Re: Couple of questions

[Adrian McMenamin <adrian@mcmen.demon.co.uk>]

On Fri, 2006-03-17 at 18:58 -0500, Lee Revell wrote:
> On Fri, 2006-03-17 at 23:51 +0000, Adrian McMenamin wrote:
> > On Fri, 2006-03-17 at 18:46 -0500, Lee Revell wrote:
> > > On Fri, 2006-03-17 at 23:29 +0000, Adrian McMenamin wrote:
> > > > 
> > > > So, just to be clear. When I am transferring a "period" I should
> > > > actually stream half a period's worth of bytes into the channel
> > > > playing
> > > > the left and half a period into the right - and can advance each dma
> > > > transfer through the buffer by half a period length on both the left
> > > > and
> > > > the right side of the buffer 
> > > 
> > > No, one period.
> > > 
> > 
> > 
> > How does that work? That would imply that there were half as many
> > periods in the DMA buffer as specified here...
> > 
> > static snd_pcm_hardware_t snd_pcm_aica_playback_hw = {
> > 
> > 	.info = (SNDRV_PCM_INFO_NONINTERLEAVED),
> > 	.formats =
> > 	    (SNDRV_PCM_FMTBIT_S8 | SNDRV_PCM_FMTBIT_S16_LE |
> > 	     SNDRV_PCM_FMTBIT_IMA_ADPCM),
> > 	.rates = SNDRV_PCM_RATE_8000_48000,
> > 	.rate_min = 8000,
> > 	.rate_max = 48000,
> > 	.channels_min = 2,
> > 	.channels_max = 2,
> > 	.buffer_bytes_max = AICA_BUFFER_SIZE,
> > 	.period_bytes_min = AICA_PERIOD_SIZE,
> > 	.period_bytes_max = AICA_PERIOD_SIZE,
> > 	.periods_min = AICA_PERIOD_NUMBER,
> > 	.periods_max = AICA_PERIOD_NUMBER,
> > };
> > 
> > 
> > As the buffer and period (in bytes) size is constant. 
> > 
> > 
> 
> That's why the maximum buffer and period is in bytes not frames.  Stereo
> has twice as many bytes per frame than mono.



Err...yes :)

I am still confused though. In mono, every time a period is played a new
period is loaded into the DMA buffer and then transferred into the
hardware's buffer.

How does this work now - wait for one period to play on the left (or
right) channel and then transfer one period's worth on the left and
right channel (ie ALSA fills the buffer up with 2 periods at a go in
stereo) or transfer half a period's worth on each channel but move the
copy pointer on a full period (ie ALSA transfers just one period's worth
which translates as half a period in either channel) 



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