Hi,
On 15 January 2013 16:59, Netzblockierer <[email protected]>wrote:
> Hi Dan,
>
> as Codec2 is a digital codec, a drop-down of quality like on analog
> transmission seems hard to archieve. Normally, a good squelch-filter with
> amplifier would work better instead of trying to modulate it again as an
> analog narrowband signal.
>
Some of the bits have less of an effect on the signal quality than others.
Jimmy KG4SGP did some testing documented at
http://www.kg4sgp.com/codec2-bit-study.html on the 1400bps codec, and I've
done some separate testing on the 1200bps codec. Errors in some bits
affect the quality of the speech without affecting the intelligibility,
while errors particularly in the Wo_E bits cause major problems. One of
the things which at lot of people don't like about digital voice modes is
that they don't degrade gracefully like analogue modes such as SSB and it
would be nice to be able to take advantage of features like bit importance
to make trade offs which bring back some of that degradation ability.
Instead of 16QAM, I would prefer 4QAM, as it is much more robust than 16QAM
> in case of an angular phase shift. That's why 64QAM and 256QAM only work
> fine in a stable connection and 16QAM is standard in DVB-T (64QAM in
> DVB-C/S).
>
The current modem uses 14 x QPSK carriers, which I think has similar
robustness to 4QAM. If this was changed to 10 x QPSK + 2 x 16QAM with the
less important bits in the 16QAM carriers then we would cut the bandwidth
requirement from 1125Hz to 975Hz with (possibly) a minimal loss in quality
over the same path.
Regards,
Dan MD1CLV.
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