Akai VS-2EGN

2) Pulse width modulation type D/A converter
CLOCK
GENERATOR
PROGRAMMABLE,
PULSE WIDTH
VARIABLE CIRCUIT
‘ (Constant)
i
i ' l
loa el {
PULSE
WIOTH Vret
Chonges
OVo(DC OUTPUT)
Fig. 7-91 General pulse width modulation type D/A converter
T,
Vo= Ts x Vref
Ty = To (Bi/2 + B2/2?...--. + Bn—1/2"-! + Bnj2™)
In the pulse train of a constant period at a constant volt-
age, duty cycle matching with the set digital input is
generated. Smoothing by the LPF is used to obtain a
D/A converted output.
5-8 SEARCH OPERATION AND AFT
1OL 5V
Ral 12K
AFT
to FRONT AFT R43 27K
rs IDL 5V
R44 27K
Fig. 7-92 AFT Comparator
Fig. 7-92 shows the AFT comparator.
When PRESET selector button is pressed and then but-
ton-B is pressed, the TV search mode is initiated, and the
front processor gradually raises the tuning voltage (VT)
via the D/A converter (*1). Since the AFT input from the
tuner changes as shown in Fig. 7-93 if there is a TV sig-
nal, present the front processor starts reducing the value
of VT at only one third of the speed curing the time that
AFT falls after the original steady increase. (*2)
However, since the VT change lags in time after the
microcomputer passes its output to DAC as a result be-
cause that VT passe suffers delay through LPF, VT con-
tinues rising for a while after the TV signal has been
recognized, and turns to falls a little later.
When the AFT output becomes HIGH, VT is again in-
creased but at a speed 1/10th the initial speed (*3), The
time
lag
of
VT
presents
no
problem
at
this
speed.
Then,
VT
is
stopped
when
the
AFT
output
has
become
LOW.
Service Manual VS-2EGN
68
Features
@® Accuracy is not affected by the variation of clock
frequency.
Low cost and high precision compared with other
D/A converters.
High-speed operation cannot be achieved since a LPF is
used.
vr
video Adjustment
Polat
agT ourPUT
Wr increose
Lvr increase (1/10 Speed)
‘Vr Decrease(i/’3 Speed)
Fig. 7-93 AFT Comparator input/output and VT
Now, there would be no significant overshoot, and the
VT potential would come within the trailing edge of AFT
input. Then, AFT is turned ON (Mute off). Video judge-
ment is performed after 0.2 second. If VID. J="H’,
“FOUND ... ” is displayed. When button -C is pressed,
an instruction is given to memorize the data. Then, the
data being output to DAC is stored in EXT RAM as VT
data of that channel.
As a result, a channel stored previously may be called by
specifying the VT data to the DAC correspoinding to TV
data to be read from EXT RAM each time a channel is
selected.
In order to make the AFT input resistant to noise, posi-
tive feedback is effected by R41, and hysteresis is pro-
vided.
*1 1 bit change in 3 ms
*2 1 bit change in 9 ms
*3 1 bit change in 30 ms