Blogs

ATP uses AFSK method to sending telemetry data over VideoTX’s audio

channel. The carrier is 2100Hz and a byte starts with one 1300hz wave(start bit), ATP sends

data bits after the start bit. and it sends minimum 10 carrier wave(zero) before a new start bit.

You can read my detailed information from this link (sorry for my English): http://www.flytron.com/pdf/audio_modem.pdf

Calibrating a PWM generator for 2100Hz carrier wave (for Atmega328)

This code generates static 2100hz square waves, put a 100nf capacitor on outside of processor and connect it on Audio in of TX, now you have a sinus on RX side ;)

You can send the bytes any time in your main code flow because AFSK protocol following waves not time sensitive protocol.
modem[] is an byte array that filled with data to send.
modem_byte is an integer for selecting byte of modem[] array
modem_bit is an char for counting sinus waves of AFSK

This code sends start bit at start and 8 bit of byte,
you should wait for 5ms for next byte for balancing sinus waves on audio stream, then you can send another byte also.

modem_bit = 0;while (modem_bit<18) //wait for all bit waves        {
                 
        if ((modem_bit%2==0) && ((PINB & (1 << 2))) ) // wait for 1 on PWM output. 
          {
          if (modem_bit==0)  // start bit
                 {
                 TCNT1H = 1;   // this values creates small delay for increasing the wave width
                 TCNT1L = 180; // and AFSK decoder can detect this big waves to 1, other small(2100hz) waves 0
                 }
                
           if (modem_bit>1)
                 {
                 if (modem[modem_byte] & (1<<((modem_bit-2)/2))) // if bit of bte is 1, change the wave width for 1 on AFSK
                   {
                   TCNT1H = 1;   // this values creates small delay for increasing the wave width
                   TCNT1L = 180; // and AFSK decoder can detect this big waves to 1, other small(2100hz) waves 0
                   }
                 }
                 
          modem_bit++;
          }

        if ((modem_bit%2==1) && (!(PINB & (1 << 2)))) // wait for 0 on PWM output  
          {   
          modem_bit++;
          }
        }

ATP Modem Software

I wrote a modem software for reading AFSK waves without any hardware (maybe a serial 100k resistor on microphone in)

File name is: SOSD_Modem.exe

Download link is: http://www.flytron.com/bootloader/xl_tools.zip

This is only software modem on the planet (or i didn't see another) :)

It is decoding AFSK codes from microphone input like an oscilloscope and giving output values from TCP for 3th party softwares. You can design your loggers, goggle earth interfaces or more with this plug.

It is using 20byte data pack for transmitting and software can separate them

First 16 byte is data bytes last 3 byte is line end and CRC

Pack of Bytes

[0-3] Latitude (4 byte = 32 bit long integer of coordinates)

[4-7] Longitude (4 byte = 32 bit long integer of coordinates)

[8-9] Altitude (2 byte = 16 bit long integer of altitude)

[10] Heading Angle (multiply it with 1.4 for finding angle value)

[11] Battery 1 voltage (divide 10 for value, 111 mean 11.1Volt)

[12] Battery 1 voltage (divide 10 for value, 74 mean 7.4Volt)

[13] Current Sensor 1 (multiply with 0.4 for value, 101 mean 40.4Ah

[14] Current Sensor 2 (multiply with 0.4 for value, 101 mean 40.4Ah

[15] RSSI (It is percentage of maximum value, 35 mean %35)

[16] Speed (km/h)

[17] Line End 1 (value 10)

[18] Line End 2 (value 13)

[19] CRC ( 8 bit totals of first 19 btye)

If you uses my original data pack Software Modem giving this type output from port 1976 (my birth year)

$TELE,lat,lon,alti,heading,voltag1,voltage2,curren t1,current2,rssi, speed, #13 #10

Or you can use your own data pack and you can decode them yourself from modem's RAW data output.

$RAWS,byte0,byte1,byte2,byte3,byte4.........,byte16, #13 #10

then you can decode your data bytes(first 16 byte)

This software only part of ATP i didn't share the codes because i want to share the protocol with all autopilot/OSD producers and i want to use their 3th party tools too.If I share the code, they can change only AFSK frequency and visuals for non compatible products.

New Features

I added direct KML file based live view for Google Earth on the new version of modem software.

You can click on plane_tracker.kml file and Google Earth connects your Modem Software to tracking the plane :)

It is already primitive because we have some connection problem on Google Earth since few months :(

Story Of ATP

If you want to read all discussions follow this link: http://www.rcgroups.com/forums/showthread.php?t=1157558

Thanks for reading

Melih