[GreenKeys] Mystery RTTY Project

[Jones, Douglas W]

From: Harold Hallikainen via GreenKeys [greenkeys at mailman.qth.net] -- Saturday, July 8, 2023 5:49 PM

> So, it seems like we'd get framing errors on a square wave

Mechanical asynch receivers like TTYs don't detect framing errors, nor do they much care about them.

Here's what I would have expected.
1 --- idle line
0 - start
1 - D0
0 - D1
1 - D2
0 - D3
1 - D4
0 - stop (framing error, but note*
1 --- idle line, note **
0 - start
1 - D0
0 - D1
1 - D2
0 - D3
1 - D4
0 - stop
1 --- idle line, note **
0 - start

Note*: yes, a framing error, but the TTY doesn't care, it's simply insensitive to the state of the line for about 31 ms
Note**: for 9 ms, the line is ignored as part of the stop bit, the remaining 13 ms of this 1 bit are seen as an idle line.

The net result?  I wouldn't have expected RYRYRY, I'd have expected RRRRRR from just a square wave.

Unless what is happening is the following:
1 --- idle line
0 - start
1 - D0
0 - D1
1 - D2
0 - D3
1 - D4
0 - stop also sensed as a start bit for the next character ***
1 - note ****
0 - D0
1 - D1
0 - D2
1 - D3
0 - D4
1 - stop (not a framing error)
0 - start

Note***: framing error, clutch remains engaged, shaft continues to spin.
Note****: 0-1 transition was 9 ms before expected end of stop bit, too little time for it to matter.

This would give RYRYRY, but it would probably be rather sensitive to the rangefinder setting.  I'd expect the RYRYRY to sound a bit syncopated, that is, more like RY RY RY RY (not printing spaces, but a little off in the timing).

          Doug Jones
          jones at cs.uiowa.edu