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PWM - natural vs uniform sampling of audio signals

Discussion in 'Electronic Design' started by Tony, Apr 13, 2004.

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  1. Tony

    Tony Guest

    Hello all,

    All the literature I've been able to find seems to indicate that generating PWM
    by natural sampling (analog input signal compared to saw or triangle wave)
    produces lower distortion than uniform sampling (sample/hold the input first).
    But all those same sources also use the case of "single-ended" PWM (where one
    edge is fixed, and the other is modulated by the signal).

    Now it seems quite logical to me that single ended uniform sampling will produce
    even order distortion products, if only because the effective pulse position
    varies with the signal, and natural sampling seems to partially correct this
    problem.

    BUT in the real world, all the natural AND uniform sampled PWM generators I have
    found effectively compare the signal to the reference on both the rising and
    falling edges of a triangle wave, so the pulse position does not shift with
    signal. And under these conditions, it seems to me that uniform sampling should
    be better.

    I've googled a lot for enlightenment on this issue, but to no avail.

    Can anyone set me straight?

    Tony (remove the "_" to reply by email)
     
  2. Ban

    Ban Guest

    Tony,
    maybe it appeals to the mind but I do not think there is much difference if
    the squarewave is centered or not around the sample points, with the
    sampling frequency being high enough.
    A dual-sided modulation allows only *half* the sampling frequency, the
    question is if this is not of disadvantage. With a good 3rd order
    noiseshaper you gain at least 20dB of S/N in the baseband per doubling of
    the sample rate, whereas a single sided modulation will certainly not loose
    that much compared to the dual sided.
    If you think you do not need a sample/hold for the triangle modulation, you
    are mistaken. Aliasing will be much worse and creates not a harmonic
    distortion but mirrored artifacts in the baseband that are degrading the
    performance.
    Nowadays for better performance a sigma-delta converting scheme is used,
    which reduces distortion much more than even the best pure PWM is able to
    do. In this case there is a delayed feedback of several samples, so that the
    question of double sided modulation becomes even less important.
    I'm working on a digital amp which uses exactly these ingredients and after
    a lot of research I gave up on analog triangle modulators, because it
    requires much better shielding and decoupling than a A/D-conversion done in
    the "dead-time", where no high-current switching occurs.
     
  3. Tony

    Tony Guest

    Could you explain why this is so?
    My intuition told me that I DO need a S/H, but this produces "uniform sampling",
    which many references claim to be inferior to "natural sampling" (ie, no S/H).
    I would like to explore a multi-order delta modulator (although AFAIK the
    "sigma" part would not be needed). Can you suggest any good references for this,
    and especially multi-order noise shapers as referred to above?

    Tony (remove the "_" to reply by email)
     
  4. Ban

    Ban Guest

    The thing is I use a counter, which limits the resolution. Obviously an
    up/down counter has to run at double the frequency, to produce the same
    resolution, because the on duration of the switch is always of even numbers
    of count. But also the triangle wave will have to slew double and the jitter
    of the comparator is higher etc. this is again the limiting factor in an
    analog modulator.
    Well, then you need a lot of filtering on the O/P
    J.C.Candy; Use of Double Integration in Sigma Delta Modulation IEEE1985
    J.C.Candy;Oversampling DS-Converters IEEE1992
    K.Chao; A High Order Topology...IEEE1990
    Oppenheim/Schafer; Discrete-Time Signal Processing; chapter 4.9
    You'll like this one:
    http://www.e-insite.net/ednmag/archives/1995/042795/09df3.htm
    JAES_V52_3_PG166.pdf (is copyrighted) if you do not find it tell me via PM
    This is what I have here at hand, but there is little literature to find.
     
  5. litw

    litw Guest

    I once tried a design with a first order sigma-delta but without any s/h
    so the result frequency was only limited by the delay in the circuit.
    Such high freq will be not too good to the inductor i guess. But from
    the simulation i found the harmonic distortion is lower than the
    conventional sawtooth wave approach.
    Those may not be relavent to what you are discussing. just my experiences.

    litw
     
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