Hello Kevin,
I took part in some PSK31 tests (receiving only) with G3YXM and I was not
very impressed by PSK31. At levels were it gave a more or less usable copy
the signal was strong enough to be copied in normal CW.
I cannot tell you about BPSK (COHERENT/AFRICA sofware) but the statement
that it by far superior to QRSS or DFCW is hard to believe, for some sipmle
physical facts :
1. due to its nature BPSK has an advantage of 6dB over a 'normal' signal at
the same baudrate
2. the SNR (signal to noise ratio) is, regardless of what mode you are
using, dependent on the bandwidth of the signal. Reducing the bandwidth to
half will improve SNR by 3dB.
From what I understand a baudrate of 10Hz is used for BPSK on LF (what
means a minimal bandwidth of 10Hz), so that means that it would have the
same efficieny as an other signal at 2.5Hz (what equals a dotlenght of 0.4
seconds or a speed of 3WPM).
The QRSS standard (in Europe) is a dotlength of 3 seconds (= bandwidth of
0.333Hz) and using DFCW a dotlength of 10 seconds (= 0.1Hz bandwidth) can
be used (at the same 'QSO speed'). The 'sliding technique' used by I2PHD in
his program SPECTRAN allows the use of the minimal bandwidth.
So even taking the 6dB advantage of BPSK into account QRSS / DFCW have a 9
to 14dB better SNR, because the can be copied at a much narrower bandwidth .
Please do not misunderstand me, I believe that BPSK is probably the best
mode in the WPM (words per minute) datatransfer range. But I do not think
that it can compete with modes with a MPW (minutes per word) datatransfer
that can be copied at a 30 to 100 times narrower bandwidth.
Besides that : while QRSS /DFCW can be used with a class E amp. (efficiency
close to 100%), BPSK needs a linear amp (efficiency 70%), a 1.5dB
disadvantage for BPSK.
73, Rik ON7YD
At 09:48 1/06/00 +0100, you wrote:
There's a lot of discussion going on at the moment on the US "LowFER"
mailing list about the relative merits of slow CW versus BPSK (in this case
relating to the COHERENT/AFRICA software by Bill de Carle, VE2IQ). The
general consensus of opinion seems to be that if you are going to use
"machine" modes, BPSK has considerable superiority over any form of slow CW
for the kind of very weak signals often encountered on the LF bands.
Estimates of the effective improvement range from 6dB to 23dB depending on
what factors are taken into account!
At the risk of causing some heated discussion, I'm curious to know the views
of members of this list - particularly in relation to future transatlantic
attempts.
Also, does anyone have an idea of the relative usage of slow CW (QRSs and
related FSK modes) versus modes like PSK31/PSK08 etc, in Europe? My
impression is that various forms of slow CW are still favoured by most
people (and perhaps rightly so, in terms of picking the best/most convenient
mode for what you are trying to do), but I haven't been able to monitor the
LF bands for some time so I don't really know what the current situation is
like.
For anyone who isn't subscribed to the abovementioned list and wishes to
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Regards
Kevin, G1HDQ
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