[GreenKeys] N3CRK stream stop bit length

[Harold Hallikainen]

I've been working on my DSP TU ( https://w6iwi.org/rtty/DspTU2/ ) and
testing with various internet streams. It is interesting that I get an
occasional error on the N3CRK 850 shift stream (all streams at
https://w6iwi.org/rtty/audio/ ).

Attached is a logic analyzer capture of RY sent by KE3BK and demodulated
by my TU (RttyRYTiming.jpg). This prints error free on my model 15. In
particular, note the short gap between characters on the top trace. This
is the 31 ms stop bit.

Also attached is the N3CRK 850 Hz stream (RttyN3crkTiming.jpg) from
http://n3crk.ddns.net:8000/850 . It is interesting to note that the
characters are sent in pairs with a 22 ms stop bit on the characters that
are run together.

I noticed the issue on the 850 Hz stream, but looking at the 170 Hz
stream, it looks the same (character pairs with 22 ms stop bit).

I believe it is standard to use 22 ms start and data bits with a 31 ms
stop bit. But, should a 22 ms stop bit work on a model 15? It SEEMS like
we'd have the following timing:

Start bit mark to space transition: 0 ms
Sample D0: 33 ms
Sample D1: 55 ms
Sample D2: 77 ms
Sample D3: 99 ms
Sample D4: 121 ms
Sample stop bit and stop: 143 ms (22 ms after middle of D4)

With a 22 ms stop bit, there would be 11 ms from the middle of the stop
bit to the beginning of the start bit.

With a 31 ms stop bit, there would be 20 ms from the stop bit sample (22
ms after the middle of D4) to the beginning of the start bit of the next
character.

It makes sense to me that the model 15 would sense the stop bit 22 ms
after sensing D4, but does it sense it later, causing a problem with a 22
ms stop bit?

THANKS!

Harold
https://w6iwi.org





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