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Playing QuickTime 3 Movie Sound Data

Q I’d like to play a movie using the QuickTime 3 function MCDoAction for a very short period of time, say, 0.10 seconds, and then stop. I can successfully play movie video data for 0.10 seconds, but movie sound data continues to play for about 0.30 ~ 0.50 seconds. Is it possible to play movie sound data using MCDoAction for such a short time period?

A The delay you are seeing when stopping your movie after 0.10 seconds is attributable to the fact that QuickTime 3 will play out the contents of the "audio fifo queue" when stopping a sound. If we didn't do that, you would never hear the last bit of audio in every movie that you play (this is particularly irritating when your movie is a very short sound, like a button-click).

Let me explain: On the Macintosh, there is a hardware fifo (fifo = "First In/First Out Queue") which contains the actual sound samples ready to be played out the speaker by the sound hardware. It is generally very small, so you won't notice the "stop delay" on the Mac. On Windows, QuickTime 3 emulates this hardware fifo with a software buffer. When DirectSound support is available from the audio driver, the default size of this buffer is about 60 milliseconds. When driver-level DirectSound support is not available, QuickTime falls back to using Windows' waveOut APIs. These APIs are pretty poor, and latency is a big problem. For that reason, the default fifo size in that case is about 0.25 seconds (and the "stop delay" is very obvious, as you have found).

The good news is that on Windows you can change the fifo size. Go to the QuickTime control panel and select the "Sound Out" pop-up. You will then see whether you are using waveOut or DirectSound. If waveOut is selected, select DirectSound instead (if it is in the list). Then (whichever device you have selected) hit the "Options" button. The DirectSound Options dialog allows you to enter the fifo size in milliseconds. The waveOut Options dialog allows you to enter the number of waveOut buffers in the fifo, as well as their individual size in milliseconds. Shrink the fifo as far as you can without getting audio break-up (note that you will need to restart MoviePlayer every time you change the fifo size, before you can actually try it out by playing some audio). Remember, you are making a trade-off between audio breakup and latency, and depending on the capability of your sound hardware/drivers, you may have to strike a compromise between the two.

[Nov 09 1998]

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