Vocoder Board Overview; Vocoder Board - Motorola ASTRO XTS 3000 Service Manual

Vocoder Board

Overview
The vocoder board (see Figure 2-1.) contains the radio's DSP and
ADSIC circuitry. It is responsible for all voice and data processing and
signalling.
The DSP performs all signalling and voice encoding and decoding as
well as audio filtering and volume control. This includes Private-
®
Line /Digital Private Line™ (PL/DPL) encode and alert-tone
generation. The IC transmits pre-emphasis on analog signals and
applies a low-pass (splatter) filter to all transmitted signals. It is
programmed using parallel programming from the microcontrol unit
and the ADSIC.
The DSP executes a stored program located in its FLASH ROM. The
code is actually moved out of the FLASH ROM to the DSP's high-speed
SRAM, and executed from there.
The DSP also controls a crystal pull circuit to adjust its oscillator
frequency, preventing harmonics from interfering with the radio's
receive channel.
The ADSIC 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 ABACUS IC) and is programmed by the
microcontrol unit SPI bus.
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