[RTTY] PSK31 is faster (Was FD RTTY Question)

[Kok Chen]

On Jun 27, 2012, at 9:39 PM, Joe Subich, W4TV wrote:

> Does cocoaModem provide a way to set the data rate for ASCII down to
> 45.45 baud?

cocoaModem has duplicate wideband 2-receiver RTTY interfaces.  One uses Baudot encoding and the other uses ASCII encodings.  You can select either interface dynamically.

The Baudot interface is restricted to 5 bits data, with stop bits selectable between 1, 1.5 or 2 bits.

The ASCII interface can use either 7 or 8 data bits (7 bits is more "standard," the 8 bit encoding uses the standard Extended ASCII).  With stop bits again selectable as 1, 1.5 or 2 bits.  

The Baud rate, Mark/Space shifts are text fields for each (Baudot and ASCII) interface and you can use any reasonable number, such as 45.45 baud or 110 baud. I.e., you can run Baudot RTTY at 110 baud and ASCII RTTY at 45.45 baud if you wanted to.  Or 23 baud ASCII, if you like.

I do not used fldigi, but if memory serves, it has only has one RTTY interface.  However, the number of data bits is selectable and when you change the data bits to 7 or 8, it will switch to using ASCII (ITA-5) or extended ASCII encoding.  I do not know if you can choose 45.45 baud with ASCII, but I suspect that you can.

MultiPSK can also choose between 7- or 8-bit ASCII but with a fixed 110 baud rate.

I believe someone has told me at one time that MMTTY can also choose 7 bits of data.  However, MMTTY keeps using Baudot encoding (together with FIGS and LTRS) by stuffing extra zeros into the most significant 2 bits (gulp).  This is not first hand knowledge, so someone else will need to confirm whether MMTTY can run ASCII as is.

> Why 110 baud?  There is a significant bandwidth and s/n penalty in
> the higher speed even if the shift were held constant at 170 Hz.

Short answer: Model 33 and Model 35 Teletype machines. 

Back in them days, the 14, 19 and 32 can only do 45.45 baud Baudot (unless you swap out the gears), and the 33 and the 35 can only do 110 baud ASCII.  There was a popular model 28, but I draw a blank on what baud rate it handles.  If I had to guess it would be 110 baud since early DEC computers used them (I don't think any modern computer used Baudot).

When TNCs implemented RTTY and ASCII, they defaulted to what the teletype machines used; thus 45.45 Baudot, 110 baud ASCII.  Today, with software modems, your baud rate is only limited by the 300 baud upper limit that the FCC limits a US ham to (below 10m).  I don't think there has been a lower baud rate limit, so you should be able to use 45.45 baud ASCII as long as your software modem allows it.

73
Chen, W7AY