Vocoder Section - Motorola ASTRO Digital Spectra Plus Service Manual

Theory of Operation: ASTRO Spectra VOCON Board
3.3.3

Vocoder Section

Refer to Figure 3-8 on page 3-18
The vocoder section of the VOCON board is made up of a digital signal processor (DSP) (U405),
24k x24 static-RAM (SRAM) (U414, U403, and U402), 256kB FLASH ROM (U404), and
ABACUS II/DSP support IC (ADSIC) (U406).
The FLASH ROM (U404) contains the program code executed by the DSP. As with the FLASH ROM
used in the controller section, the FLASH ROM is reprogrammable so new features and algorithms
can be updated in the field as they become available. Depending on the mode and operation of the
DSP, corresponding program code is moved from the FLASH ROM into the faster SRAM, where it is
executed at full bus rate.
The ADSIC (U406) is basically a support IC for the DSP. It provides among other things, the interface
from the digital world of the DSP to the analog world. The ADSIC also provides some memory
management and provides interrupt control for the DSP processing algorithms. The configuration
programming of the ADSIC is performed by the MCU. However some components of the ADSIC are
controlled through a parallel memory mapped register bank by the DSP.
In the receive mode, The ADSIC (U406) acts as an interface to the ABACUS II IC, which can provide
digital output of I (In phase) and Q (Quadrature) data words directly to the DSP for processing. Or
the data can be filtered and discriminated by the ADSIC and data provided to the DSP as raw
discriminator sample data. The latter mode, with the ADSIC performing the filtering and
discrimination, is the typical mode of operation.
In the transmit mode, the ADSIC (U406) provides a serial digital-to-analog (D/A) converter. The data
generated by the DSP is filtered and reconstructed as an analog signal to present to the VCO and
Synthesizer as a modulation signal. Both the transmit and receive data paths between the DSP and
ADSIC are through the DSP SSI port.
6881076C25-D
and your specific schematic diagram.
3-17
October 28, 2002