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 > _______________________________________________ > Alsa-devel mailing list > Alsa-devel@xxxxxxxxxxxxxxxx > http://mailman.alsa-project.org/mailman/listinfo/alsa-devel > _______________________________________________ Alsa-devel mailing list Alsa-devel@xxxxxxxxxxxxxxxx http://mailman.alsa-project.org/mailman/listinfo/alsa-devel
