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10.4. Caller ID Detect Circuit
10.4.1. Function (FSK signal)
The caller ID is a changeable ID which the user of a telephone circuit obtains by entering a
contract with the telephone company to utilize a caller ID service. For this reason, the operation
of this circuit assumes that a caller ID service contract has been entered for the circuit being
used. The data for the caller ID from the telephone exchange is sent during the interval between
the first and second rings of the bell signal. The data from the telephone exchange is a modem
signal which is modulated in an FSK (Frequency Shift Keying) format. Data "0" is a 1200 Hz sine
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