Hi John,
The problem with most (but not all) digital voice systems is the RX
receives 'all or nothing'. If the SNR drops below a certain threshold,
most of them will not give any copy at all.. much in contrast to the
good old analog modes, which -depending on the operator's skill- still
allow communications. A system which automatically adjusts the
compression / quality to the path conditions would only work on a
two-way, at least half-duplex link which may be impractical for the
'portable' underground station...
73,
Wolf .
Am 03.12.2013 14:29, schrieb John Rabson:
I think I understand the term “dispersion” in the optical context and how there
could be a similar mechanism at VLF. My main reasons for posting are:
1) an interest is reduced-bandwidth speech systems at cave radio frequencies
(typically 87 kHz)
2) the resilience of such systems when used in the presence of impulsive interference
such as LORAN – a problem which is widespread in north-west Europe
3) there are also systems which work at VLF and are intended for subterranean
location. A typical carrier frequency appears to be of the order of 3 kHz, and
I wonder if a simple SSB system on 3.3 kHz USB system might give useful range
(a few hundred metres) for voice communication.
4) Some Australian amateurs have been experimenting with digital voice systems
(codec2?) on HF and it seems to be possible this approach might also be useful.
John F5VLF
On 3 Dec 2013, at 10:16CET, hvanesce <[email protected]> wrote:
It might be informative to consider exploitation and mitigation of
dispersion in approaches to compression for VLF links.
Jim AA5BW
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