Telephone DTMF

Country Loon · Oct 29, 2004

I know a little about DTMF in phones and decided to make my own. I created
the dual sine waves for all the numbers on a PC - digitally. I then fed the
audio output to a phone line via the PC speaker and a phone - nothing
happened. I expected it to dial the number. I know the frequencies are
right - does the duration have to be a certain time for the exchange to
accept? In the end I just used a modem and fed ascii commands to it but am
still wondering what went wrong.

Tom

John Fields · Oct 29, 2004

On Sat, 30 Oct 2004 12:19:51 +1300, "Country Loon"
<fitlikemin@mearns.fairmers.co.uk> wrote:

I know a little about DTMF in phones and decided to make my own. I created
the dual sine waves for all the numbers on a PC - digitally. I then fed the
audio output to a phone line via the PC speaker and a phone - nothing
happened. I expected it to dial the number. I know the frequencies are
right - does the duration have to be a certain time for the exchange to
accept? In the end I just used a modem and fed ascii commands to it but am
still wondering what went wrong.

---
The duration has to be >= 40ms, but the amplitude has to be high
enough to "break dial tone", something you surely won't be able to
achieve with a PC speaker acoustically coupled to the phone, never
mind the distortion you're generating!

I don't recall offhand what the amplitude and distortion specs are,
but you might want to Google for "DTMF encoder" or "DTMF transceiver"
and get them from some manufacturer's literature; Zarlink comes to
mind for transceivers...

--
John Fields

Frank Bemelman · Oct 29, 2004

"Country Loon" <fitlikemin@mearns.fairmers.co.uk> schreef in bericht
news:1099092009.542218@ftpsrv1...

I know a little about DTMF in phones and decided to make my own. I created
the dual sine waves for all the numbers on a PC - digitally. I then fed
the
audio output to a phone line via the PC speaker and a phone - nothing
happened. I expected it to dial the number. I know the frequencies are
right - does the duration have to be a certain time for the exchange to
accept? In the end I just used a modem and fed ascii commands to it but
am
still wondering what went wrong.

I'd think that 200ms tone and 500ms pause ought to work. Increase speed
to see if you can make it faster. Or perhaps you frequencies weren't right
after all, or the volume too low. It's all guessing of course.

--
Thanks, Frank.
(remove 'x' and 'invalid' when replying by email)

Joerg · Oct 29, 2004

Hi John,

The duration has to be >= 40ms, but the amplitude has to be high
enough to "break dial tone", something you surely won't be able to
achieve with a PC speaker acoustically coupled to the phone, never
mind the distortion you're generating!

I don't recall offhand what the amplitude and distortion specs are,
but you might want to Google for "DTMF encoder" or "DTMF transceiver"
and get them from some manufacturer's literature; Zarlink comes to
mind for transceivers...

AFAIK the DTMF level has to be between -12dBm and 0dBm at 600Ohms. Quite

a lot.

Regards, Joerg

http://www.analogconsultants.com

Country Loon · Oct 30, 2004

"Frank Bemelman" <f.bemelmanx@xs4all.invalid.nl> wrote in message
news:4182d5a7$0$36860$e4fe514c@news.xs4all.nl...

"Joerg" <notthisjoergsch@removethispacbell.net> schreef in bericht
news:8yAgd.15276$6q2.8502@newssvr14.news.prodigy.com...
Hi John,

The duration has to be >= 40ms, but the amplitude has to be high
enough to "break dial tone", something you surely won't be able to
achieve with a PC speaker acoustically coupled to the phone, never
mind the distortion you're generating!

I don't recall offhand what the amplitude and distortion specs are,
but you might want to Google for "DTMF encoder" or "DTMF transceiver"
and get them from some manufacturer's literature; Zarlink comes to
mind for transceivers...

AFAIK the DTMF level has to be between -12dBm and 0dBm at 600Ohms. Quite
a lot.

I guess so. There's been a time that gadgets were sold that served as
an automatic dialer. Hold it against the mouthpiece and press a button.
Those weren't extremely loud.

--
Thanks, Frank.
(remove 'x' and 'invalid' when replying by email)

I used to have one for an old answering machine so the volume does not have

to be loud at all. It was a small hand held device that I entered the right
numbers to gain access to my recordings.

Tom

John Fields · Oct 30, 2004

On Sat, 30 Oct 2004 21:05:55 +1300, "Country Loon"
<fitlikemin@mearns.fairmers.co.uk> wrote:

I used to have one for an old answering machine so the volume does not have
to be loud at all. It was a small hand held device that I entered the right
numbers to gain access to my recordings.

---
That's _vastly_ different from using acoustic DTMF to dial the phone
number which connects the answering machine to the PSTN.

--
John Fields

James Meyer · Oct 30, 2004

On Sat, 30 Oct 2004 12:19:51 +1300, "Country Loon"
<fitlikemin@mearns.fairmers.co.uk> wroth:

I know a little about DTMF in phones and decided to make my own. I created
the dual sine waves for all the numbers on a PC - digitally. I then fed the
audio output to a phone line via the PC speaker and a phone - nothing
happened. I expected it to dial the number. I know the frequencies are
right - does the duration have to be a certain time for the exchange to
accept? In the end I just used a modem and fed ascii commands to it but am
still wondering what went wrong.

Tom

Don't forget the "twist". The relative amplitude of the high and low
group tones is important too.

Jim

Dbowey · Oct 30, 2004

Tom posted:

I know a little about DTMF in phones and decided to make my own. I created
the dual sine waves for all the numbers on a PC - digitally. I then fed
the
audio output to a phone line via the PC speaker and a phone - nothing
happened. I expected it to dial the number. I know the frequencies are
right - does the duration have to be a certain time for the exchange to
accept? In the end I just used a modem and fed ascii commands to it but
am still wondering what went wrong.
--

I sent you an email last evening, but I received a failure notice. I don't
know if the US and UK DTMF systems are the same, but.....

I have the ANSI Standard that sets the requirements for DTMF in the local
channel, at the NI. If you wish, I will copy the two pages and email them -
what format do you like? I won't post the pages as it is copyrighted.

Don

John Fields · Oct 30, 2004

On 30 Oct 2004 16:58:58 GMT, dbowey@aol.com (Dbowey) wrote:

I sent you an email last evening, but I received a failure notice. I don't
know if the US and UK DTMF systems are the same, but.....

I have the ANSI Standard that sets the requirements for DTMF in the local
channel, at the NI. If you wish, I will copy the two pages and email them -
what format do you like? I won't post the pages as it is copyrighted.

---
If it's copyrighted and you don't have permission from the owner of
the copyright to make and distribute copies, in any amount, I believe
you'll be in violation of that copyright if you do.

--
John Fields

Dbowey · Oct 30, 2004

John posted:

On 30 Oct 2004 16:58:58 GMT, dbowey@aol.com (Dbowey) wrote:

I sent you an email last evening, but I received a failure notice. I don't
know if the US and UK DTMF systems are the same, but.....

I have the ANSI Standard that sets the requirements for DTMF in the local
channel, at the NI. If you wish, I will copy the two pages and email them
-
what format do you like? I won't post the pages as it is copyrighted.

---
If it's copyrighted and you don't have permission from the owner of
the copyright to make and distribute copies, in any amount, I believe
you'll be in violation of that copyright if you do.
--

You are correct.

Don

Dave VanHorn · Oct 31, 2004

DTMF is designed to work over a huge range of amplitude.
Twist (level of high vs low tone) is less tolerant. Ideally, you want your
high tone 1-3dB louder than the low tone. By the time it reaches the CO,
they will be about equal.

50mS on, 50mS off is typical, and should work 100%.
If it's not, something's very wrong.

--
KC6ETE Dave's Engineering Page, www.dvanhorn.org
Microcontroller Consultant, specializing in Atmel AVR

Joerg · Oct 29, 2004

Hi Frank,

I guess so. There's been a time that gadgets were sold that served as
an automatic dialer. Hold it against the mouthpiece and press a button.
Those weren't extremely loud.

I still have a couple of these. They were meant to operate a voice mail

system when you were overseas, such as somewhere in Europe where the
phones wouldn't always have DTMF. But you could not really dial with
these. Answering machines were more sensitive. Actually mine was so good
that it even worked on phone lines in the boonies where you could barely
hear it answering through all the crackle. It still read the tones right.

Regards, Joerg

http://www.analogconsultants.com

Joop · Nov 1, 2004

"Country Loon" <fitlikemin@mearns.fairmers.co.uk> wrote:

I know a little about DTMF in phones and decided to make my own. I created
the dual sine waves for all the numbers on a PC - digitally. I then fed the
audio output to a phone line via the PC speaker and a phone - nothing
happened. I expected it to dial the number. I know the frequencies are
right - does the duration have to be a certain time for the exchange to
accept? In the end I just used a modem and fed ascii commands to it but am
still wondering what went wrong.

Tom

Did something like that way back in 1985 or so with a C64 when our

town started to support tone dialling. I was a student in those days
and we spend many hours calling in to quiz radio shows being pretty
quick compared to other listeners. Usual numbers under a button, stuff
like that. Still have some tape with our airtime, about 90 minutes all
together. Oh, where did time go...

Anyway, we started with 1 second tones and tried to reduced the
duration to the minimum. I found that the first digit had to be a bit
longer than the following to get the best results.
But if speed is not an issue I would try with longer tones to see if
your telco system accepts it. Only then reduce it to typical phone
memory dial speed.

B.t.w. the C64 was connected to the speaker of an ordinary dial (I
mean dial!) phone. Thus coupling it purely AC because of the built in
transformer.

Good luck

Joop

Dbowey · Nov 2, 2004

Mac posted:

<< On Sat, 30 Oct 2004 01:40:41 +0200, Frank Bemelman wrote:

"Country Loon" <fitlikemin@mearns.fairmers.co.uk> schreef in bericht
news:1099092009.542218@ftpsrv1...
I know a little about DTMF in phones and decided to make my own. I created
the dual sine waves for all the numbers on a PC - digitally. I then fed
the
audio output to a phone line via the PC speaker and a phone - nothing
happened. I expected it to dial the number. I know the frequencies are
right - does the duration have to be a certain time for the exchange to
accept? In the end I just used a modem and fed ascii commands to it but
am
still wondering what went wrong.

I'd think that 200ms tone and 500ms pause ought to work. Increase speed
to see if you can make it faster. Or perhaps you frequencies weren't right
after all, or the volume too low. It's all guessing of course.

500 ms pause? I don't even have a 500 ms pause when I dial by hand, and
speed dial does 7 digits in, I would guess, 2 seconds or less.
500 ms is a long time, but not harmful. The requirement is on the minimum
times. The sum of the pulse duration and the interdigit interval (known as
the "cycle time," cannot be less than 100 ms.

Don

Jim Thompson · Oct 29, 2004

On Fri, 29 Oct 2004 18:35:20 -0500, John Fields
<jfields@austininstruments.com> wrote:

[snip]

---
The duration has to be >= 40ms, but the amplitude has to be high
enough to "break dial tone", something you surely won't be able to
achieve with a PC speaker acoustically coupled to the phone, never
mind the distortion you're generating!

[snip]

Back in the mid-70s I discovered that the least distortion way to
"acoustically" couple to a telephone handset was to use a coil driving
the EARPIECE! So I did a single-muff "acoustic" coupler, microphone
for receiving and coil-drive for transmission.

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice

480)460-2350 | |
| E-mail Address at Website Fax

Jonathan Kirwan · Oct 30, 2004

On Sat, 30 Oct 2004 12:31:36 -0500, John Fields <jfields@austininstruments.com>
wrote:

If it's copyrighted and you don't have permission from the owner of
the copyright to make and distribute copies, in any amount, I believe
you'll be in violation of that copyright if you do.

I believe that fair use provisions of the copyright law allow for limited
copying or distribution of published works without the author's permission -- if
the copy is used for teaching, and better still distributed without charge, then
the copy is more likely to be considered as "fair use." Even better, if the
copy was made spontaneously, is for temporary use, and isn't part of an
"anthology." "Research and scholarship" are recognized in the area of
technology (as well as some other areas such as science and scholastics) are
recognized "fair use."

Of course, I'm just pairing up my beliefs with yours. I have no special
knowledge. But I certainly a significant difference between copying two pages
to be sent to a specific individual and posting those same two pages on the
internet. Whether or not copyright law sees any difference is a matter for
those with better knowledge than me.

Jon

Telephone DTMF

Country Loon

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John Fields

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Frank Bemelman

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Joerg

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Country Loon

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John Fields

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James Meyer

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Dbowey

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John Fields

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Dave VanHorn

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Joerg

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Joop

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Jim Thompson

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