Rx Tone Pl/Digital Pl Decode Filtering; Audio Amp; Internal And External Speaker - Motorola EP350 Service Manual

4-2
• De-emphasis: This circuit restores the original state of signal of which high-frequency
component has been emphasized by the pre-emphasis.
• Expander: This circuit expands the signal compressed twice by the Compressor in dB scale to
restore the original signal state.
• VR4: This circuit controls the volume for adjusting the RX output level.
Adjustment range: -18.0dB, -4.5dB to +4.5dB in 0.25dB steps.
• SMF: Smoothing filter to eliminate the high-frequency and clock components generated in the
Audio processor IC.

4.1.1.2 RX Tone PL/Digital PL Decode Filtering

• Sub-audio Programmable LPF: Low-pass filter to eliminate components of the RXA1 signal.
This circuit is controlled by the internal registers and by the audio processor IC for cut-off
frequency.
• VR5: This circuit controls the volume for adjusting the output level from the sub-audio LPF
signal. Adjustment range: -6.0dB, +6.0dB in 0.5dB steps
The sub-audio tone of U102 Pin 18 output pass through switchable high-pass filter U105. This filter
(U105-A, B) has 4 different high pass cut-off frequencies which can be selected accordingly to the
sub-tone audio. This filter (U105-A, B) filters the unwanted sub-tone. The tone of U105-B output
passed through U105-C comparator, which acts as a squaring circuit. This output signal is then sent
to microprocessor Pin 3. The micro-processor then decodes CTCSS and CDCSS from this square
signal.
4.1.2

Audio Amp

The de-emphasized audio signal from Pin 21 of audio processor IC passes through volume control
(SW/VOL1) and is amplified by U601BTL audio amplifier to a sufficient level to drive a loud speaker.
U601 has mute/un-mute function controlled by audio-mute control (Pin 83) of CPU. When U601
Pin 1 is low, the audio amp goes to active (un-mute) mode. When U601 Pin 1 is high, the audio amp
goes to mute mode.
4.1.3

Internal and External Speaker

When no external speaker is plugged into J601, the RX audio is passed through to the internal
speaker. When an external speaker is plugged into J601, the internal speaker is cut-off from the
rest of the circuit and RX audio is passed through the external speaker.
Controller Theory of Operation: RX Audio Circuit