Apco, Crosspatch, And Audio Jack Receive Audio; Wireline Receive Audio - Motorola APX 7500 Detailed Service Manual

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2.3.15.3 APCO, Crosspatch, and Audio Jack Receive Audio

Receive analog audio originating from Codec OUTP3 (RX_FILT_AUDIO) is the single-ended audio
source for APCO receive audio, crosspatch receive audio, audio jack receive audio, and wireline
receive audio. The audio coming from Codec OUTP3 splits off for APCO, crosspatch, and audio jack
receive audio and first passes through muting gate U1810 which is controlled by OMAP output
RX_TONES_CTRL (U400 pin Y15). It then splits again and passes through two of the op amps in
quad buffer op amp package U1802. The output of U1802 pin 14 (RX_AUDIO_BUFF) is sent to the
low speed connector board's Audio Jack Connector P100 pins 2 and 3 (the same receive audio is
output at both the right and left speaker connections). This audio jack receive audio is at a fixed level
of approximately 300 mVrms relative to a 1.0 kHz tone at 60% FSD. The output of U1802 pin 8
(RX_AUDIO) is sent to both the high speed connector board's Crosspatch Connector J24 pin 1 and
the low speed connector board's Accessories DB25 connector J104 pin 4 (the A(rx) portion of the
Consolette APCO Interface). Both of these audio paths are also at a fixed level of approximately 300
mVrms relative to a 1.0 kHz tone at 60% FSD. Additionally, for the crosspatch configuration, there is
a channel active output which indicates when receive audio is present. CP_CHAN_ACTIVITY*
(driven low by open drain MOSFET Q1813 when valid receive audio is present) is located on the
high speed connector board's Crosspatch Connector J24 pin 2. When a Consolette is connected in a
crosspatch configuration, this CP_CHAN_ACTIVITY* output will drive the active low PTT input and
key up whatever communication device (most likely another Consolette) it is connected to. For the
Consolette APCO interface, a channel active solid state relay (K1800) is provided. This is especially
useful when the customer's setup has a signal with a high voltage or if the customer wants to switch
in a ground signal. The relay circuitry has a maximum load voltage of 350V and a maximum load
current of 120 mA. Access to the input and output of the relay are found on the low speed connector
board's DB25 Accessories Connector J104. CHAN_ACT_RELAY_IN (J104 pin 21) is the input to the
relay and CHAN_ACT_RELAY_OUT (J104 pin 25) is the relay's output. The relay is normally open
and controlled by OMAP output CHAN_ACT (U400 pin P20). When valid receive audio is present,
CHAN_ACT turns on transistor Q1803 which causes a contact closure of the relay.

2.3.15.4 Wireline Receive Audio

The audio coming from Codec OUTP3 (RX_FILT_AUDIO) is also split off for RX wireline audio
(RX_FILT_AUDIO_WL). It first passes through the line adjust circuit made up of op amp U1818 and
EEPOT U1803. This circuit allows the notched, filtered audio level to be adjusted via EEPOT U1803
which is controlled by the OMAP's I2C bus; moreover, this is done by varying the "Wireline Out (RX)
Gain Setting" in the Consolette Configuration, thus adjusting the audio level to the wireline. This
allows compensation for line losses to obtain the desired audio levels. The adjusted audio is then
split into opposing phases in the line driver circuit (quad op amp package U1808, Q1801 and
Q1802), where receiver audio is applied to the line transformer T1801 and the wireline. With two-
wire selected, the line driver audio is also applied to the input of the transmit audio/tones detection
path through T1801 pin 4. A portion of the line driver audio is also fed into the input amplifier of the
transmit audio/tone detection path, via the output of U1808 pin 7 and switch U1805; this audio is
approximately 180° out of phase with the received audio at T1801. This is done in order to, at least
partially, cancel the receive audio so that a tone from a control console may be more easily detected
in the presence of receiver audio. With four-wire selected, this cancellation is not necessary since
switch U1805's connection to the two-wire interface is open, while U1805's connection to the four-
wire interface is closed.
September 9, 2011
Theory of Operation: System Communications Overview
68009482001