*edit* link to file with a few more tests, including x264 and DTS
Since I also experience some lip sync problems on many videos I decided to encode a variety of test files to see just how good/bad it is, from an audio perspective. In other words, I kept the video stream simple and constant, so that any differences are only due to the audio formats. Also these tests have nothing to do with lip sync problems like the ones related to FFW/RW etc… , these are constant throughout any video file encoded with these settings, and independent of FFW/resuming etc… the playback. I used the current firmware for my testing (1.01.024), it would be interesting to see if other ppl with this firmware could check these results, and also if it gets any better or not in the prerelease version.
I made a simple xvid video stream, with PAL resolution @25fps, showing 5 screen flashes in 2s intervals, and synchronized it to 5 pulses of audio encoded in a variety of audio codecs (MP3 2.0, AC3 2.0, AC3 5.1, AAC 2.0) and muxed it in different containers (AVI, MKV, MP4). The purpose of the tests was to see if with each file I could tell if either the audio pulses or the screen flashes came before the other; if you can you would most definitely see lip sync problems with a video encoded with a similar audio track. What I didn’t expect to see as well was the behavior of the WDTV with some combos. Anyhow, these were the results I got, and I think it would be interesting if other people could download these files and see if they get similar results, either with the current or the prerelease firmware (it’s just a 1.15 MB rar file).
All the files play perfectly in sync on a PC with different sound players.
MP3 2.0 (all OK)
In AVI/MKV/MP4: good sync
AC3 2.0 (good sync, unexpected decoding)
In AVI/MKV/MP4: good sync
(all of these were decoded in the WDTV to PCM and sent to the receiver as such instead of being bitstreamed as AC3, even though Digital is selected in the sound options)
AC3 5.1 (bad sync or unexpected decoding)
In AVI/MKV: (bad sync, sound has about 80/100ms negative delay with video, though it was bitstreamed as AC3)
In MP4: (good sync, but now it was sent as PCM again)
In MKV/MP4: hard to tell, but it seems like there is a small positive delay
I don’t think it is supported in either AVI/MP4 except by cheating, so I just tested it in MKV, and it seemed fine.
So… basically it seems like a mixed bag, with different problems for different types of files. MP3 seems to be the safest -though lowest quality- bet, however it gets hairy when you play stuff encoded with AC3, and possibly with AAC as well, though to a lower degree. You either get a very noticeable lip sync problem with 5.1 AC3 tracks (negative audio delay, which is more objectionable than positive one), or no lip sync problems with stereo tracks, but those are never bitstreamed, just sent as PCM. And if you mux AC3 in a MP4 container it seems that even surround tracks get decoded/downmixed, so it’s even worse than with MKV.