HFC-4S Cologne Chip AG, HFC-4S Datasheet - Page 204

• Manufacturer Part Number: HFC-4S
• Description: Isdn HDLC Fifo Controller With 8 (4) Integrated S/t Interfaces
• Manufacturer: Cologne Chip AG

HFC-4S
HFC-8S
9.1 DTMF detection engine
The transmission of dialed numbers on analog lines is normaly done by DTMF (Dual Tone Multi-
Frequency). This means that pairs of two frequencies are used to determine one key of a keypad like
shown in Table 9.2.
Thus there are 4 low tones and 4 high tones and therefore 16 combinations of 2 tones. Because the
ISDN network has several interfaces to the old-fashioned POTS analog network, in-band number
dialing with DTMF can take place. To decode this DTMF information the HFC-4S / 8S has a built
in DTMF detection engine.
The detection is done by the digital processing of the PCM input data by the so-called Goerzel Algo-
rithm
where
is a constant for each frequency and
every 125 s for 16 or 32
The start condition is
After processing equation (9.1) for
The calculation of equation (9.1) is done for every new PCM sample value (for all 8 frequencies)
every 125 s. Optionally also the second harmonic (double frequency) is also investigated. The
factors are values concerning to the DTMF frequencies. If the DTMF calculation is implemented in
integer arithmetic, it is useful to multiply
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The DTMF engine must be enabled by setting bit V_DTMF_EN in register R_DTMF0. How many
iterations are calculated with the Goerzel algorithm is determined by the register value V_DTMF1 in
the register R_DTMF1. A good compromise between bandwith of the Goerzel filter and the length
of the investigation is a value of 102. A DTMF detection can be done on a continuous base. However
204 of 273
values are listed in Table 9.3.
Ï
Ò
·½
is a coefficient calculated from the 2 previous coefficients
Ï
¼
1209
Ï
1
4
7
£
Ò
Ï
·½
Table 9.2: DTMF tones on a 16 keys keypad
¾
½
values.
1336
Ï
2
5
8
0
Æ
Ï
Keypad
Ò
Ã
Ü
¼
·½
.
times the real power amplitude is
Æ
¾
is a new PCM value every 125 s. Equation ( 9.1) is calculated
1477
·
DTMF controller
¾ Ó× ¾
3
6
9
#
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Ï
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Data Sheet
Æ
with
¾
¡
Ï
1633
½
A
B
C
D
Ò
¾
½
¡
Ã
Ï
to exploit the whole 16 bit value range. These
high tones (
Frequencies
697
770
852
941
¼¼¼ ÀÞ
¡
Ò
Ï
½
Æ
·
¡
Ï
low tones
Ü
(
Æ
ÀÞ )
ÀÞ )
½
Ï
Ò
and
March 2003 (rev. A)
Ï
Ò
½
. The factor
Cologne
Chip
(9.1)
(9.2)
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