MSAN-108 Zarlink Semiconductor, Inc., MSAN-108 Datasheet - Page 10

• Manufacturer Part Number: MSAN-108
• Description: Applications of The MT8870 Integrated DTMF Receiver
• Manufacturer: Zarlink Semiconductor, Inc.

MSAN-108
"
between the low and high group tones. It is specified
in dB as:
Twist is usually caused by the frequency response
characteristic of the communication channel. Along a
telephone line higher frequencies tend to roll off
faster than the lower ones so the line response is
usually compensated for by applying pre-emphasis
(negative twist) to the originating DTMF signal. In
extreme
compensation. This could be realized with a filter
arrangement utilizing the input op amp.
Any communication path that can pass the human
voice spectrum is eligible for DTMF signalling.
Therefore a variety of ”front-end“ interfaces may be
applicable in a given control system.
commonly used media are copper wire and RF
channels. An optical fibre could carry a light beam
modulated by DTMF. Although this would incur a
large overhead in terms of bandwidth utilization,
optical fibres do offer isolation from external electro-
magnetic interference.
data signals must be sent near a high power
transmission line environment, strong electric and
magnetic fields could have a devastating effect on
signals transmitted over wires.
A-54
Twist
where V
and V
ZENERS ARE 15V 250mW
RESISTORS ARE 1% 1/4 W (unless otherwise stated)
RING
TIP
"
H
is known as the difference in amplitude
is the amplitude of the high frequency tone.
L
is the amplitude of the low frequency tone
cases
TWIST = 20 log
630 V
630 V
0.01 f
0.01 f
the
110 K
1W
110 K
1W
For example, if control or
10
receiver
V
V
Figure 10 - MT8870 Front End Protection Circuit
H
L
DTMF over fibre-
may
require
More
110 K
110 K
optics could easily be employed as a highly reliable
communications method in a harsh interference
infested environment.
In modern digital switching equipment the MT8870
can easily be interfaced to a digital PCM line by
using a codec as an input interface (Fig. 11).
Actually, all that is required for the interface is a PCM
decoder. In fact, the output filter that normally is
associated with PCM decoders is not required since
the high group DTMF bandpass filter has an upper
cutoff frequency low enough to meet the required
roll-off of the PCM filter.
DTMF In Mobile Radio Applications
DTMF
distributed communications systems, such as multi-
user mobile radio (Fig. 12). It is a
two-way radio environment since it slips neatly into
the center of the voice spectrum, has excellent noise
immunity
implementation are currently available.
directly
simplifying automatic phone patch systems.
Several
currently use DTMF signals to control radio
repeaters.
identification, selection of appropriate repeater links,
selection
repeater status, and for completing automatic phone
patch links.
If available in a system of this type, audio from a
long distance communications link (microwave,
K
52
signalling
compatible
emergency
K
68
of
and
repeater
Functions
highly
220
K
plays
with
medical
frequencies,
integrated
an
are,
1
2
3
4
telephone
Application Note
V
Ref
important
MT8870
service
typically,
"
natural
methods
reading
signalling,
It is also
networks
role
"
mobile
in the
of
of
in