[RTTY] Jitter

[Hank Garretson]

Good Evening David,

Sorry I'm so slow to respond. Had some unfortunate stuff going on.

I have replaced the Edgeport pseudo COM port I was using for MMTTY FSK with
a real PCIe serial port.

Stop-bit length is now 32 ms, right where it should be.

Thank you for setting me straight.

TinyFSK is still on my to-do list.

Diddle Exuberantly,

Hank, W6SX


On Tue, Feb 27, 2018 at 1:13 AM, David G3YYD <g3yyd2 at btinternet.com> wrote:

> 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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