[RTTY] Jitter

[David G3YYD]

Hank

The 54mS (milliseconds) you see is not timing Jitter that is the stop bit
length. But that stop length is miles too long. The best stop bit length is
33mS. Every character you are sending takes 186mS while it should be taking
165mS for a standard RTTY character of 1 start bit + 5 data bits + 1.5 stop
bits.

If you sent a standard RTTY character that would save 13% of the
transmission time. 

Jitter on the bit timing would have to be extremely poor to show up on the
2Tone stop bit time measure which is what this figure is showing. 2Tone
averages the last 4 characters to derive this figure. Some sending systems
have really poor stop bit length timing and then it will show up as a
considerable variation in this number. Some propagation induced variation is
to be expected at times.

If there is a setting for stop bit it should be set to 1.5. MMTTY has a
setting for this. 2Tone is already set for 1.5 stop bit length and has no
operator adjustment.

73 David G3YYD  

-----Original Message-----
From: RTTY [mailto:rtty-bounces at contesting.com] On Behalf Of Hank Garretson
Sent: 27 February 2018 01:13
To: David G3YYD; ed at w0yk.com; RTTY Reflector
Subject: Re: [RTTY] Jitter

G3YYD said:

The TX bandwidth for 45.45 baud FSK is dependent on the radio rather than
> the system used for keying within reason. Obviously if the keyer is 
> producing lots of short transients then it will widen the bandwidth 
> unless the FSK is well filtered within the radio.
>

That is what I thought, but wanted expert confirmation. Thank you David and
Ed.


> As for timing jitter, why keep it when it can be avoided? This will 
> save having to repeat an exchange when it would have been copied first 
> time if the TX was jitter free.
>
l
My excuse is that even with a wire antenna at only 46-feet high I do pretty
well and get very few repeat requests. Plus I have a lot of more pressing
projects on my plate.

Another big factor is that my second-story shack is located directly under
my antenna. One 40-meter inverted-V leg end is only four feet from the shack
outside wall. Needless to say I have huge RF-in-the-shack issues. So,
addressing jitter by using AFSK is problematic. Every extra cable adds
another level of RFI susceptibility.

2Tone says my jitter is 54 ms.

W7AY says:

If the signal starts with a very good SNR (your neighbor's RTTY signal),
> then your peak-to-peak jitter can be 11 millisecond before you see 
> degradation for a 45.45 baud RTTY signal.
>

Can the derogation be quantified for my 54 ms?

If you are using 75 baud, the peak-to-peak jitter of 6.5 milliseconds will
> cause errors.  This is why most people consider bit-banged FSK from a 
> computer to be unusable for 75 baud RTTY.
>

Not unusable here. I work plenty of stations in the RSGB 75-baud contest.
Very few repeat requests.

In any case, I really should go to TinyFSK.

Diddle Exuberantly,

Hank, W6SX
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