Luminance Signal Recording System; Luminance Signal Recording Frequency; Pre-Emphasis Characteristics; White And Dark Clip Level - JVC HR-S10EG Service Manual

1.3 LUMINANCE SIGNAL RECORDING SYSTEM
PRE-
rrrwa | wiD cur} fem. mop. HPF REC AMP
Azimuth video
Video input
signal
(Color TV signal)
Color signal recording system = ——
head + 6°
Fig. 1-3 Luminance signal recording system
Frequency modulation (FM) is used for the luminance signal
recording system. A simplified block diagram of the system
is shown in Fig. 1-3.
A lowpass filter (LPF) removes the color component and
Passes only the luminance component of the input color TV
signal. At the next stage pre-emphasis circuit, the high fre-
quency portion of the luminance signal is enhanced in order
to improve S/N during FM recording. Since excess pre-
emphasis could lead to black/white reversal due to the
shortened recording wavelength, a white/dark clip circuit
cuts the overshoot and undershoot components which ex-
ceed certain positive and negative levels.
The frequency modulator (FM MOD) converts the AM
luminance signal to FM, which goes through a highpass filter
(HPF) to the recording amplifier. These circuits amplify the
signal with the proper frequency characteristic, after which
it is mixed with the down converted color signal and sup-
plied to the video heads.
1.3.1 Luminance Signal Recording Frequency Character-
istic (LPF)
As shown in Fig. 1-4, when the video input is a color TV
signal, a lowpass filter removes the color component and the
luminance signal, with a bandwidth of from about 30 Hz to
3.0 MHz, is used. With some VHS models, when the input is
a black and white TV signal, it bypasses the LPF, allowing
a wider bandwidth to beyond 4 MHz.
Luminance signal recording band
(Color TV signal)
Color subearrier
(4.433619 MHz)
1 2 3 4 5 6
$$ Frequency (MHz)
Fig. 1-4 Luminance signal recording band
1-4
1.3.2 Pre-emphasis Characteristics
15
4
13
12
"
10
9
(1)
(2)
= Response (48)
O2Nnesuare
OoF OT 02 Od +2 345
———— Frequency (MHz)
c T=Cx Rb= 1,3 psec * 0.05 ysee
—_
x= BB gso3
Ra
"— Ras OUT
IN Fb
(1) T=1.35 psec (2) T= 1.25 usec
x24 X=37
Fig, 1-5 Pre-emphasis characteristics
1.3.3 White and Dark Clip Level (See Fig. 1-6.)
White clip level : 160739. % measured from sync tip
Dark clip level : 40 + 10% measured from sync tip
Note: The level from sync tip to white peak is 100%.
1.3.4 FM Carrier Frequency and Deviation (See Fig. 1-6.)
Synctip :3.840.1 MHz
White peak : 4.8 +0.1 MHz
Deviation : 1.0+0.1 MHz
1.3.5 FM Signal Recording Frequency (HPF)
As indicated in Fig. 1-6, when the video input is a color TV
signal, it goes through an HPF for vacating the area for the
down converted color signal. With some VHS models, when
the input is a B/W TV signal, the HPF can be bypassed to
extend the bandwidth to the DC area.