Agreed Don! But then again, I only did a 80 meter single band gig.
Thanks for the contact! I hope you did well. It was a great contest.
73
Jim W7RY
On 10/2/2013 10:31 AM, Don AA5AU wrote:
I don't recall hearing any "slow" RTTY signals at all this weekend. Maybe it's
not as widespread as some think.
73, Don AA5AU
________________________________
From: Kok Chen <chen@mac.com>
To: RTTY Reflector <rtty@contesting.com>
Cc: "ws7ik7tj@gmail.com WS7I" <ws7ik7tj@gmail.com>
Sent: Wednesday, October 2, 2013 12:28 PM
Subject: Re: [RTTY] Looking for an interface - a few simple requirements
On Oct 2, 2013, at 8:26 AM, Jay WS7I wrote:
In fact I had it once using MMTTY at a place where I contest on occasion.
Could the recent spate in Slow RTTY be caused by the open sourcing of MMTTY,
and people unknowingly modifying the code and thus inserting latencies to the
characters?
There are three kinds of "slow RTTY," some are more obvious than the others.
The first kind "sounds" normal, with correct character rate and bit rate,
except extra diddles are inserted when they need not be, as if it were a slow typist.
The ones with extraneous diddles also tend to come with a lot of V characters from these
stations under severe multipath conditions (the V are simply LTRS shift characters -- the
diddles -- that has the start bit smearing into the LSB of the Baudot character. So,
lots of diddles = lots of V.
The second is the K3/paddle guys -- you can spot these signals a mile away. While the baud rate is
constant (otherwise, Baudot decoders would not be able to copy them), the character rate is not
consistent. They sound weird, but prints correctly. The stop bit stretches wildly from 33
milliseconds to hundreds of milliseconds. This kind of "slow RTTY" is caused by the
firmware in the K3 not issuing a diddle when it is needed, and leaving the FSK state in the Mark
condition while waiting for the paddling to finish. Those of you with a K3 can easily hear this on
the monitor by sending somethings like WA0AOI. The zero and the O will likely suffer from
stretched stop bits unless you can paddle *really* fast -- way, way faster than 60 WPM, since the
"PARIS" Morse speed measures average speed, not when the Morse character has lots of dah
elements. Since few people can paddle beyond 80 WPM, it is the norm to get this (the K3 will not
send FSK with
a hand key, just through a pa
ddle, by the way).
If you think this is rare, you should see the email I get from K3 users who
actually uses this :-). Many are teetaletolers in RTTY or they are just having
fun while operating their K3 portably. If a rare DX pops up and you don't have
a laptop with you, you can still work the DX -- the decoded Baudot appears on
the VFO display of the K3. It is hard to discourage them from the practice,
since ham radio is all about having fun, and these guys are probably having
more fun than people using function keys. But Elecraft really should fix their
firmware.
The third happens all the time with FSK (and rare with AFSK) and the majority
of people don't even know it -- 2Tone has an indicator for the stop bit
duration, otherwise in the past you will need to plot out the decoded waveform
to see it.
I think it happens because the "next" character is not sent to the UART fast
enough, thereby stretching the stop bit. This happens more often than you think: stop
bits would stretch between 22 ms to perhaps 60 ms (not as long as the K3/paddle case,
though). Proper software should not allow this to happen even through misconfiguration
(and with AFSK, it really can't happen unless the software is pretty broken).
The most common cause is that the software is controlling the character timing,
and using the UART only to control the bit timing. (nd in the K3 case, the
operator is causing the character latency :-).
I have even seen this type of stretched stop bit with MicroHAM devices when the
software misuses the character buffer in the interface -- even though the
interface has a buffer of more than one character, the software still only uses
it as a one character buffer. It is not a hardware problem, and with proper
algorithms on the computer end, they do not happen -- all that needs to happen
is to keep at least two characters the MicroHAM interface (or any UART based
interface that has a buffer, for that matter), and let the firmware in the
interface perform the character timing.
(The "usual" argument for only using a single character UART buffer is so that
you can quickly flush an exchange. But that should not be a problem with macro-driven
contest exchanges, and delaying a flush by 180 ms won't really hurt much, and only on the
occasions when you decide to stop a macro mid-stream.)
Since 2Tone has a stop bit duration indicator, you have a tool today to monitor
your own signal to see if this is happening with your own transmissions with
whatever UART interface that you are using. Try it. I think a lot of people
who use FSK are going to be surprised by what they see reported back by 2Tone.
This will, as time goes by, is not just a nuisance, but become more important as modems
start adopting "pseudo-synch" techniques to get through flutter conditions
better.
Folks who have used the "numerical flywheel" in RITTY will know what I mean.
With these techniques, you are using the start-stop information from more than a single
character to derive the character frames.
Start-stop errors contribute to about 3 times more errors than errors in data
bits because a character synch/frame error will propagate until the decoder
finally finds the true start bit again. If you can use even just two
character's worth of start-stop information, the print errors drop
dramatically. By the time you use four characters worth, the start-stop
errors are pretty much out of the picture and you only have to deal with random
errors.
Naturally, techniques like RITTY's numerical flywheel requires very accurate
character (thus stop bit) timing (or the technique includes some magic to
account for changing stop bit durations -- yeah, I have been looking into that
too and can tell you that it is a real pain when stop bits jitters :-).
With the techniques that tries to make start-stop RTTY behave more like
synchronous FSK (Amtor FEC is synchronous FSK, by the way, and it is quite
robust), the baud rate has to be rather precise (one percent baud rate error
over 30 bits becomes a large error), and also be constant (it is hard to track
character clock when the bit clock itself jitters). In the future, it is
possible that if you run the wrong baud rate or generates too much jitter (for
example by using EXTFSK), your transmission may not be able to take advantage
of these techniques.
Buy up the FSKits while they are still around since they provide FSK with good
timing, HI HI. (Or modify a SignalLink to include the FSKit chip per the QST
article earlier this year.)
With FSKit, you are using AFSK timing, so the jitter is moderately low all the
way to 75 baud.
A little birdie tells me that it is developing another AVR design which might
become publicly available to convert FSK from the computer to FSK for the
transmitter. I am told it reclocks the data to 45.45 baud, and cleans up
EXTFSK jitters :-).
73
Chen, W7AY
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