VK3ABK > 9K6 16.06.07 06:51l 37 Lines 1612 Bytes #999 (0) @ WW
BID : 58418_VK3HEG
Read: DG8NGN GUEST DL3MHT DC9RD DD1OV DG4YDF DL7NDR
Subj: High Speed Packet
Sent: 070616/0521Z @:VK3HEG.#WEV.VIC.AUS.OC #:58418 [Ballarat] $:58418_VK3HEG
To : 9K6@WW
Using high-speed packet used to be a step-up some years ago in Australia,
but now it seems to be only used for satellite packet; I made up the G3RUH
modem to use 9k6 data with a PK-88 TNC. Some new interest may be on the way
as my sysop has plans for 9k6 packet, although a 2k2 node may be a problem!
Years ago, Nyquist, and Shannon, determind the maximum bit rates that could
be supported on a given frequency/channel. Nyquist showed that if you want
to send data on a carrier, then the carrier frequency must be at most half
the data rate. This can be shown by sketching a few sine waves, and drawing
verticals representing data at intervals along the time axis. You can then
visualise that sufficient information will be sent and eventually all of the
full sine wave will be modulated.
Shannon gave the relationship between circuit capacity, bandwidth and signal
to noise ratio....C = W log(base 2) of S/N. This may not worry us on packet,
but it is important for our picture transmission and reception of weather
satellite maps, slow scan, and of course fast scan TV.
The usual methods for using Amateur type radios for high speed packet, are
aimed at by-passing the audio stages by applying the modulation to the
varactor and taking data from the discriminator in FM mode. As you will see
from the sine-wave sketch, a harmonically related data rate may not be timed
to cover the whole sine-wave and some errors can occur.
I would be interested in news of high speed packet being used for other than
73. Dick. VK3ABK.
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