Before
we move on to the theory behind this, just take a look at these sound clips. 
GOTO clip1: http://www.genave.com/audio/dtmf-page-normal-S00P0501P99.wav
GOTO clip2: http://www.genave.com/audio/dtmf-page-slow-CP4D34SS.wav
DTMF- Dual Tone Multi Frequency
Hz
|
1209
|
1336
|
1477
|
697
|
1
|
2
|
3
|
770
|
4
|
5
|
6
|
852
|
7
|
8
|
9
|
941
|
*
|
0
|
#
|
Did you ever think of how your response (pressing a
particular number) is identified by the receiving side? For an example let’s
consider a situation where you are in an Interactive Voice Recording (i.e.
calling to customer care). They asked
you to press 1 or 2 or any number to identify your response. Pressing a key
will send a sinusoidal tone for each of the two frequencies (i.e. 697 and
1209Hz for “1”). See the image below. The multiple tones are the reason for
calling the system multi frequency and it is called as dual tone because of the
use of two frequencies. Two frequencies are there to make sure that frequencies
generated accidently will not be identified as a press. Columns will have
higher frequencies than rows. In normal dialing also we can use DTMF, but only
for special purposes (if needed MSC can identify the pressed keys).
Interactive Voice Response (IVR) - is a technology that automates interactions with telephone
callers.
Figure
1:
DTMF
|
Pulse dial system was used earlier and it is
a slow and older method. For optimum use, push-button phones utilize
DTMF signaling, which was also known as touch-tone dialing.
Figure 2: Push
Button Phone
Now you know the theory, and let’s see how it is implemented
Mark and Space
Mark refers to the duration a DTMF tone is produced (the time which a DTMF digit tone is actually producing sound), and Space refers to the duration of the silence between individual digits.Notations
- Mark/Space : Pronounced as "Mark and Space”. Refers to the durations of mark and space.
- 40/40: The decoders expect the DTMF tones to exist for at least 40 milliseconds and in silence for 40 milliseconds between each DTMF digit.
References: Some of the images, details and sound clips were
taken from the following website. http://www.genave.com
