[RTTY] High Speed RTTY again

[Kok Chen]

On Jan 29, 2010, at 1/29    5:33 PM, k3mm at verizon.net wrote:

> I can't understand the logic of running ASCII vs. BAUDOT.  That's 2  
> extra bits per character without considering shifts.


With a contest type exchange, say something like:

KF3P 599 03 AA6TY

and with USOS turned on, the exchange becomes this in Baudot:

KF<FIGS>3<LTRS>P<space><FIGS>599<space><FIGS>03<space>AA<FIGS>6<LTRS>TY

So, starting with a string of 17 characters, we had to add 6 (!)  
Baudot shift characters.

Lets say both of them use 2 stop bits, the 7-bit ASCII idiot has to  
send 17*10 = 170 bits, while the Baudot genius sends 23*8 = 184 bits.

Gasp!!!

Even if he sticks to using just 45.45 baud, the ASCII guy takes less  
time to transmit a moderately typical contest exchange!  (OK, I am  
only feigning my surprise; having done this exercise before, I knew  
the result ahead of time :-)

Not to speak of LTRS/FIGS errors where a single error can cause  
multiple character errors in the Baudot case.

BTW, there is another property of async communications (missed start  
bits) that causes fewer characters to win out.  I.e., like LTRS and  
FIGS characters, a misidentified start bit causes more errors than  
just a single character.

> Running higher speed and extra bits also brings increased chances  
> for errors on weak signals.

As shown above, adding extra bits to a character don't directly  
translate to more chances of errors, because you are comparing it  
against the case where you are adding more characters, and not just  
bits.

> If we are going to do that, we'd be much better off running PSK at  
> double speed, which isnt a bad idea anyway...

Although PSK31 is better than 45.45 baud Baudot or ASCII for quiet  
propagation conditions, the FSK modes survive better through selective  
fading and flutter.  I.e., as conditions become poor, RTTY beats out  
PSK31.

I remember one time pulling my hair out when a JA was operating PY0FT,  
trying to copy his PSK31 signal through equatorial flutter; a few  
minutes later, he QSYed to RTTY on the same band and his print was  
pristine for an easy QSO.

Character for character, 7-bit ASCII is longer than 5-bit Baudot.   
However, for contest type exchanges, as I illustrated above, at the  
same baud rate, ASCII beats out Baudot in terms of speed and therefore  
also have fewer bits for noise to hit.

For long ragchews, as long as you don't send too many punctuations, I  
think Baudot is a little faster than ASCII.  But what you might lose  
in speed, you also make up with being able to use lower case instead  
of shouting all the time.

In short, if you want RTTY-like sensitivity and noise characteristics,  
just use 45.45 baud ASCII radioteletype (or 50 baud, if you want it to  
go just a little faster).  45.45 baud ASCII will be about the same  
effective speed (and even a little faster, 7.5% in the above example)  
than Baudot RTTY.

You don't need to go to crank it up to 110 baud unless you want to  
race with the PSK125 speed demons.

73
Chen, W7AY