Pioneer tuning fork Service Manual page 38

4 . 1 Recording
4 . 1 . 1 Automatic gain control ( A G O
A clamper fixes the sync tip level and makes the video sig­
nal amplitude constant.
Recording
Y
Y
AGC LPF
Video
ACC
input BPF
C-signal
/ s : 3.58MHz
carrier
In
1H (63.5/is) delay
Vi
(a) Circuit
Silica
Input
Piezoelectric
element
lb) Delay-line
4 . 1 . 3 Y-signal emphasis
White noise is unavoidable in F M . The noise level of this
kind increases in proportion to frequency. The noise, there­
fore, is suppressed by preemphasis before modulation and
deemphasis after demodulation. 8 m m V C R a n d 1/2"
VCRs in 0 I I & III and VHS E P modes employ the dynamic
emphasis system by which the a m o u n t of emphasis is
decreased as the input signal level increases to prevent over-
modulation. T h e Y-signal is then clipped and FMed.
36
FMed Y-signal
h 3
FM
HPF
modulator
urn
Freq.
LPF
converter
Down-converted
C-signal
Fig. 13 Simplified Block Diagram of VCR
Y o u t
Vd
C out
Piezoelectric
element
Output
Supersonic w a v e
Fig. 14 Y-C Separation by Comb Filter
4 . 1 . 2 Y-C separation
The Y- a n d C-signals are separated by Y-C separator to
avoid intermodulation distortion because they have been
in the same frequency band.
The input signal is added to or subtracted from the 1H-
delayed signal. The addition and subtraction yields Y- and
C-signals respectively because of the phase difference be­
tween adjacent lines (NTSC). Ref. Fig. 2, p l 3 .
Playback
HPF
A m p
LPF
Bandwidth
FMed Y-signal Deviation
yiMHz]
Y-component
Filters
C-component
Filters
Y-component
(c) Characteristic
Fig. 15 An Example of Dynamic Preemphasis i/3)
Freq.
LPF
demodulator
Mix
Freq.
BPF
converter
C-component
7
For weak signal
For strong signal
Frequency (MHz)
Y
:C
Video
out