Ori Pessach wrote: > > Hi, > > Here are the patches against xawtv-3.35. Be aware that this bit of hackery is > far from perfect: it causes the capture rate to be visibly jerky at times, > since the video capture thread is trying to sync to a time line that advances > in rather large steps, and is at the mercy of the system's scheduler. The > audio time line is just inherently jittery. > > In fact, the more I think about it, the more I'm convinced that a > multithreaded capture architecture is simply a bad idea, and can't be made to > work reliably. A two thread design, with a realtime priority thread doing > video and audio capture, and a low priority thread doing the encoding, should > work much better. In my opinion, ffmpeg's capture code would be a good > starting point to get a decent video capture app up and running with minimum > hacking. > > Anyway, there are two patches - for some reason, the only way I could get the > source to patch correctly was to use: If you want a quick-and-dirty synch technique that works very well, try qtrec (http://www.ee.up.ac.za/~justin/v4l2/). The current version may not even compile, but the synch technique works great. How it works: 1) Calculate exactly how many audio samples there should be per video frame. 2) Wait for a valid video frame. 3) The moment the frame is complete use GETISPACE on the dsp to determine how much audio data is available. 4a) If the are fewer samples available than are required per frame, then read what samples are available, and pad the rest by duplicating the last value. 4b) If there are more than twice the number of samples required, then drop one frame worth of audio. 4c) if (!4a || 4b) read one frame's worth of audio data. 5) Write video and audio to output. (Note that the maximum audio correction at one time is 20ms - which is barely audible.) As I said, this is quick-and-dirty, but it works very well. Last night I recorded a 1 1/2 hour movie without any noticeable synch problems (and no noticeable audio defects). In fact, to test it, I have forcibly set the dsp rare off by as much as + and - 25% - the audio sounded as though it was pitch-shifted, but was still pretty good. -justin
