G'day Andy,

>I think VK2ZTO has already carried the multi-tone idea to one of it's
logical conclusions, with seperate tones for >each different
alphabetical characters - see 'FDK' on his web pages
http://www.qsl.net/vk2zto.

The first version wasPiccolo as designed in the 1950s by the UK Foreign
Office for diplomatic comms.  32 tones corresponding to the 32 states of
the 5 bit RTTY code transmitted at 100ms per tone.  Various amateur
reincarnations appeared over the years, with the number of tones ranging
from six to 32 with assorted coding schemes.   A modern 12 tone varient
is often heard on 14MHz and has been around for the last year.

I see why you think that FDK is a variant of Piccolo, but to me it is
completely different.   The characters are encoded not in the tones, but the
difference between the two tones, so it is not a coding scheme in the sense
mentioned above.    This allows some further enhancement which is simply not
applicable to the other method, for example by center frequency tracking,
the software can better pick the tone pairs at the limit of S/N and also
truncate the scan band.    Also I can't think of a another way of getting a
better bit/baud ratio.

BTW, the only place I have seen this used is by NASA for deep-space craft
health status information.    They apparently use two or three tone pairs to
indicate various health states of the craft.   The difference technique
allows them to de-doppler shift a very, very weak signal.   I was told about
this about 6 months after I proposed my FDK scheme which is coming close to
three years old now (and I am stilling pushing it onto a reluctant audience
[-).

73s Steve Olney (VK2ZTO/AXSO - QF56IK : Lat -33 34 07, Long +150 44 40)
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HomePage URLs:
http://www.qsl.net/vk2zto
http://www.zeta.org.au/~ollaneg

Containing:-
ULF, ELF, VLF & LF Experimentation
InfraSonic Experimentation
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