Rx Audio Circuit; Figure 2-4. Rx Audio Block Diagram - Motorola CP185 Basic Service Manual

Theory Of Operation: Major Assemblies 2-5
2.2.4

RX Audio Circuit

The RX audio circuit consists of Audio Processor IC, Audio amp, speakers & Sub-tone system.
Audio Processor IC (AK2347)
RXA1 RX LPF TX/RX HPF
Audio IN Scrambler / De-
VR3 Expander VR4 SMF
(from IF IC) Descrambler emphasis
pin 24
-4 to +3.5dB /
-18, -4.5 to + 4.5dB /
0.5dB
0.25dB
pin 21
U102
Sub audio
Programmable -6 to +6dB /
Audio Amp.
LPF 0.5dB
Vol1
INT SPK. J601
VR5
IN-
OUT-
IN+
pin 18
Sub audio
OUT+
U105-A,B
SVR
U601
HPF
EXT SPK.
Compar
To CPU
ator
Audio Mute
(tone detect)
control
U105-C

Figure 2-4. RX Audio Block Diagram

The RX Audio from U201 is channeled to Audio processor IC. VR3 controls the received
demodulated signal level from -4.0 dB to +3.5 dB in 0.5 dB steps. RX LPF eliminates high-frequency
audio components > 3 kHz. TX/RX HPF eliminates low-frequency audio components lower < 250Hz.
Descrambler (if ON) inverts the spectrum distribution of audio signals with respect to scrambling
frequency. De-emphasis (if ON) restores high-frequency component of audio signal which has been
emphasized by the pre-emphasis circuit in transmitting radio. Expander (if ON) expands audio signal
by 0.5 dB to restore the original signal compressed by transmitting radio. VR4 amplifies RX audio
level by -18.0 dB, with -4.5 dB to +4.5 dB in 0.25 dB steps adjustment range. Smoothing filter (SMF)
eliminates high-frequency and clock components, generated by ASIC.
Sub-audio Programmable LPF totally eliminates voice audio from Audio signal to extract sub-audio
tone. VR5 regulates the output level of extracted sub-audio tone and sends it to a high pass filter
(U105-A,B) with 4 selectable cut-off frequencies and finally passes through a comparator (U105-C),
to square the signal and sends it to the MCU.
The output audio signal of Audio Processor IC is directed to volume control switch (SW/VOL1) which
is controlled by user and is finally amplified by U601BTL Audio Amplifier to a sufficient level to drive
either the external or internal speaker.