Technical Q&As


QTW 54 - Audio and Video Go Out of Sync (1-Aug-95)


Q We are trying to use QuickTime for Windows 2.0.2 and Cinepak compression with our product. We send a sync frame every second, but the audio and video keep going out of synchronization. As the CD plays, the audio and video go further and further out of sync. Every 15 seconds or so, the playback hesitates and then seems to sync up, but then it gets progressively worse again. What would cause this?

A It sounds as if the movie's audio was recorded at a non-MPC sampling rate. Open the movie in Movie Player, click on "Movie|Get Movie Info...", and find the Sound Quality numbers. All cross-platform or Windows-specific audio tracks should be recorded at 11025, 22050, or 44100 samples per second. If the track was recorded on a Macintosh, the rate might be 11127, 22254, or something else.

When playing an audio track, QuickTime for Windows queries the sound driver to determine if it can play back at the recorded rate. Unfortunately, many sound cards and/or sound-card drivers answer "yes" and then do a less-than-spectacular job of actually playing back at the desired rate.

The ideal solution is to re-record or resample the audio track at 11025, 22050, or 44100 samples per second, and then make a new movie using the new track. As an alternative, you can add the following lines to your QTW.INI file:

    [Sound]
    requestedrate0=22050
    actualrate0=22050

If the audio-sampling rate of the movie is 22050, changes to the [Sound] section of QTW.INI won't help, unless the samples are 16-bit. If this is the case, try:

    [Sound]
    ChannelControl=8

This option, which is new in QTW 2.0.2, forces 8-bit sound output and should prevent DMA-channel conflicts between the sound card and the CD-ROM drive. If this helps, the problem is probably an interrupt or high-DMA channel conflict between these two devices. Use MSD to assess the situation, and then run the CD-ROM and sound card setup programs to correct any problems.

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