Frame accurate synchronizing of subtitle files with MPEG video using DirectShow

Question

This is a problem I have been dealing with for a while, and haven't been able to get a good answer (even from Microsoft). I'm using the generic dump filter to write hardware compressed MPEG files out to disk. In the graph, I also have a SampleGrabber filter that gets called on every frame. From the SampleGrabber callback, I get a subtitle, along with the DirectShow timestamp and write them out to a SAMI (.smi) subtitle file. This all seems to be working, as the SAMI file contains the correct subtitles for every frame. However, I have a few problems:

1. The first few (usually 3 or 4) DirectShow timestamps are all 0. If I'm getting callbacks from the SampleGrabber, shouldn't these timestamps be incrementing?

2. When I begin playback, the first timestamp shown is about 10-20 subtitles into the SAMI file. I'd assume the first frame would show the first timestamp in the file.

3. This is probably related to #2, but the subtitles are not synchronized to the appropriate frames in the file. They can sometimes be up to 40 frames late.

I'm using DirectShow via C++, capturing with a Hauppauge HVR-1800 under Windows XP SP3 (with latest drivers 09/08/2008), and playing back under Media Player Classic 6.4.9.0. Any ideas are welcome.

Answer 1

Are you using getting the incoming IMediaSample's GetTime or GetMediaTime. GetTime is what you want as it respresents the streams presentations time.

Be sure to also check the incoming IMediaSample's isPreRoll function. Preroll samples should be ignored as they will be output again during playback. Another thing I would do is make sure that your sample grabber is as far downstream in your filtergraph as it can be. Preferably after any demuxer's and renderers.

Also see the article on TimeStamps in the DirectShow documentation. It outlines the other caveats of using timestamps.

Of course, even after all of the tips above, there is still no absolute guarantee as to how a particular DirectShow filter is going to behave.