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Recording DC-20Hz via soundcard

Discussion in 'Electronic Design' started by [email protected], Feb 26, 2007.

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  1. Guest

    The solution presented at the following link describes how to
    frequency modulate a DC-20Hz signal onto a relatively high carrier
    frequency to get through the soundcard's AC-coupling capacitor, and
    then demodulate the signal in software using Matlab.

    http://www.elecdesign.com/Articles/Index.cfm?AD=1&ArticleID=2641

    Unfortunately I don't have Matlab, only Cool Edit Pro. If possible,
    what sound editing functions are needed to demodulate the FM signal,
    and what would the procedure be?

    For example if I were to demodulate an AM signal in CEP, I would use
    the "Distortion" function to chop the negative half cycles, then use
    the "Filter" function to filter out the remainder of the carrier.
     
  2. John Barrett

    John Barrett Guest

    FM in CEP -- I dont think so -- you have a transformation from tone to
    voltage that you cannot "program" into CEP, as its based on the values of
    the components used to adjust the VCO

    Being a programmer, I would write code to sample the sound card directly and
    implement a zero crossing detector.. since its a pure tone, that will give
    me the frequency, and the math given in that article will give you the
    inverse transform to convert that to a voltage. There is plenty of sample
    source code out there for accessing the mic input as raw 8 or 16 bit data
    using DirectSound, and with sample rates up to 44khz, you could run the VCO
    from 100hz to 20khz range without dropping data needed to detect the zero
    crossings.. then the only question is, how long a period of time do you
    count crossings before converting -- the longer you count, the more accurate
    the conversion.

    If you have ever worked with C# -- I could scare up the capture routines
    from one of my RF to Internet bridge applications .. get you going in the
    right direction :)
     
  3. Wrong way around.
    First use a low pas gradual filter,
    then you are left with an amplitude modulated RF signal
    (amplitude depends on frequency).

    It is called 'flank' detection.

    If you 'rectify' that you have the original sort of.
    But it is not very linear.

    Better to do it in software yourself.
     
  4. scada

    scada Guest

    Check out www.dplot.com .
     
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