MT8888C/MT8888C-1
applications, leaving R1 to be selected by the
designer. Different steering arrangements may be
used to select independent tone present (t
GTP
) and
tone absent (t
GTA
) guard times. This may be
necessary to meet system specifications which place
both accept and reject limits on tone duration and
interdigital pause. Guard time adjustment also allows
the designer to tailor system parameters such as talk
off and noise immunity.
Increasing t
REC
improves talk-off performance since
it reduces the probability that tones simulated by
speech will maintain a valid signal condition long
enough to be registered. Alternatively, a relatively
short t
REC
with a long t
DO
would be appropriate for
extremely noisy environments where fast acquisition
time and immunity to tone drop-outs are required.
Design information for guard time adjustment is
shown in Figure 6. The receiver timing is shown in
Figure 7 with a description of the events in Figure 9.
mode has been selected. DTMF signals cannot be
detected if CP mode has been selected (see Table
7). Figure 8 indicates the useful detect bandwidth of
the call progress filter. Frequencies presented to the
input, which are within the ‘accept’ bandwidth limits
of the filter, are hard-limited by a high gain
comparator with the IRQ/CP pin serving as the
output. The squarewave output obtained from the
schmitt trigger can be analyzed by a microprocessor
or counter arrangement to determine the nature of
the call progress tone being detected. Frequencies
which are in the ‘reject’ area will not be detected and
consequently the IRQ/CP pin will remain low.
LEVEL
(dBm)
-25
AAAAAAAAAAAAA
AAAAAAAAAAAAA
AAAAAAAAAAAAA
AAAAAAAAAAAAA
AAAAAAAAAAAAA
Call Progress Filter
A call progress mode, using the MT8888C/
MT8888C-1, can be selected allowing the detection
of various tones, which identify the progress of a
telephone call on the network. The call progress
tone input and DTMF input are common, however,
call progress tones can only be detected when CP
0
= Reject
250
500
750
FREQUENCY (Hz)
= May Accept
AAAAAAAAAA
AAAAAAAAAA
= Accept
AAAAAAAAAA
AAAAAAAAAA
Figure 8 - Call Progress Response
EVENTS
t
REC
V
in
A
t
REC
B
t
ID
C
D
t
DO
TONE
#n + 1
E
F
TONE #n
t
DP
t
DA
t
GTP
TONE
#n + 1
ESt
t
GTA
V
TSt
t
PStRX
St/GT
RX
0
-RX
3
DECODED TONE # (n-1)
#n
t
PStb3
# (n + 1)
b3
b2
Read
Status
Register
IRQ/CP
Figure 7 - Receiver Timing Diagram
4-95
MITEL [ MITEL NETWORKS CORPORATION ]
