Sierra Video Tahoe Series 16 - Manual (Page 53)

Tahoe Series 16
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and C3 are a snubber network which controls peaking on the crosspoint bus. The gain of
the output amplifier is adjusted by a 15 turn control (R7). R9, a 75 ohm 1% resistor
provides the required output line driving impedance.
Control Circuits
The 504044 is usually controlled by a centralized control processor module which may be
in the same frame as the 504044 or in another frame in the overall system. Each of the
data inputs to the 504044 is in series with a 4.7K resistor. This serves two functions. It
protects the logic circuits on the 504044 CMOS inputs from static over voltage or static
damage. Second, it protects the bus in the system from a failure on an individual 504044
module.
U3 is a 4 line to 16 line decoder. Its output determines which bus on the 504044 is being
addressed by the central controller. The strobe input (J1-6S) is centered timing wise in
the data time that appears on the other logic lines to the 504044.
Power Supply
Low voltage AC enters the 504044 on J3 pins 27 and 28. If dual supplies are used, low
voltage AC from the second supply enters the 504044 on J3 pins 25 and 26. The low
voltage AC is rectified (D1 through D8) and filtered (C7 and C8) to unregulated DC. U11
provides +5VDC for the logic circuits. U12 is a 7805 and provides +5VDC to the analog
circuits. U10 is a 7905 and provides -5VDC to the analog circuits.
Audio Crosspoint Module 504045
The 504045, 16 x 16 audio crosspoint module is a plug in module that is a complete
audio switching module containing; input buffers, crosspoint array, output amplifiers,
control logic and power supply. It is the basic building block for the audio portion of the
Tahoe series 16 routing switchers. The 504045 employs balanced audio switching. The
advantages include; higher internal levels to achieve very high dynamic range (over
100db), lower crosstalk, no audio ground current, and no switching transients. The
504045 has a bandwidth of about 500KHz. This allows it to be used for switching time
code at even the fastest machine shuttle speeds.
Input Buffers
Each one of the 16 audio inputs is buffered by a dual low noise op-amp (U1). The circuit
provides 40db of common mode rejection. The circuit gain is set by 1% resistor values.
Crosspoint Matrix
The crosspoints on the 504045 module are configured from individual 4 by 1 IC
crosspoint chips. Each of the sixteen buses has four 4 by 1 IC’S on it. Each switching IC
has 3 control inputs. The “A” and “B” inputs select “1 of 4” inputs for that IC. The third
control input is a CS (chip select). By using the chip select input, multiple 4 by 1 IC’S can
be linked to form a larger bus.
Output Amplifiers
The output amplifier circuit is based on another dual low noise op-amp. The circuit is also
configured to reject up to 40 db of common mode signal. This includes common mode
crosstalk (which is immeasurable below 15 KHz) and switching transients. The output
circuit has a gain of 10db to offset the deliberate loss introduced by the input network.
Control Circuits
The 504045 is usually controlled by a centralized control processor module which may be
in the same frame as the 504045 or in another frame in the overall system. Each of the