Difference between revisions of "FramesPeriods"
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[https://bugtrack.alsa-project.org/wiki/wikka.php?wakka=FramesPeriods original] created
[https://bugtrack.alsa-project.org/wiki/wikka.php?wakka=FramesPeriods original] created 2006-09-24 by [[User:JamesCourtierDutton]]
Revision as of 19:32, 11 August 2007
A frame is equivalent of one sample being played, irrespective of the number of channels or the number of bits. e.g.
- 1 frame of a Stereo 48khz 16bit PCM stream is 4 bytes.
- 1 frame of a 5.1 48khz 16bit PCM stream is 12 bytes.
A period is the number of frames in between each hardware interrupt. The poll() will return once a period.
The buffer is a ring buffer. The buffer size always has to be greater than one period size. Commonly this is 2*period size, but some hardware can do 8 periods per buffer. It is also possible for the buffer size to not be an integer multiple of the period size.
Now, if the hardware has been set to 48000Hz , 2 periods, of 1024 frames each, making a buffer size of 2048 frames. The hardware will interrupt 2 times per buffer. ALSA will endeavor to keep the buffer as full as possible. Once the first period of samples has been played, the third period of samples is transfered into the space the first one occupied while the second period of samples is being played. (normal ring buffer behaviour).