On 06/06/2018 11:37 PM, David Rowe wrote:
Hello List,
OK, so we have FreeDV 700D which is competitive to SSB at low SNRs.
This is comprised of the Codec 2 700C speech, powerful Forward Error
Correction (FEC), and the new OFDM modem.
I am now taking a look at improving speech quality with some new Codec
2 modes, the idea is to drop these new modes into the LDPC/OFDM modem
framework that is working so well.
Modes I am considering:
i) A higher quality mode in the 2400 bit/s range. Maybe wideband
audio. We could get this into 2000 Hz of RF and SNRs of less than
10dB. 2400 bit/s would also be useful for UHF/VHF work, if competitive
to FM and AMBE.
ii) Improving the 700C Codec quality, in particular robustness to
different microphones and speakers.
Here is a first pass, comparing Codec 2 1300 (the codec used for
FreeDV 1600), a simulation of SSB at 10dB SNR, and a prototype of a
new 2400 bit/s (ish) mode:
http://rowetel.com/downloads/codec2/proto/
It sounds very good. I can see this compete with FM on VHF/UHF.
(The current modes compete very well with SSB, but still are no match
when compared to the average FM transmission)
Assuming a 40ms frame size, adding a 16 bit sync word and keeping about
8 bit as 'guard' it would make a nice 3000A mode as a successor to 2400A.
(The varicode bits aren't really needed in the voice frames as you can
put those in dedicated data frames)
I am guessing it would be just a bit to wide to use manchester and still
fit through the audio filters on the average handheld.
But probably it could be used on those sets with access to the
discriminator and modulator (like the FT817).
Looking forward to getting it to work here and hopefully get some more
interest in experimenting on our local repeater.
73,
Jeroen PE1RXQ
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