I'm looking for some fundamentals on writing software decoders
for signals. I've found alot of things for windows (scancat,mmtty
etc) however not alot for linux. So since I use linux I'd like
to get a discussion going on writing some command line decoders
that will take the input from my soundcard and output the characters
for something like ACARS, TTY etc.

Any help would be great
TIA
der dogdog

Samstag, 18. März 2006 08:23 (US Central Time)
wrote in rec.radio.scanner:

I'm looking for some fundamentals on writing software decoders
for signals. I've found alot of things for windows (scancat,mmtty
etc) however not alot for linux. So since I use linux I'd like
to get a discussion going on writing some command line decoders
that will take the input from my soundcard and output the characters
for something like ACARS, TTY etc.

You might wanna drop the acars thing ... we already have it at
www.acarsd.org.

Talk to you later ... Hans from Germany
--
2006/03/18 17:20
EDDS 181720Z 09008KT CAVOK 04/M02 Q1015 NOSIG

On Sat, 18 Mar 2006 14:23:34 GMT, wrote:

to get a discussion going on writing some command line decoders
that will take the input from my soundcard and output the characters
for something like ACARS, TTY etc.

Grab an old TTY decoder program and you'll see how it's done ...
pretty trivial, really. The rough part (for me, at least, since I've
never tried it) is getting the sound card to tell you which frequency
it's hearing. But once you have the digital train decoding ASCII is
no job at all (0100 0001 is upper case A - no "decoding" needed), and
decoding Baudot isn't much more complicated (keep the current shift
state in a boolean and look up the character in a table).

On Sat, 18 Mar 2006 12:24:14 +0000, Hans Muecke wrote:

Samstag, 18. März 2006 08:23 (US Central Time)
wrote in rec.radio.scanner:

I'm looking for some fundamentals on writing software decoders
for signals. I've found alot of things for windows (scancat,mmtty
etc) however not alot for linux. So since I use linux I'd like
to get a discussion going on writing some command line decoders
that will take the input from my soundcard and output the characters
for something like ACARS, TTY etc.

You might wanna drop the acars thing ... we already have it at
www.acarsd.org.

Talk to you later ... Hans from Germany

Will do on that Hans. I found the ACARS stuff and its nice. A little
more than I wanted but it has alot of features.
thanks
dogdog

On Sat, 18 Mar 2006 13:43:45 +0000, Al Klein wrote:

On Sat, 18 Mar 2006 14:23:34 GMT, wrote:

to get a discussion going on writing some command line decoders
that will take the input from my soundcard and output the characters
for something like ACARS, TTY etc.

Grab an old TTY decoder program and you'll see how it's done ...
pretty trivial, really. The rough part (for me, at least, since I've
never tried it) is getting the sound card to tell you which frequency
it's hearing. But once you have the digital train decoding ASCII is
no job at all (0100 0001 is upper case A - no "decoding" needed), and
decoding Baudot isn't much more complicated (keep the current shift
state in a boolean and look up the character in a table).

Have any web locations for older decoders with source? I've been
looking but cant find the source, realizing its mostly in
visual basic I think. But moving it over to something else shouldnt
be that hard.

TIA
derdogdog

On Sun, 19 Mar 2006 13:50:54 GMT, wrote:

Have any web locations for older decoders with source? I've been
looking but cant find the source, realizing its mostly in
visual basic I think. But moving it over to something else shouldnt
be that hard.

Most of what I was thinking of was written in 8080 assembly. I
doubt that I could find my old 8" floppies or, even if I could, if I
could find anything that could read them.

But here's the program in English - translate it to whatever language
you need:

For ASCII, input the bits, starting with the start bit. Keep the next
8 bits. Assemble them into a byte. You're done - that's the
character.

Oh, there's a stop bit - either use it as a verification bit - if it's
missing, skip the character because something's wrong - or throw it
away.

You can find a bit-banger routine he
http://www.ganssle.com/articles/auart.htm - look at Cin. BITTIM and
HALFBT are only needed if you don't have a timer with fine granularity
- a few microseconds would be good.

For Baudot (or Gray or Murray code), it's 5 bits (and the first bit is
1-1/2 bit times or 2 bit times long - depending on whose Teletype
machine is transmitting - Teletype corporation made them both ways,
depending on the customer - so you might want to give the user the
choice of 1-1/2 or 2 stop bits). Grab the next *5* bits, assemble
them into a right-justified byte (the left 3 bits are 0) (stop bit
here too - same as for ASCII) - the bit banger will work here too -
look to see if you're in Figs or Ltrs and look it up in the
appropriate table. If it's all 5 bits (31), it's Ltrs, so set your
shift indicator to Ltrs. If it's Figs (27) set your shift indicator
to Figs. (Ltrs and Figs are transmitted characters.) Give the user
the choice of "Unshift on space". If it's chosen, and the character
is a space (4), convert to 32 (ASCII for space) and set the shift
indicator for Ltrs. If not, just convert to 32.

You can find the entire Baudot chart at
http://home.austin.rr.com/kinghome/signpage/baudot.html.

Oh - if you're in Ltrs shift and receive Ltrs, that's normal. Old TTY
operators would "diddle" the Ltrs key, to let you know that they were
thinking of what to say. Those of us who used real Teletype machines
back in prehistoric times still do that. And, when using paper tape,
punching Ltrs erased any error, so if someone is transmitting an old
tape, you could get a string of Ltrs..

Now, as far as what happens between the Line In jack and the program,
you're on your own - I've never done sound card programming.

what the hell for?
Get a job.