Tx Audio Circuit - Hytera MD78X Service Manual

Baseband Section Service Manual
4.5.2

TX Audio Circuit

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. After
9.3V is divided by R255 and R264, a bias voltage of about 7V is provided to the front panel MIC. The
external MIC is connected to the MIC_HED of the codec as a uni-directional input. Both MICs are
controlled by Q232. When the internal MIC is active, EXT_MIC_EN is at low level, bias voltage of
external MIC is disconnected, and the ADC in U231 samples voltage at the MICIN_HND. When the
external MIC is active, EXT_MIC_EN is at high level, Q232 is turned on, bias voltage is connected,
and the ADC in U231 samples only audio data at MICIN_HED.
Audio signals input from the MMP interface or the accessory jack, are fed to the codec (gain of codec
is programmable via the CPS) and then are converted by the ADC into 16-bit PCM digital audio, which
will be fed to U302 via the SSI interface.
When the radio is set to operate in loudspeaker mode, audio signal is generated by the internal MIC,
and then feeds to U231, and finally is output via SPK2. The output audio signal is amplified, and then
is output as the two paths of audio PUB_ADDRESS1 and PUB_ADDRESS2.
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