[alsa-devel] Alsa principles

[Takashi Iwai]

It looks like a good idea to have these as a real devel-FAQ on
Wiki...


Takashi

At Mon, 13 Aug 2007 09:03:59 +0200,
Clemens Ladisch wrote:
> 
> LCID Fire wrote:
> > First - I didn't find any complete description about interleaved and
> > non-interleaved transport. What is actually (in a nutshell) the
> > difference and what the consequences of either approach?
> 
> When using interleaved access, the sample values that are to be sent at
> the same to each channel arestored  right after another in the buffer,
> i.e., like this:
> 
> > Channel1
> > Channel2
> > Channel3
> > Channel4
> > Channel5
> > Channel6
> > Channel1
> > Channel2
> > .
> > .
> > .
> 
> In ALSA, the set of samples for all channels is called a frame.  In this
> example, a frame would consist of six samples.
> 
> 
> When using non-interleaved access, you have effectively one buffer for
> each channel, i.e., the entire buffer would look like this:
> 
> c1 c1 c1 ..... c2 c2 c2 ..... c3 c3 c3 .......
> 
> > snd_pcm_hw_params_set_format:
> > So this is setting some kind of format!? I mean first off - which format
> > should one set? There are a few dozens to choose from and the hardware
> > will most likely not support all of them.
> 
> When using the "default" device that can automatically convert sample
> formats, it will support all (well, most) of them.
> 
> > So how does one retrieve a list of the hardware supported formats?
> 
> snd_pcm_hw_params_test_format() or snd_pcm_hw_params_get_format_mask()
> 
> > And which one should be considered superior?
> 
> Whatever is easiest to handle in your program.
> 
> > I assume it does affect the buffer size eventually needed!?
> 
> Yes, if you measure the buffer size in bytes.
> 
> > snd_pcm_hw_params_get_channels_max:
> > This one should IMO output the channels - but for a capture device
> > (where often there is just a single stereo input) this would normally
> > just return 2, right?
> 
> The "default" device can give you as many channels as you want.
> 
> > snd_pcm_hw_params_set_periods:
> > And we are at the REAL confusing stuff. Here one can set "periods". But
> > what is meant by a period?
> 
> It's a part of the buffer that gets transferred between two interrupts.
> The OSS API calls this "fragments".
> 
> >And there's a count of periods set - what is this count used for?
> 
> It's a different method of setting the period size.  The three
> parameters are related like this:
> 
> periods = buffer_size / period_size
> 
> The buffer and period sizes are measured in frames.
> 
> You could also set the buffer and period _length_, which are the same
> values, but measured in microseconds.
> 
> > snd_pcm_readi:
> > Since the buffer is a void* I assume that the format does affect the
> > data needed for the buffer.
> 
> Yes.
> 
> > But having a buffer in userspace and a buffer set in alsa means it
> > does copy at least twice.
> 
> ALSA's buffer is the hardware DMA buffer; there is only one copy
> involved.
> 
> 
> HTH
> Clemens
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