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'[PIC]:interfacing PIC to a phone line'
2001\10\20@021626 by BENEDICIC SAMO

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       Hi!

I have a problem: how could I connect PIC output pin to a phone line in
order to dial one or more numbers using DTMF dialing? TIA,
                                                                                                                       Samo

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2001\10\23@080323 by James Padfield

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>
>I have a problem: how could I connect PIC output pin to a phone line in
>order to dial one or more numbers using DTMF dialing? TIA,
>

Try this as a starting point:

http://www.geocities.com/kenboak/rat_ring.html


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2001\10\24@045244 by Vasile Surducan

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Or: www4.tpg.com.au/users/talking/TaxiPhonePage1.html
Vasile


On Tue, 23 Oct 2001, James Padfield wrote:

{Quote hidden}

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2001\10\25@181729 by =?iso-8859-1?B?Tm9tZWwgoA== ?=

Is it critical to use dtmf?

You could always use pulse dialing which would be much easier...just pulse one then off an amount of times equal to the number you want to dial...1 time frame inbetween pulses and for pulse length...three time frames inbetween numbers I think ti is...you cam look it up...The only drawback would be that it wouldn't work for touch tone services such as voice mail and menu selections.

You could always try to use a table lookup method for the tones. Set loop for how long the tone, look up the pause, or number of instructions to loop through, for the freq, like for 1900 Hz you would pause for whatever amount of time and make then pin high then pause for whatever amount of time and make the pin low. The time would be around 1/2f. To make it somewhat a sine wave, which I doubt is very critical since the harmonics of any of the frequencies wouldn't affect each other, you could filter the output so the cuttoff freq is at the highest DTMF frequency. This should kill most of the harmonics. I would test to see if a square wav would work reliably first. You could do this by getting an old phone and hooking an op-amp in comparator mode (with non-inverting input at ground) up to the speaker terminals and press a key to get a nice square wave output. You could then hook a speaker up to the output and put it by the mic on a phone hooked to the phone line. You should be able to dial numbers this way.

{Original Message removed}

2001\10\25@225842 by Bob Ammerman

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----- Original Message -----
From: "Nomel  " <nomelspamKILLspamTECHIE.COM>
To: <.....PICLISTKILLspamspam.....MITVMA.MIT.EDU>
Sent: Thursday, October 25, 2001 3:09 PM
Subject: Re: [PIC]:interfacing PIC to a phone line

<snip>

> You could always try to use a table lookup method for the tones. Set loop
for how long the tone, look up the pause, or number of instructions to loop
through, for the freq, like for 1900 Hz you would pause for whatever amount
of time and ...

<snip>

Note: DTMF means DUAL TONE MULTIFREQUENCY. There are two tones produced at
the same time. You probably could do a wavetable synthesis of the high- and
low- tones, sum them inside the PIC and then output them via PWM.

Bob Ammerman
RAm Systems

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2001\10\26@083257 by Olin Lathrop

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> Note: DTMF means DUAL TONE MULTIFREQUENCY. There are two tones produced at
> the same time. You probably could do a wavetable synthesis of the high-
and
> low- tones, sum them inside the PIC and then output them via PWM.

Since both tones are sine waves, you could also just have a table of one
sine cycle (or 1/4 cycle if you're willing to do more addressing
calculations) and index thru it with two pointers at different rates, then
sum the results.


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2001\10\26@085612 by Drew Vassallo

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>Since both tones are sine waves, you could also just have a table of one
>sine cycle (or 1/4 cycle if you're willing to do more addressing
>calculations) and index thru it with two pointers at different rates, then
>sum the results.

I've always been curious whether or not you could use a square wave and have
it recognized by a DTMF receiver.  Would make things a lot simpler,
obviously.  Anyone know?

--Andrew

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2001\10\26@091953 by Don Hyde

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To get the eventual product FCC approved, it will need to meet out-of-band
specifications, which in this case means having a low-pass filter to reduce
the harmonics below the specified levels while keeping the tones up to the
full amplitude.

The dual-frequency tones were designed so that the harmonics of the
low-frequency ones are not very close to the high-frequency ones, so good
detectors might not mind having a pretty crude approximation to a sine wave.

"Real" DTMF products use 4 or 5-bit DAC's, so it can be done with
not-very-fine signals.  If you have 4 or 5 PIC pins, it's easy to turn them
into a DAC with an R-2R ladder, taking advantage of the nearly-symmetrical
drive available from PIC output pins.  The analog filter would have fewer
parts for a signal starting with the output of a 4-bit DAC than a 1-bit DAC
(i.e. square wave), so you can trade digital resources versus analog parts.

Another thought might be to output the low-and high-band square waves on
separate pins, with separate low-band and high-band low-pass filters to keep
low-band harmonics out of the high band, then sum them with a couple of
resistors.

> {Original Message removed}

2001\10\26@120633 by Olin Lathrop

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> I've always been curious whether or not you could use a square wave and
have
> it recognized by a DTMF receiver.  Would make things a lot simpler,
> obviously.  Anyone know?

I don't know, but since the first harmonics of a square wave start at 3
times the fundamental frequency you might just get away with it.

That brings up another idea, though.  Generate the square waves in software,
then apply a little low pass filtering to each one and add the results.  A
simple one or two pole low pass filter should suffice.  These are very easy
to do in software and take few cycles, especially when the filter fraction
is 1 / 2**N so you can use bit shifting for the divide.


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Olin Lathrop, embedded systems consultant in Littleton Massachusetts
(978) 742-9014, olinspamspam_OUTembedinc.com, http://www.embedinc.com

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2001\10\26@204736 by Bob Ammerman

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The formal specs for DTMF are rather restrictive. The tones have to be
pretty clean. However, that doesn't mean a receiver, or even most receivers,
might be more forgiving. Remember, however, that in addition to requirements
on what the receiver must accept, there are also requirements on what the
receiver must reject. It may be that square waves tend to fall into the
latter category because of the excessive energy at the harmonics.

Bob Ammerman
RAm Systems

{Original Message removed}

2001\10\26@211126 by Jinx

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> The formal specs for DTMF are rather restrictive. The tones have
> to be pretty clean. However, that doesn't mean a receiver, or even
> most receivers, might be more forgiving. Remember, however, that
> in addition to requirements on what the receiver must accept, there
> are also requirements on what the receiver must reject. It may be
> that square waves tend to fall into the latter category because of
> the excessive energy at the harmonics.
>
> Bob Ammerman
> RAm Systems

Assuming an ordinary copper system with capacitance, transformers
and the like, wouldn't there be a fairly good roll-off of the higher
harmonics of a square wave ?

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2001\10\26@211734 by Ned Konz

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On Friday 26 October 2001 05:59 pm, Jinx wrote:
> > The formal specs for DTMF are rather restrictive. The tones have
> > to be pretty clean. However, that doesn't mean a receiver, or even
> > most receivers, might be more forgiving. Remember, however, that
> > in addition to requirements on what the receiver must accept, there
> > are also requirements on what the receiver must reject. It may be
> > that square waves tend to fall into the latter category because of
> > the excessive energy at the harmonics.
> >
> > Bob Ammerman
> > RAm Systems
>
> Assuming an ordinary copper system with capacitance, transformers
> and the like, wouldn't there be a fairly good roll-off of the higher
> harmonics of a square wave ?

The third harmonic of 697 Hz is about 2100 Hz, and the highest legit
frequency of a DTMF tone is 1633 Hz. Add to this the fact that typical voice
lines have a bandwidth almost to 3KHz, you're going to be passing quite a bit
of 2100Hz energy.

Now, whether or not a given receiver will see those harmonics and do
something with them (like reject the tones because there's too much
out-of-band "noise"), I don't know.

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2001\10\26@214610 by David VanHorn

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At 08:43 PM 10/26/01 -0700, Bob Ammerman wrote:
>The formal specs for DTMF are rather restrictive. The tones have to be
>pretty clean. However, that doesn't mean a receiver, or even most receivers,
>might be more forgiving. Remember, however, that in addition to requirements
>on what the receiver must accept, there are also requirements on what the
>receiver must reject. It may be that square waves tend to fall into the
>latter category because of the excessive energy at the harmonics.
>
>Bob Ammerman
>RAm Systems


The specs pretty much require at least a 4 bit DAC with some post filtering.
While you may get it to recognize with square waves, you'll never get telco
approval.
The detectors are pretty much all energy detectors, so too much energy out
in the harmonics will hose you.



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2001\10\26@215024 by Jinx

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> Now, whether or not a given receiver will see those harmonics and
> do something with them (like reject the tones because there's too
> much out-of-band "noise"), I don't know.

When you get down to it, there's really only one way to find out, eh ?

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2001\10\27@170457 by uter van ooijen & floortje hanneman

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> I've always been curious whether or not you could use a square wave and
have
> it recognized by a DTMF receiver.  Would make things a lot simpler,
> obviously.  Anyone know?

According to the requirements for DTMF a receiver is surely not REQUIRED to
recognise such a distorted signal (distorted compared to the sum of two
sines), but I am not sure it is FORBIDDEN to recognise it. I do not have the
requirements at hand, so I can not check it.

Wouter

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2001\10\29@135431 by Harold M Hallikainen

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       For the high tones, at least, the harmonics should be outside the
spectrum of the LPF before the A/D at the central office. The third
harmonic of the low tones could be a problem though. A clever technique
to avoid "talk off" in DTMF decoders is to do "in band" to "out of band"
comparisons. If ONLY the two DTMF tones are present and nothing else in
the audio spectrum is (other than low level harmonics, intermod, and
noise), we pretty much know it's DTMF and not someone talking. This,
ideally, would be done with a combination of band pass and band reject
filters. The output of the band rejects would be summed and that level
compared with the output of the band pass. A similar technique was used
to detect the old 2600 Hz disconnect tone without it being talked off.
You could, I suppose, to a comparison of the outputs of all 8 BPFs for
the different tones and make sure that one high tone and one low tone are
so many dB above the outputs of all the other filters. This'd be a cheap
simulation of the in-band to out of band comparison. But then, you could
also just DSP the whole thing and go back to in-band versus out-of band.

Harold


On Fri, 26 Oct 2001 20:43:04 -0700 Bob Ammerman <RemoveMErammermanTakeThisOuTspamADELPHIA.NET>
writes:
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2001\10\31@125627 by Peter L. Peres

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> For the high tones, at least, the harmonics should be outside the
> spectrum of the LPF before the A/D at the central office. The third
> harmonic of the low tones could be a problem though.

Yesterday night I ran a couple of quick Spice runs with a square wave
source on one of the DTMF frequencies (1633) and several simple passive
LPF filters, doing transient and fourier analysis to 8kHz. None of the
designs got anywhere near passing. Best results were obtained with a LC
lowpass using 5uF and 1.4mH I think, and a 10R series damper on each to
get the Q under sontrol. This still did not pass. I think that a
20dB/octave filter is required to reach a somewhat usable signal spectrum.

Peter

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