[GreenKeys] Why and When 7.00 vs. 7.42 unit code?

[Duncan Brown]

Dave,

Nick's original query was about the different stop bits, not the data 
bits.  The "standard" stop bit being 1.42 times the data bit came about 
almost 100 years ago for compatibility between the Morkrum system and a 
competing Bell system.  The Bell equipment never went anywhere, but the 
7.42 code carried on (until the US military went to 7.00 c1965).

Although the size of the stop bit effects the WPM rate, it does not 
effect the baud rate or data bit size.

A machine running 67 wpm (50 baud) will have 20 ms data bits.  But the 
machine's selector mechanism only needs a few ms to make the M/S 
determination, so a 45.5 baud machine can read a 50 baud signal with 
little trouble. (The range will be reduced and may have to be reset for 
optimal copy.)  If you have copied the German WX station DDK on your 
45.5 baud machine, you are copying a 50 baud signal. (But don't try it 
on a "glass TTY" - they are optimized for only one baud rate at a time!)

Have fun,

Duncan


On 10-Mar-14 10:22, David Burns wrote:
> The difference between a machine running 7.00 and 7.42 is the length 
> of the stop-bit.  The data-bits are exactly the same length (22ms) on 
> both machines (hence they can interoperate), but the stop-bit is about 
> 1 1/2 times (1.42) a standard data-bit time span on the 7.42 machines.
>
> When I received a WU Model 100 being retired from service in 1966, I 
> was told it was a "66WPM machine" but would run OK with a "60WPM 
> machine".  True.
>
> It would seem a 31-millisecond stop-bit (on a 7.42 machine) affords a 
> bit more latitude in synchronizing machines on a sloppy circuit at the 
> expense of moving data at a slightly slower rate than a machine with a 
> 22-millisecond stop-but (7.0).
>
> For the gory details, see Table II in Irv Hoff's introduction:
>
> http://www.samhallas.co.uk/repository/telegraph/introduction_to_RTTY.pdf
>
> -Dave in Boston
>
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