Re: creating a 57 KHz signal from 19 KHz reference

Christophe,


See my message to Andy,


I managed to get this working by using the 'PLL Ref Out' block which
does produce a nice 19 KHz tone. Then filter (BFP) and multiply by three
using a 3-input multiply block, to get a 57 KHz reference signal.

Next, use a complex conjugate to turn negate the frequency and
mix/multiply with the 57 KHz RDS carrier.


Next step:
try to find a way to use the 19000/8 (=2375 Hz) reference for PSK
demodulation.
But I first need to read-up on  PSK demodulator
(https://wiki.gnuradio.org/index.php/Guided_Tutorial_PSK_Demodulation),
especially the part on timing recovery.



Cheerio! Kr, Bonne,


On 15/11/19 10:03, Christophe Seguinot wrote:
>
> 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 cos^3 (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 =a^3 {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,
>>
>>