Hi Gullik yes, the adaption of FreeDV for FM use is a bit of a bastardization, but nevertheless, useful.
FreeDV is designed for HF circuits. Putting aside the papr requirements though, OFDM is a good way to optimally fill the available BW for an FM baseband system. The frequency and phase response is pretty rough but for only 1000 bps, an equalizer is trivial. 1000 bps is easy for a 25kHz spaced FM system. No special techniques are required. The gotcha is that with a a non-linear demod process, like FM demod, channel errors (freq and phase response) that occur at IF (IE pre-detection) are VERY difficult to fix post detection. This is what designers run into when pushing the data rate on a baseband FM system. OFDM (or other multicarrier system) is useful because the channel changes little over the bin bandwidth, and hence equalization is not required for each sub channel. However, such nasties like assymetrical sideband IF response cause distortions post-demod that manifest themselves as IMD and the SNR available can be quite low in reality. glen On 15/12/2014 11:26 PM, Gullik Webjörn wrote: > Isn't this really about how legacy FM radio can be used with codec2 in a > simple > way? > > Just to add to the confusion, couldn't a simple n/m bit encoding be > sufficient? If we take 8b/10b coding, > it has some interesting characteristics. > > i) There are enough transitions to enable good clock recovery > ii) There is a maximum run length of same bit, in effect forcing a > minimum frequency to the signal. > iii) out of the possible 1024 symbols just 256 are used for data, > leaving a lot for control > iv) It is simple to encode / decode ( just lookup tables) > v) it is 80% efficient vs NRZ (8/10) > > With the low codec2 bitrate, it should give ample margins for decoding. > > The benefits of something along these lines is that almost all legacy > AND modern radios would be compatible, > althought IF filters would be suboptimal, unless replaced. > > Using a "baseband coding" the bit shape could be adjusted using a simple > FIR filter, again in a very simple > microprocessor or in a shift register for that matter (aka G3RUH). With > careful deviation adjust it could > be GMSK. > > Just my 2 c worth, > > Gullik > > ------------------------------------------------------------------------------ > 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 > -- - Glen English RF Communications and Electronics Engineer CORTEX RF & Pacific Media Technologies Pty Ltd ABN 40 075 532 008 PO Box 5231 Lyneham ACT 2602, Australia. au mobile : +61 (0)418 975077 ------------------------------------------------------------------------------ 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
