The microphone pushbutton switch (S3) is used to turn on the mic channel by pushing it in, and to mute the mic channel by leaving it out. The mic LED is powered on when the mic is on by using the other half of the switch.
The phone line input is meant to monitor both ends of a phone conversation. An external local phone connected to the output phone jack is needed to per- form dialing and to take the phone line “off hook” in order to answer a call.
The phone input can be disconnected from the phone lines and muted by us- ing the phone pushbutton switch. Like the mic switch, mute is “out” and on is “in”. The phone signal is filtered for unwanted high frequency noise by C3 and C4; this is called low-pass filtering (LPF). It is also filtered for unwanted low frequency noise by C5, which is called high-pass filtering (HPF). This cap is also used to block the high DC voltage on the phone line from passing through the isolation transformer, T1. Because of this DC voltage which normally is
50 VDC off-hook, and can even have spikes upwards of hundreds of volts, the first three caps in the circuit (C3, C4, C5) were chosen to be able to handle 1 kV each. R30 performs some signal attenuation and isolation, and then the isolation transformer T1 further helps prevent unwanted interference between the phone line circuit and the PPM3 circuitry. T1 also takes the differential phone audio signal and converts it to a single-ended signal (which means one side is grounded). C31 performs some more LPF, and C16 performs some more HPF. R31 is one more protection against the PPM3 circuit interfering with the phone line circuits and vise-versa. The two diodes, D1 and D2 pro- vide a final protection against high-level signals reaching the PPM3 circuitry and potentially doing damage. They provide an upper voltage limit of 0.7 volts and a lower voltage limit of –0.7 volts. Any signal outside these limits will be shorted to ground. At this point, the signal is amplified with the same type of pre-amp that mic circuit used, but with a different compression level. The problem with monitoring both sides of a phone conversation is that the signal from the local phone is usually much higher is amplitude than the signal from
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