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From: John Rabson <john.rabson07@gmail.com>
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Date: Tue, 3 Dec 2013 19:29:01 +0100
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Subject: Re: LF: RE: Article on VLF
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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 =96 =
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 <dl4yhf@freenet.de> wrote:

> Hi John,
>=20
> 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...
>=20
> 73,
>  Wolf .
>=20
> Am 03.12.2013 14:29, schrieb John Rabson:
>> I think I understand the term =93dispersion=94 in the optical context =
and how there  could be a similar mechanism at VLF. My main reasons for =
posting are:
>>=20
>> 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 =96 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.
>>=20
>> John F5VLF
>>=20
>> On 3 Dec 2013, at 10:16CET, hvanesce <hvanesce@comcast.net> wrote:
>>=20
>>> It might be informative to consider exploitation and mitigation of
>>> dispersion in approaches to compression for VLF links.
>>>=20
>>> Jim AA5BW
>>>=20
>=20
>=20


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