Yet another post about audio sync being off. I’ve got lots of MKV files, and on the majority of them, which play FINE in VLC media player on Windows, they play out of sync on the WD TV Live. However, unlike most “laymen” on this site, I know a lot about file formats and how sync works. I work at Microsoft as a developer in the audio/video department, and I know exactly what it takes to make audio and video play back in sync. **bleep**, I wrote a good chunk of the code.
Folks, almost any media that you would rip from, in order to obtain an MP4, MKV, AVI file, etc, has what are called “presentation timestamps” in the source media (DVD, blu-ray). These presentation timestamps tell the computer exactly when to play the media, so the audio and video don’t get out of sync. The computer reads and decompresses the source footage, sometimes well ahead of where it’s playing, and waits on each stream, video and audio, for the right time to play its’ decompressed footage. Frequently the timestamps aren’t included with every packet of audio or video, but are interspersed within the streams, like every 2 seconds or so. Your “rip” software SHOULD take this into account when producing it’s output file, but even so, the output streams in the MKV or MP4 file should ALSO have presentation timestamps. They might be being written at different points than in the source footage, but trust me, the presentation timestamps should be there, or your ripping software ain’t worth squat.
Secondly, any professional source footage (dvd, blu-ray) should be authored according to a “master clock”. The audio presentation timestamps should exactly match up with where they should be, given that all audio on the DVD/blu-ray should be delivered at a master format of however many Khz samples per second. In other words, if the audio says it’s 48,000 Khz, then at 48,000 samples into the audio, the presentation timestamp **bleep** well better be 1 second exactly. Video, likewise, is authored on the DVD according to the master NTSC clock, at *exactly* however many frames per second. DVD players wouldn’t be able to keep sync correctly if BOTH the DVD wasn’t authored according to a master clock and the presentation timestamps are missing or are wrong.
Okay, so assuming the PC can play back the MKV file or the MP4 file exactly correctly means either one of two things: (a) the presentation timestamps are wrong and the media player is against all the odds playing it exactly right (1 million to 1 shot), or (b) the file was ripped correctly and the presentation timestamps are correct.
Given that it’s very likely (b) above, and knowing that I have 100’s of MKV files on my PC that play correctly on the PC, the fact that the WD TV Live box does NOT play them back in sync means the bug is in the WD code, not the source files.
Simply put: on SOME files, not all of them, the WD TV Live box is not obeying the presentation timestamps correctly, for whatever reason. Given that the WD box doesn’t play a lot of MP4 or M4V files in sync either, leads me to believe there is simply a sync bug in the WD box that they haven’t found or tracked down or understand how to fix, yet. the fact that it happens on some files and not others, but nobody knows exactly which files or what formats cause the issue, is disturbing from a customer service standpoint. There should be a batch of test files in a suite which are tested against, for each new version of the firmware that comes out, with some way to automatically detect if the audio and video are in sync. Why, when I was just a wee tad at Microsoft 15 years ago, I wrote an automation tool like this which watched the video output and listened to the audio output, and verified the sync was actually in sync.
I’d be happy to talk to a WD representative about timestamps…