[RTTY] Jitter

[David G3YYD]

Hank

Sorry for the delay have been operating in the Commonwealth Contest. This is
the oldest amateur contest still going and is of course CW only. It was
originally called the British Empire Radio Union contest, but that was the
pre-WW2 political situation. Today it's the Commonwealth Contest but we all
still call CQ BERU.

Glad you now have the correct stop length. 2Tone does not show timing jitter
due to the averaging over 4 character times. But it does show the stop bit
length accurately. 

If anyone does see 2Tone stop length jitter then whatever you are using
needs to be replaced with something better. Simplest and cheapest is audio
2Tone DOOK being the narrowest and does not sacrifice any decode performance
at the far end. My testing says it is about 0.4dB better than AFSK/FSK but
that difference is not noticeable.

See you in the BARTG contest next weekend? Solar/geomagnetic forecasts do
not look too good but we will find out. Best bands here will be 20/40. Will
be lucky to make many Qs on 10 if any.

73 David G3YYD/M7T

-----Original Message-----
From: RTTY [mailto:rtty-bounces at contesting.com] On Behalf Of Hank Garretson
Sent: 09 March 2018 21:20
To: David G3YYD
Cc: Tim Gennett; RTTY Reflector
Subject: Re: [RTTY] Jitter

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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