GAI-Tronics ITR1000-001 Model ITR1000-001 Basic Tone Remote Deskset with Line Select User and Installation Manual - Manual (Page 25)

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Theory of Operation
Digital Circuits
On power-up, microcontroller U1 reads switch SWA through tri-state buffer U3. Switch readings
determine certain operating characteristics of the desk set.
Analog-to-Digital Conversions
Each keyboard button is associated with a particular voltage level at microcontroller inputs AN0 and
AN1. These inputs are periodically scanned to determine whether a key is being pressed or released.
The microcontroller determines whether the handset is on-hook, off-hook, or whether the handset PTT
bar is being pressed by reading the voltage level at input AN2.
Serial Peripheral Interface
The microcontroller uses a synchronous serial interface to control LEDs, provide audio control logic, and
to interface non-volatile memory U2. LEDs are turned on or off by the outputs of serial-to-parallel latch
U6. The microcontroller manages audio circuits through the outputs of serial-to-parallel latch U4.
Operating parameters such as function tone frequencies, etc., are stored in serial NVRAM U2.
Audio Circuits
Audio from the wire line is scaled through amplifier U12a and fed through the line audio adjustment,
POT3. Then, it is high-pass filtered through U12b to remove wire line hum. The resulting signal is gated
to the compressor circuit consisting of U11, Q8, and associated components.
Audio at TPC1, the compressor output, is notch-filtered to ensure that LLGT is not heard while listening
to transmissions from parallel desk sets. The notch filter consists of U10a, U10b, U10c, POT1 and
associated components. Wire line audio at TPN, the notch filter output, is then gated to amplifier U13b
where it is summed with any required tones (such as volume beeps) from the microcontroller. At the
output of U13b, audio is fed to speaker amplifier U15. Also, the signal can be gated to the handset
earpiece. Earpiece volume is adjusted through POT5.