Re: creating a 57 KHz signal from 19 KHz reference

[Christophe Seguinot]

Hi Kristoff

I already tried this in the past but did not saved the corresponding grc file. I remember thath I did not succeed in getting a non rotating constellation for the RDS signal which normally is achieved when using your method. I also tried to demodulate L-R using the square of the 19Khz pilot (in the original GRC example, L+R demodulated signal has wrong phase and wrong amplitude so that is it not possible to recover L or R audio channel)

In fact cos³(x)= {cos(3x)+3cos(x)}/4 which after filtering lead to cos(3x)/4

However, in our case the 19KHz pilot amplitude (a) is very low so that when cubing (a cos(x))³

You get  (a cos(x))³=a³ {cos(3x)+3cos(x)}/4 which can be close to noise level

Normally, even if your 57Khz is low, it should be there.

Can you provide your grc file for further investigation?

Regards, Christophe

On 14/11/2019 23:39, Kristoff wrote:

Hi all,


I'm again experimenting with GNUradio, now with a flowgraph I found ("rds_rx") to demodulate FM/RDS.

One facts about FM/RDS:
After demodulation a FM broadcasting signal, the signal contains multiple sub-signals, (among others) a 19 KHz 'stereo pilot' and a PSK (RDS) signal at 57 KHz.
57 KHz is three times 19 KHz.

Question:
I am trying to find a way to use the 19 KHz stereo-pilot signal to create a 57 KHz carrier (to down-convert the FM/RDS carrier at 57 KHz).
How do I do this?


I tried using a PLL carrier-tracking block to extract the 19 KHz carrier (which seams to work) and direct that carrier to a 3 input multiplier block, but that does not seams to work.
Even after filtering out DC and the carrier at  -19 KHz (negative frequency), the output of the multiplier is always zero. (*)

Or is there a better way to generate a 57 KHz complex sine-wave from a 19 KHz reference signal?


(*)
When I just connect the output of a signal-source block (that contains a 19 KHz complex-number signal) and connect this to all ports of a 3-input multiplier, I do get a carrier at 57 KHz.
So I am puzzled why this seams to work, but using the same technique to multiply the output of a PLL carrier-tracking block does not result in anything useful.


Cheerio! Kr, Bonne,