples at each transition. Most likely the audio will get behind the video because there will probably be one last AC3 frame that creates up to 1535 samples of silence beyond the length of the video (that is how SoundCode works.)

I have not used DVD Studio Pro, but what you want to do is place each AC3 file so that it starts at the first video frame it is associated with. if you can do that then you will probably get proper synchronization. However, you will be at the mercy of the DVD player's demuxing and synchronizing circuitry that will have to resync the AC3 decoding at each transition if the transition happens at a non 4.0 second multiple. Dolby Digital decoders are pretty good at smoothing out transitions (sort of a 256 sample crossfade), but it very much depends on the audio contained in the AC3 streams as to how it sounds.

The best/safest thing to do is encode the entire movie soundtrack to create a single AC3 file from the start.

Paul Neyrinck
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  #13  
Old 03-12-2007, 11:08 PM
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Default Re: AC3, is it Sample Accurate?

Quote:
The best/safest thing to do is encode the entire movie soundtrack to create a single AC3 file from the start.
this is exactly what i told them to do.

thanks, Paul! you're a huge asset to this forum! your posts are chock-full of great info. when i grow up, i want to be as smart as you!
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  #14  
Old 03-13-2007, 11:57 AM
KK Proffitt KK Proffitt is offline
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Default Re: AC3, is it Sample Accurate?

Quote:


I have not used DVD Studio Pro, but what you want to do is place each AC3 file so that it starts at the first video frame it is associated with. if you can do that then you will probably get proper synchronization. However, you will be at the mercy of the DVD player's demuxing and synchronizing circuitry that will have to resync the AC3 decoding at each transition if the transition happens at a non 4.0 second multiple. Dolby Digital decoders are pretty good at smoothing out transitions (sort of a 256 sample crossfade), but it very much depends on the audio contained in the AC3 streams as to how it sounds.

The best/safest thing to do is encode the entire movie soundtrack to create a single AC3 file from the start.

Paul Neyrinck
www.neyrinck.com
Your last statement is true.

Authoring for DVD deals with GOPs (groups of pictures) rather than video frames. You can get into trouble if markers, etc. are not aligned with GOPs. Even though you may be dealing with QT movies in DVDSP, once you start to encode, markers that break the GOP rule (in case you're using one to tell where to lay back the audio) will get tossed. I wouldn't want to trust what DVDSP would do to the audio at that point.

Also with AC3 breaks, it's good to have a second or two of black for the decoder to switch on some machines.
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