I think you need to consider why anyone would want codec2 on VHF/UHF. We
have an uphill battle there. It seems to me that a 4800 Baud modem would
not provide much, if any, advantage over D-STAR. Why would the typical
Amateur Radio buyer not just stay with D-STAR?
A 2400 modem might be able to offer much narrower bandwidth that D-STAR
and better range. You might, indeed, consider a 1400 modem.
Does adding 1/3 FEC /really /help our VHF/UHF channel, or are we just
/assuming/ that it helps, versus just interpolating or having mild
errors (not squeals, our codec doesn't squeal, but the wrong sound here
or there that your ear and brain correct for). Can you show the effect
with a channel simulator or real-world tests?
Widening the bandwidth to include more error correction also reduces the
available power for the real data and increases the signal-to-noise. Is
it really the case that the FEC would help us through fades? Or are the
fades too long, and too deep, for any amount of FEC to help us without
introducing seconds of latency to the speech. Might be fine for paging
and broadcasting, but not what we do.
Thanks
Bruce
On 05/20/2012 02:04 PM, Gullik Webjörn wrote:
Granted, a 2400 baud modem would optimise the RF side.
However, I would cast my vote for 4800, since it could be
possible to turn this into a "2-slotted" data protocol,
and it would make "subrepeaters" possible, i.e. retransmitting
at a frame in the interpacket gap between voice frames
(at somewhat lower than 2400 bd).
4800 baud would easily fit available radio.
There could be many different implementations as time goes by.
SM6FBD /Gullik
On Sun, 2012-05-20 at 16:24 +0200, Kristoff Bonne wrote:
Hi,
We are looking into the possibility to create an additional modem for
codec2, but for VHF/UHF frequencies.
As a first "proof-of-concept", this would be to convert my gmsk modem to
a new format to carry codec2 voice. This would have the advantage that
the hardware requirement to run this would be minimal.
Sofar, I see two options:
- A 2400 bps modem:
This would be quite sufficient to contain codec2 voice (at 1400 bps),
additional syncronication patterns + some additional data and even an
options 2/3 FEC for the voice part.
- A 4800 bps modem:
This would then give even more headroom, e.g. tu use a 1/3 FEC for voice
(which would make it much more robust) and still have headroom for
syncronisation patterns and some additional data
The 2400 bps modem would have the advantage of the lower bandwidth
(better S/N ratio, better suited for e.g. bands with limited bandwidth:
10 meter, 4 meter).
The 4800 bps modem would have the advantage of the better FEC.
Does anybody have a comments on this?
73
Kristoff - ON1ARF
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