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CIRCUIT DESCRIPTION
When the audio received must be routed to the
external speaker or speaker/microphone, Local Audio
Mute Gate U114D is enabled by U111/latch U108.
The audio is passed to local audio output amplifier
U132. The gain of U132 is adjusted by the local audio
volume control and the on/off switch.
6.12.7 COMPANDOR OPTION
The compandor option enhances the receive and
transmit audio when used in conjunction with the
Telephone Interface Card (TIC) in LTR systems.
The filtered Receive Audio passes through the
Receive Mute Gate U113C to the expander input on
A301, pin 1. The expand output of A301, pin 2 is
coupled to the audio outputs by U114C.
The TX-VOICE on P100, pin 32, passes through
the TX Voice Gate U158A to the expander input on
A301, pin 5. The compressed output of A301, pin 4 is
passed to the Transmit Audio Buffer.
6.12.8 TRANSMIT AUDIO
PTT switch (Q101/Q102) provides local micro-
phone Push-To-Talk (PTT) indication to U105. U105
then tells U111 via the data bus that the local micro-
phone PTT has been activated.
U164A amplifies the microphone audio signal to
provide the correct input level to U164B. Local
Microphone Mute Gate U117C is controlled by A/D
processor U111/latch 106. The function of U117C is
to mute the local microphone audio when the local
microphone PTT switch is pressed. This prevents
interference if the microphone remains live when the
PTT switch is pressed. Buffer U164B combines the
microphone audio signal from U164A with the audio
signal from the Repeat Gate U153C.
U127B/C form a high-pass filter that attenuates
frequencies below 300 Hz to prevent interference with
the LTR data applied at U129B. Pre-emphasis at 6 dB
per octave is provided by an RC combination before
the signal is fed to the Limiter U127D.
Limiter U127D and rectifiers form a precision
limiter which prevents over modulation caused by
high-level input signals. With normal input levels, the
output of a bridge rectifier follows the input of the
October 1999
Part No. 001-2009-201
bridge. When a high-level signal is applied to the
bridge, the bridge opens and the output of the bridge is
limited to a specific level.
The output of the limiter passes to a composite 6-
pole splatter filter formed by U127A, U128D and
U128A separated by buffers U128B and U128C.
The output from U128A is fed to Normal Modu-
lation Mute Gate U118B that is controlled by A/D
processor U111/latch U106. When enabled, the gate
passes transmit audio to EEPOT U149. U149 is an
electronically adjustable potentiometer that adjusts the
gain of transmit audio amplifier U129C. The gain of
U129C can only be adjusted through the software,
therefore, a computer must be attached to the MAC
card when levels are set.
The output of U129C is fed to summing amplifier
U129B where it is combined with LTR transmit data
and CWID when present. The gain of audio and data
are the same so unity gain is produced. The output
signal is fed to the TCXO where it frequency modu-
lates the transmit signal.
6.12.9 TRANSMIT AUDIO PROCESSING
Transmit voice from the backplane comes into
the MAC on P100, pin 32. When used this signal
passes to the transmit voice amplifier U130A. The
output level of the amplifier is adjusted by a level con-
trol. The output of U130A is applied to another trans-
mit voice amplifier U130B and Transmit Voice Gate
U158A. U158A is controlled by A/D processor U111/
latch U107. When enabled, the gate passes the voice
to Transmit Option Gate U158C and on to the transmit
audio buffer U164B. Transmit Voice amplifier U130B
is adjusted by a level control. The output is fed to
Transmit Net Gate U153B. Gate U153B is controlled
by A/D processor U111/latch U155.
6.12.10 TRANSMIT DATA AND CWID PRO-
CESSING
The data signal is produced by A/D processor
U111 on Transmit Data and Transmit Shape outputs.
The transmit shape output is normally the opposite
logic level of the transmit data output when data is
transmitted. However, the bit before a logic transition
occurs, the transmit shape output is the same logic
level as the transmit data output. This results in a
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