[RTTY] RTTY Screwed by FCC?

[Kok Chen]

On Oct 15, 2006, at 9:33 AM, Bill Turner wrote:

> If CW is digital, it then follows that dit-dah and dit-dit have the
> same numeric value and are therefore the same character.

 From the channel encoding sense, amateur "CW" is a one-bit code (on  
or off).

It is source encoding ("bit timing") that maps these two states (not  
two bits, but one bit) into more states: viz, dit element, dash  
element, inter-element spacing, inter-character spacing.

A further stage of source encoding maps a letter A into a dit  
followed by and inter-element spacing followed by a dash, and  
followed by an inter-character spacing.

When you send GG instead of "going" it is yet another level of source  
encoding.

The "CW modem" that is used to match Morse to the HF channel is  
simply one bit on-off keying of a single carrier.

With RTTY, the modem is FSK and it has two non-overlapping carriers.   
Baudot is just a source encoder (so is ASCII).  And when you type 73  
for "best wishes" (or "73's" for "best wisheses") that is yet another  
level of source encoding.

You can pass Baudot as a single on-off keyed carrier (as the early  
amateurs did), but the use of two widely spaced carriers is a better  
channel encoder ("modem") for the HF channel because FSK can counter  
selective fading (just look at the behavior of the amplitudes of your  
crossed ellipse display).  Many software modems (and hardware modems  
like the ST-8000) can be operated as Mark-only or Space-only and you  
can see how much the use of two carriers bring to the table.

73
Chen, W7AY