Message-ID: <200208051611.g75GBw900788@block.iamanidiot.com>
To: rsgb_lf_group@blacksheep.org
Subject: Re: LF: Confused over Jason coding
In-reply-to: Your message of "Mon, 05 Aug 2002 16:22:16 BST."             <7D653C9C42F5D411A27C00508BF8803DCB9B63@mail.dstl.gov.uk>
Date: Mon, 05 Aug 2002 12:11:58 -0400
From: "David Willmore" <davidwillmore@iamanidiot.com>
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> Must have missed something, but in looking at the coding scheme for the
> frequency shifts in Jason (as a preliminary to writing some PIC code to
> drive a DDS chip for a beacon)  I came across a snag.
> 
> The 64 characters are coded 4 bits at a time (allowing for the MSB
> identifier) as 16 frequency shifts.  OK so far, 16 shifts = 4 bits.  But,
> what happens if it is impossible to get more than 8 bits of shift?   For
> instance, say the frequency from the last character ended up at exactly
> midway - frequency 8.  Then it is impossible to code any value greater than
> 8 without going outside the defined frequency limits.
> 
> I'm sure this was all explained last year when the mode was being developed
> but can't find any reference to it now.  And the Tech Notes that come with
> V0.94 don't cover this aspect.
> 
> A thought has just occurred while writing this - does the direction of
> frequency shift matter, which can then be taken modulo 16, giving 16
> possible shifts from 17 tones?  Which means the sideband in use is
> sensitive.  Have I answered my own question ?

You 'wrap' around at the top of the 'band'.  So, all frequencies are available
at all times.  So, if you're at (zero based) tone 8 and want to shift +16, you
will end up on tone 8+16 mod 17 or tone 7.  The 'next tone' formula is just
current tone + desired shift mod 17.  Shifts are always thought of as 'positive',
but the band is just a little mobius strip. :)

Cheers,
David N0YMV