AC3 delay when using passthrough.

[Rob_Quads]

I have a IBM T40 laptop. I use a Xitel USB HiFi Pro link which gives an digital output to play audio via my amp.

I am trying to play an AVI with AC3 audio - 448kb/s.

If I play the file and use AC3Filter and have it setup just to output the sound in stereo the audio plays fine and IS in sync with the video.

If I set AC3Filter to just passthrough the signal to my digital out the audio is delayed.

The problem only seems to occur with some AVI files, others I cam passthrough the AC3 stream without any problems and it is in sync (not sure on thier bitrates)

Anyone got any ideas how to sort this? Is it due to my sound card resampling/up/downsampling the digital signal?

Cheers for any help in advance.

Rob

[Prospero424]

I don't think that this has anyting to do with the sound card resampling the stream. Like you said, it's just passing the signal through. perhaps those movies that do play properly have a lower bitrate AC3 stream, you might want to check on this because it could be important.

I have a question: what happens when you try to pass through the signal while watching a DVD movie rather than an avi?

Have you tried watching these AVIs with a dedicated DVD player such as PowerDVD or WinDVD, which have their own AC3 decoders?

What I'm getting at is that we need to isolate where the bottleneck is. It could be that this USB device, due to the slower speeds inherent in the USB format, just can't keep up with a full 448 stream. It may be that AC3Filter is the culprit, introducing addittional latency.


Let me know what you find out.

[Rob_Quads]

If you are watching a DVD with AC3 it works fine.

In someting like PowerDVD etc you cannot watch an AVI etc and get AC3 passthrough working. This is due to USB SPDIF passthrough not working the same way as if you had it on a PCI sound card.

The only way I could get the AC3 Passthrough was to make up a custom graph for the file and then use Zoom Player Pro to play it.

USB isn't a problem as it pays other stuff fine.

The strange bit is that it plays some AVI's fine which have 5.1 encoded in them (once I have made a graph up for them)

I need to check further as I think the problem might actually be down to Zoom Player and thier Pause function as it appears as long as I do not pause the film its plays in sync (only watched a minute or two, really need to sit and watch a good 30 minutes to check) but if i pause the film its instantly out of sync so it must not be stopping the streams at the same point?

[ungajava]

I am also having the same problem. I am also playing avi's with ac3 audio, and using spdif passthrough with ac3 filter, but i have also tried ac3acm and ffdshow all using their passthrough option. They play fine when downmixing to 2 channel and playing through the computer speakers. I don't know if this is where the problem is different or not, but it seems that at the beginning of the movie the sync is ok, but they will slowly lose sync after a period of time. If I hit pause then play again they will be in sync again. I didn't mean to hijack the thread, it just seemed our problems were similar.