Another little piece of the story is that in start-stop operation the
receiver samples the incoming signal at the place where it expects the
center of the bit to be. Thus it is tolerant of signals that are too
fast or too slow. Of course today it is easy to get the speed very
precise; but in the early days it was a matter of motors with centrifugal
speed governors.

With synchronous transmission the receiver knows where the bit boundaries
are going to be, so it is possible to sample near the end of each bit time
when all of the energy in the received signal has come in. Start-stop
has to throw away roughly half of the energy in each bit because of the
center sampling. Hence synchronous transmission has an advantage in
signal-to-noise ratio.

In the days of mechanical teleprinters, synchronous operation had a much
greater advantage. A mutilated STOP pulse would allow the receiving
shaft to continue rotating, and then several characters would be
received in error as a result of that single bit error. With electronic
reception there is no rotating shaft, so it is possible to reset the
receiver to the starting position instantly.

It is also possible with electronics to achieve a quasi-synchronous
operation with start-stop signals. The idea is that instead of having
the STOP pulse be arbitrarily long, it is of fixed length and an idle
character is sent if there is nothing to send from the keyboard. This
is usually called "diddle". With the incoming data stream being a
steady stream of printable characters and nonprinting idle characters
it is synchronous for the duration of the transmission. The detector
can synchronize to this signal and take advantage of all of the energy
in each signal pulse. The K6STI RITTY software (no longer marketed)
operates on this principle.
--

jhhaynes at earthlink dot net

Howard Goldstein ) writes:
On Wed, 22 Sep 2004 16:04:02 +0000 (UTC), Mike Andrews wrote:
: When you add start and/or stop bit, things get a lot easier, and
: that is the case with most serial communications: you can reset the
: character and bit-time clocks per-character. When no sync bits are
: present, you have to derive the bit timing and character timing from
: the data-bit transitions in the data stream, and things can get a bit
: iffy. Telco circuits have hardware that requires K transitions per N

For time like this ghod invented the digital PLL (DPLL). The best
discussion of the DPLL I'd ever seen was in an old (olde) Osborne &
Associates book with wonderful text and even better line graphics.

(I imagine that invoking Osborne materials shows my age)

Yes, because I can't figure in which book they'd be talking about PLLs.
Osborne books tended to be about software, and when it was hardware
it was about CPUs. I do have one Osborne book about video controller
ICs. But I don't remember there being general electronic books from
the company.

Michael VE2BVW

Howard Goldstein ) writes:
On Wed, 22 Sep 2004 16:04:02 +0000 (UTC), Mike Andrews wrote:
: When you add start and/or stop bit, things get a lot easier, and
: that is the case with most serial communications: you can reset the
: character and bit-time clocks per-character. When no sync bits are
: present, you have to derive the bit timing and character timing from
: the data-bit transitions in the data stream, and things can get a bit
: iffy. Telco circuits have hardware that requires K transitions per N

For time like this ghod invented the digital PLL (DPLL). The best
discussion of the DPLL I'd ever seen was in an old (olde) Osborne &
Associates book with wonderful text and even better line graphics.

(I imagine that invoking Osborne materials shows my age)

Yes, because I can't figure in which book they'd be talking about PLLs.
Osborne books tended to be about software, and when it was hardware
it was about CPUs. I do have one Osborne book about video controller
ICs. But I don't remember there being general electronic books from
the company.

Michael VE2BVW

Thanks for all the great information guys.. that helps a lot...

So, where can I get a good comm program that has RTTY capability
(ideally freeware, to get started with) for my computer?

Thanks!
Dave

Thanks for all the great information guys.. that helps a lot...

So, where can I get a good comm program that has RTTY capability
(ideally freeware, to get started with) for my computer?

Thanks!
Dave

In article ,
David Harper wrote:
So, where can I get a good comm program that has RTTY capability
(ideally freeware, to get started with) for my computer?

You couls start with a google search for "rtty12g" which is a very
old program written in BASIC to run under DOS. That's a program I
have used to make a TTY machine play in a museum situation.

Or there's the modern free MMTTY program, which is intended for running
RTTY on the air, but it has an output to FSK key the transmitter and you
can use that to drive a TTY machine (through a suitable driver circuit
of course) What it doesn't do, and I wish it did, is while receiving
to output "cleaned up" TTY signals so they could go to a printer.
--

jhhaynes at earthlink dot net

In article ,
David Harper wrote:
So, where can I get a good comm program that has RTTY capability
(ideally freeware, to get started with) for my computer?

You couls start with a google search for "rtty12g" which is a very
old program written in BASIC to run under DOS. That's a program I
have used to make a TTY machine play in a museum situation.

Or there's the modern free MMTTY program, which is intended for running
RTTY on the air, but it has an output to FSK key the transmitter and you
can use that to drive a TTY machine (through a suitable driver circuit
of course) What it doesn't do, and I wish it did, is while receiving
to output "cleaned up" TTY signals so they could go to a printer.
--

jhhaynes at earthlink dot net

So, where can I get a good comm program that has RTTY capability
(ideally freeware, to get started with) for my computer?

Go here for sound card programs. As mentioned mmtty is very good to start
with for rtty.

http://www.muenster.de/~welp/sb.htm#zvei

Here is another place to checkout. It mentions packet, but the interface is
for all sound card modes.

http://www.packetradio.com/

So, where can I get a good comm program that has RTTY capability
(ideally freeware, to get started with) for my computer?

Go here for sound card programs. As mentioned mmtty is very good to start
with for rtty.

http://www.muenster.de/~welp/sb.htm#zvei

Here is another place to checkout. It mentions packet, but the interface is
for all sound card modes.

http://www.packetradio.com/