Akai VS-2EGN

5-7 D/A CONVERTER PERIPHERAL CIRCUITS
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1) Fig. 7-89 shows D/A converter peripheral circuits.
VS-2 employs a D/A converter (ICS) using the pulse
width modulation system. ICS consists of four D/A con-
verters, three of which are used.
DAC-1 is a VT 13-bit signal, DAC-3 is a mute control 6-
bit signal, and DAC-4 is a fine data 6-bit signal.
DAC-1 output passes through the smoothing circuit
(R60, C16, C17), and then the main component of 2 ms,
which is the repetition period of the DAC output, is re-
moved by the Twin-T type filter.
This has a long repetition period compared to Fine
(DAC 4), and the number of bits is large. Therefore, un-
less DC conversion is performed thoroughly, the use of
13 bits is wasted, while if the time constant is set at a
large value, an unduly long time would be required until
VT becomes stable. The above is required for the ef-
fective solution of the problem.
The range of VT change is from 0.3140.02V to 3
£1.8V. It is designed so that the VT change/bit is small
when the VT potential is low, and that the VT change
increases as the VT potential rises, by using a variable
time constant during charge/discharge. (Refer to Fig.
7-90). A tuning oscillator variable capacitor diode ex-
hibits the characteristic where VT changes more when its
potential is low than when it is high. The above action is
taken, which more or less compensates for it.
DAC-3 output is used as a mute signal. Since a 1-bit
I MUTE
1
Fig. 7-89 D/A Converter Peripheral Circuits
cle 0.01
Vari-Cap
—>VT
Fig. 7-90 VT change
spike of noise is output once in 32 us when Mute is
switched off, it is removed by C14 anda resistor on the
tuner side,
There is a clock divide output in IC5 (MB88301). The
4 MHz clock is subjected to division by 256 down to
15625 Hz. The resultant signal is picked up at pin @
and differentiated by C24 and R64. Only the leading
edge is sent to the video circuit by D16. This is used as
a trigger for the horizontal frequency to make the chara-
cter appear at the left top part of the screen even when
there is no video signal.
ee
Service
Manual
VS-2EGN
—-—
$$
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