On Fri, 23 Jan 2009 14:55:36 -0200, Gabriel Genellina wrote: > En Fri, 23 Jan 2009 14:36:46 -0200, Peter Pearson ><ppear...@nowhere.invalid> escribió: >> On Thu, 22 Jan 2009 20:58:14 -0200, Gabriel Genellina wrote: >>> En Mon, 19 Jan 2009 21:57:04 -0200, Peter Pearson >>> <ppear...@nowhere.invalid> escribió: >>> >>>> The following code uses ossaudiodev to read 1000 values from >>>> my sound card at a rate of 12,000 samples per second: >>>> >>>> When I select a sample rate that is not a power of 2 times >>>> 3000 samples/second, a strong and very regular sawtooth is >>>> superimposed on the signal. At some sampling frequencies, >>>> it appears as a rising sawtooth, and at other sampling >>>> frequencies it is a declining sawtooth, so I'm presumably >>>> lost in some aliasing wilderness. As best I can tell, >>>> it's a 48 KHz sawtooth. >>> >>> That could be a hardware and/or driver limitation. By example, an >>> AC97-compliant chipset may provide a fixed rate of 48000 samples/second >>> any sample rate conversion must be done by other means, if possible at >>> all. >> >> Oh! As a matter of fact, my "soundcard" *is* AC97-compliant >> ("VT8233/A/8235/8237", according to Sysinfo). So . . . this >> sawtooth might result from some software, somewhere in the bucket >> brigade between me and the hardware, attempting to perform >> sample-rate conversion? The 48 KHz coincidence seems very >> significant. > > Yep. Google found this for me: > http://mail-index.netbsd.org/port-i386/2003/02/18/0003.html > > Using a sample rate that is a sub-multiple of 48000 is perhaps a safe > approach.
Back around the time I installed Ubuntu's Hardy Heron, several copies of this line appeared in /var/log/user.log.0: Jan 4 21:32:53 eleodes pulseaudio[6437]: alsa-util.c: \ Device front:0 doesn't support 44100 Hz, changed to 48000 Hz. So I suppose I'll just restrict myself to well-behaved frequencies. I'm not sure what "sub-multiple" means, but all the successful frequencies I've found over 1 KHz have been multiples of 1.5 KHz that evenly divide 48 KHz. Frequencies showing large sawtooths include 2, 4, 8, 10, and 16 KHz. -- To email me, substitute nowhere->spamcop, invalid->net. -- http://mail.python.org/mailman/listinfo/python-list