Lee Revell wrote:
On Wed, May 27, 2009 at 9:47 PM, Cameron Stone camerons.lists@cse.unsw.edu.au 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.
Try resampling the .wav file before with sox.
Best regards, Clemens