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From: | Jean-Michel FRIEDT |
Subject: | Re: [Discuss-gnuradio] sound card output issue |
Date: | Thu, 19 Jul 2018 16:17:10 +0200 |
User-agent: | Internet Messaging Program (IMP) H3 (4.3.7) |
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 afloating 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 < 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 ---------------------------------------------------------------- This message was sent using IMP, the Internet Messaging Program. _______________________________________________ Discuss-gnuradio mailing list address@hidden https://lists.gnu.org/mailman/listinfo/discuss-gnuradio
--JM Friedt, FEMTO-ST Time & Frequency/SENSeOR, 26 rue de l'Epitaphe, 25000 Besancon, France
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