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

What will you display on?

wrote in message
oups.com...

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

My PC will be the receive station.

nana wrote:
What will you display on?

wrote in message
oups.com...

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

But, if you are displaying it on a PC, won't the sound card be inside it?


wrote in message
oups.com...
My PC will be the receive station.

nana wrote:
What will you display on?

wrote in message
oups.com...

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

Yes, RECEIVING with the PC. Transmitting via a PIC. The PIC will be
airborne. My PC will not.

nana wrote:
But, if you are displaying it on a PC, won't the sound card be inside
it?

Finally, Dave is beginning to give us the complete story.

Now, Are you thinking of generating audio tones using the pic in order
to transmit AFSK, audio frequency shift keying? This will basically be
filtered and sent to the microphone input of a transmitter. Or are you
going to use the pic to control a tone generator? What you use as a
transmitter has a lot to do with how complex the PIC program will be an
which PIC you will use.

Several years ago my company hired an engineer to develop a fax/modem
phone line switch using a 16 bit PIC. I don't remember the number.
Anyway, the circuit he designed and the software the programmer
developed watched for the phone ring tones and for the tones sent by
fax machines. It also generated the ring tones to ring the telephone on
the line it had selected. Just as with your project, it never had to do
all at the same time, although,I think the timer generating the tone
could continue until the PIC turned it off.

Have you looked at all the data sheets and sample programs available
for the PIC from the manufacturer? It's been a long time since I looked
there, but I think there were sme good examples of what you want to do.
Paul, KD7HB

nana wrote:
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.

W1AW sends bulletins in 110 baud ASCII.

http://www.arrl.org/w1aw.html

--
Jim Pennino

Remove -spam-sux to reply.

W1AW sends bulletins in 110 baud ASCII.

http://www.arrl.org/w1aw.html

--
Jim Pennino

Well, they'd be about the only ones!

Nana

wrote:
Finally, Dave is beginning to give us the complete story.

Hey. :-) In all fairness, I did say "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." Although it was several
messages back.

Now, Are you thinking of generating audio tones using the pic in
order
to transmit AFSK, audio frequency shift keying? This will basically
be
filtered and sent to the microphone input of a transmitter. Or are
you
going to use the pic to control a tone generator?

Probably the latter. I'd like to be able to use a standard handheld
with a PTT. I might have the tone generator be an external circuit
outside the handheld... still not sure.

Have you looked at all the data sheets and sample programs available
for the PIC from the manufacturer? It's been a long time since I
looked
there, but I think there were sme good examples of what you want to
do.
Paul, KD7HB

I've checked a couple, but I don't plan on generating the tones with
the PIC... I'd like to take as much processing off the pic as possible.
Thanks for the help!
Dave

nana wrote:
Aaahhhh! Okie Dokie Dave. Little lights started to flicker on. You
sure are
a cryptic devil, aren't you?

So, why not just generate the 5 unit code at CMOS levels then run it
into an
XR2206 tone generator like everyone else? It will save you hours of
heartache. http://www.klm-tech.com/technicothica/xr.html

Wow, nice chip. I like the datasheet on that one so far. That might
be a great option...

Are you attempting to make a beacon, or something for APRS? What is
the
purpose of the one-way acft borne RTTY transmitter?

Mainly temeletry. Most of it would be GPS information, but some would
be temperature and status information.

I ask because there are
pic type units made that take a GPS position, format it to packet
1200bd
APRS and transmit same on VHF. (search APRS)

Is this what you have in mind?

Somewhat, but I was kinda hoping to have a lot of control over what is
sent... and it'd be educational to design my own RTTY generator.

It'd be going on a high altitude balloon. I've launched one in the
past using pre-recorded audio sounds (numbers and the pilot-alphabet),
but it's slow and consumes more battery power. I'd also like to get
more frequent transmissions.
www.geocities.com/achilles03/hab.htm

Thanks for the info!
Dave