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Using a microcontroller as an audio mixer?

Discussion in 'Electronic Design' started by John - kd5yi, Oct 30, 2005.

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  1. John - kd5yi

    John - kd5yi Guest

    These questions are for my education rather than for an existing device.

    I have a microcontroller containing an analog amplifier whose input can be
    switched between pins of the micro. Suppose I have 8 audio inputs. The
    amplifier has a gain of 1 and a 5 MHz GBW product. Suppose further that I
    now select each of the inputs rapidly (at, say, a 40 kHz rate) so that they
    are time-division multiplexed on the output. This is a low-fidelity thing.
    There is a filter on the amplifier's output such that the signal is 6dB
    down at 4 kHz.

    Will it sound crappy? Do I need to mulitplex faster? My thought is that I
    must switch at least 8 times faster than my highest audio frequency of
    interest. Am I correct?

    Thanks in advance for your replies.

  2. Guest

    16 times faster. That is, each input channel must be sampled at least
    twice the frequency of interest. This is the Nyquist frequency. Google
    that up for an explanation.

    Nyquist frequency is easy to remember since it is simply 2x(frequency
    of interest). But this is the minimum really. The rule of thumb for
    good sampling is 2.5 to 3 times the highest frequency you want to scan.

    4kHz is good for sampling speech so you need to sample it at 8kHz
    minimum. If you sample at 4kHz, the played back sound will only be 2kHz
    in quality at best.

    Also, I think just switching is not enough. You must read all 8 inputs
    together every 125us (8kHz) and somehow 'merge' them. Maybe do some
    averaging or take the highest value etc. Then output the result.
  3. J. David

    J. David Guest

    I would say you should multiplex a little bit higher than twice the highest
    audio frequency. But higher frequency multiplexing would help to filter out
    more effectively the aliases if you use 6dB per octave filter.
    And your sound will not be crappy as you said.

  4. Donald

    Donald Guest

    What is a little bit higher ??

    If twice is 200% of the highest freq, is 210% OK or 300% a little bit ???



    I am not an audio guy or analog guy for that matter.
  5. Guest

    Like in my earlier post, my signal processing lecturers keep repeating
    the usual rule of thumb of 250% to 300% of the highest frequency. Of
    course, the more sample the better but you'd also need faster/more
    capable hardware. And in terms of sampling for recording, you'll end up
    with larger file sizes. But it all depends on your application.

    For audio you can use even less than 250% since most people can't
    really appreciate the tinny sound of the higher frequencies anyway. An
    example of this is audible sound is roughly up to 20kHz and what most
    people consider good sampling (CDROM, MP3 etc) is at 44kHz. That is
    about 220%. Although I've seen recordings using 48kHz which is 240% of
    human hearing range.
  6. Joerg

    Joerg Guest

    Hello John,
    With a uC a totally regular switching might not really work if it has to
    do any other jobs (which it usually does). It's not impossible though.
    Any imperfections in switching will modulate into the output, along with
    some noise from its other innards.

    Those SoC controllers have never floated my boat. First, they were
    expensive. Then whenever I looked at the performance of the amps and
    stuff on them I somehow had to yawn. Often a simple LM324 would run
    circles around them.

    Regards, Joerg
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