[RTTY] K or carriage return

[Al Kozakiewicz]

Yeah, point taken.  I was thinking pure symbol rate and not actual information.

And given that the last model 15 or KSR33 departed the scene decades ago, why aren't we using ASCII, anyway?

Al
AB2ZY

________________________________________
From: rtty-bounces at contesting.com [rtty-bounces at contesting.com] On Behalf Of Kok Chen [chen at mac.com]
Sent: Wednesday, February 16, 2011 2:37 PM
To: RTTY contest group
Subject: Re: [RTTY] K or carriage return

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

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