On 22 April 2012 06:06, Mike Eber <[email protected]> wrote: > I have some 2 meter 2400 bps FFSK modems and was wondering if a serial > stream would work.
Hi Mike, As you might have seen, I'm working on documentation for a real time implementation using 4800 GMSK modems (D-Star like). With 2400, you could probably use the 1500bps codec mode, using the remaining 900bps for FEC, or data. Or, if you just want to test it out, pad out the 900bps.There's 60 bits per frame in the 1500bps codec. So, for every 60 bit frame, you will have 36 bits to pad out, or otherwise use. However, the issue you will run into with radio is sync. From time to time, you need to include a known bit pattern that the receiver can "lock on" to. So if data is lost, and the placing is lost - it can quickly get back into place. Without this, as soon as the placing is lost, the rest of a transmission will be unrecoverable. The problem is, you must fit in this sync pattern while still sending the 1500bps codec2 data. My suggestion here, if you don't want to include any other data with the transmission is to on the first (and every nth frame) have a 24 bit sync pattern + 23 bits protected data (making 12 bits out) + 48 unprotected bits = 95 bits. So, just pad a zero on the end. On all other frames, send 3x23 bits protected data (69 bits) (making 36 bits out) + 24 bits unprotected = 93 bits. So just pad 3 zeroes on the end. Or even better make the last 3 bits some pattern that if incorrect could be a sign of sync loss. But, for sure it's possible. I suspect that decoding data to audio will be as simple as decoding the FEC, and piping the output into the codec2, then taking the raw audio output from codec2 into the audio device (configured for 8k, 16bit, mono). Encoding, I am not sure if this will have timing issues you will need to work with. In theory, if you sample audio from the audio device at the 8k, 16bit, mono rate. Pipe it into codec2, you should end up with codec2 data at 1500bps, producing 60bit frames you can apply the FEC/etc to. So, it might "just work". Somehow I reckon it'd need more work. So, I guess my point is, it's not as simple as piping codec2 into the modem. There's the above to consider too. Best regards, Peter. ------------------------------------------------------------------------------ For Developers, A Lot Can Happen In A Second. Boundary is the first to Know...and Tell You. Monitor Your Applications in Ultra-Fine Resolution. Try it FREE! http://p.sf.net/sfu/Boundary-d2dvs2 _______________________________________________ Freetel-codec2 mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/freetel-codec2
