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

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


You've got it all correct Dave, but for one multiplication error. Eight bits (1
start bits, 5 character bits, and 2 stop bits) of 22ms works out to 176ms, not
196ms.

73, de Hans, K0HB

Thanks, I realized after I posted... it's Friday.

Dave

I think you will find an "idle keyboard", no key pressed, will be a
continuous mark tone.

Paul, KD7HB

wrote:
Thanks, I realized after I posted... it's Friday.

Dave

wrote in message
oups.com...
I think you will find an "idle keyboard", no key pressed, will be a
continuous mark tone.

Paul, KD7HB

Quite right. The 'diddles' you hear are often Letters character being sent
to stop the machine from getting bored.
By the way, I saw it mentioned that a Stop bit was 2 units long? I've always
thought it was 1.5.

Nana

Thanks for the info. The "diddles" (LETTERS char) I was hearing wasn't
a continuous mark tone, which was what I referenced in my original
post. I assume this is to help the RX station stay in sync.

Dave

Hi, Dave.
I think we need to define what you are trying to do. What do you mean
by RTTY? Are you sending Baudau (SP!) or ASCII? If ASCII, are you
sending 7 bit characters with parity or 8 bit with parity?


All RTTY that amatuers generate is asynchrounous, meaning that each
character is a self-contained package. Synchronizing occurs when the
start bit (space) is received and ends when the stop bit is received.
Both sender and receiver must be set to handle the total lenght of the
"package". Actual stop bits or "marking" at teh end of the package may
be any length, but as Nana pointed out, must be at least the minimum
specified for the protocol. Either 1.5 or 2 units minimum.

Areyou using all software to send the RTTY, or are you using a
UART/USART chip to actually convert from paralell to serial?
Paul, KD7HB

I'm still open to what type of RTTY to use... and as far as
transmitting, I'm not using available software or a UART/USART chip.
My goal is to program a PIC so that it can control a FSK circuit and
transmit to my PC, which is receiving with MMTTY or something similar.
Thanks for the info all!
Dave

But Dave, the soundcard is doing that already!

Most Ham RTTY is in Baudot (Paul, take note), I've never seen any ASCII on
air although it was experimented with when machines became available. The
actual code is called Murray Code. Baudot is a bit of a misnomer as it
really refers to timing.

MMTTY will do all the timing/decoding of the characters in RTTY, the sound
card will perform the audio processing. For a good old fashioned TU, even
one in a PIC, you will need some good audio filtering and processing in
order to clean up the tones. My old unit was called an ST6 and might appear
on Google. It consisted of some audio filtering, then two tuned filters
using toroids (88mH) then a slicer and a level shifter.

This unit could only handle two frequencies 2125 and 2295Hz for a 170Hz
shift signal. Since RTTY is also being sent on the ham bands with 200Hz
shift, you will need to account for this. Since then, new chips became
available from Exar to decode the audio tones, based on PLL's, but it must
be said, the audio performance was never quite as good under noisy
conditions. The DSP in new soundcards now run rings around this old process.

To assist you in your quest I suggest you read some of the notes with MMTTY
as he refers to the signal processing in them. Also, there was a Greenkeys
website around a couple of years ago with a lot of information about RTTY
signalling which might help.

Nana (aka Brad VK2QQ)

wrote in message
ps.com...
I'm still open to what type of RTTY to use... and as far as
transmitting, I'm not using available software or a UART/USART chip.
My goal is to program a PIC so that it can control a FSK circuit and
transmit to my PC, which is receiving with MMTTY or something similar.
Thanks for the info all!
Dave

nana wrote:
But Dave, the soundcard is doing that already!

Yes, indeed it is. But I wouldn't be looking to transmit via a PIC if
I could take a PC along. I'm looking for a PIC-based solution in order
to make it flight-weight.

Dave