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,
