Hytera MD78X, Service Manual

Page: 345 / 519

Circuit Description
UHF5 (806MHz
–
941MHz)
22
5.2.3
Audio Module
The audio module is used to output RX audio and input relay audio and TX audio, which serves as the
RX audio path and TX audio path.

RX Audio Path
U302 sends digital audio signals to the audio processor codec (U231) via the SSI bus of
MCBSP1.This bus is composed of CLKX, FSX, DX and DR. It sends the 16-digit demodulated PCM
audio signal to the codec (U231), and adjusts the signal to appropriate amplitude according to the
volume (RMS should be 80 mV when frequency is 1 kHz and deviation is 3 kHz). Then the codec
(U231) converts PCM data into analog audio data via DAC.
U231 provides two outputs: SPK1 and SPK2. SPK1 is amplified by U238 and then is fed to the PA
U201, to derive two outputs of received audio, which will be applied to the front panel speaker and
further development interface respectively. SPK2 is amplified and divided into the four paths of signal
Handset_audio, RX_audio, PUB_Address1 and PUB_Address2. The first two paths are amplified by
U235, and the last two paths are amplified by U236. Output of Handset_audio
,
PUB_Address1 and
PUB_Address2 are controlled by Q231, Q235 and Q233 respectively.

TX Audio Path
The audio processor U231 has two MIC inputs: the internal MIC and external MIC. The internal MIC
is connected to the MICIN_HND of U231, and is combined with AUX1 to provide differential input.