[GreenKeys] Teletype M14 - Marion, VA

[Duncan Brown]

I was just reading about the two-headed TD in TM 11-2222 (that Nick just 
put up on his site).  A mechanical TTY only needs about 5-10% of the bit 
time  to make  the mark/space decision. So on a good circuit, there is a 
lot of extra time per bit going to waste.  The range finder sets the 
time between the start pulse and when the first bit is read. On a 60wpm 
circuit, the first half of each bit was used for one channel and the 
second half of the bit for the second channel. This gave a throughput of 
120wpm on the circuit!

What got me thinking of the XD tape readers was that Steve's M14 with 
the external range display seemed to have two separate sections of range 
display, like you might want if you needed switch between two different 
range settings for two different circuits.

Steve -- What does the external range finder plate look like???

-...-

I didn't say that the range finder "changed "  the speed,  just that it 
could allow the receiving of different speeds.  And the signal does not 
need to be "CLEAN".  I have often copied the German WX station DDK 
(10.100 Mc) on 60 wpm machines. The range setting is less, but the copy 
is as good as if it were a 67wpm machine. (The range setting is going to 
have little or no effect on QRM, QSB, etc)

Remember that the M14 & M15 were designed to operate on long, DC, 
telegraph lines and they had to be able to put up with all the 
distortion to the pulse shapes caused by these line. With the advent of 
the wireline & radio modems, most TTYs had DC circuits of only a few 
feet (maybe a 100ft or so in a big comm center). So the distortion 
caused by long DC lines was no longer a problem.

Have fun,

Duncan
K2OEQ

On 29-Mar-15 17:26, COURYHOUSE at aol.com wrote:
> Jim! - thanks  -that is a pretty interesting  system!  had not  heard 
> of   doing this before.
> Good  way to get double use out of a circuit!
> Ed#
> In a message dated 3/29/2015 1:04:14 P.M. US Mountain Standard Time, 
> jhhaynes at earthlink.net writes:
>
>     What Duncan is talking about is a WW-II expedient to increase traffic
>     capacity.  A two-headed XD was made with the usual start and stop
>     distributor segments and the other five divided in two. One read
>     head was connected the the early segments and the other to the late
>     segments.  For receiving you would adjust the range finder on a
>     printer
>     so that it would sample in the early half of the pulse time or in the
>     late half.  Thus you have two-channel multiplex, assuming the circuit
>     was good enough to deliver sufficiently distortion-free signals
>     to the printers.
>



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