Audio Processing And Digital Volume Control; Audio Amplification Speaker (+) Speaker (-) - Motorola GM-950 Manual

CH ACT is routed to the µP pin 25 (pin 40 from version 0102726B09_Cntl on) while SQ DET adds
up with LOCK DET, weighted by resistors R0113, R0114, and is routed to one of the µP´s ADC
inputs U0101-43. From the voltage weighted by the resistors the µP determines whether SQ DET,
LOCK DET or both are active.
SQ DET is used to determine all audio mute/unmute decisions except for Conventional Scan. In this
case CH ACT is a pre-indicator as it occurs slightly faster than SQ DET.
6.2

Audio Processing and Digital Volume Control

The receiver audio signal enters the controller section from the IF IC U5201-28 (UHF) / U5201-28
(VHF) / U2201-28 (MB) on DET AUDIO and passes through RC filter, R0203 and C0208 which
filters out IF noise. The signal is AC coupled by C0207 and enters the ASFIC via the PL IN pin
U0201-J7.
Inside the ASFIC, the signal goes through 2 paths in parallel, the audio path and the PL/DPL path.
The audio path has a programmable amplifier, whose setting is based on the channel bandwidth
being received, then a LPF filter to remove any frequency components above 3000Hz and then an
HPF to strip off any sub-audible data below 300Hz. Next, the recovered audio passes through a de-
emphasis filter if it is enabled (to compensate for Pre-emphasis which is used to reduce the effects
of FM noise). The IC then passes the audio through the 8-bit programmable attenuator whose level
is set depending on the value of the volume control. Finally the filtered audio signal passes through
an output buffer within the ASFIC. The audio signal exits the ASFIC at RX AUDIO (U0201-J4).
The µP programs the attenuator, using the SPI BUS, based on the volume setting. The minimum /
maximum settings of the attenuator are set by codeplug parameters.
Since sub-audible signalling is summed with voice information on transmit, it must be separated
from the voice information before processing. Any sub-audible signalling enters the ASFIC from the
IF IC at PL IN U0201-J7. Once inside it goes through the PL/DPL path.
The signal first passes through one of 2 low pass filters, either PL low pass filter or DPL/LST low
pass filter. Either signal is then filtered and goes through a limiter and exits the ASFIC at PL LIM
(U0201-A4). At this point the signal will appear as a square wave version of the sub-audible signal
which the radio received. The microprocessor (U0101-10) will decode the signal directly to
determine if it is the tone/code which is currently active on that mode.
6.3 Audio Amplification Speaker (+) Speaker (-)
The output of the ASFIC's digital volume pot, U0201-J4 is routed through a voltage divider formed
by R0401 and R0402 to set the correct input level to the audio PA (U0401). This is necessary
because the gain of the audio PA is 46 dB, and the ASFIC output is capable of overdriving the PA
unless the maximum volume is limited.
The audio then passes through C0401 which provides AC coupling and low frequency roll-off.
C0402 provides high frequency roll-off as the audio signal is routed to pins 1 and 9 of the audio
power amplifier U0401.
The audio power amplifier has one inverted and one non-inverted output that produces the
differential audio output SPK+ / SPK- (U0401-4/6). The inputs for each of these amplifiers are pins 1
and 9 respectively; these inputs are both tied to the received audio. The audio PA's DC biases are
not activated until the audio PA is enabled at pin 8.
Theory of Operation
Receive Audio Circuits
5C.3-15