Re: [Discuss-gnuradio] sound card sampling rate question

[Marcus Müller]

Hi JM,

guessing in the dark: You are likely running a modern linux
distribution. Many of these tend to emulate an Alsa device, which in
fact is but an Pulseaudio server, which in turn speaks to the real
device. They set that as the default audio device in the system.
Have you tried using the explicit device name (aplay -L) of your
physical device in the audio sink?

Greetings,
Marcus

On 25.06.2014 10:40, jmfriedt wrote:
> 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
>