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Radio Modulation Technology

Discussion in 'General Discussion' started by Hughie, Dec 9, 2014.

  1. Hughie

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    Can anyone in the know comment on what modulation technology DJI use.

    I had assumed it was spread spectrum modulation (SSM) because it is 2.4Ghz and that seems to be the norm for that frequency these days. Also, the transmitter has to bind to the receiver - which suggests they are agreeing on a PN code pattern for the dialogue - which fits with SSM.

    However, something has been nagging away at me since I noticed a recent RC transmitter f/w update whose description referred to the "PPM link". Pulse Position Modulation is quite old (30+ years I think). I have read that it was superseded by Pulse Code Modulation which in turn was superseded by the 2.4Ghz SSM group of protocols. These narrow band methods (PPM/PCM) are prone to interference from other users on the same frequency. So I am curious why they would use the term PPM in the description of the firmware update.

    Can anyone clear up what DJI use for their modulation?

    Thanks
     
  2. noiseboy72

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  3. Hughie

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  4. wildpalms

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    I believe that article is now quite old ()model reviews one) and Spektrum also use frequency hopping (DSMX).

    PPM in this case has nothing to do with radio transmissions.

    A normal servo is driven by a pulse every 20ms. The pulse being between 1 and 2ms in width. If you connected a receiver to a flight controller you would ordinarily have 6 leads. Each with a particular channel.

    PPM allow you to have 8 channels on a single wire. Each channel sent as a pulse with a gap and then the next channel. Can fit 8 into the normal 20ms window as it were. If you have receiver outputting PPM then a single wire to the flight controller can carry all your channels. PPM often used in the trainer port as you can have all those channels carried over a single signal wire.
     
  5. garrock

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    DJI's 5.8ghz transmitters are based on FHSS. They send a packet of digital data; select the next hopping channel; wait for the next time interval; then do it all over again.

    A packet has two parts: Control and Payload.

    • The Control portion of the packet contains the transmitter address.
      This is what the receiver binds to; the address.
      The Payload portion has digital values for each RC Channel.
      An RC Channel being each Joystick direction or toggle switch.

    The receiver converts the payload data into a D-Bus (aka S-Bus) data stream.
    The NAZA control unit inside the P2V constantly listens to the D-Bus data stream which contains the RC Channel information.

    At no time is there a PPM or PCM series of digital pulses. That is Last Century.

    ---------------------------------

    DJI's 2.4ghz transmitters are similar. The only difference is how they use the spectrum to send data packets.
     
  6. Hughie

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    Interesting, and thanks garrock and wildpalms.
     
  7. ianwood

    ianwood Taco Wrangler
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    The regular P2 uses a 2.4GHz control system based on Futaba FASST. FASST is spread spectrum using 36 hopping frequencies spread across the entire 2.4 ISM band. Entirely stock setups have been effective up to 2 miles. I don't dare fly that far but I use a Futaba radio with the stock receiver and antennas in the noisiest of urban environments without any signal issues.
     
  8. LandYachtMedia

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  9. ianwood

    ianwood Taco Wrangler
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    GFSK is a type of modulation. FASST is the specific modulation protocol.
     
  10. garrock

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    As for commonality between P2 and P2V...

    The small receiver board-module in the P2 drone (be it either the P2 2.4ghz or P2V 5.8ghz flavor) are the same form factor and send the same serial data stream to the NAZA controller. The serial data stream is called D-Bus which I assume is the same as the Futaba S-Bus data stream.
     
  11. Hughie

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    Interesting. Thanks.