Indeed I have since checked any source of crosstalk and removed all links from
source to audio sink but the one going to input number 3 and still the same
issue is observed. However remplacing 90000 Hz output (at 192000 Hz sampling
rate) with 90000.1 Hz solves the issue of the unexpected shape at the output
of the sound card. It seems to me we have a strange relation when incrementing
the NCO phase, but I fail to identify the cause of the problem, and why
moving the frequency as the flowgraph is running solves the output shape.
I thought that incrementing and decrementing the frequency defined as a
floating point number might lead me to a value slightly different from
90.0 kHz,
but at least printf displays exacly 90 kHz.
The sound card is indeed nice for VLF. The documentation claims
(https://uk.focusrite.com/usb-audio-interfaces/scarlett-18i8/specifications)
103 dBFS or 17 useful bits. From eastern France I collect TDF, DCF (and time
the signals with respect to GPS 1 PPS) but the British eLORAN station is too
far. I have not (yet) tried MSF.
JM
> 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 <
> jean-michel.friedt@femto-st.fr> 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
>>
>> ----------------------------------------------------------------
>> This message was sent using IMP, the Internet Messaging Program.
>>
>>
>> _______________________________________________
>> Discuss-gnuradio mailing list
>> Discuss-gnuradio@gnu.org
>> https://lists.gnu.org/mailman/listinfo/discuss-gnuradio
>>
>
--
JM Friedt, FEMTO-ST Time & Frequency/SENSeOR, 26 rue de l'Epitaphe,
25000 Besancon, France
----------------------------------------------------------------
This message was sent using IMP, the Internet Messaging Program.
_______________________________________________
Discuss-gnuradio mailing list
Discuss-gnuradio@gnu.org
https://lists.gnu.org/mailman/listinfo/discuss-gnuradio
No comments:
Post a Comment