Just a crazy digital radio idea - ?

Discussion in 'General Electronics Chat' started by parkland, Oct 30, 2013.

  1. parkland

    Thread Starter New Member

    Dec 1, 2012
    I've just been fooling around with digital radio modes.
    I'm just curious about why the audible tone is commonly used to transfer information, rather than the carrier itself?
    Are there any data radios that rely on phase shift of the carrier alone?
    How hard would it be to detect? Is that the main issue?

    The data could be transmitted by reversing the carrier output from the transmitter.

    Lets pretent this is the output from a transmitter:
    1 is the peak of the waveform, and 0 is the bottom.

    Then to transmit data, reverse the carrier every so often"
    00000011111111000000000011111111111111100000000111 - binary data

    Is there an easy way to detect if the phase has shifted like that?
    It seems like that would be the most effective way to transmit data, and use the least amount of bandwidth too.

    The transmitter seems like it would be easy to me, just add a circuit to reverse the polarity on demand from a computer signal. The receiver is what I can't fathom how it could work.
  2. Papabravo


    Feb 24, 2006
    Shifting the phase or frequency of a carrier signal has quite a few undesirable side effects on the spectrum of the signal created by the modulation technique. If the RF carrier is maintained in a constant and stable way then the spurs of the modulation technique can be dealt with in a variety of other ways.

    For example in PSK31 it is not uncommon to multiply the audio carrier that is being phase shifted by a cosine function at twice the bit rate so that the zeros of the cosine function correspond to the places where the audio carrier changes phase. This minimizes the spurs caused by phase changes with a discontinuous derivative. The resulting signal is well within the passband of a typical SSB receiver meaning that no additional components are required in the radio to do digital modes.

    The same thing is true with two-tone RTTY or FSK441 or I suspect JT65 and its relatives. These digital modes require only a computer, and any standard SSB transceiver.
  3. #12


    Nov 30, 2010
    I have actually seen this on an oscilloscope.

    I can not say where or the men in black might visit me.
  4. russian


    Sep 1, 2012
    Sorry for the offtopic - parkland, please PM if you want to discuss your previous idea :)
  5. ronv

    AAC Fanatic!

    Nov 12, 2008
    It's called phase encoding. In days of old it was used to record data in tape drives. In that system we wrote 40 zeros at the beginning and end of each block so we could establish the correct phase. Then run a pll and compare the phase to the clock.
  6. alfacliff

    Well-Known Member

    Dec 13, 2013
    if you use a ssb radio, the tone is the carrier. the reason for using the tones is that you would have to modify your radio to get teh carrier to shift. it is called afsk, you put a fsk keyed tone into a ssb radio and out comes real fsk carrier from transmitter
    this is the advantage of using the soundblaster type digital modes.
  7. w2aew

    Active Member

    Jan 3, 2012
    I assume you're referring to digital modes like PSK31, etc. where you're using a audio tone to create the transmitted signal, and to receive the signal. The truth is, it's all the same thing! The reason you are dealing with an audio tone is because you're dealing with a SSB transmitter and receiver. Carefully consider the following:

    If you have a SSB transmitter at 14.070MHz set to USB and transmit an audio tone at 1kHz, the resulting RF signal will be an unmodulated RF signal at 14.071MHz. If the audio tone at 1kHz is modulated, the the RF output will be a signal at 14.071MHz that is modulated exactly the same way. In other words, if the audio tone is BPSK modulated, then the RF output is also BPSK modulated (just as you suggested).

    By setting the reciever to 14.070MHz, the modulated 1kHz tone is recovered. If the receiver was set to 14.0705MHz, the audio tone would be 500Hz modulated the same way. At 14.071MHz, the modulation would be centered around DC. At the end of the day, the RF signal is simply a carrier (at 14.071MHz) that is BPSK modulated. We set the receiver so that the RF signal is placed in the upper sideband, resulting in an audio tone.

    Using an audio tone to do the modulation and demodulation makes it easy to use the sound card and software to act as the modem (modulator-demodulator).
  8. Dave_UYZ

    New Member

    Jan 16, 2014
    I think PSK31 uses this technique