Due to multiple requests, I decided to put together what I have and release it as is. As you can see from the video below it works! However there is still work to be done, including cleaning up the design a bit.
Archive for the ‘AFSK Modem’ Category
OSD and Audio Modem Design Files
Friday, July 10th, 2009OSD Demo
Saturday, March 21st, 2009The OSD functionality is now operational. A short demo can be seen in the video below. The OSD and audio telemetry functionality is now combined on the same board. I am planning to use this board with the AttoPilot autopilot. I will be now working on my new airframe (Telemaster Electro) and integration with the AttoPilot.
Telemetry using audio channel of video transmitter
Wednesday, March 11th, 2009In all my FPV setups the audio channels remains unused, so I decided to use it for telemetry. The idea is quite simple and can be seen in the block diagram below.
The boxes in orange are the only custom made parts, the rest are existing components. A board with a Propeller chip is used to read the GPS data and send them through the transmitter audio channel. Audio generation with the Propeller can be achieved by using PWM on a single pin and then low pass filtering the output.
The modulation technique used is also quite standard and implemented in only one COG. I decided to use the Bell 202 AFSK modulation, on top of that the AX.25 protocol is implemented. The GPS data are encapsulated into an unnumbered AX.25 frame and send at 1200 bps line speed. Error detection, framing and synchronization is already build in to the AX.25 protocol. This protocol is quite popular and used in HAM radio, the main advantage however is the availability of applications that can be used to receive AX.25 packets. The ground station basically requires no additional hardware, a PC with a sound card is all that is needed. The audio channel of the receiver is connected to the audio input of a PC and an application called AGW Packet Engine is used to demodulate the data. A custom application was finally made to receive the payload (GPS) data and display them.
The following video shows the whole system together. My next step is to use the same board to implement OSD functionality, the complete system will be used to display and downlink the telemetry output from an autopilot I will be soon be experimenting with.