1.9. OPERATIONAL MODES AND CONTROLS. (CONT)
INCOMING CALL WITH MODE SELECTOR SWITCH IN AC SUPV POSITION
When the telephone set is installed for operation with a switching facility and the MODE SELECTOR SWITCH is in AC SUPV position,
the called telephone set is signaled by a 570 Hz ring signal. The following functions occur in the order given below:
The 570 Hz signal is received on the receive terminals of the telephone set. The receive terminals are inductively coupled to the input
of the 570 Hz amplifier in the ring acknowledge tone detector circuit. However, the output of the circuit does not energize the integrator
circuit because the 570 Hz signal is modulated.
The capacitor on the input to the integrator acts as a short circuit to the modulation frequency. The detector output is used to energize
The 1633 Hz oscillator switches on and off at modulation frequency. The output of the oscillator is coupled to the tone ringer circuit
receive element. The telephone set rings at a frequency of 1633 Hz and the ringing signal is stopped at the modulation rate of the
incoming ring signal.
The call is answered by lifting the handset from its cradle. When the handset is lifted from its cradle, the hookswitch contacts change
the polarity of the input to the release timer circuit. This energizes the release timer, the output of which goes to the release oscillator
circuit and the send amplifier.
The send amplifier sends the 2550 Hz ring-trip-tone to the switching facility. The switching facility acknowledges the ring-trip-tone,
stops sending a ringtone, and sends a steady 570 Hz ring-trip-tone to the telephone set.
The ring-trip-tone is received on the receive terminals of the telephone set which are coupled to the 570 Hz amplifier. Here the signal is
amplified and detected.
The ring-trip-tone is amplified and the output of the detector energizes the integrator circuit. The integrator circuit provides a dc output
which energizes the latch circuit. The latch circuit, when energized, stops the ring-trip-tone and energizes the receive amplifier and the
sidetone gate circuit.
The receive amplifier is now active and a voice path is established between the telephone set and the switching facility.
When the handset is placed in its cradle at the end of a call, a release tone is sent to the switching facility.