Toshiba D-VR3SU Service Manual page 133

Circuit Operating Descriptions
2) ACC (Automatic Color Gain Control) Circuit
The ACC is used as burst ACC in the LP mode, however it is also used for peak ACC in the SP/SLP mode.
The purpose of using two different ACC operations is to improve the overall Chroma S/N ratio during playback.
In SP and SLP, there is H-sync alignment. This indicates that there is bust alignment as well. Whenever two video
tracks overlap or a video head picks up crosstalk from an adjacent track, beats are produced during playback.
Perhaps the most noticeable beats are produced by H-sync and burst. But in SP and SLP, these beats occur right at
H-sync and burst and are out of the picture. In LP, however, there is no H-sync alignment and these beats can be
seen in the picture. To keep the beats at a minimum in LP, we keep the burst level constant so that the beat inten-
sity is constant. We know that ACC acts to improve the color S/N,and in LP, the ACC detector locks at the burst
level,and keeps it constant. Thus we have ACC operation with the least beats. In SP and SLP, the beats caused by
burst overlap are out of picture, so we don't really mind if the burst level changes or not. To improve the color
S/N ratio even more,we use peak ACC in SP and SLP.That is,if the chroma level is too low to record, the amplifi-
cation degree is increased by 3dB. However, the chroma level is sufficient for recording, this peak ACC is changed
to burst ACC to avoid over amplification. By changing the ACC according to picture color content, the burst level
may vary. The color ratio improvement is based on the color content itself during SP and SLP provides a some-
what better S/N ratio.
3) Four (4) Phase Rotation
CH1 is advanced 90° every channel, while CH2 is delayed 90°. When the frequency is set to 629KHz, if phase is
shifted by +/-90 it becomes 629KHz +/-90. The 40fh+/-90 (=629KHz +/-90)is balanced modulated via fsc
(3.58MHz) depending on which side band is detected. That is, the fs +40fh+/-90 (4.2MHz +/-90) of total frequen-
cy is supplied to the main converter. In record mode, the signal operates same as in play back mode. During play-
back, the phase is returned to original state.
4) AFC (Automatic Frequency Control) Circuit
Luminance signal is input to H-sync separator. The H-sync is separated and supplied to phase comparator.
This signal can be described as fh (Horizontal Sync frequency of input video signal). However, VCO oscillates at
320fh (5.035MHz). This 321fh is counted down by 1/8 and 1/40 and resultant fh is supplied to phase comparator.
fh and fh are supplied to the phase comparator for comparison of their phases.
After comparison, the phase
differences is output to VCO (320fh) in terms of error voltage. Therefore, the oscillation frequency of VCO is
controlled by this error voltage. That is, if the fh phase is changed by H-sync signal fh, error voltage is changed
accordingly and if the phases of fh and fh are met due to change of VCO oscillation frequency, error voltage does
not feedback. 320fh VCO is oscillated in accordance with phase sync at fh. Therefore, 40fh input to sub converter
by phase shift is always sync horized with phase.
The AFC loop performs the same operation during record and playback. In recording, phase of VCO is in accor-
dance with H-sync signal of current video signal.
Which in playback, the phase sync of VCO is consistent with H-sync signal which is separated from the video
signal.
7-33