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Question about RDS (Radio Data System)

Discussion in 'Electronic Basics' started by pozz, Jul 2, 2003.

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  1. pozz

    pozz Guest

    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?
     
  2. 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.
    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.
     
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