Yes Kevin is on the right track - it's an integer multiple of the 75 baud symbol rate of each carrier in the cohpsk modem. Lot of trade offs in designing a modem waveform, especially for a tough channel like HF.
The choice of Fs = 7500Hz made life easier for the modem internally. It may not stay that way forever..... See codec2-dev/octave/cohpsk_frame_design.ods for more info on the waveform design. Resampling is cheap in cycles and should be distortion free. It's taken care of by the Fs = 8000 Hz interface in the freedv api so the 7500 Hz sample rate is invisible to end users. Cheers, David On 04/02/17 04:28, Kevin Otte wrote: > I see in the FSK code that "Sample freq must be divisible by symbol > rate". I don't know if the PSK modems are similarly impacted, but: > > 8000 / 750 = 10.6666... (ouch) > 48000 / 750 = 64 (ahh :) > > It's been my experience (anecdotal though it may be) that sound cards > prefer working at the higher sample rates anyway. Running all our modems > at 48000 helps with that and gives us internal consistency, but there > are some potential drawbacks: > > - This could impact the complexity for the STM platform in the SM1000. > - There are plenty of sound cards out there that top out at 44100, so > some resampling would still have to occur there too. Of course, any > artifacting on that would likely be far enough up the spectrum that it > wouldn't be noticable in what we're doing. > > Even so, running from 48000 back down to 8000 is also a much nicer divisor. > > 73 de Kevin N8VNR > > (reposted from the correct source address) > > On 02/03/2017 10:20 AM, Helmut wrote: >> Well, maybe that’s the reason. I already wonder about a modem sampling >> rate of 7500 Hz, as today’s most audio applications are using integer >> divisors/multipliers of 48000 Hz. > > ------------------------------------------------------------------------------ > Check out the vibrant tech community on one of the world's most > engaging tech sites, SlashDot.org! http://sdm.link/slashdot > _______________________________________________ > Freetel-codec2 mailing list > [email protected] > https://lists.sourceforge.net/lists/listinfo/freetel-codec2 > ------------------------------------------------------------------------------ Check out the vibrant tech community on one of the world's most engaging tech sites, SlashDot.org! http://sdm.link/slashdot _______________________________________________ Freetel-codec2 mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/freetel-codec2
