Re: LF: Re: WSPR Dx

[Andy Talbot <andy.g4jnt@googlemail.com>]

Not quite as low a coincidence as you'd think.   IF, and this is the main thing, a chunk on interference has to get past the convolutional / Viterbi  data decoder.  If your interference has sidebands, then there is a probability all its sidebands will wobble together.   If this wobble is interpreted as valid FSK that passes the decoder test, then all will result in the same site of bits being decoded.

 

Now, the source encoding and removal of all input redundancy means than just about any random set of 72 bits that get generated will end up with a valid looking callsign, locator and a power level.   And of course, all sidebands will give the same result if they've wobbled together.

 

There are probably far more decodes of the interference that get rejected, its just those that the laws of probability say must happen occasionally, with their sidebands, that generate the multiple hits.

 

Up to 650 Watts now the rain has evaporated from the house roof and the ground is a bit drier.    And not a wisp of smoke or a flame in sight

 

Andy
www.g4jnt.com

On 5 October 2010 00:08, James Moritz <james.moritz@btopenworld.com> wrote:

Dear Andy, LF Group,

Well, this is what 137.5k +/- 100Hz has looked like for the past 22 hours or so - quite a bit of impulsive and other noise here!

I am seeing quite a lot of the peculiar "double false decodes", i.e. the same ficticious callsign decoded twice during a single transmission period, at different frequencies and with different delta-T. This has happened about 50 times during the period shown on the screen shot, and makes up a large majority of the total number of false decodes. I imagine the probability of this happening actually as a result of random coincidence must be fabulously tiny - so perhaps some artefact of the decoding algorithm, or a software bug?

Cheers, Jim Moritz
73 de M0BMU

----- Original Message ----- From: "Andy Talbot" <andy.g4jnt@googlemail.com>
To: <rsgb_lf_group@blacksheep.org>
Sent: Monday, October 04, 2010 11:09 PM
Subject: LF: WSPR Dx

Jim -

You're decoding some wonderful DX on WSPR, including an M6 station at 19Mm.
Impulsive interference or carriers?

Andy
www.g4jnt.com