Hi, Audio over a standard NFM channel has the advantage of compatibility with the huge installed base of transceivers that hams already have. This enables amateurs with an NFM transceiver, a computer, and a basic sound card <-> radio interface to experiment with digital speech (and internet linked repeaters, "text" messaging, etc). I think that if it is easy for hams to get their foot in the door so to speak and experiment, that they will then be more likely to buy or build new radio transceivers that are purpose built for digital modes.
May I suggest that something like 1200bps MSK (eg 1500 Hz carrier, + pi/2 phase shift for a 1 and - pi/2 phase shift for a 0) which will fit through the audio pass-band of a NFM voice transceiver and can be coherently demodulated at the receiver. I also think that it would be a good idea for each frame (or super-frame?) to include a synchronisation burst that has good autocorrelation properties (eg a short maximal length sequence) - in addition to robustly establishing frame sync, this known sequence could be used by the receiver to correct linear distortion in the channel (eg from pre- and de-emphasis audio filtering). This technique is used in other radio systems, eg 2g-GSM and STANAG-4285. Regarding the 7dB degradation in performance of FSK over NFM vs FSK over RF (SSB), I wonder what effect would be had by increasing the bit rate and using stronger forward error correction? For example, dropping the requirement for compatibility with voice NFM transceivers (but working with 9k6 FSK data transceivers) if say 9k6 baud GMSK with a r=1/8 convolutional code was used to carry 1200 bps payload, how would the BER/SNR performance compare to 1200bps Bell202 AFSK over an NFM voice channel? 73 ZL2WRW Ross Whenmouth ------------------------------------------------------------------------------ Download BIRT iHub F-Type - The Free Enterprise-Grade BIRT Server from Actuate! Instantly Supercharge Your Business Reports and Dashboards with Interactivity, Sharing, Native Excel Exports, App Integration & more Get technology previously reserved for billion-dollar corporations, FREE http://pubads.g.doubleclick.net/gampad/clk?id=164703151&iu=/4140/ostg.clktrk _______________________________________________ Freetel-codec2 mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/freetel-codec2
