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2. Volume adjustment, sound field processing and EQ adjustment
circuits.
All channel signals are sent to N402 inside which the independent volume adjustment,
EQ adjustment and all sound field modes process are performed.
The sound field processing and EQ adjusting circuit is mainly processing the L&R
channel signals. According to the schematic diagram, the L&R channel signals are
added simultaneously to the pins 15, 17, 13 and 16 of N402. When the unit mode is in
the Hi-Fi mode, the internal circuit of the pins 17&16 is connected and the other input
signals are cut off. At this time, only the L&R channel volume can be adjusted and only
the pins 31&32 send out signals. Therefore, the unit is in the 2CH output mode. The
unit mode is not in the Hi-Fi mode, other input signals are connected but the pins
17&16 signals are cutoff. At this time, all channel volumes can be adjusted
independently and the sound field processing or EQ adjusting of the L&R channels
can be performed. Finally, all channel signals pass out from the pins 31, 32, 33, 34,
35&36. The SW channel signal from the pin 36 reaches the amplified speakers to be
amplified through the SW output terminals. Other channel signals reach to the power
st
amplifying circuit to be amplified. The L&R channel signals will go through 1 grade
LPF and MIX amplification (Karaoke signals are overlapped into L& R channels).
3. Input signals detect, search and frequency spectrum sampling
cir
cuits
3.1 Input signals detect and search circuit: The six channel signal lines of the input IC
N402 are connected with 100K sampling resistors R533, R534, R657, R676 and R678
respectively. The signals are mixed by these resistors and added to the opposite-phase
input terminal to be amplified. VD431 and C481 connected to N403B's output end
constitute half-wave rectifying filter circuit. Then the signals reach the voltage comparer
composed of N403A. The output end of N403A (SEARCH)is connected to the pin 28 of
CPU. This control signal is the search and detect signal: when it is low level, it enters the
search mode; when it is high level, it stops searching. Its works as follows:
3.1.1 When this unit is getting started, the A&B control signals from the pins 38&39 in
the domination of the CPU's inter program are added to the input select circuit to
search circularly once. When there are no signals in these four input connectors, the
VCD mode stops automatically. When there are signals in one of the four connectors,
AC signals will appear in all channels of the input N402. These AC signals are
amplified by N403B and rectified and filtered by VD434 and C481 to become DC
signals. At this time, the opposite-phase voltage of N403A is 0.01V. When this DC
voltage surpasses 0.01V, the output end of N403B sends out a high level (SEARCH)
close to positive power supply voltage (A+6V) which reaches the pin 28 of CPU. CPU
keeps searching in the connector in which there are input signals and the unit will play
normally.
1.2.2 When press the SEARCH on the front panel, CPU sends out A&B control
signals again to start searching. Meanwhile, the pin 27 (EX) sends out a high level
which makes V446 inductive. The emitter of V446 sends out a high level which passes
through R498 which makes the opposite-phase voltage of N403A to be 0.4V. That is
to say, if you want to stop searching of CPU, the gained voltage after the input signals
are rectified and filtered must exceed 0.4V. This voltage is higher than 0.01V when
this unit is getting started in order to avoid that the CPU receives signals mistakenly
and stops searching due to the large external interference signals. If the input signals'
amplitude is not high enough, CPU will continue searching. When the amplitude is
high enough, N403A sends out high level to the pin 28 to stop searching. The pin 27
(EX) will become low level again and the opposite-phase voltage of N403A will also
returns back to 0.01V. The whole searching process is finished.
