ah yes. well, take a leaf out of how ADSL works: (this email written for everyone, not just the technical conscetti)
With ADSL the wideband channels is measured and where the signal to noise is highest, it is taken advantage of with more bits per symbol. Now, in voiceband FM we all know that preemphasis and deemphasis is there because the output noise spectrum for an FM demod is triangle in shape- that is the noise energy density per unit frequency bandwidth increases with increasing frequency away from zero frequency. We apply a deemphasis to FM receivers to try and produce a constant noise density versus frequency. Without preemphasis, we'd get a rather dull end to end audio response. In practical terms, there is more noise in the 2500 to 3000 Hz segment than in the say 250Hz to 750Hz segment. So, with more signal to noise at some frequencies, one can take advantage of this. Where there is more signal to noise, we squeeze more bits per symbol . With QPSK, we get 4 bits per symbol. But 256QAM we get 8 bits per symbol. Therefore one might end up with something like this 250-500Hz : 256QAM 500-1000 : 64 QAM 1000-2000 : 16 QAM >2000 QPSK etc etc. now, of course if this is an 'adaptor ' for existing rigs, preemphasis and demphasis have to be assumed and the output levels and input gains shaped to suit. BUT all the Free DV modem simplicity goes out the window, so it is probably a poor application. forget it ! :-) I'd like to see some active PAPR control go into FreeDV (that is having a receiver that knows about the transmitter's PAPR behaviour and reduction means , as not to cost too much MER/SNR) . No time right now... another baby coming. There is volumes written about it in IEEE Transactions on Broadcasting (which I read) since there is appreciable commercial gain from needing a 50kW PEP versus a 150kW PEP transmitter.... 73 glen VK1XX On 15/12/2014 9:45 AM, David Rowe wrote: > Hi Glen, > > That was a bit fast for me! Could you expand on that a little? > > For example do you mean: > > 64 QAM -> low freq audio carrier -> FM mod > > Thanks, > > David > > On 15/12/14 05:44, glen english wrote: >> Is possible to exploit the FM noise triangle, and use higher QAM >> densitites at lower frequencies (say 64QAM) and lower qam densities(say >> QPSK) at higher frequencies. (have done). For audio FMed, it is >> rather a waste (though simple) to use the same constellation density >> over the wideband . For AM, not the case (constant noise density v freq) . >> >> 'other' qam densities enables the use also of various PAPR reduction >> schemes like moving constellation points to reduce PAPR. >> >> Turn up the deviation and watch the SNR versus carrier #. >> >> glen VK1XX >> >> On 15/12/2014 3:13 AM, Steve wrote: >>>> Here in my neck of the woods for me to play with this I would need to have >>>> this fit into a 10khz Channel. >>> No problem, just set your deviation to 1.1 to 1.5 kHz. >>> >>> ------------------------------------------------------------------------------ >>> 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 > -- - 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
