Introduction

Eventually, this system was replaced by a system called "DTMF", also known as Touch-Tone. This system uses specific noises ("tones") to send one of 16 different codes.

Although originally intended for telephone use, DTMF tones are now routinely used when code numbers must be sent through channels that were originally intended for audio data.

Among other things, DTMF tones can be recorded and played back later.

DTMF Tones

The following table shows the common telephone keypad in blue. The red column shows additional keys that are not usually present on telephones, but are part of the DTMF specification. The green labels indicate the tones used for each row and column.

1	ABC 2	DEF 3	A	697 Hz	low tones
GHI 4	JKL 5	MNO 6	B	770 Hz
PRS 7	TUV 8	WXY 9	C	852 Hz
*	0	#	D	941 Hz
1209 Hz	1336 Hz	1477 Hz	1633 Hz
high tones

Example: When the "9" button is pressed, the 852 Hz and 1477 Hz tones are sent.

SSI202 DTMF Decoder Chip

Since DTMF is so useful, there have evolved a number of chips that make detecting and decoding DTMF easy.

We will use as our example the SSI202 DTMF decoder chip. By the time that you read this, there will likely be better, faster, cheaper solutions. But the SSI202 makes a nice example.

The SSI202 listens for DTMF tones and outputs a binary code, corresponding to the combination it hears. In the following chart, D1-D8 are the binary bits resulting from the DTMF tones. There are 16 possible combinations of 4 bits that are on or off.

	DTMF tones		digital output
key	low tone	high tone	D8	D4	D2	D1
1	697	1209	0	0	0	1
2	697	1336	0	0	1	0
3	697	1477	0	0	1	1
4	770	1209	0	1	0	0
5	770	1336	0	1	0	1
6	770	1477	0	1	1	0
7	852	1209	0	1	1	1
8	852	1336	1	0	0	0
9	852	1477	1	0	0	1
0	941	1336	1	0	1	0
*	941	1209	1	0	1	1
#	941	1477	1	1	0	0
A	697	1633	1	1	0	1
B	770	1633	1	1	1	0
C	852	1633	1	1	1	1
D	941	1633	0	0	0	0

You can choose to decode the 4-bit output into one of 16 different commands that can be sent via a tone, or hook each of the outputs to a different device.

Please note that the binary output of the SSI202 DTMF decoder is latched. When you send a burst of tones to send a binary output, the output stays there until you replace it with a different value.

Assorted DTMF Chips