(\
\)
AUDIO BOARD
AUDIO BOARD CIRCUIT
DESCRIPTION
C)
The audio board receives signals from the receive
board, the headsetmicrophone, intercom input and
,auxiliary input, and outputs
to
the transmit board,
the headsetearpiece, an external speaker, intercom
outand 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
of
thesedevices iscontrolled bythe audiolevel
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.
MicrophoneAmplifier
From the headsetconnector the microphone signal
is amplified byU103. Depending upon which type
of
microphone is used, S102 is set by the user
to
dynamic orelectret. 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 inputsignal is reduced.
Visual indication
of
peak limiting is provided by
D608 which is driven by U105 when thevoltage at
pin
6exceedshalf avolt. D
111
(version3and later)
/isolatesthe time constant
of
the amplifierfrom'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 1and 2the LED willremain lit for several
seconds. VR604 is adjusted by the user for maxi-
mum inputwith minimal limiting.
QI04
is
used
to
tum the microphone on and offand is driven by the
push-to-talklatch circuit. When
SIlO
is pressed in
and held, thestate
of
Ul16Bfollows the switch and
tums on Q110 and Q111 via D110. This lights
D609 and gates on the microphone audio. Releas-
ing S110 tums offthe microphone.
3-27
U116A does not change state under these condi-
tions because
C~82
is discharged when theclock is
pulsed.
If,
however, S110 is depressed twice in
quick succession, C182 will be charged for the
second clock pulse and this will setU116A. Now
the microphone is latched on via D109 and will
remain on until S110 is depressed once again,
resetting U116A.
Auxiliary
Input
Amplifier
Audio from
JlOl
is attenuated and converted from
balanced to single-ended by U101A. U101B,
UI02A, Q101,
QI02
and
QI03
form apeak limit-
ing amplifier identical to that described for the
microphone above. S
101
not only enables the
audio path for the auxiliary input but also lights
D607
to
half-brilliance
to
indicate active auxiliary.
When peak limiting occursU 105Ashorts outR120
and D607 is brought
to
full brilliance.
Intercom Driver
UI07
Ais 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. Asingle load resistor is
usually located in the power supply for the wired
intercom. On versions 3and later, UI07B is used
to
drive the lowerend
ofTIOl.
This section is not
acurrent source and was added to provide more
headroom.
To
this end, U107 is operated from the
higher voltage ahead
of
the regulator. S104 is a 9
section
DIP
switch which is used to configure the
base for compatability with aspecific wired inter-
com. S103 functions as either
an
intercom onloff
switch.(Telex, Clearcom standard) or as achannel
AlB switch (RTS, Clem'com TW). In the case
where the intercom interface is disconnected by
S103, R186 functions as the load resistor. Thefull
secondary
ofTlOl
is used for 300 ohm intercoms;
the tap is used for 200 ohm systems. On version 2
andlater, S
112
allowsselection
of
eitherlow output
(100 mY) or high output (800 mY).