RTTY format
 

I'm trying to write a program (for my own education) that will transmit
RTTY. I'm having some difficulty determining the exact format. From
what I've observed from MMTTY, an idle keyboard has a 22ms space tone
followed by a mark tone until the next space tone. I would have
assumed that a standard letter/8bit transmit would last 196ms (22*8),
but after recording it with a wav recorder and viewing it, it appears
to be shorter (168ms or so, +/- 10). I'm not sure if the recorder is
not accurate with timekeeping, or if I don't fully understand the
format.

I would like to believe the standard format for a letter/8bit transmit
is a start bit (space), data bits (marks/spaces), and two stop bits
(space), *ALL* of which are 22ms long. Is this statement correct?
Thanks in advance,
Dave

On 21 Jan 2005 11:37:19 -0800, wrote:

I'm trying to write a program (for my own education) that will transmit
RTTY. I'm having some difficulty determining the exact format. From
what I've observed from MMTTY, an idle keyboard has a 22ms space tone
followed by a mark tone until the next space tone. I would have
assumed that a standard letter/8bit transmit would last 196ms (22*8),
but after recording it with a wav recorder and viewing it, it appears
to be shorter (168ms or so, +/- 10). I'm not sure if the recorder is
not accurate with timekeeping, or if I don't fully understand the
format.

I would like to believe the standard format for a letter/8bit transmit
is a start bit (space), data bits (marks/spaces), and two stop bits
(space), *ALL* of which are 22ms long. Is this statement correct?
Thanks in advance,
Dave

No. It's a 5-bit code, plus start/stop bits. google for Baudot and you
can find a code chart.

The stop bit may be 1.0, 1.42 or 1.5 units long, with 1.42 being the
value based on the most common mechanical printers.

Hence, you will often see it called a 7.42 unit code.


Jack K8ZOA

wrote in message
ups.com...

I would like to believe the standard format for a letter/8bit transmit
is a start bit (space), data bits (marks/spaces), and two stop bits
(space), *ALL* of which are 22ms long. Is this statement correct?

As Jack pointed out, it is a five bit code. Even more interesting, it is a
stateful code. There are two shift characters, LTRS and FIGS, which change
the meaning of the codes that follow.

Years ago I was teaching a college assembly class and the Baudot code made
for an interesting exercise g

...

Hi,

Further - the RTTY code is asynchronous where the stop bit may
be any length over the standard 1.42 or 1.5 bit minimum. It is
therefore almost standard practice to pass the key-strokes into a
buffer and to output them from there so that the final result is
almost synchronous. If this buffer should be emptied then some
neutral character such as a 'letters shift' is continually output
(called I think, 'diddling') to keep the circuit alive and to
remove any DC component caused by a constant mark during 'hunt
and peck' keying.

Don't forget to include the other common commercial standard
of a 20ms bit-size (50 baud) in your program.


Cheers - Joe

In both Baudot and ASCII an idle keyboard produces a constant MARK signal
(loop current 'on' for a TTY machine), thus no signalling changes. In
Baudot/RTTY
it's common practice however to periodically emit what is called a 'diddle'
when
the keyboard is idle. Normally a LTRS character is used as the 'diddle'.
This
helps the receiver stay better synchronized to transmitter. Different
software
can be programmed to emit constant 'diddles' or to emit them about once a
second.

-- Tom



wrote in message
ups.com...
I'm trying to write a program (for my own education) that will transmit
RTTY. I'm having some difficulty determining the exact format. From
what I've observed from MMTTY, an idle keyboard has a 22ms space tone
followed by a mark tone until the next space tone. I would have
assumed that a standard letter/8bit transmit would last 196ms (22*8),
but after recording it with a wav recorder and viewing it, it appears
to be shorter (168ms or so, +/- 10). I'm not sure if the recorder is
not accurate with timekeeping, or if I don't fully understand the
format.

I would like to believe the standard format for a letter/8bit transmit
is a start bit (space), data bits (marks/spaces), and two stop bits
(space), *ALL* of which are 22ms long. Is this statement correct?
Thanks in advance,
Dave