Hello Gullik, Nice work on testing quantisation of the modulator output. Sometimes fixed point can give surprising results, e.g. less bits are required than first imagined.
A better test would be measure the BER at a given channel SNR, plot that against number of bits used for quantising the modulator signal. It would also be useful to test the demod BER at different input levels, while keeping SNR constant. Cheers, David On Sun, 2012-07-15 at 14:27 +0200, Gullik Webjörn wrote: > I started a discussion about the sensitivity of > the fdmdv modem to A/D resolution. I have finally gotten > down to testing this and the result is amazing (to me). > > I wanted to understand if a mcu with a 12 or 10 bit a/d > converter would be sufficient for a modem. > > To test this I have taken a dv encoded file, and running > it through Davids fl_fdmdv GUI program I get the spots > on the scatter map about 1.5 mm as displayed on my laptop > without maximizing the screen. Thus the SNR as judged from > the scatter map is about 50:1, almost 37dB. (1.5 mm in a 80 mm square) > > So, I wrote a small snippet of code that did the following: > > real[i] = (real[i] + 8) & 0xfff0; > > initially with 16, but then with 32 etc all way up to 1024. > > Very little expansion of the scatter "spots" occur until I have actually > reduced the resolution from 16 bits down to 6 (!) bits. > > My conclusion from this experiment is that reducing the final wordlength > down to this small size does NOT deteriorate the SNR significantly, > so most probably an 8-bit D/A and 8-bit A/D would be sufficient. > Thus it could even be done with an R-2R ladder. > > Spot size IS 1.5 mm, indicating that the "math" processing of the signal > although *good* is not perfect, i.e. since I never get a single pixel > the scattering is created by math "imperfections". > > So, halving the word size has minimal effect on the result. > I even had to do a od -x | more and inspect the wav file to convince > myself this is actually true. > > Regards, > > Gullik /SM6FBD > > > > ------------------------------------------------------------------------------ > Live Security Virtual Conference > Exclusive live event will cover all the ways today's security and > threat landscape has changed and how IT managers can respond. Discussions > will include endpoint security, mobile security and the latest in malware > threats. http://www.accelacomm.com/jaw/sfrnl04242012/114/50122263/ > _______________________________________________ > Freetel-codec2 mailing list > [email protected] > https://lists.sourceforge.net/lists/listinfo/freetel-codec2 ------------------------------------------------------------------------------ Live Security Virtual Conference Exclusive live event will cover all the ways today's security and threat landscape has changed and how IT managers can respond. Discussions will include endpoint security, mobile security and the latest in malware threats. http://www.accelacomm.com/jaw/sfrnl04242012/114/50122263/ _______________________________________________ Freetel-codec2 mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/freetel-codec2
