User guide

18 RealCT Direct API Developer Guide
Chapter 2: Digit Handling
Defining DTMF Signals
DTMF digits are the ones on your telephone keypad. Figure 4
shows the combination of two out of eight possible frequencies
that make up each digit. All digits in a given row share the same
low frequency. All digits in a given column share the same high
frequency.
Figure 4. Frequencies in DTMF Digits
For example, if you press ‘7’, your phone sends a dual-frequency
tone made up of 852 Hz and 1209 Hz to the CO. When you dial
out, the CO uses the digit as part of an address. During a call,
applications recognize DTMF tones as a way to navigate
through information. All Brooktrout boards can recognize and
send DTMF digits.
DTMF digits include the standard twelve keypad digits, along
with additional ‘A’, ‘B’, ‘C’, and ‘D’ signals. Phone systems use
these additional signals to assign priority to a call.
12
3
54
6
78
9
0
#
*
A
B
C
D
1209 1336
1477
1633
697
770
852
941