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Will mini MP3 recorders/players do subaudio?

Discussion in 'Electronic Design' started by Robert Martin, Apr 3, 2004.

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  1. Can anyone advise if the capsule shaped 128MB USB connect MP3
    recorder/players, commonly sold for under $100, will record and
    playback frequencies in the 5-20 Hz range.

    Of course, headphones would not be used in this application.

    If not, what would be the most cost effective design for doing this
    on-board and where might I find a schematic?

    Robert
    SCU
     
  2. artie

    artie Guest

    The problem isn't with the playback side, but with the encoding side.
    It sounds like you want to use this for data recording. If you're
    doing a single sine wave, pretty much full scale as far as amplitude
    goes, and varying in frequency, I'd guess that most mp3 encoders will
    encode it fine.

    You could do some experiments using freeware/shareware audio editing
    software. On Windoze, Cool Edit can generate tones, and save them as
    mp3 files. Try encoding some test signals and then decoding them.
     
  3. I would use a Philips PCF8591 chip, connected with 4 wires to the parport
    of the PC (scl, sda in, sda pulldown).
    You can write the soft to do 30 samples / second for example, using the PC
    timer, and record on the PC.

    In 32kB you can then store 262144 bits / 12 = 21845 samples.
    21845 / 30 = 728 seconds, so say 12 minutes.

    If you need standalone small portable, a PIC with AD and some SRAM at
    30 samples/s say 12 bits even for a 32 kB RAM would be 16 x 1024 seconds,
    / 100 so good for 160 seconds, 2.5 minutes.
    More SRAM more minutes...
    And would use little power, and cost next to nothing (a few $).
    Youd have to make a 25 Hz lowpass in record to get rid of aliasing if
    other frequencies are present.
    If I did the math right.
    JP
     
  4. On a sunny day (Sun, 04 Apr 2004 08:06:50 +1000) it happened Robert Martin

    OK, I wanted to write (messed up the text):

    I would use a Philips PCF8591 chip, connected with 4 wires to the parport
    of the PC (scl, sda in, sda pulldown).
    You can write the soft to do 30 samples / second for example, using the PC
    timer, and record on the PC.

    If you need standalone small portable, a PIC with AD and some SRAM at
    30 samples/s say 12 bits even for a 32 kB RAM.
    In 32kB you can then store 262144 bits / 12 = 21845 samples.
    21845 / 30 = 728 seconds, so say 12 minutes.

    More SRAM more minutes...
    And would use little power, and cost next to nothing (a few $).
    Youd have to make a 25 Hz lowpass in record to get rid of aliasing if
    other frequencies are present.
    If I did the math right.
    JP
     
  5. Ian Stirling

    Ian Stirling Guest

    Maybe.
    At this sort of frequency, you run into problems with capacitive
    coupling, and the encoder deciding that you can't hear 5Hz, so
    leaving it out.

    Buy one for evaluation, if it works, use it.
    Some with other than MP3 voice encoding (ADPCM, ...) might suit
    better.
    I'd test the one I have here, but it'd mean searching for the line-in
    jack, which I've misplaced.
     
  6. James Meyer

    James Meyer Guest

    If you use a voltage to frequency converter between the source and the
    MP3 unit, you could extend the response down to zero Hz.

    With a zero offset so that zero volts in gives you 10 KHz out and a
    scale factor of 1 KHz per volt you might get pretty good recording from zero Hz
    to 5 KHz. An anti-aliasing would be rrequired as well as a matching frequency
    to voltage converter for playback.

    Jim
     
  7. Still goofed here, for 20 Hz in you need to sample at 50 Hz, so 7 minutes
    because of Niquist.
    Must have been tired last night...
    Watched too much alien movie perhaps ;-) and that was a good one :)
    JP
     
  8. Ian Stirling

    Ian Stirling Guest

    I would expect that voltage to amplitude modulation at say 2Khz might be
    more suitable.
    I suspect that you'd tend to get distortion caused by the quantisation
    of the frequency bins.
     
  9. Nico Coesel

    Nico Coesel Guest

    I think they will, but you may need to do some minor modifications. I
    suppose the mini MP3 players will have RC filters that may not pass
    these low frequencies. These filters can be modified quite easely.
     
  10. James Meyer

    James Meyer Guest

    Maintaining calibration of an amplitude modulated scheme might be more
    problematical than a frequency modulated scheme. An amplitude based system
    would suffer degradation if the amplitude of the signal out of the MP# recorder
    wasn't constant. On the other hand, a 10 KHz signal is 10 KHz even if it is
    distorted and severely attenuated.

    I was envisioning a (more or less) continuous, analog,
    modulator/demodulator system.

    Jim
     
  11. Ian Stirling

    Ian Stirling Guest

    But, you've got an A/D, a complex codec, a D/A and a complex decoder in there.
    I was talking about the codec having problems with a slewing frequency,
    which is practically never present in any normal audio signal, and much
    less likely to have problems with a simple AM signal, as that's
    practically what it's been designed to encode.
     
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