Sierra Video Manzanita 161VS - Manual (Page 5)

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Sierra Video Systems • [email protected] • http://www.sierravideo.com
MODEL 161 INSTRUCTION MANUAL
diagram. Each audio input is configured for a
balanced signal with two signal connections. R5/
R6 and R7/R8 divide the signal level in half and
provide a 40K ohm input impedance to the audio
signal.
U3 is a dual op-amp with each op-amp connected
to one of the input divider outputs. U3's only
function is to reduce the impedance from 5K
ohms (the parallel equivalent of each input di-
vider) to less than a few ohms. The low imped-
ance is needed to keep the unselected audio inputs
from crosstalking into the desired channel. The
outputs of the input buffer op-amps are connected
to two eight input dual channel analog switches,
U4 and U5. U4 and U5 have their outputs con-
nected together and the control connected to
configure them as a sixteen by one balanced
switch. The balanced switching technique has a
number of advantages:
• Double the signal level for 6dB more dynamic
range.
• Common mode crosstalk is completely canceled
in the output amplifier.
• Using balanced audio and unbalanced video
prevents their interacting when they must exist in
the same environment.
• Switching transients are rejected by the output
amplifier resulting in a silent switch.
The output of the analog switches feeds U6 which
is both a common mode rejection circuit and a
6dB amplifier to make up for the loss of the input
attenuators.
Control circuits. The four latch circuits on the
503135 are each the same. The LSB latch will be
discussed here. Refer to page 3 of the 503135
schematic diagram. The circuit is made up of
U8A, U8F and its associated components. The
two hex Schmidt gates are connected in series to
form a non-inverter with R45 as positive feed-
back. With no input the circuit forces itself to
either 0 or +12V. When a control input is active
the controlling device puts a low impedance 0 or
+12V on the input to U8A. This overrides the
The output of the 16 crosspoints are tied to the
single pull down emitter resistor R13 and to an
output amplifier consisting of U2. U2 is a 50MHz
wide band video amplifier with a very low output
impedance (about 0.1 ohms). R15 in conjunction
with R14 + variable R11 sets the gain of the entire
503135 to 2X. R16 raises the output impedance
to 75 ohms required to interface to external 75
ohm coax cable environment.
The output of U2 is also fed to the sync separator
circuit Q4. Q4 is turned on by R19 except during
the negative extreme of sync on the video signal.
This results in 12 V P-P inverted sync at the
collector of Q4. U7B and U7C are connected
with R20 and C11 as a low pass filter which
separates vertical interval from the composite
sync output of Q4.
U7D functions in two modes depending on
whether or not sync is present. When sync is
present it is inverted by U7D and its trailing edge
is made into a switching pulse by U7E. When
there is no sync U7 pin 4 stays high. This is AC
coupled through C10, so the DC level at U7 pin 4
is not important. U7D now functions as a free
running oscillator. D2 and D3 allow the duty
cycle and period of the oscillator to be close to
vertical sync to prevent U7D from changing state
as an oscillator if it is driven by a signal at U7 pin
4.
The output of U7 pin 10 is a short positive pulse
at vertical interval line 6. This is used to clock the
latch portion of U1. U1 is a 4 bit latch and de-
coder. The 4 line control input to U1 is decoded
by U1 to select the desired 1 of 16 video
crosspoints.
Audio circuits. The 503135 has two identical
audio channels. One channel has its input and
output connections on edge connector J2. The
other channel uses exactly the same pin numbers
on J3. Only channel one is discussed here. The
audio circuits are on page 2 of the schematic