Gullik Webjörn wrote on 2012-07-15 14:27: > 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). > > 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.
Hello Gullik! Nice work! This is not so surprising to me, and I think you should be careful with your conclusions: *) The modulation is a phase modulation, that is primarily not working with amplitude shifts. That makes it more robust against amplitude noise. It's quite the same with the difference between AM an FM, where AM is very sensitive to atmospheric noise. The downside of FM is bandwidth usage, with a phase modulation it is kind of the minimum sampling rate. *) You probably tested with a perfect signal. Would be interesting what is happening if you add white noise to your samples. Sox can do that for you for example. *) You would not want to implement a real-world system with less than 8 bits, and 8 bits still is not really that much. You cannot sample a signal perfectly: You have more amplitude variations than just from your signal, like background noise, QRM, other signals. Especially other strong nearby signals will result in big amplitudes at your ADC, which in turn saturates at its highest and lowest sample value, and you start to loose information. In German that's 'Grossignalfestigkeit', in English I think it's called large signal immunity or large signal rejection. You can get along with less bits, but you have to apply very narrow and exact filtering before ADC. *) SNR to bits resolution has a know relation: SNR = (1.76 + n*6.02) dB where n is the number of bits. For 8 bits you should get 749.92 dB SNR. ADCs are never perfect and add some noise, which lowers your SNR. Thus you get a certain effective SNR, which corresponds to certain effective (non-integer) bits resolution. Even good ADCs loose some 1 bit resolution. Regards Patrick -- Engineers motto: cheap, good, fast: choose any two Patrick Strasser <patrick dot strasser at tugraz dot at> Student of Telematics, Graz Univ. of Technology, Austria ------------------------------------------------------------------------------ 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
