Function:
The tone dialing circuit consists of a DTMF (Dual Tone Multi Frequency) signal generator
(outputted from) for tone dialing, and also a circuit for outputting the signal to line.
The DTMF circuit indentifies inputs from the 12 keys (1, 2, 3, 4, 5, 6, 7, 8, 9, 0, and ) by
means of a total of seven frequencies, that is four low frequencies (Low group) and three high
frequencies (High group).
Signal Pass:
Refer to SIGNAL ROUTE ().
The signal combination and frequency corresponding to each dial key are shown below.
12.4. Pulse Dial Circuit
Circuit Operation: ( )...Line 2
The dial pulses are generated by the CPU IC801, and reach the Telephone Line via the following
path;
pin 68 (pin 66) of IC801 PC8 (PC7) Q130 and Q131 (Q230 and Q231) D131 and D130
(D231 and D230)
Telephone Line
12.5. Speakerphone Circuit
Function:
The circuit controls the automatic switching of the transmitted and received signals, to and from
the telephone line, when the unit is used in the hands -free mode.
Circuit Operation:
The speakerphone can only provide a one-way communication path.
In other words, it can either transmit an outgoing signal or receive an incoming signal at a given
time, but cannot do both simultaneously. Therefore, a switching circuit is necessary to control
the flow of the outgoing and incoming signals.
This switching circuit is contained in IC401 and consists of a Voice Detector, TX Attenuator, RX
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