[GreenKeys] [External] Re: M28 ASR Print Issue

[Jones, Douglas W]

From: John, W9DDD [w9ddd at tapr.org] -- Monday, June 3, 2024 6:20 PM

> My impression is that dot cycle is designed to test a circuit for bias.

I believe this is correct.  Note that first-generation TTY equipment was run using relays as repeaters along the line.  No individual current loop was longer than about 5 miles.  Relays every 5 miles amplified the signal and allowed continent spanning networks.

If you're using polar signalling, the time to shift from mark to space should naturally be the same as the time to shift from space to mark, but with classic telegraphy, it wasn't polar, mark was current flowing, space was no current.  Many relays have slightly different delays to open than they have to close.  If you have a string of these on a long line, a square wave going in one end of the line won't be square at the other end.

I'm not big on RTTY, but my guess is that many early modulator/demodulator sets (we now call the modems) had similar properties, slightly different delays between onset of a mark and onset of a space -- again, use of polar signalling (with one frequency for mark, a different frequency for space) would be likely to give equal delays, while use of CW signalling (tone versus no tone) would tend to skew.

Expecting to get a clean RYRY from a continuous square wave input seems more a matter of luck than a matter of intended behavior.

            Doug Jones