AC3 interleaving in AVI

len0x · 8th April 2004, 14:20

The other day I was trying to think why recommended value for interleaving is 96 ms. And it turned out that I don't quite understand how audio is interleaved in AVI anyway. Consider this: 1 video frame is probably the smallest data chunk which cannot contain audio data inside, right? So 1 frames = 40ms for PAL for instance. So you can only have 40,80,120ms interleaving. Then how come values like 96ms are valid interleaving values?

P.S. Probably I don't get very simple, but important point, but I just can't figure out which

alexnoe · 8th April 2004, 14:23

One AC3 frames lasts 32ms.

If you split AC3 frames apart, the decoders b0rk, meaning that you have to write pieces of k*32 ms.

Since the decoders are even more b0rked, k must be between 2 and 5...

The simple but important point you don't get is that no one says that the number of video frames between 2 audio blocks has to be constant.

len0x · 8th April 2004, 17:05

Quote:

Originally posted by alexnoe
The simple but important point you don't get is that no one says that the number of video frames between 2 audio blocks has to be constant.

still don't quite get it - do you mean that 96ms is an average and not exact interleaving ?

alexnoe · 8th April 2004, 17:11

Exactly. You put one audio chunk after 120 ms, the next one after 80 ms etc. Important for AC3 is the audio chunk duration of k*32 ms, not the way it is put between the video (at least not more than for any other format...)

AVI-Mux GUI included an RIFF tree viewer. Just mux a file with some settings and look at it in the viewer, then you'll see. VD also has such a tree viewer, but it gets terribly slow on large files.

len0x · 8th April 2004, 17:16

ok, that makes sense.

Another question then: does interleaving overhead is linear proportional to average interleaving ms ?

alexnoe · 8th April 2004, 17:20

Overhead is 16 (Open-DML) or 24 (non-Open-DML) bytes per audio chunk. So for NanDub, the answer is yes (interleave = chunk size), for avi-mux gui, you can separately set interleave and chunk size.

If you set 96 ms in NanDub, you get 24 bytes per 96 ms of overhead.

-> http://www-user.tu-chemnitz.de/~noe/...l#overhead_avi

len0x · 8th April 2004, 17:32

ok. thanks!

8th April 2004, 14:20	#1 \| Link
len0x I'm afraid we've to stop Join Date: Mar 2003 Location: Amongst mad people Posts: 5,398	AC3 interleaving in AVI The other day I was trying to think why recommended value for interleaving is 96 ms. And it turned out that I don't quite understand how audio is interleaved in AVI anyway. Consider this: 1 video frame is probably the smallest data chunk which cannot contain audio data inside, right? So 1 frames = 40ms for PAL for instance. So you can only have 40,80,120ms interleaving. Then how come values like 96ms are valid interleaving values? P.S. Probably I don't get very simple, but important point, but I just can't figure out which __________________ Gordian Knot Family: Gordian Knot: website, download Auto Gordian Knot: Website and download, tutorial, FAQ

8th April 2004, 14:23	#2 \| Link
alexnoe Banned Join Date: Aug 2002 Location: Germany Posts: 1,737	One AC3 frames lasts 32ms. If you split AC3 frames apart, the decoders b0rk, meaning that you have to write pieces of k32 ms. Since the decoders are even more b0rked, k must be between 2 and 5... The simple but important point you don't get is that no one says that the number of video frames between 2 audio blocks has to be constant. Last edited by alexnoe; 8th April 2004 at 14:47.*

8th April 2004, 17:16	#5 \| Link
len0x I'm afraid we've to stop Join Date: Mar 2003 Location: Amongst mad people Posts: 5,398	ok, that makes sense. Another question then: does interleaving overhead is linear proportional to average interleaving ms ? __________________ Gordian Knot Family: Gordian Knot: website, download Auto Gordian Knot: Website and download, tutorial, FAQ

8th April 2004, 17:20	#6 \| Link
alexnoe Banned Join Date: Aug 2002 Location: Germany Posts: 1,737	Overhead is 16 (Open-DML) or 24 (non-Open-DML) bytes per audio chunk. So for NanDub, the answer is yes (interleave = chunk size), for avi-mux gui, you can separately set interleave and chunk size. If you set 96 ms in NanDub, you get 24 bytes per 96 ms of overhead. -> http://www-user.tu-chemnitz.de/~noe/...l#overhead_avi Last edited by alexnoe; 8th April 2004 at 17:24.

8th April 2004, 17:32	#7 \| Link
len0x I'm afraid we've to stop Join Date: Mar 2003 Location: Amongst mad people Posts: 5,398	ok. thanks! __________________ Gordian Knot Family: Gordian Knot: website, download Auto Gordian Knot: Website and download, tutorial, FAQ

8th April 2004, 17:11	#4 \| Link
alexnoe Banned Join Date: Aug 2002 Location: Germany Posts: 1,737	Exactly. You put one audio chunk after 120 ms, the next one after 80 ms etc. Important for AC3 is the audio chunk duration of k*32 ms, not the way it is put between the video (at least not more than for any other format...) AVI-Mux GUI included an RIFF tree viewer. Just mux a file with some settings and look at it in the viewer, then you'll see. VD also has such a tree viewer, but it gets terribly slow on large files.

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