[RTTY] What 2Tone measures

[Kok Chen]

On Oct 14, 2013, at 4:57 PM, Jim W7RY wrote:

> With all of this flying around, I was wondering if we are actually taking/making the "measurements" with accurate "test equipment".

Keep in mind that there are multiple issues with FSK timing -- one is with Stop bits that are not 22ms, 33ms or 44ms.  For that, the 2Tone stop bit display reports a reasonably accurate stop bit duration as long as the data and start bits average out to be 22ms each.  

A different timing issue has to do with jitter (both data bit jitter and stop bit jitter).  Since 2Tone aggregates the start and data bits, bit jitter (such as from EXTFSK) will not show up in the 2Tone "stop bit" display.

To see jitter, you need a scope that is synch'ed to the character frame, or some equivalent instrumentation in the software modem to see the data "eye" -- see the last three figures here for one example of such instrumentation:

http://www.w7ay.net/site/Technical/RTTY%20Sidebands/sidebands.html

When there is jitter, the zero crossings will move around from one character frame to the next.

(Notice that if the transmission uses 45.0 baud instead of 45.45 baud, 2Tone will "measure" a stop bit that is stretched by a little more than 1 ms, when it really has stretched only by about a quarter to half of a millisecond).

73
Chen, W7AY