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AVR & RFM70 based wireless guitar system - two RFM70 in parallel toincrease the bandwidth?

Discussion in 'Embedded' started by wzab, Jan 15, 2011.

  1. wzab

    wzab Guest


    I've just finished my RFM70 based wireless guitar system

    (I hope to publish it soon, but code and documentation needs some
    - the system is designed to be very cheap - just CS5343/4 as ADC,
    and RFM70 in the transmitter, and RFM70 with ATmega32u4 in the
    The system is visible as USB MIDI/audio device so you can connect it
    "alsa_in" to "jakcd" in Linux and then to your virtual guitar amp/
    soubd processor
    - e.g. rakarrack or guitarix. Additionally the transmitter contains 4
    switches and
    4 potentiometers, which are mapped to MIDI controls)

    The system works acceptably with 24bit/48kHz without acknowledge/
    Anyway some packets are dropped and this increases the noise level, so
    the acknowledge/retransmission will be needed.
    Other users reported that 500 kb/s is achievable with ACK.

    I think, that I'll need to use two RFM70 in parallel in the Tx and in
    Rx to provide
    the bandwidth sufficient for may needs (OK. maybe I'll reduce the
    resolution to 20bits,
    which gives 960kb/s).

    My questions are. Is it reasonable to expect, that two RFM70 will work
    together at distance of ca. 10 cm?
    How should I allocate the frequency to them?
    I have 83 1MHz channels. At 2mb/s air bitrate each RFM70 occupies
    2MHz. Should
    it be OK to separate both channels by 4MHz or it is better to assure
    the distance
    of ca. 40 MHz? (But then I'll more significantly spoil the 2.4MHz band
    in my viccinity ;-) ).
    What do you think about it? Any hints are appreciated.
    wzab, Jan 15, 2011
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  2. wzab

    linnix Guest

    No sure how much you can adjust these modules, but you will have
    serious interferences with two transmitting at the same time. For
    higher data rate, you might need higher carrier frequency (i.e. 5GHz).
    linnix, Jan 15, 2011
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  3. wzab

    wzab Guest

    Hmmm, in fact I'm rather afraid if one module will be able to receive
    the acknowledge, when the another one is transmitting.
    I'm afraid, that the input RF amplifier may get saturated by the
    sent by the neighbouring module, even though it is tuned to another
    The big advantage of RFM70 is their good availability and very good
    It can be done at 2.4GHz, as Nordic offers nRF24Z1
    ( http://www.nordicsemi.com/index.cfm?obj=product&act=display&pro=86 )
    and AUREL offers the audio links 16b/44.1 kHz
    ( http://aurelwireless.com/rf-wireless-modules/product-info.asp?id=303
    http://aurelwireless.com/rf-wireless-modules/product-info.asp?id=302 )
    working in this band.
    Unfortunately I'm not able to buy the nRFZ24Z1 breakout board in
    and AUREL's modules are quite expensive (and you need to solder
    wires to the chip pins to extract the signal in a digital I2S form
    from them).
    wzab, Jan 15, 2011
  4. wzab

    linnix Guest

    Yes, you need some settling delays, which would cut into your data
    rate further. You need to either compress your data or go to the 5GHz
    ISM band.
    linnix, Jan 15, 2011
  5. wzab

    Jim Stewart Guest

    Diversity receive is the usual solution to this
    sort of problem. You might contact the manufacturer
    and see if their protocol can be adapted to it.
    Jim Stewart, Jan 17, 2011
  6. wzab

    wzab Guest

    Thanks. For now I have simply reduced the sampling rate
    by factor of two (quick & dirty solution - simply
    discarding every second sample without antialiasing filtering,
    but my source is supposed not to contain signals above 12 kHz),
    and the resolution to 20 bits.
    Now it is possible to send the bitstream via RFM70
    with autoacknowledge/retransmission.

    The noise level has decreased significantly because
    no packets are lost.
    wzab, Jan 19, 2011
  7. wzab


    Feb 5, 2011
    Likes Received:

    I don't know if I break any existing rule about reopening an old post, couldn't find the rules.
    I wanted to ask you about RFM70 as I'm using some of them to transmit data (audio data is my intention). What is the maximum speed have you achieved? I can't manage to work with the auto retransmit mode! Using this mode in the transmitter and without receiver the transmitter doesn't notify me about maximum retransmit count! Any tips?

    Thank you very much, I appreciate any help you could give to me.

    davidgf, Feb 5, 2011
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