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eprom to generate pink noise

Discussion in 'Electronic Design' started by [email protected], Mar 16, 2006.

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  1. Guest

    Okay, so I need to generate pink noise, with a rolloff (cutoff point?
    for lack of a better term) of at least of 20khz (its for people to
    listen to). I have made several posts regarding the well known 1 bit
    audio playback method of digitizing sounds into eproms as a bitstream,
    and encoding the sounds to match the response of an RC filter for
    playback. My question now is, can this be done for pink noise? I.e.,
    can I just take pink noise, encode it with my little bitstream/RC
    filter encoder, and it will work? This, instead of using a random bit
    shift register with a wierd ass pole-cancelling filter?

    Pink noise is just a sound right?

    But how to I calculate what bitrate and RC filter time constant I will
    need for pink noise up to 20khz?
  2. Mochuelo

    Mochuelo Guest

    I think it should.
    Pink noise is low-pass filtered white noise. White noise is noise with
    equal amount of noise at all frequencies.
    I recommend an active filter, to have a higher frequency selectivity.
    The order of the filter depends on the bit rate. The frequency
    response of the filter should be such that 20 kHz passes by with
    little attenuation (less than 3 dB) and your switching frequency
    passes with as much attenuation as possible (at least 40 to 60 dB).
    First you choose your bit rate (according to your noise shaping
    scheme) and then the filter appropriate for that bit rate. It would be
    best if you could run some simulations (Matlab, Simulink,
    Simplorer...) before building anything.

    BTW, do you sleep well? :) I say this because I tried pink noise a
    couple of nights, long time ago, to mask noise from my neighbors
    (human and not human), but couldn't stand it. However, _if_ I could
    get used to it, I think it would help. I used a CD, though.

  3. Mochuelo

    Mochuelo Guest

    That was provided that the 1-bit modulator can run at the very high
    frequency required to generate baseband signals which are free of
    noise up to 20 kHz. This will probably limit you, because you may end
    up needing some GHz.

    Also, ask yourself if you really need 20 kHz, or whether 10 kHz is

    I haven't tried listening to pink noise that has suffered an MP3
    compression. MP3 was not optimized for that. If it sounds sufficiently
    well for you, use a dirty cheap MP3 player (it will not need a high
    SNR :).
  4. Ken Smith

    Ken Smith Guest

    Pink noise is white noise that has been mildly low pass filtered. You can
    take advantage of this in the design of the EPROM playback system. You
    didn't say how low you wanted to go with the bottom end of the frequency
    responce. Just for fun, lets assume you want to go down to 20Hz. Lets
    also assume that you want to use only a single RC filter.

    I'd place the RC pole at:

    F(rc) = sqrt(20 * 20K) = 632Hz

    Assuming R on the order of 10K, I get C=0.025uF. If this is surface
    mount, it would be time to stop and re-think because that is too big for
    a good NPO capacitor.

    Now comes a question you need to answer. How accurate do you want the
    top end of the frequency responce curve? There is an extra noise in the
    system from quantization. This noise has an RMS value of about

    To make the math simple, lets assume that this noise is uniformly spread
    up the Nyquist. You need to move Nyquist up until the amplitude of this
    added noise is low enough that it doesn't mess up your amplitude curve at
    the top end.
  5. Keith

    Keith Guest

    Yes, white noise has a constant amplitude over frequency. Pink noise is
    white noise passed through a 1/f filter.
  6. Guest

    Very interesting! But how do I figure out what the bitstream needs to
    be and what bitrate I need to use?
  7. Rich Grise

    Rich Grise Guest

  8. Ken Smith

    Ken Smith Guest

    You have to follow that with a "pink filter" to make the white noise into
    pink noise. I explained how to make the contents of a PROM and a single
    RC work for the job. The extra complexity of the PROM saves quite a few
    parts on the PCB.

    Another way to do it may be to use chaos to fill in for the randomness.
    There is a circuit that uses all 4 parts of a quad op-amp like the LM324
    to make chaos. I wonder if anyone has done a spectrum on its output.
  9. David Harmon

    David Harmon Guest

    On Mon, 20 Mar 2006 21:35:50 GMT in, Rich
    I like the one in the "CMOS cookbook".
  10. Joseph2k

    Joseph2k Guest

    That will depend on what kind of D/A converter you use and your sample rate.
  11. Ken Smith

    Ken Smith Guest

    I think the OP is long gone. He was hoping to use a single RC filter on a
    "one bit ADC". Since he is only trying to make pink noise, it is likely
    this would work. I think he found he was in over his head.
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