Video Circuit; Bnc Input Connector - Trap Filter Circuit; Nc-Sync Generator Circuit; Agc Circuit - Sony UP-895 Service Manual

4-1. Video Circuit

4-1-1. BNC Input Connector - Trap Filter Circuit

A composite video signal is input from the BNC connector (VIDEO IN) to the MA-99 board. 75 Z
termination is turned on and off using a DIP switch. After that, the input signal branches into three paths.
One is passed through a trap filter (FL1) for NTSC, and another is passed through a trap filter (FL2) for
PAL. The other is not processed at all. These signals are input to analog selector 1 (IC1). Each signal is
selected by a system control and input to video amplifier 1 (IC2).

4-1-2. nC-SYNC Generator Circuit

A nC-SYNC signal is generated from the SYNCOUT signal that is output from video amplifier 1 (IC2).
The nC-SYNC signal is also used in an AGC circuit.

4-1-3. AGC Circuit

The signal from an analog selector 1 (IC1) branches into two paths. One is input to video amplifier 1
(IC2), and the other is input to the AGC circuit for detecting the peak of a signal. The gain is controlled
by a contrast amplifier in video amplifier 1 (IC2). When an AGC function is set to ON, the peak voltage
of the input video signal obtained by the AGC circuit is controlled by a system control (IC304) so that it
is output from analog selector 2 (IC5). When it is set to OFF, the reference voltage is controlled by a
system control (IC304) so that it is output from analog selector 2 (IC5). The gain is controlled by
inputting the voltage to video amplifier 1 (IC2).

4-1-4. Brightness and Contrast Control

The video signal output from video amplifier 1 (IC2) is input through a buffer (Q220) to video amplifier 2
(IC6). For contrast control, the gain is controlled by inputting the voltage obtained using the volume on
the front panel to video amplifier 2 (IC6).
The video signal output from video amplifier 2 (IC6) is input through a low-pass filter to the brightness
control amplifier (Q7). For brightness control, the gain is controlled by inputting the voltage obtained
using the volume on the front panel to the brightness control amplifier (Q7).

4-1-5. A/D Conversion and D/A Conversion

The video signal output from video amplifier 2 (IC6) is converted into a digital signal using an A/D
converter and stored in memory by a memory and head control circuit.
The A/D-converted signal is converted into a video again using a D/A converter, and a sync signal is
added using analog selector 3 (EE signal). Moreover, the sync signal is sent to analog selector 4 and
output from the BNC connector (VIDEO OUT) to the outside. A relay (RY1) is used to switch THRUE
and EE signals and controlled by a system control.
4-3
UP-895/(E)