On Feb 16, 2011, at 10:45 AM, Al Kozakiewicz wrote:
> 1. PSK31 is 31.25 baud and RTTY (in ham applications) is 45.45 baud. RTTY
> is thus about 45% faster then PSK31.
The latter statement is not quite true.
RTTY uses 7 to 8 bits to transmit a character, if there is no LTRS/FIGS
transitions. The "7 to 8 bits" is because some people use 1 stop bit, some use
1.5, some use 2 and some mechanical teletypewriters used 1.4.
(Oops, I can just see it now, the bit misers are going to start a "use 1 stop
bit" mantra to shave an extra quarter second per exchange, and the rest of us
can continue to get a good laugh :-).
A character at a LTRS/FIGS transition takes 14 bits to 16 bits per character to
send.
There are lots of LTRS/FIGS transition in a contest exchange.
Unless you are RAEM, there are at least two LTRS/FIGS transition just to send
your call sign. For my call sign, instead of sending 4 Baudot symbols, I need
to send 6 Baudot symbols -- a 50% increase. People who have call signs like
7L4IOU will be transmitting 10 Baudot symbols instead of 6 Baudot symbols for
an even larger percentage overhead!
PSK31 on average uses 6.33 bits per character for English. It is even less if
you only use lower case (veteran PSK contesters do that). You can see my
analysis of DominoEX and PSK31 character rates here:
http://homepage.mac.com/chen/Technical/DominoEX/Measurements/index.html
Having little interest in contesting, I have not done an alphabet frequency
analysis of the PSK31 Varicode for the typical contest exchanges.
Indeed, as we had discussed about a year ago, 45.45 baud ASCII produces shorter
exchanges for many contest exchanges than 45.45 baud Baudot even though ASCII
is a 7 bit code and Baudot is a 5 bit code (and ASCII don't have the 123 QWE
problem, so potentially fewer repeats).
The FIGS/LTRS scheme of M. Baudot (to reduce the code for a character from 6
bits to 5 bits) is not really a good compromise; the saving in percentage is so
little when you add on the start and stop bits. Fortunately, we don't see that
false economy repeated in subsequent amateur digital modes.
73
Chen, W7AY
_______________________________________________
RTTY mailing list
RTTY@contesting.com
http://lists.contesting.com/mailman/listinfo/rtty
|