Re: [Discuss-gnuradio] sound card output issue

[Brad Hein]

I see a lot of inputs to the sound card sink in the flowgraph.. could the signal be getting mixed with itself among the many inputs to the card and causing new frequency components as it gets mixed? Can you try sending the flow graph signal to just one sink input at a time (perhaps using a constant source on the others if needed) to try and rule out this possibility?

Also this looks like a good card for vlf, hows the noise floor?

[Sent from mobile device]

On Thu, Jul 19, 2018, 4:49 AM Jean-Michel FRIEDT <address@hidden> wrote:

I have a funny problem with the output of a sound card fed from a signal
source.
The hardware setup is a Scarlett 18i8 sound card sampled at 192 kHz (for VLF
   signal analysis) with the signal generated from the front output number 1
   (channel number 3 in the GNURadio audio sink).
The sound card is controlled from ALSA.
   gr::log :INFO: audio source - Audio sink arch: alsa
The minimalistic flowgraph to reproduce the problem is at
   jmfriedt.org/untitled_grc.png
When first launched with an output frequency set to 90 kHz, the sound card
output looks like jmfriedt.org/snd1.png, ie with some spectral component
around 6 kHz whose origin I am unable to identify. If, using the slider, I
change the frequency and set it back to its original value of 90 kHz, I get
the fine output of jmfriedt.org/snd2.png which looks as good as I can
expect from an output frequency so close to the Nyquist frequency.

Since I want to automate data acquisition over a long duration (ie launch
the python script, collect a few seconds of data, kill the script and launch
it again a few minutes later, and repeat for a few months or years) the option
of manually setting the frequency is not acceptable. I cannot think of a way
to start debugging the problem, whether on ALSA source or the signal generator
side. Both seem reasonable

Any hint at which block could be the cause of the issue (there's only
two of them left in the minimalistic GRC graph !). I've read both source
codes and cannot see the difference between the initialization and the
callback function when changing the frequency.

Thanks, JM

--
JM Friedt, FEMTO-ST Time & Frequency/SENSeOR, 26 rue de l'Epitaphe, 
25000 Besancon, France

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