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Date: Tue, 03 Dec 2013 18:47:18 +0100
From: wolf_dl4yhf <dl4yhf@freenet.de>
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Subject: Re: LF: RE: Article on VLF
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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 <hvanesce@comcast.net> wrote:
>
>> It might be informative to consider exploitation and mitigation of
>> dispersion in approaches to compression for VLF links.
>>
>> Jim AA5BW
>>