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PICList Thread
'Help for telephone project'
1998\05\01@212344 by Louis Grau

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Hi!

I would like to design an electronic system that plugs between a
phone/fax/etc. and the phone line.
This system should just change some digits of the number being dialed
(usually only the first digit).
The digits should only be changed at specific time ranges and specific days
of the week.

Therefore this project needs the following parts:
- Real time clock/calendar
- DTMF decoder
- DTMF generator
- Analog circuitry (mainly for initially disabling the DTMF tones from the
phone and then connecting
the phone to the phone line)

That first looks like a simple project. However, I would like to use as few
parts as possible. My question is:
is is realistic to have the clock, DTMF decoding and encoding tasks all
done by a PIC microcontroler?
I already spent several hours searching on the Internet and found solutions
for DTMF tone generating.
Doing a clock/calendar with timer/software doesn't look very complicated
(at first glance). But DTMF
decoding looks a lot more complicated (A/D converter + DFT?). A Fourrier
transform must obviously
eat up a lot of PIC ressources (execution time and code memory). Maybe
someone has found a trick
for detecting DTMF tones in a simpler way. Who knows?

Concerning the analog circuitry, the more I think about it, the more
complicated it looks to me. I see
two different approaches for changing the first digit of the dialed number:
1 - The phone is permanently connected to the phone line. But I need to
find a way to intercept (at least
the first two) DTMF tones. Then the PIC has to dial the new changed
digit(s) while eventualy getting additional
tones from the phone.
2 - The phone is temporarily disconnected from the phone line (with a
relay). But here, how can the dial
tone, busy tone, operator voice (etc..) be heard by the person (or fax,
modem) who makes the call.

Actually I have no solution to the problems above. I would apreciate any
good ideas from experienced
PIC developers. I haven't design anything since the 68705 and 8748
microcontrolers were very popular!

Louis Grau.
France

1998\05\01@230033 by David VanHorn

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That first looks like a simple project. However, I would like to use as few
parts as possible. My question is:
is is realistic to have the clock, DTMF decoding and encoding tasks all done
by a PIC microcontroler?


Certainly. You've got 60 hz available, which gives you an extremely accurate
timebase (long term)
DTMF decoding might be workable on the pic, but why not use external DTMF
encoder/decoder
chips. That's what they do. :)    The phone interface, to do this right,
won't be real simple. If you take it
in logical blocks it will be easy, but not simple.

Intercept the line during active time, provide a constant current source
with about 50mA and 24V.
Dial tone isn't strictly needed, but you can generate it easily.
Grab the number, change what you want, loop the outside line, dial it, then
connect the fax, and drop out of the connect.
Watch line current into the fax to determine on/off hook status.

2 - The phone is temporarily disconnected from the phone line (with a
relay). But here, how can the dial
tone, busy tone, operator voice (etc..) be heard by the person (or fax,
modem) who makes the call.


You just reconnect after you dial, say 10-100ms. The fax does what it
usually does.  Ideally, you're only adding about 1-2 sec to the call, so it
shouldn't irritate the dialing timeouts.

For bonus points, Have your output DC polarity mimic the incoming line, so
as to avoid accidental disconnect or large
thump noises when you transfer.   Automatically detect tone or pulse dial
outside lines, and allow the fax to use both as well.
Speed up your tone dialing, It's possible to get about a 30% increase in
speed by doing something a little non-obvious with the dialing.
DO NOT shorten the tone bursts below 50mS that rarely works.

Very little pic problems here really, mostly phone problems :)
If you can find it, a copy of "Understanding Telephone Electronics" from
radio shack is about indespensible.
It's about the best book on this sort of thing that money can buy.

1998\05\02@131733 by hansen

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David VanHorn wrote:
> Very little pic problems here really, mostly phone problems :)
> If you can find it, a copy of "Understanding Telephone Electronics" from
> radio shack is about indespensible.
> It's about the best book on this sort of thing that money can buy.

Does anyone know where this can be obtained these days?  It's
not in the RS catalog anymore.  It is there another source for
this type of information?

John Hansen
spam_OUThansenTakeThisOuTspamfredonia.edu

1998\05\02@152817 by andre

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Louis:
I would not worry about DTMF decoder because it is very simple to implement.
I use motorola part to decode the analog signal. on output  I get 4 bit data
and data available pin. I simply interface it to pic and all done.  I use
MC145436ap
If you want to use an other chip for decoding and dialing I would recommend to
use
MITEL products. This company make only telephone communication IC's.

Hope this helps.

Andre Abelian


{Quote hidden}

--

1998\05\02@164554 by Marc Heuler

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Hi Louis (Louis Grau), in <.....199805011811_MC2-3BAF-DD10KILLspamspam@spam@compuserve.com> on May 1 you wrote:

> But DTMF
> decoding looks a lot more complicated (A/D converter + DFT?). A Fourrier
> transform must obviously
> eat up a lot of PIC ressources (execution time and code memory). Maybe
> someone has found a trick
> for detecting DTMF tones in a simpler way. Who knows?

There ARE ways to detect DTMF in a PIC, but they consume lots (almost all)
of CPU time and still do not meet the DTMF specs.  They distinguish the
digits, but can't tell DTMF from noise or voice.  Also, developing such an
(poor) algorithm that fits into a PIC is a job of several manweeks.

Better take one of the DTMF encoder/decoder chips, such as MITEL 88xx
series.  You can _reliably_ detect and send quality DTMF tones in minutes!

> 2 - The phone is temporarily disconnected from the phone line (with a
> relay). But here, how can the dial
> tone, busy tone, operator voice (etc..) be heard by the person (or fax,
> modem) who makes the call.

I think MITEL has hybrid circuits that "simulate" a phone line, and they
definately have circuits that convert the outgoing line into Audio IN and
Audio OUT (MH88422 and others).  Probably one of the most expensive
solutions, but easy to develop with.  When you use ready made hybrid
modules you might even get FCC approval without much hassle (check with the
manufacturer).

1998\05\03@190701 by Frank Mckenney

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John Hansen <hansenspamKILLspamOAK.AIT.FREDONIA.EDU> wrote:
> David VanHorn wrote:
> > Very little pic problems here really, mostly phone problems :)
> > If you can find it, a copy of "Understanding Telephone Electronics" from
> > radio shack is about indespensible.
> > It's about the best book on this sort of thing that money can buy.
>
> Does anyone know where this can be obtained these days?  It's
> not in the RS catalog anymore.  It is there another source for
> this type of information?

John,

I just finished some research to see what it would take to add "busy" LEDs
to twelve switch boxes sharing the same four lines (don't ask (;-)).You
might find the following useful:

   FCC Part 68 Direct Connection to the U.S. Telephone Network
   http://www.ce-mag.com/tel95a1.html

   Telephone interfacing circuits
   www.mmainteractive.com/electronics/phone/audio.htm
   http://www.hut.fi/Misc/Electronics/telephone.html

   TELECOM Digest and Archives
   http://hyperarchive.lcs.mit.edu/telecom-archives/

Oh, and if anyone knows of a relatively inexpensive "quad" IC designed for
direct phone connection, I'd like to hear about it (;-).


Frank McKenney            / OS/2 Advisor (OS2BBS)
McKenney Associates       / Richmond, Virginia / (804) 320-4887
Internet: .....rrs0059KILLspamspam.....ibm.net / TalkLink: WZ01123

1998\05\03@200419 by Dennis Plunkett

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At 06:10 PM 1/05/98 -0400, you wrote:
{Quote hidden}

What you are proposing in not as simple as one might think, there are
several issuses to consider, most of them involve timing and level control
and impedance. For outgoing calls the solution is fairly simple, as the
outgoing DTMF stream will have to be interrupted. But this may sound simple,
but is not! As you will have to interrupt the 2 wire circuit to the phone
and exchange, and also provide the battery feed to the FAX. On an incomming
call you will have to have a ring detector that connects the line to the FAX
(90VRMS on -48Volts (Typical)).

The basic soultions offered thus far from MITEL will work, you will need one
SLIC (COT) and one phone end (I forget what theu call this but it detects
the ring, matches the line impedance and provide hybrid balance). Then you
will also need some form of DTMF detection and generation and a micro to
control it all (All of the Mitel devices in this situation work with
Analogue signals). The soulution is not cheep (Your own hybrid and gyrator
relays and stuff will be cheeper) but will work. Do you need more infomation?


Dennis

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