Toshiba D-VR3SU, Руководство по обслуживанию

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Circuit Operating Descriptions
7-28
7-9 VCR Video
(1) Luminance Signal Recording System
Fig. 7-18 Luminance Record Process
IC301 LA71207M
VIDEO 1N
AGC
DET
VIDEO
AGC
F. B
CLAMP
6dB
AMP
QV/QH
INSERT
LPF
YNR Y/C
COMB
CCD
3 MHz
LPF
AMP
28
69 70
clock
data
chip select
53
54
55
VIDEO OUT
26
FM
MOD
MAIN
EMPH
REC
FM EQ
W/D
CLIP
NL
EMPH
DETAIL
ENH
REC
CURRENT
AMP
SP H'D
CLAMP
39 34
FM
AGC
REC COLOR
22
21
66
73
+
SLP H'D
1) Outline
Fig. 7-18 shows the video signal recording system.
The selected video input signal goes to pin 28 of Luma/Chroma processor IC (IC301). And then it enters VIDEO
AGC circuit. The gain of AGC circuit is controlled by AGC detector so that the output is constant (approx. 2Vp-p).
The output signal of AGC is clamped by the FBC(Feed Back Clamp) circuit. This signal appears at pin 26, after
being amplified at the internal video amp and driver.
The output signal from the clamp circuit enter the detail enhancer circuit. In the detail enhancer circuit, the low
level high frequency video signal is emphasized to improve the original signals frequency characteristics.
onlinear emphasis circuit is employed to improve S/N and frequency response characteristics together with the
following main emphasis. Noise effects the FM wave at a higher frequency, so the S/N can be improved by
emphasizing the higher frequency before recording and by suppressing the play signal during demodulation.
The difference of non linear emphasis from main emphasis is that the emphasis characteristics change is depend-
ing on the input level. The gain of the emphasis circuit is inversely proportional to the level of the high frequency
component of the signal. That is, if the high frequency portion of the signal is low the main emphasis circuit will
amplify the signal.