Thanks Glen, We are stuck with the 7500Hz for the moment (an integer ratio of the modem symbol rate, details in octave/cohpsk_frame_design.ods).
Yes I had in mind something like your latter suggestion, up sample to 16 kHz, then linear interpolation. It would need to be tested in terms of modem implementation loss. However I know what needs to be done, brainstorming that is the fun part. What would be wonderful is someone stepping forward to carefully code and test. Any volunteers? I'll work with you to help code, review, and test. You need C coding skills with an interest in DSP, some time, and determination. Thanks, David On 03/08/15 09:09, glen english wrote: > David > for resampling... > > so you want to go from 8000 to 7500 Hz ? > > I've always picked my modems to be integer divisors of 48k . makes life > easy. > However, if we are stuck with 7500... > > 1) SRC using 120kHz as the common sample rate (polyphase resampler > interpolator/decimator) > cleanest, highest MIPS usage, easiest to understand. > > 2) Farrow interpolator. > Best if passband is a small fraction of the sample rate. > Most used for fractional sample rate conversion for mismatched clocks. > > A counter is used counting samples in and out, the fraction determined, > and the resampling FIR coefficients are calculated on the fly. > > given a big chunk of the Nyquist BW is used, perhaps upsample to 16k , > convert to 15k, > and then crash-downsample/undersample to 7500 (cheap). > > > -- > > I can do this. > --- > > > > On 3/08/2015 9:20 AM, David Rowe wrote: >> Further to this thread (started on the digitalvoice list) James has >> provided a patch to implement his suggested changes to the FreeDV API, >> which is now checked into codec2-dev. >> >> So wonderful for me to have some one step up and contribute some code. >> >> Thanks James! >> >> Now I need to go to work on the FreeDV GUI program to support these changes. >> >> Down the track I'd like to see the FreeDV API accept a 8000 Hz sample >> rate for the 700 mode (its currently 7500Hz). I have some ideas on how >> to do this if anyone would like to work on the C code. But right now my >> focus is on tuning the FreeDV 700 mode. >> >> On that subject I'm getting some great reports from people working >> across the Atlantic on FreeDV 700 on as little as 5W - thanks Gerhard, >> Walter, and Mel! The new modem is really working well on the HF fading >> channel, and we have several examples of 80% DV copy when SSB was >> unusable. I feel the speech codec needs some work, but we will get there. >> >> Exciting times for HF DV! >> >> - David >> >> ------------------------------------------------------------------------------ >> _______________________________________________ >> Freetel-codec2 mailing list >> [email protected] >> https://lists.sourceforge.net/lists/listinfo/freetel-codec2 >> > ------------------------------------------------------------------------------ _______________________________________________ Freetel-codec2 mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/freetel-codec2
