Table of Contents
Advertisement
( \
\
)
AUDIO BOARD
AUDIO BOARD CIRCUIT
DESCRIPTION
C)
The audio board receives signals from the receive
board, the headset microphone, intercom input and
,auxiliary input, and outputs to the transmit board,
the headset earpiece, an external speaker, intercom
out and auxiliary out (see block diagram).
Receive Expanders
Audio from the receive board is routed through
Jl08, Jl09, JllO and Jlll to U1l2 and U1l3. The
gain ofthese devices is controlled by the audio level
at the rectifier pins .(4 and' 16). As the level in-
creases, so does the gain, and this results in a 2: 1
linear expander characteristic. This complements
the processing done in the transmitter. The audio
is then buffered and delivered to the remote select
switches.
Microphone Amplifier
From the headset connector the microphone signal
is amplified by U103. Depending upon which type
of microphone is used, S102 is set by the user to
dynamic or electret. The electret position provides
an 11 dB drop in gain to compensate for the extra
output.
The amplifier is peak limiting, and
responds to either positive or negative peaks ex-
ceeding about 1.1 volts in amplitude. Peak detec-
tors Q106 and Q107 charge C120 and C121,
driving QI05 toward pinchoff. This reduces the
gain momentarily until the input signal is reduced.
Visual indication of peak limiting is provided by
D608 which is driven by U105 when the voltage at
pin 6 exceeds half a volt. D111 (version 3 and later)
/isolates the time constant of the amplifier from'that
of the LED driver. The latter is much shorter than
the former, and causes the LED to extinguish im-
mediately upon removing the input signal. On
versions 1 and 2 the LED will remain lit for several
seconds. VR604 is adjusted by the user for maxi-
mum input with minimal limiting. QI04 is used to
tum the microphone on and off and is driven by the
push-to-talk latch circuit. When SIlO is pressed in
and held, the state of Ul16B follows the switch and
tums on Q110 and Q111 via D110. This lights
D609 and gates on the microphone audio. Releas-
ing S110 tums off the microphone.
3-27
U116A does not change state under these condi-
tions because
C~82
is discharged when the clock is
pulsed. If, however, S110 is depressed twice in
quick succession, C 182 will be charged for the
second clock pulse and this will set U116A. Now
the microphone is latched on via D109 and will
remain on until S110 is depressed once again,
resetting U 116A.
Auxiliary Input Amplifier
Audio from JlOl is attenuated and converted from
balanced to single-ended by U101A.
U101B,
UI02A, Q101, QI02 and QI03 form a peak limit-
ing amplifier identical to that described for the
microphone above.
S101 not only enables the
audio path for the auxiliary input but also lights
D607 to half-brilliance to indicate active auxiliary.
When peak limiting occurs U 105A shorts out R 120
and D607 is brought to full brilliance.
Intercom Driver
UI07A is configured as an AC current source
whose output CUlTent equals the input voltage at
C125 divided by the value of R158. Thus the
voltage gain is dependent on the value of the load.
In this way many such intercom stations can be
connected in parallel without each successive sta-
tion loading the line. A single load resistor is
usually located in the power supply for the wired
intercom. On versions 3 and later, U I07B is used
to drive the lower end ofTIOl. This section is not
a current source and was added to provide more
headroom. To this end, U107 is operated from the
higher voltage ahead of the regulator. S 104 is a 9
section DIP switch which is used to configure the
base for compatability with a specific wired inter-
com. S103 functions as either an intercom onloff
switch.(Telex, Clearcom standard) or as a channel
AlB switch (RTS, Clem'com TW). In the case
where the intercom interface is disconnected by
S 103, R 186 functions as the load resistor. The full
secondary ofTlOl is used for 300 ohm intercoms;
the tap is used for 200 ohm systems. On version 2
and later, S 112 allows selection of either low output
(100 mY) or high output (800 mY).
Advertisement
Table of Contents
