MSAN-108 Zarlink Semiconductor, Inc., MSAN-108 Datasheet - Page 7

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

Application Note
it reduces the probability that speech will maintain
DTMF simulation long enough to be considered
valid.
immunity because dropout (longer than t
noise pulses will restart t
environments, t
absent guard time, t
time allowed between successive DTMF signals. A
dropout shorter than t
and will not register as a successive valid tone
detection. This guards against multiple reception of
a single character. Therefore, lengthening
increase noise immunity and tolerance to the
presence of an unwanted third tone at the expense
of decreasing the maximum signalling rate.
The intricacies of the digital detection algorithm have
a
performance. It is here that the initial decision is
made to accept the signal as valid or reject it as
speech or noise.
Trade-offs must be made between eliminating talk off
errors and eliminating the effects of unwanted third
tone signals and noise.
conflicting events. On one hand valid DTMF signals
present in noise must be recognized which requires
relaxation of the detection criteria.
hand, relaxing the detection criteria increases the
probability of receiving
significant
MT8870
MT8870
The trade-off here is decreased noise
St/GT
V
St/GT
V
(a)
ESt
(c)
DD
ESt
DD
GTP
impact
R
1
should be decreased. The signal
GTA
R
V
C
GTA
C
"
V
DD
hits
DD
, determines the minimum
on
GTP
R
will be considered noise
"
t
t
2
GTP
GTA
due to talk off errors.
. Therefore, for noisy
the
[t
t
t
R
These are mutually
GTP
GTA
GTP
= (RC)In(V
= (RC)In(V
P
= R
= (R
= (R
> t
overall
1
R
GTA
1
P
2
C)In(V
C)In(V
/(R
On the other
]
DD
DD
Figure 6 - Guard Time Circuits
1
DA
/V
/[V
+ R
DD
DD
TSt
t
DD
) due to
GTA will
receiver
2
/V
/[V
)
)
-V
TSt
DD
TSt
)
-V
])
TSt
])
MT8870
Many considerations must be taken into account in
evaluating criteria for noise rejection.
telephony environment two sources of noise are
predominant.
which generally comes from dial tone harmonics,
and band-limited white noise .
complex
excellent immunity to voice, third tone and noise
signals which prevail in a typical voice bandwidth
channel.
The algorithm used in the MT8870 combines the
best features from two previous generations of Zarlink
digital decoders with improvements resulting from
years
environment. The algorithm has evolved through a
combination of statistical calculations and empirical
"
reliable decoder.
Applications
The proven reliability of DTMF signalling has created
a vast spectrum of possible applications.
recently, many of these applications were rendered
ineffective due to cost or size considerations. Now
that a complete DTMF receiver can be designed with
merely a single chip and a few external passive
components one can take full advantage of a highly
developed signalling scheme as a small, cost-
effective signalling solution.
tweaks
St/GT
V
ESt
DD
(b)
"
of
R
to result in the realization of an extremely
1
digital
practical
V
C
DD
These are, third tone interference,
a) Tone present and absent guard
b) Tone present less than tone
c) Tone present greater than tone
R
times equal.
absent guard time.
absent guard time.
2
averaging
Note: Typically V
use
[t
t
R
t
GTP
GTA
GTP
P
= R
= (R
= (R
< t
within
1
R
GTA
P
1
2
algorithm
TSt
C)In(V
C)In(V
MSAN-108
(R
]
=
In the MT8870 a
1
1
2
+ R
V
the
DD
DD
DD
2
/[V
/V
)
TSt
DD
telephone
)
-V
provides
TSt
In the
])
Until
A-51