David Harper wrote:



Is anyone familiar with the hardware details of the FSK process?
Specifically what components are involved in transforming a frequency
into a bit?

Thanks!
Dave

I think you already have a couple of messages on how the frequency can be
generated from a bit.

It appears to me you are asking for the opposite.

Basically, the modem runs the receive frequencies through narrow filters. If
the frequency received matches the passband of the filter, the audio tone
gets through the filter. The tone can then drive something as simple as a
transistor amplifier or darlington pair from one state to another (e.g.
from Vc=5v to Vc=0.5v). These outputs can then drive gates in the right
combination to get your serial output.

What has changed over the past two decades is how the filters have been
implemented and how the serial signal is generated.

What used to be narrow filters built with big inductors and critical
capacitors became filters implemented using op amps which, in turn, became
filters using dsp techniques.

Serial signal generation started out with straight off and on pulses to rtty
ksrs, moved to simple uarts built with logic gates, which became uarts on
ic's.

The basics are the same, however. Narrow filters for the tones and logic to
generate the serial signal.

73,

tim a0bwr

tim gorman ) writes:
David Harper wrote:



Is anyone familiar with the hardware details of the FSK process?
Specifically what components are involved in transforming a frequency
into a bit?

Thanks!
Dave

I think you already have a couple of messages on how the frequency can be
generated from a bit.

It appears to me you are asking for the opposite.

You're right, I completely missed that.

Michael VE2BVW

(Michael Black) wrote in message ...
tim gorman ) writes:
David Harper wrote:



Is anyone familiar with the hardware details of the FSK process?
Specifically what components are involved in transforming a frequency
into a bit?

Thanks!
Dave

I think you already have a couple of messages on how the frequency can be
generated from a bit.

It appears to me you are asking for the opposite.

You're right, I completely missed that.

Michael VE2BVW

Thanks all, that was informative and I appreciate the insight.

Dave

(Michael Black) wrote in message ...
tim gorman ) writes:
David Harper wrote:



Is anyone familiar with the hardware details of the FSK process?
Specifically what components are involved in transforming a frequency
into a bit?

Thanks!
Dave

I think you already have a couple of messages on how the frequency can be
generated from a bit.

It appears to me you are asking for the opposite.

You're right, I completely missed that.

Michael VE2BVW

Thanks all, that was informative and I appreciate the insight.

Dave

Ok, I have one more additional question. :-)

For a communications protocol such as RTTY, I know the mark and space
frequencies indicate 0 and 1 values of a (usually) 5-bit character.
But how does the receiving side synchronize with the transmitting
side? How does the receiver continue to properly allocate the
incoming bits? After, say, the 30th bit value, how does the receiver
know that it *IS* the 30th bit value? Especially with three 1's or
three 0's consecutively and no frequency changes...?

Is the receiver just very accurately timed? When it occurs, do the
transitions from 0's to 1's (and vice versa) serve to resynchronize
the receiver with the transmitter?

Sorry for the storm of questions, but thanks in advance!
Dave

----- Original Message -----
From: "David Harper"
Subject: FSK technical question


For a communications protocol such as RTTY, I know the mark and space
frequencies indicate 0 and 1 values of a (usually) 5-bit character.
But how does the receiving side synchronize with the transmitting
side? How does the receiver continue to properly allocate the

OK, remember the whole start and stop bit thing?

The line sits at mark when idle. When a character comes, the line drops to
space for one bit time. This is the start bit. Then the 5 or 8 bits are
transmitted, then one, 1.5 or two stop bits, which are really nothing more
than the minimum time between characters. So the receiver is guaranteed *at
least* one bit time of mark followed by exactly one bit time of space
between characters. The receiving side does need to be reasonably accurate,
but only accurate enough to not garble a character. It never has to keep in
sync for more than 10 bits worth (8 data bits plus a start and stop bit).
If the protocol specifies more than one stop bit, from the receiver's
perspective that is nothing more than additional time the transmitter has
allotted to do end of character processing.

On your earlier question about receiving FSK, the various posters answered
what you would do if you wanted to use an SSB or AM rig, or an audio FM rig
to receive FSK. However, a purpose-built FSK receiver would probably use an
FM discriminator, and simply recover data, rather than audio, from the
discriminator. Remember that an FM discriminator has an output that is
related to the frequency. If you fed the discriminator two frequencies, the
output would be two voltages.

...

"David Harper" wrote in message
m...
Ok, I have one more additional question. :-)

Sorry, I skipped something on the previous response.

I answered for ASYNCHRONOUS serial such as RTTY or async ASCII.

Some protocols, such as packet, use SYNCHRONOUS serial. Synchronous serial
is a lot harder to receive. There are no start and stop bits, so the
protocol doesn't involve that part of the overhead that async uses.

There are several synchronous protocols, but they mostly involve two
characteristics.... first, there is some mechanism for the receiver to
recover the clock. Frequently, the clock is embedded in the data, although
is could be sent over another channel. This allows the receiver to know the
bit boundaries. Every so often (typically every data packet) a special
pattern is sent that allows the receiver to identify the character
boundaries. In the common protocols, such as X.25 (or AX.25), there is also
a prohibition against sending too many of the same bit in a row. Special
procedures are invoked if this happens in the data.

...

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

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/