Motorola ASTRO DIGITAL XTS 3000 series Basic Service Manual page 40

also required for functionality. It is programmed using parallel
programming from the microcontrol unit and the DSP-support IC.
The DSP-support IC performs analog-to-digital and digital-to-analog
conversions on audio signals. It contains attenuators for volume,
squelch, deviation, and compensation, and it executes receiver
filtering and discrimination. The IC requires a 2.4MHz clock to
function (generated by the digital back-end IC) and is programmed by
the microcontrol unit SPI bus.
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