Re: Starting the device manually

[Eric Peters <epeters@graphics.cs.uni-sb.de>]

Clemens Ladisch wrote:
> Eric Peters wrote:
>>I'm a bit confused by the alsa documentation. I want to fill up a buffer
>>of a alsa device with snd_pcm_writei and then if the buffer is full, I
>>want to be able to start the playback manually. By looking to the
>>documentation, I found the method snd_pcm_sw_params_set_start_threshold.
>>In some other messages on this list there was the hint to set this
>>threshhold larger than the max_buffer_size and then the device shouldn't
>>start automatically. So I've done. But unfortunately the playback starts
>>again automatically.
> 
> What are the contents of /proc/asound/card?/pcm0p/sub0/sw_params during
> playback?

tstamp_mode: NONE
period_step: 1
sleep_min: 0
avail_min: 1
xfer_align: 4096
start_threshold: 2703204728
stop_threshold: 16384
silence_threshold: 0
silence_size: 0
boundary: 1073741824

Can you explain me these values and what they're for? I think, I know 
the sense of start and stop threshold. But the other values...

> 
>>/* calculating the number of frames of a buffer and a period */
>>buffer_frames = buffer_time * sample_rate;
>>period_frames = period_time * sample_rate;
> 
> Is this correct?  Aren't the xxx_time variables in microseconds?

I don't know. ;-)
How is a frame defined?

> 
>>snd_pcm_sw_params_set_start_threshold(..., 2*buffer_frames)
> 
> This is OK, but it is common to use just the boundary value instead of
> some explicitly calculated value.

The boundary value in the output above? How can I determine this value 
in my application?

> 
>>- I think, if I don't call snd_pcm_start there should be no output if
>>the start threshold is larger than the max_buffer_size. Is this correct?
> 
> ... if the start threshold is larger than the current buffer size, in
> frames.  Yes.
> 
>>- Is it possible that the starting behaviour depends on the hardware I'm
>>using?
> 
> No, the software parameters are hardware independent.
> 
> 
> Regards,
> Clemens


Thanks Clemens for your fast answer.

Best regards,

Eric

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