Motorola MTM700 Detailed Service Manual page 78

4.2 - 22
The Audio system consists of the following components:
• RX and TX path multiplexer
• RX and TX programmable potentiometer
• CODEC
• DSP
• Audio PA
TX Path
The speech audio is fed to the internal microphone input (INT_MIC), the external microphone input
(EXT_MIC) or the TX audio input (TX_AUDIO). The internal and external microphone inputs have a
DC bias prepared for connecting a microphone. The TX audio input has a DC blocking capacitor.
Multiplexer U0201 selects one of the possible inputs and feeds the signal to amplifier U206-4. The
gain of this amplifier is switchable via gate U0202. The programmable potentiometer U0203 attenu-
ates the signal according to the radio mode of operation. Finally, the CODEC FL0200 converts the
analog signal into digital format and transfers it to the DSP via the serial audio port (SAP). The DSP
performs the functions of audio filtering, ACELP speech compression, digital modulation, and trans-
fers the data to the RF section. When the radio is operating in the telephone interconnect mode, the
DSP performs the required tasks such as echo and noise reduction.
RX Path
The receiver feeds the digital output signal via the baseband port (BBP) to the DSP which performs
the functions of digital demodulation, ACELP speech de-compression, and audio filtering. After
processing, the DSP feeds the signal via the serial audio port (SAP) to the CODEC FL0200. The
CODEC converts the digital audio format to an analog signal. One of the CODEC's analog output
signals is amplified by U0206-1 and then fed to the RX_AUDIO pin on the 20 pin accessory connec-
tor J0400. The second output signal from the CODEC is fed to the programmable potentiometer
U0204 which attenuates the analog signal according to the radio mode of operation or the setting of
the volume control. The multiplexer U0201 selects between the signal from the potentiometer or the
signal SP_AUDIO from the 40 pin expansion head connector J500 and routes the audio to the audio
power amplifier (PA) U0205 and the amplifier U0206-2. The amplified audio from U0206-2 is availa-
ble on the HANDSET_AUDIO pins on the 20 pin accessory connector J0400 and the 40 pin expan-
sion head connector J0500. The audio PA has one inverted and one non-inverted output that
produces the differential audio output signals SPEAKER+ and SPEAKER-. Both signals are availa-
ble on the 20 pin accessory connector J0400 and the 12 pin control head connector J0550.
MTM700 Mobile Radio / Detailed Service Manual THEORY OF OPERATION (800 MHz)