Hello Ross, Thanks for your comments,
Re your MSK suggestion: I think a MSK signal at an audio carrier frequency of 1500Hz, sent over a FM radio, will have a wide RF bandwidth so have the same performance hit as FSK over FM. Cheers, David On 14/12/14 14:27, Ross Whenmouth wrote: > 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 > ------------------------------------------------------------------------------ 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
