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GND
47nF
Interlace (15KHz)
254
10uF
259
+
254
PN2222
257
100K
247
33K
256
200K
.047uF
318
355
1/4
LM339
13
11
10
+
356
3.92K
1.5-2VDC
59,D6
357
.05VDC
7V pp 60,C5
Vs,Hs
255
246
200K
1K
358
H
s
355
355
Horizontal
Sync
2
VC
1
Vertical
Sync
322
6.8K
6.8K
GND
V
s
270Ω
270Ω
1.8K
1.8K
270Ω
331
325
326
330
1/4
LM339
1/4
LM339
327
22K
323
2
4
5
1
7
6
3
12
.14-.16V
+
+
321
328
0Ω
GND
+12V
VC
318
0Ω
257
1.8K
355
1/4
LM339
14
9
8
+
353
15.8K
354 360
364
366
7.15K
7.15K
2.1-2.4VDC
4.6Vpp 58,D6
Hs
15.8K68.1K
Horizontal Sync
1K
246
+12V
Vertical Sync
FBP
PN2222
0Ω
364
VERTICAL AND HORIZONTAL SYNC CIRCUIT DESCRIPTION.
Vertical Sync
Horizontal Sync
Sync Interface
2 Comparators
Vertical Sync To Horizontal Cycle Synchronization
and Composite Sync Decoder
For Interlaced Vertical Sync.
{
Composite
Sync
To LA7851 pin 19
To LA7851 pin 1
Composite sync or separate vertical and horizontal sync
are buffered by two comparators in the sync interface
circuit. A vertical sync synchronization circuit is used to
insure a stable raster and functions as a sync separator.
The synchronization circuit is bypassed, for interlaced
vertical sync, because this circuit rejects the half
horizontal line time variation used to generate the
interlaced vertical raster.
The sync interface comparators are biased to .15 volts,
by resistors 323 & 327 , to permit receiving low level
sync signals such as RS170. For low level composite
sync, the vertical and horizontal lines are tied together
and jumper 328 is left off. For normal amplitude sync,
(greater than 2.3 volts) resistors 325 & 326 form an
attenuator to protect the sync interface comparators and
normalize the sync amplitude. This combination also
reduces noise sensitivity since the sync voltage amplitude
is low at the comparator input which slows the
comparator response and acts as a low pass filter.
For the interlaced sync case, the pullup resistor 321 is
left off and the voltage divider resistors 246 and 257 act
as the pullup. Also the vertical sync synchronization
comparators are disabled by changing the input resistors
to bias the comparators in the high output state and
resistor 366 is left off. Capacitor 254 acts as a sync
separator for composite interlaced sync. Capacitor 259
and jumper 364 are used to couple the composite sync to
the LA7851 vertical sync input pin 19.
The vertical sync synchronization window comparator
generates a pulse, a little after the midpoint of each
horizontal cycle. This pulse is shorted to GND by
transistors 255 except when vertical sync is active. The
two transistor circuit permits using either positive or
negative pulses for vertical sync.
Capacitor 318 couples the vertical sync pulses to
transistors 254 & 255 . When no sync pulse is present,
transistor 255 is turned on by resistor 246 . For a
negative vertical sync pulse, transistor 255 is turned off
by the negative pulse applied to resistor 257 and the
window comparator pulse is allowed to be the vertical sync
pulse. For positive vertical sync pulse, transistor 254 is
turned on by resistor 247 & 256 , which shorts the base
of transistor 255 to GND also allowing the window
comparator pulse to act as the sync pulse.
A sawtooth waveform is produced on integrating
capacitor 358 by applying the flyback pulse to resistors
360 & 357 . This sawtooth waveform is connected to two
comparators which are biased by resistors 353 ,
356 , 354 , & 360 such that both comparator outputs are
high between 1.8 volts to 2.3 volts. This circuit would
produce a pulse on both the positive and negative slope
parts of the sawtooth waveform. Resistor 357 eliminates
the output pulse on the negative slope by introducing part
of the flyback pulse to pin 8 which keeps the comparator
from going high at this time. Resistors 364 & 366 act as
a pullup for the window comparator and apply a 6 volt bias
to the vertical sync input, LA7851 pin 19. At 6 volts, the
vertical sync input is inactive. It becomes active only
when the window comparator output and the ± sync
transistors are all high.
+
323
321
246
327
328
326325
255
257
259
254
257
255
247 256
255
254
255
318
353
254
246
255
360356
354
360 357
358
357
364 366
78
366
364