Toshiba D-VR3SU, Service Manual

Page: 134 / 217

Circuit Operating Descriptions
7-34
(4) Chroma Signal Playback System
Fig. 7-21 Chrominance Playback Process
IC301 LA71207M
91 74
91 73
91 72
91 67
66
65
57
58
CH1
AMP
AMP
AMP
AMP
CH2
S P
H'D
SLP
H'D
CH2
CH1
H'D S/W
H'D AMP S/W
1.3 MHz
LPF
ACC AMP
MAIN
CONV
BURST
DE-EMPHA
3.58MHz
BPF 1
YNR Y/C
COMB
PB AMP
COLOR
N.C
Y/C MIX
3.58MHz
BPF 2
CCD
91 26
39
34
VIDEO OUT
1) Outline
Fig. 7-21 shows the chroma signal playback system.
The FM signals picked up by the CH-1 and CH-2 video heads are supplied to the pre-amp block.
The FM signal from CH-1 and CH-2 are alternately selected by the switch and output signal as a continuous
signal. Goes to the ACC amp through the 1.3MHz LPF. The 1.3MHz LPF is used for passing only down converted
629KHz chroma signal in the playback mode. The ACC amp stabilizes the 629KHz color signal level.
The output color signal from amp then enters the main converter circuit. In the main converter circuit this signal
is mixed with the 4.21MHz phase shifted carrier signal and converted into 4.21MHz + 629KHz signals.
2) Main Converter
Inside of IC, the main converter converts the 629KHz rotational chroma signal to a 3.58MHz non-rotational signal.
The two inputs of this main converter are the 629KHz signal, which comes from the output of the ACC, and a
4.21MHz which has the same rotational phase as the 629KHz signal. It is important that the rotational phase of
the 4.21MHz signal is the same direction as the 629KHz playback chroma signal. To obtain the 3.58MHz non-rota-
tional stable signal, the same direction rotational signal should be mixed with the rotational chroma signal.
During the conversion process, the phase is also mixed by the frequency. Therefore, when 629KHz is subtraced
from 4.21MHz,the result is the non-rotational 3.58MHz stable signal. The output signal of the main converter goes
to the 3.58MHz BPF. In the 3.58MHz BPF,the conversion noise(4.21MHz+629KHz=4.8MHz) is rejected and the
3.58MHz color signal goes to the comb filter.
In the comb filter, the crosstalk components due to the adjacent track are eliminated and the color signal is
applied to PB-AMP, BURST De-Emphasis, Killer and are applied to LUMA and CHROMA mixer input through
the CNC block.