Selcal HF Technical info ????

Date:         03 Oct 99 18:18:26 
From:         airnig@bigfoot.com (N.Smith)
Organization: Customer of Planet Online
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Does anyone know the specs for the multi tone selcal signal used on
HF, and the way it is composed into two tone bursts from a 4 letter
code ???

Many thanks
Nigel Smith
airnig@bigfoot.com