Calling Line Identifier

FX602

© 1998 Consumer Microcircuits Limited

19

D/602/7

Note that the 50ms monitoring period of the CONFIRM_QUIET state when added to the 0.5 to 10ms de-

response time of the FX602 in Tone Alert Detect mode and the 0 to 5ms delay in switching to FSK mode

results in a valid CAS detection occurring at between 50 and 65 ms after the end of the CAS, leaving at least

35ms for muting the local handset fully, testing for off-hook extensions and initiating the dtmf ACK transmission

while still meeting the 100ms specified for CIDCW from end of the dual-tone burst to the start of the ACK. See

also note 3 below.

When the transaction has been completed then remove speech muting, reset the timer, set the FX602

to Tone Alert Detect mode and return to the INITIAL state.

6. WAIT_DET_LOW state

This state is entered from the MEAS_DET_HIGH state when the FX602 DET output has been high for 65ms

i.e. for too long to be a valid CAS.

Wait for the DET output to go low then reset the timer and return to the INITIAL state.

Times T1, T2 & T3

The values given below have been selected to give an extremely low incidence of false CAS detections while

maintaining a high probability of decoding correct CIDCW initiation signals by taking advantage of the specific

profile of FX602's responses to typical speech and CAS signals. Two options are given, the choice being

determined principally by how easily the local speech can be muted.

a)

When the CIDCW circuits are built into the telephone set so that locally generated speech can be

muted quickly and without injecting noise then it is recommended that:

T1 should be zero, i.e. muting should be initiated as soon as the FX602 DET output goes high.

T2, the time for local speech muting to take effect, should be as short as possible and in any

case not more than 5ms.

T3, the minimum length of a valid DET output high time, should be 15ms.

b)

When the CIDCW circuits are housed in an adjunct box so that muting is only possible by interrupting

the 2-wire connection to the telephone set, then it is recommended that:

T1 should be 15ms i.e. speech muting should only be initiated after the DET output has been

high for 15ms.

T2, the time for speech muting to take effect, should be as short as possible and in any case

not more than 5ms.

T3, the minimum length of a valid DET output high time, should be equal to T1 plus T2 plus

10ms, i.e. between 25 and 30ms.

Option (b) reduces the frequency of short speech mutes by a factor of about 5 at the expense of a small

increase in the number of missed CAS signals.