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Thread: Pulse Density Modulation with I2S

  1. #1
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    Pulse Density Modulation with I2S

    Hi , a friend had the idea that the I2S can also be used for other purposes . This time nothing digital , but " retro " Analog .
    I2S should be fast enough to achieve a few hundred kilohertz with Pulse Density Modulation (<-not "PWM").
    The ~100-1000KHz sine PDM, AM-modulated it should give a Longwave/Mediumwave-Transmitter (with audio D/A resoulution of a few bits), as output-object for the audiolibrary and with minimum external components.
    Ideally, the only external component should only be a short wire as antenna, but maybe it is a good idea to add a low-pass filter to "smooth" the PDM?

    I have no experience with something like this, but i think it is fun to try it.
    A range of a few meters would be perfect.

    My question is, if someone is willing to help with the electronics or can give tips regarding the antenna and the low-pass filter.

    Edit: https://en.wikipedia.org/wiki/Pulse-density_modulation
    Last edited by Frank B; 06-16-2015 at 06:05 PM.

  2. #2
    Senior Member PaulStoffregen's Avatar
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    If you try coupling an antenna using only passive parts, be careful of standing wave ratios. You could easily damage the Teensy pin.

    My not an RF or antenna expert, but my gut feeling is you may need some sort of impedance matching, from the relatively high impedance of the Teensy pin (3 volts, only a few milliamps) to the relatively low impedance of an antenna.

    My guess is radio folks may cringe at the incredible harmonic content around 5+ MHz for a few hundred kHz carrier. Maybe? There's a huge difference between what's "possible" and what's "legal" when it comes to transmitting radio signals.

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    Yes, i know that is problematic... i don't want a range of more than 10 meters - and with a few milli amps there is no risk (i think).
    The low pass filter should prevent the incredible harmonic content ...? :-)

  4. #4
    Senior Member PaulStoffregen's Avatar
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    You should probably think of antennas in terms of impedance.

    You don't get to control the milliamps separately from controlling the volts. Their ratio is fixed, due to the physical shape of the antenna. If you don't like that ratio, you need an impedance matching circuit or transformer.

  5. #5
    Senior Member PaulStoffregen's Avatar
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    I added PDM microphone support to the Audio library today.

  6. #6
    Senior Member+ manitou's Avatar
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    Re: AudioInputPDM

    FWIW, I did simple AudioAnalyzePeak tests with T3.2, serial plotter, and Adafruit PDM mic
    https://www.adafruit.com/product/3492
    3v3 GND, CLK to pin 9, OUT to 13, SEL NC

    works for me

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