DTMF FAQ - TELEPHONE TONE DIALING CHIPS V1.06
* About the Author
* Why I wrote this FAQ
* Short introduction to DTMF
* Decoding DTMF
* List of DTMF-receiver chips
* An alternative approach DSP
* If you live in USA read this section !
* Conclusion/About the Author
ABOUT THE AUTHOR
Author : T.H. Tsim
Date : August 25th, 1994
E-mail : email@example.com
Contributors: Terry Lalonde, Dbsnow, Andrew R. Ghali, Brad Yearwood,
Gary Schaum, Arkady Horak-Systems
WHY I WROTE THIS FAQ
Hi! After reading sci.electronics for a couple of years, I've
discovered that many people are interested in decoding DTMF-signals
for various applications. A frequently asked question (FAQ) concerning
this subject is: "What IC should I use in order to decode DTMF
SHORT INTRODUCTION TO DTMF
DTMF means: Dual Tone Multiplexed Frequency, the tones you might have
heard while pressing the keys on your telephone. Each key produces a
slightly different signal, which is a mixture of two frequencies, i.e.
pressing '1' will send a tone made of 1209 Hz and 697 Hz to the other
end of the line.
(From the Editor)
Here's a more detailed explanation of what DTMF is:
Q: What frequencies do touch tones use for which numbers?
A: The touch tone system uses pairs of tones to represent the various
keys. There is a "low tone" and a "high tone" associated with each
button (0 through 9, plus * (star) and # (octothorpe or pound symbol).
The low tones vary according to what horizontal row the tone button is
in, while the high tones correspond to the vertical column of the tone
The tones and assignments are as follows:
1 2 3 A : 697 Hz
4 5 6 B : 770 Hz
7 8 9 C : 852 Hz
* 0 # D : 941 Hz
---- ---- ---- ----
1209 1336 1477 1633 Hz
1209 Hz 1336 Hz 1477 Hz 1633 Hz
697 Hz 1 2 3 A
GHI JKL MNO
770 Hz 4 5 6 B
PRS TUV WXY
852 Hz 7 8 9 C
941 Hz * 0 # D
When the 4 button is pressed, the 770 Hz and 1209 Hz tones are sent
together. The telephone central office will then decode the number
from this pair of tones.
The tone frequencies were designed to avoid harmonics and other
problems that could arise when two tones are sent and received.
Accurate transmission from the phone and accurate decoding on the
telephone company end are important. They may sound rather musical
when dialed (and representations of many popular tunes are possible),
but they are not intended to be so.
The tones should all be +/- 1.5% of nominal. The high frequency tone
should be at least as loud, and preferably louder than the low
frequency. It may be as much as 4 db louder. This factor is referred
to as "twist." If a Touchtone signal has +3db of twist, then the high
frequency is 3 db louder than the low frequency. Negative twist is
when the low frequency is louder.
Q: What are the A, B, C and D touch tone keys used for?
Why are they not found on touch tone phone sets?
A: These are extensions to the standard touch-tones (0-9, *, #) which
originated with the U.S. military's Autovon phone network. The
original names of these keys were FO (Flash Override), F (Flash), I
(Immediate), and P (Priority) which represented priority levels that
could establish a phone connection with varying degrees of immediacy,
killing other conversations on the network if necessary with FO being
the greatest priority, down to P being of lesser priority. The tones
are more commonly referred to as the A, B, C and D tones respectively,
and all use a 1633 Hz as their high tone.
Nowadays, these keys/tones are mainly used in special applications
such as amateur radio repeaters for their signalling/control. Modems
and touch tone circuits tend to include the A, B, C and D tones as
well. These tones have not been used for general public service, and
it would take years before these tones could be used in such things as
customer information lines; such services would have to be compatible
with the existing 12-button touch tone sets in any case.
There are many ways to detect and decode these DTMF tones. One idea
could be an eight sharp-tuned filter combination with detection
circuits. Needless to say, this is very impractical, considering the
various ICs (Integrated Circuits or 'chips') made by different
manufacturers all over the world.
Most of these ICs do not require more than one (inexpensive) 3.58 MHz
x-tal or resonator and the power circuitry. Usually the output is
4-bit binary + 1 strobe.
l l- d3
l DTMF l- d2
signal in -l Decoder l- d1 4-bit binary out
l chip l- d0
l l- strobe
Note: This figure is merely for decoration, it does not imply package
Nice text to read about DTMF and telephony can be found in:
"Understanding Telephone Electronics"
Howard W.Sams & Co
LIST OF DTMF-RECEIVER CHIPS
The companies are listed in a random order:
Silicon Systems Inc.
SSI202 DIP-18 [*]
[*]: For those in the USA, see the addresses section for this chip.
[filipg: addition follows]
We have a pinout for the SSI-202 from WDUNCKEL1 (firstname.lastname@example.org):
Data 1 =| |= Data 2
HEX/B =| S |= DATA 4
ENABL =| S |= DATA 8
IN1633 =| I |= CLRDATA VALID
VP =| |= DATA VALID
NC =| 2 |= ATB (CLOCK OUT)
GND =| 0 |= X IN
X ENBL =| 2 |= X OUT
ANALOGIN=| |= GND
A 10 Meg resistor accross the colorburst crystal is neccessary. Pins
5,2,3,8 are tied high and 4,15,9,7,10 are tied to ground. Data output
on d1-d8 will be valid when data valid goes high.
[filipg: back to the original FAQ]
All req. 3.58 MHz (3.579545 MHz) crystal
A neat and well documented design + PCB layout by Tom Miller KA1JQW,
can be found in "The ARRL handbook for radio amateurs", ISBN:
Chrystal Semiconductor Corp.
MSM6843 DIP-28, 3.58, 4b1s
Application note in OKI Telecom LSI data book
(From Arkady Horak-Systems)
I spent about the last 14 months of my life working on an integrated
DTMF receiver here at Motolora and I would like to have you add the
part to the DTMF-list. The receiver is part of a 6805 8-bit
microcontroller and is called the MC68HC05F5. Here is a quick feature
* MC68HC05 based core micro
* Memory mapped I/O registers
* 5632 bytes of user ROM
* 224 bytes of RAM
* 15-bit multi-function timer
* Power saving STOP, WAIT and Data-Retention modes
* Mask optional COP (computer operating properly) or watchdog timer
* Self check ROM
* Fully static operation with no minimum clock speed
* 30 bidirectional I/O pins
* On chip oscillator for use with external crystal or ceramic
resonator 2 or 4 MHz operation only while DTMF rec. is functioning
* DTMF receiver
* 40 pin plastic dip or 44 pin plastic leaded chip carrier packages
Being an avid electronics hobbyist I feel this part will see a lot of
service in hobby applications. This part is, to the best of my
knowledge the first general purpose microcontroller with an integrated
DTMF receiver. Currently only a ROM based version is available but as
sales grow a 705 or EPROM version may be manufactured. Also, this part
is compatible with the 6805 development tools and software.
Motorola CSIC Microcontroller Division
M957-0x DIP-22 or SOIC-24, 4b1s, 5 and or 12 Volt (read below)
CAUTION: M957-02 and M957-01 differ in operating voltage!, M957-01 can
operate from 5 - 12V, M957-02 operates 5V only (not suitable for 12 V
A ready to build schematic by R.G. Krijgsman based on this Teltone
M957-2 chip can be found in: "Het Elektuur telefoonboek" ISBN
90-70160-94-3, (Dutch language)
KT3170 DIP-18, minimal ext. parts ?
UMC, United Microelectronics Corp.
UM9203 DIP-18 4b1s and 2of8
UM92870A/B/C DIP-18 (three variants)
Examples of line-interfaces and app-notes in the UMC communications
You can get application notes (in PostScript) from the Mitel WEB
server: http://www.semicon.mitel.com/prod_sum_dtmf.html or FTP site:
[*]: For those in the USA, see the addresses section for this chip.
1. Mitel has an Application Note - MSAN-108, titled "Applications of
the MT8870 integrated DTMF receiver" in their Digital/Analog
Comunications Handbook ISSUE 9. From my experience this app. note
is valid for all Mitel tone recs.
2. Teltone seem to carry this type-number as well.
TC35301AP DIP-28, 4b1s
You can get Application Notes
its/Section_08/index.html) on the following chips from WWW. They are
in PostScript so be ready to print or download them.
CD22202 DIP-18, 4b1s
CD22203 DIP-18 4b1s and 2of8
CD22204 DIP-14 or SOIC-24, 4b1s
* 3.58 = an inexpensive 3.58 MHz needed
* 4b1s = 4-bit 1 strobe output (Hexoutput)
* 2of8 = 2-of-8 code
AN ALTERNATIVE APPROACH: DSP
Another way to decode/produce DTMF signals is by applying a DSP, a
Digital Signal Processor. Practical examples of such an approach can
be found in the TMS320 BBS, which is currently being mirrored at
ftp://ti.com/mirrors/tms320bbs/. You may also wish to see the whole
Files of interest concerning this faq are:
TEL-2-1.EXE 57K | DTMF Encoder/Decoder for C3x by DSP
TEL-2-2.EXE 53K | DTMF Detection Data Sheet by Ensigma Ltd.
DTMF10.EXE 24K | TMS320C10 based DTMF decoder
DTMF10E.EXE 19K | TMS320C10 based DTMF encoder
DTMF_C17.EXE 126K | TMS320C17 based DTMF encoder/decoder
IF YOU LIVE IN USA READ THIS SECTION !
(From the Editor)
If you are designing a device that will attach to the phone line and
you are situates in USA, you will need to make the FCC happy. You can
either do this by submitting your completed circuit to them for
compliance testing (which can run a few thou$and) or you can purchase
a small device that is preapproved by the FCC. It is called DAA for
Data Access Arrangement. It is a small, usually potted, module that
isolates (literally) your circuit from the phone line.
The FCC section you are trying to satisfy is part 68. You should visit
your local library and read up on it whether your get the preapproved
part or try to have the FCC [affiliated lab] do the testing .. or just
plain hope you don't get caught. It's pretty dry reading but that's
There are several different modules on the market that fit into this
category. They are:
* XE0068 from:
374 Turquoise St
Milpitas, Ca. 95035
"They carry quite a few different daa modules. Some with Touch
tone encode/decoder built in which sells for about $45. And a
basic daa for about $25."
* 73M9001 from:
TDK/Silicon Systems (SSi)
[no address or distributor - call DigiKey] "It is a 30 pin
socketed dual sided surface mount PCB. All protection is on the
module and includes a speaker driver and additional relay drivers.
Can be used up to 14.4KBPS V32.bis speeds."
* Various from:
* CH1840 from:
"The chip plugs into phone lines, splits the signal between input
and output, has pins for ringing, off-hook, etc. The chip has
FCC-transferable Part 68 registration, so it's 100% legal to
connect whatever you want to it. There's an international version
that should work down there in Oz."
Someone was obviously not too happy using Cermetek parts. This is
included here NOT because I'm biased but as a general precaution
to designers. These things inherenly impose regulations. Need I
say more?.. he writes:
I once worked on a telephone-interface project with the
unfortunate selection of the Cermetek parts. They were originally
chosen for their small size and supposed FCC ease, but it turns
out that a DDA interface from discrete parts was much cheaper and
much better quality.
+ They have HORRIBLE noise problems. I don't know why, but they
seemed to pick up every electric field in the machine and
amplify it to obnoxious levels. Probably something to do with
their "solid-state" transformer.
+ The "Pre-FCC-approved" parts have to include a "squelch"
circuit to comply with FCC maximum level limitations. The FCC
specifies the maximum as some number of dB over a one second
period. Well, the squelch circuit does not take time into
account, so it clips any loud transient sound. This
frequently clips normal-level voice signals, and causes a
real problem when speaking loudly.
+ The "pre-FCC-approved" part is also pretty big, about 1"x2",
if I remember. With modern parts, a good circuit can fit in
the same space.
+ The FCC approval was just as hard with the part as without.
We had to include MOVs and other stuff between the Cermetek
and the phone line, and still go through all the testing
required of a discrete circuit.
+ Since they're epoxy-potted modules, they can't be fixed, and
we had a serious problem with broken parts. Almost one in
three was unusable.
+ The sidetone level (audio loopback) was much to high. It was
OK once the speaker on the Cermetek end became used to it,
but it bothered new users and would probably make data
So, if you have the time and the need, invest the effort to build
a decent interface from parts. If you just need one for hobbyist
needs, the Cermetek might work, but I got better performance from
a phone interface ripped from a 1200 baud modem.
There are just my experiences, and they were with early-rev parts,
so they may have improved. Still, it would have saved us several
months of engineering time if we had avoided Cermetek from the
(This from Steve in Vigra - you know who you are :-)
* Kit from:
6212 SW 8th Place
Gainesville, FL 32606
(FYI, this information posted by Dan Hoehnen and dated Sep 11,
They sell a partial or full kit or an assembled unit for MPC-2. It
provides an FCC approved interface (if you buy the pre-assembled
unit) that gives you acces to audio in/out, ring detect and
off-hook detect connections.
Circuit board only is $9.00, Complete MPC-2 kit is $19.95, MPC-2
that is assembled and FCC approved is $29.95
The above mentioned company also sells an upgrade board that
basically conditions the outputs a little better for pc
The upgrade can be bought for $69.95 kit, inlcludes an assembled
MPC-2. Or assembled for $99.95.
* B.G. Micro, Inc.
P.O. Box 280298
Dallas, TX 75228
* Dallas Semiconductor
* MITEL Semiconductor
Head office in Canada:
360 Legget Dr.
P.O. Box 13320
Kanata, Ontario, Canada K2K 1X3
Tel: 1-800-561-0860 (maybe)
Tel: 1-613-592-2122 (ask for Semiconductor Applications)
UK Tel: 044 291430000
Fax: 044 291436389
Germay Tel: 49 7117701522
Fax: 49 7117701524
CONCLUSION/ABOUT THE AUTHOR
Well, that's the latest I've found on DTMF receivers. There are no
doubt other companies producing these kind of chips, so please let me
know and I'll improve this list (when I've got the time)
Unfortunately, I'm not that experienced yet to answer any difficult
technical questions about this subject.
For suggestions, improvements, info, flames, waterbuckets (?) please
e-mail me at: email@example.com.
I graduated July 1st 1994, so I won't continue writing updates for
this faq (no time, no space, no money :)
I like to thank all of you who think this faq was usefull, especially
the ones who wrote me e-mail . I was very suprised to see this faq
appear in the most strange places on the Net :)
Anyway...have fun with those di-du-dit-di-duh tones :)