Question about RDS (Radio Data System)

[pozz]

I know that RDS base-band signal is amplitude modulated
with a 57kHz carrier locked with the third armonic of the
19kHz carrier used to modulate (in FM) stereo audio signal.

I think that 57kHz carrier must be in phase with 19kHz (third
harmonic) so RDS demodulator can extract 19kHz from audio
signal and generate the third harmonic (with a PLL?), 57kHz,
that can be used to demodulate RDS signal.

But I read RDS standard and I noticed that an RDS decoder
is composed by a band-pass filter centered at 57kHz
feeded with MPX audio. At the output of that filter I have
RDS signal alone, without audio signal and *without* 19kHz
too. The 57kHz carrier used to demodulate RDS signal is
extracted from RDS signal at the output of band-pass filter!!
But in what kind of manner I can extract a carrier from
a DBSC-SC (suppressed carrier!) modulated signal?

And, if I can extract 57kHz from RDS signal alone, why 57Khz
must be in phase with 19kHz if I don't use this to extract
that?

 

[Ricardo Matos Abreu]

"pozz" <pozNOzuSPgnAMo@libero.it> wrote in message
news:3jEMa.141428$lK4.3943827@twister1.libero.it...

I know that RDS base-band signal is amplitude modulated
with a 57kHz carrier locked with the third armonic of the
19kHz carrier used to modulate (in FM) stereo audio signal.

I think that 57kHz carrier must be in phase with 19kHz (third
harmonic) so RDS demodulator can extract 19kHz from audio
signal and generate the third harmonic (with a PLL?), 57kHz,
that can be used to demodulate RDS signal.

But I read RDS standard and I noticed that an RDS decoder
is composed by a band-pass filter centered at 57kHz
feeded with MPX audio. At the output of that filter I have
RDS signal alone, without audio signal and *without* 19kHz
too. The 57kHz carrier used to demodulate RDS signal is
extracted from RDS signal at the output of band-pass filter!!
But in what kind of manner I can extract a carrier from
a DBSC-SC (suppressed carrier!) modulated signal?

First, I confess I don't know nothing about RDS.
Second, after band-pass filtering you may demodulate signal with an envelope
detector, which is the easiest and cheapest way to do it. RDS has low baud
rate, I suppose.

And, if I can extract 57kHz from RDS signal alone, why 57Khz
must be in phase with 19kHz if I don't use this to extract
that?

Perhaps this is done this way in order to let the demodulator's designer
choose between coherent and non-coherent design. Coherent design gives best
results but is more expensive.