[RTTY] TinyFSK modem for 45 and 75 baud

Kok Chen chen at mac.com
Sat Oct 12 16:56:57 EDT 2013


On Oct 12, 2013, at 10:28 AM, Bill Turner wrote:

> Why so many I have no idea. Can someone explain this? There
> must be some tradeoff between them, but what is it?

If all else were equal, and with proper demodulating software, a 1.5 stop bit transmission is the easiest one to recover from after a loss of frame sync (pretty intuitive too if you just look visually on a 'scope).

Two stop bits come next in terms of recovery time, and the one stop bit case takes the longest to recover after a loss of frame sync.

In terms of ISI errors, stop bits that are precisely 1 (22 ms), 1.5 (33 ms) and 2 (44 ms) all provide zero ISI as long as the demodulation filter is the beta=1 Raised Cosine,  see:

http://en.wikipedia.org/wiki/Raised-cosine_filter 

or filters that are derived from the Beta=1 Raised Cosine, see Figure 8 in:

http://w7ay.net/site/Technical/Extended%20Nyquist%20Filters/index.html

Notice that the impulse response in the above cases all go through zero at T=1, 1.5 and 2.    I.e., these stop bit durations will cause no harm to the following start bit.

T=1.5 is quite bad when using anything but the beta=1 Raised Cosine (see the curves that are not green in the Wiki page), but I do not know of any existing software modem that does not use the beta=1 Raised Cosine or its derivative.  (I have no knowledge of what MMTTY uses as filters, however.)

Originally, 2Tone had used a beta of around 0.25, but after some comparisons, David has since changed to using the 2nd order Raised Cosine from my article.

The worst stop bit period in terms of ISI (when the stop bit's ISI stomps on the following start bit at the receiving end) is a period that is close to halfway between 22ms and 33 ms, or halfway between 33ms and 44 ms.  Avoid using stop bit periods that are near to 28 ms or near to 38 ms.

ISI from the stop bit is not a small problem since it causes maximal noise to the start bit that follows it.  If you lose that start bit, you lose frame sync.  And when you lose frame sync, it causes 3 times greater error rate than random bit errors.

73
Chen, W7AY



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