Hi Wolf,
I agree about the threshold effect. The problem we encounter in cave radio is
that ordinary SSB on 87 kHz USB (the commonly used frequency in Europe) is not
always robust in the presence of strong interference such as LORAN. Even if
digital methods turn out to be no better (or even worse) than analogue SSB, it
would be worth having some idea of how much. I have received some suggestions
about an adaptive LORAN-canceller, but I do not know anybody who has actually
tried it.
An alternative way round the LORAN problem is to use a different frequency, and
John Hey has produced a Heyphone that runs on 38.4 kHz and works well. The
problem is interworking with existing equipment as there is a substantial
installed user base of 87 kHz sets. When the frequency-agile System Nicolas III
is readily available this may well be the solution but it is always worth
investigating other approaches – such as the VLF system on 3.3 kHz USB.
Unfortunately, for medical reasons I can no longer do serious caving, and in
this part of France there does not seem to be any interest in improving cave
communications. Local cavers, understandably, are quite content for SSF to
bring their radios when there is an emergency.
73,
John
On 3 Dec 2013, at 18:47CET, wolf_dl4yhf <[email protected]> wrote:
> 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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