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| 型號： | MT8870 |
| 廠商： | Mitel Networks Corporation |
| 英文描述： | () |
| 中文描述： | （） |
| 文件頁數(shù)： | 7/23頁 |
| 文件大?。?/td> | 177K |
| 代理商： | MT8870 |
Application Note
MSAN-108
A-51
Figure 6 - Guard Time Circuits
V
DD
V
DD
St/GT
ESt
R
R
1
R
2
C
MT8870
V
DD
V
DD
St/GT
ESt
MT8870
C
R
1
R
2
V
DD
V
DD
St/GT
ESt
MT8870
C
t
GTP
= (RC)In(V
DD
/V
TSt
)
t
GTA
= (RC)In(V
DD
/[V
DD
-V
TSt
])
(a)
(b)
[t
GTP
> t
GTA
]
t
GTP
= (R
1
C)In(V
DD
/V
TSt
)
t
GTA
= (R
P
C)In(V
DD
/[V
DD
-V
TSt
])
R
P
= R
1
R
2
/(R
1
+ R
2
)
[t
GTP
< t
GTA
]
t
GTP
= (R
P
C)In(V
DD
/V
TSt
)
R
P
= R
1
R
2
(R
1
+ R
2
)
t
GTA
= (R
1
C)In(V
DD
/[V
DD
-V
TSt
])
a) Tone present and absent guard
times equal.
b) Tone present less than tone
absent guard time.
c) Tone present greater than tone
absent guard time.
Note: Typically V
TSt
=
V
DD
1
2
(c)
it reduces the probability that speech will maintain
DTMF simulation long enough to be considered
valid. The trade-off here is decreased noise
immunity because dropout (longer than t
DA
) due to
noise pulses will restart t
GTP
. Therefore, for noisy
environments, t
GTP
should be decreased. The signal
absent guard time, t
GTA
, determines the minimum
time allowed between successive DTMF signals. A
dropout shorter than t
GTA
will be considered noise
and will not register as a successive valid tone
detection. This guards against multiple reception of
a single character. Therefore, lengthening
t
GTA
will
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 significant impact on the overall receiver
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. These are mutually
conflicting events. On one hand valid DTMF signals
present in noise must be recognized which requires
relaxation of the detection criteria. On the other
hand, relaxing the detection criteria increases the
probability of receiving
"
hits
"
due to talk off errors.
Many considerations must be taken into account in
evaluating criteria for noise rejection. In the
telephony environment two sources of noise are
predominant. These are, third tone interference,
which generally comes from dial tone harmonics,
and band-limited white noise . In the MT8870 a
complex
digital
averaging
excellent immunity to voice, third tone and noise
signals which prevail in a typical voice bandwidth
channel.
algorithm
provides
The algorithm used in the MT8870 combines the
best features from two previous generations of Mitel
digital decoders with improvements resulting from
years of practical use within the telephone
environment. The algorithm has evolved through a
combination of statistical calculations and empirical
"
tweaks
"
to result in the realization of an extremely
reliable decoder.
Applications
The proven reliability of DTMF signalling has created
a vast spectrum of possible applications. Until
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.
相關(guān)PDF資料 |
PDF描述 |
|---|---|
| MT8880C | ISO2-CMOS Integrated DTMFTransceiver |
| MT8880CE-1 | ISO2-CMOS Integrated DTMFTransceiver |
| MT8880CC | ISO2-CMOS Integrated DTMFTransceiver |
| MT8880CE | ISO2-CMOS Integrated DTMFTransceiver |
| MT8880CN | ISO2-CMOS Integrated DTMFTransceiver |
相關(guān)代理商/技術(shù)參數(shù) |
參數(shù)描述 |
|---|---|
| MT8870BE | 制造商:Panasonic Industrial Company 功能描述:RECEIVER |
| MT8870D | 制造商:MITEL 制造商全稱:Mitel Networks Corporation 功能描述:ISO2-CMOS Integrated DTMF Receiver |
| MT8870DC | 制造商:MITEL 制造商全稱:Mitel Networks Corporation 功能描述:ISO2-CMOS Integrated DTMF Receiver |
| MT8870DC-1 | 制造商:MITEL 制造商全稱:Mitel Networks Corporation 功能描述:ISO2-CMOS Integrated DTMF Receiver |
| MT8870DE | 制造商:Microsemi Corporation 功能描述: |
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