Digital signal processing, Tx audio – GAI-Tronics MRTI 2000 (No. PL1877A) Microprocessor Radio Telephone Interconnect Installation & Service Manual User Manual
Page 164
Theory of Operation
PL1877A Microprocessor Radio Telephone Interconnect
12/10 158
Digital Signal Processing
Receive audio enters the module through U18a. U24b and U18, configured as a digitally controlled gain
stage, are under processor control via data lines MPD8-15. The leveled audio signal passes through an
anti-aliasing filter, U24c, and onto the encoder portion of CODEC U20.
The CODEC is a device that “quantizes” (samples) the analog audio signal according to “mu”-law coding
conventions (i.e., companding). Output at U20-11 is a serial 8-bit code suitable for conditioning by the
DSP algorithms. The conditioned receive audio data, in serial form from U10, is input to the receive data
(DR input, pin-6) of the CODEC. There it is restored to analog by the decoder portion of the CODEC.
The CODEC expands the signal (decompression) back to its original dynamics, reversing the
compression that took place in the encoding process. This process is known as companding.
The restored audio signal is without much of its original undesirable attributes, such as noise, hum, etc.
Also if de-emphasis is selected by parameter 1, audio is restored to flat response. Reset to the DSP
processor, asserted on power up to assure proper initialization (provided by U4), is under control of the
HOST processor.
TX Audio
The audio processing that occurs to the TX (phone line to transmitter audio), is similar to the process that
occurs to receive audio. However, different algorithms are used to detect VOX, dial-tone and call
progress tones, and DTMF tones. Audio enters the DSP module and is directed to U19-19.
The process of audio signal leveling is essentially the same to that used in the receiver path. Once auto-
leveled, the analog transmit audio is routed through CODEC U21, where companding takes place as in
the case of receive audio. From there, the PCM signal is handled by the DSP processor, and
reconstituted as analog audio by the CODEC.