[GreenKeys] Diddle, Real Metal, 1.0 Stop Pulse

[Bob Camp]

Hi

A machine that is designed for a stop bit length of 1.0 probably 
engages the clutch well before the end of a 1.0 width pulse. I have not 
spent enough time spinning a 28 slowly and watching what is going on to 
know when the clutch fires on them. Certainly the 28 was designed well 
after the 15 and may have been based on an improved knowledge of what 
was going on.

If both the sending and receiving machines are running synchronous 
motors on the same power grid you can conceivably go for quite a while 
before you get into trouble.

Your point about the machine sounding different when it is fed a 1.0 
length stop pulse is telling. I suspect the clutch is not engaging at 
all in that case. This may or may not be a good thing for wear and tear 
on the machine ....

With an ATC/DTC type TU the odd stop bit length biases the switch point 
off of center just a little bit when you have a weak signal. This has 
nothing to do with the TTY but it will have an effect on the quality of 
the copy. Of course if you send a bunch of LTRS characters all the time 
that will move things a bit off center as well.

	Enjoy!

		Bob Camp
		KB8TQ




On Feb 25, 2005, at 9:59 AM, Jerry wrote:

> Surprisingly enough I recall some 30+ years ago when Bill Carver
> (k6olg/w7aaz now) and Jim Haynes (w6jve) and I think Larry Laitinen
> (wa6jyj/w7jyj) were running a lot of tests on RTTY using just a 1.0 
> stop
> pulse.  All of us were using model 28 machines as I recall.  The model
> 28's ran 'real busy' when receiving the 1.0 stop pulse and sounded like
> they were going much faster than the 60wpm normal speed using a 1.42
> stop pulse.
>
> What was really amazing was the copy was noticeably better.  Now that I
> think about it, I seem to recall that Bill (k6olg/w7aaz) had designed a
> new TU and it was processing the info much better with the 1.0 stop
> pulse and a full set of diddles.  These tests were run over some period
> of time on several occations, and the copy was always much better.
>
> I just went out and looked in my 'stash' and yep, I still have my
> original lash up of Bill's TU sitting on a shelf.  I guess a true
> 'rtty'er never gets rid of anything, chuckle!
>
> Jer -n6jp-
>
> ----- Original Message -----
> From: "Bob Camp" <ham at cq.nu>
> To: "Veenstra, Lester" <Lester.Veenstra at intelsatgeneral.com>; 
> "Greenkeys
> ((E-mail))" <greenkeys at mailman.qth.net>
> Cc: "Jerry" <gh1lockett at bak.rr.com>
> Sent: Friday, February 25, 2005 6:16 AM
> Subject: Re: [GreenKeys] Diddle, Real Metal, & Indeterminate Line 
> Length
>
>
> : Hi
> :
> : You are correct at least as far as the machines I have really dug 
> into
> : (15's). The mechanical felt pad clutch engages at the end of a 1.0
> : length stop pulse. The only problem with this is that it (as least
> from
> : 20 year old memory) is a full 1.0 length pulse. If the sending 
> machine
> : is running just a bit fast the receiving machine will fall out of
> : synchronization every so often.
> :
> : As far as I can see there is no advantage to diddle on a mechanical
> : TTY. There certainly is an advantage on certain types of TU's but not
> : on the TTY it's self. If anything diddle wears out the machine a bit
> : faster.
> :
> : The biggest problem I see with "synthetic sync" operation using 
> diddle
> : is that you do not have a defined stop bit length. Normal clock and
> : data recovery circuits rely very much on the fact that they know
> : exactly what they are looking for. Each weird stop bit length (1.31,
> : 1.42, 1.5) effectively is a different clock rate you have to watch
> for.
> : I have never seen a sync system that uses odd width pulses in the bit
> : stream.
> :
> : This isn't to say you can't watch for all the possible rates and 
> guess
> : which one is being used. The issue is that you can guess wrong if you
> : have  three closely spaced stop pulse widths. If you really want to 
> go
> : wild you can always go to a Veterbi decoder and do convolutional 
> guess
> : work until you run out of CPU clock cycles. I suspect that at 45 baud
> : that a lot of of bits (> 2 characters) . It would still work better 
> if
> : you knew the stop pulse width ahead of time since you would have 
> fewer
> : buckets to examine and less chance of error.
> :
> : We're getting a bit far afield from my original intention here which
> : was simply to caution about the use of 1.0 length stop bits if 
> there's
> : a mechanical TTY on the other end ...
> :
> : Take Care!
> :
> : Bob Camp
> : KB8TQ
>
>