At Thu, 28 May 2009 09:28:57 +0200, I wrote: > > At Thu, 28 May 2009 08:52:44 +0200, > Clemens Ladisch wrote: > > > > Lee Revell wrote: > > > On Wed, May 27, 2009 at 9:47 PM, Cameron Stone > > > <camerons.lists@xxxxxxxxxxxxxxx> wrote: > > > > I've got a USB microphone application that I want to sample at 256000 8 > > > > bit samples per second so I can listen to bats. I know that my device > > > > can get close to that rate, but alsa seems to restrict sampling rates to > > > > 192kHz. > > > > > > > > Is it possible to push the maximum sampling frequency higher? If so, > > > > where should I start looking? > > > > > > For starters, try adding a 256K sample rate to include/sound/pcm.h: > > > and sound/core/pcm_native.c: > > > > This is not necessary; drivers can define whatever sample rate they > > want. 1,792,000 Hz works just fine with the Bt878 driver. > > > > > $ arecord -v -Dplughw:1,0 test.wav -d 1 -r 256 > > > Recording WAVE 'test.wav' : Unsigned 8 bit, Rate 256000 Hz, Mono > > > ... > > > Slave: Hardware PCM card 1 '8-Mic AVR Adaptor' device 0 subdevice 0 > > > Its setup is: > > > ... > > > rate : 256000 > > > > This indicates that the record PCM stream _does_ use the correct rate. > > > > > $ aplay -r 256 test.wav > > > Playing WAVE 'test.wav' : Unsigned 8 bit, Rate 256000 Hz, Mono > > > Warning: rate is not accurate (requested = 256000Hz, got = 192000Hz) > > > > The automatic sample rate converter never uses a frequency higher than > > 192 kHz. > > > > This maximum frequency is defined in alsa-lib/include/pcm_plugin.h, but > > you cannot change it without changing LINEAR_DIV and the algorithm in > > alsa-lib/src/pcm_pcm_rate_linear.c. > > It's anyway better to user other resampler like pph or samplerate > plugins. And these have no restriction, I guess. > > It'd be nicer if the rate plugin itself provides the min/max rates > optionally. It shouldn't be hard -- just extend the pcm_rate_info_t > to contain the new info. A version check would be required to make > the backward compatibility, though. OK, here are two patches. One for alsa-lib and one for alsa-plugins. The linear plugin still limits the rate up to 192kHz. But other plugins should work (hopefully). Takashi ---
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