Hi All,Earlier in the list, I had described finding a sound card that seemed to work fine for the 1600 mode, but had some intermittent errors on 700D, even on a very high SNR back-to-back test between computers.
I did some diagnostic recordings, comparing the device to one that doesn't present the same symptoms, and sent them to David. He identified a subtle difference between the behavior of the two devices, and using that as a starting point, I have continued to investigate what exactly is the difference between the two devices -- and that investigation is still ongoing.
However, during that conversation, I asked David a couple of questions that really belong on the list here, so I'm hoping David will be willing to provide a little commentary on either or both of these:
1. What details about the 700D mode's modem layer make it more dependent on things like frequency linearity, frequency stability, phase stability, etc., above and beyond those of older modes like 1600? 2. I noticed that the keying spectra of the 700D mode contains much more energy outside the 1kHz to 2kHz frequency "envelope" than the 1600 mode did. I can't post pictures to the list, but below are a couple of links to the different measured spectra. Is the 700D mode dependent on having this additional spectral energy to demodulate the symbols properly? E.g., is this similar to the difference between the hard-keyed RTTY spectrum distribution vs. that of a wave-shaped PSK signal? Or is there some other reason why the mode appears "broader"? 1600 Spectrum: http://www.kk5jy.net/snr-freedv/codec2-1600.png 700D Spectrum: http://www.kk5jy.net/snr-freedv/codec2-700D.png Thanks ahead of time to David for taking a look at these. 73, Matt
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