Re: [Discuss-gnuradio] sound card output issue

[Andy Walls]

> From:         Jean-Michel FRIEDT
> Date:         Thu, 19 Jul 2018 16:17:10 +0200

> 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,
> 
[snip]

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

Having the benefit of hindsight, I'll guess it is a beat frequency,
since GCD(192000, 90000) = 6000.

If a QT Freq Sink (with 8192 points and a Kaiser window) doesn't show
the 6 kHz component, I'd blame the sound card.

-Andy