[Discuss-gnuradio] sound card sampling rate question

[jmfriedt]

I am facing a funny issue: I want to use the sound card for analyzing the 
spectral response
of a quartz tuning fork at 32768 Hz. I just happened to discover that my laptop 
(Panasonic CF-19)
has a sound card able to sample a signal at 192 kHz. I checked with audacity 
(Generate -> Tone at
32768 Hz and a frequency counter gives the right output frequency while an 
oscilloscope displays
a clean sine wave).
Now I want to do the same with gnuradio-companion: Signal Source sampled at 192 
kHz, output frequency
at 32768 Hz, directly connected to the audio sink manually set to 192 kHz. To 
make a long story short:
at low frequency (<20 kHz output) the output is at the right frequency, so the 
sampling rate is
properly understood. Above 24 kHz I get a clean sine wave output at f-24 kHz, 
so it looks like an
aliasing effect with a sampling frequency of 48 kHz, which is not consistent 
with my first observation.
And setting an output frequency of 24.xx kHz (xx=300 or 400 Hz) generates on 
the oscilloscope a funny
low frequency beat signal which must be related to the antialiasing filters of 
the card.

What I cannot understand is where gnuradio fails to initialize the sound card 
the way audacity does.
Reading the source code, I find in gr-audio/lib/alsa/alsa_sink.cc the following 
intialization

      // sampling rate
      unsigned int orig_sampling_rate = d_sampling_rate;
      if((error = snd_pcm_hw_params_set_rate_near(d_pcm_handle, d_hw_params,
                                                  &d_sampling_rate, 0)) < 0)
        bail("failed to set rate near", error);

      if(orig_sampling_rate != d_sampling_rate) {
        fprintf(stderr, "audio_alsa_sink[%s]: unable to support sampling rate 
%d\n",
                snd_pcm_name(d_pcm_handle), orig_sampling_rate);
        fprintf(stderr, "  card requested %d instead.\n", d_sampling_rate);
      }

which does not seem to test whether the sampling rate is above or below 48 kHz 
(as found in the
pull down menu of the Audio Sink block of gnuradio-companion), and I get no 
error message when
running my application.
Any idea what could be going wrong ?

Thanks, JM

-- 
JM Friedt, FEMTO-ST Time & Frequency/SENSeOR, 32 av. observatoire, 25044 
Besancon, France