4 . 1 . 8 Frequency down-conversion
T h e C-carrier separated by a Comb-filter is frequency-
converted from 3.58MHz to about 7 0 0 ± 500kHz. The rea
sons why t h e C-carrier is down-converted are:
a) Tape recording system is liable to m a k e jitter.
b) Phase-modulated high-frequency signal is highly affect
ed by jitter.
The down-converted C-carrier is then filtered by Low-pass-
filter ( L P F ) .
4 . 1 . 9 Mixing
I n this process the low frequency C-carrier is mixed with
the FMed Y-carrier. T h e Y-carrier is set at a level so that
its played-back level becomes the maximum. The Y / C mix
ing level ratio should be at a value where resultant spuri
ous is kept within specifications. The Y-carrier works as
a bias current for C-signal.
4 . 2 Playback
Playback process is the reverse of the above.
4 . 2 . 1 Y-signal
The Y-signal passes H P F , Limiter and then Demodulator.
T h e amplitude fluctuation of the signal is suppressed by
the Limitter. The upper sideband of the signal is re
produced. T h e D e m o d u l a t o r is of the pulse count type of
low carrier leak. The demodulated Y-signal is filtered in
L P F . The modulation noise is suppressed by Deemphasiz-
er. Dropouts are compensated, and the Y-signal is then
mixed with C-signal.
4 . 2 . 2 C-signal
The down-converted C-signal passes L P F , its amplitude
is made constant by A C C , the burst level is reduced to the
original level. The C-carrier is reconverted to 3.58MHz.
A n A u t o m a t i c Phase C o n t r o l ( A P C ) loop controls the
Voltage Controlled Oscillator (VCO), which puts out local
carrier, with the phase difference between those of the burst
signal and the signal of quartz oscillator to absorb play
back jitter. T h e C-signal is filtered by a 3.58MHz Band
pass filter (BPF) and Comb-filter to become pure C-signal,
deemphasized and then mixed with Y-signal.
6.
H i g h D e n s i t y R e c o r d i n g
1/2" a n d 8mm VCRs employ guard-bandless system t o
save the t a p e . Fig. 21 shows video track p a t t e r n s . In (a)
fast-speed m o d e , the tape allows guard b a n d s or vacant
spaces between tracks because the head-gap length is short
er t h a n the track pitch. In (b) medium-speed m o d e , the
tracks are contiguous t o each other leaving n o space for
guard b a n d s .
Further, in (c) slow-speed mode, tracks are overlapped each
other. The width of tracks recorded by the overlapping be
comes narrower t h a n the head-gap length. In playback the
signal on adjacent tracks are picked up together with the
necessary signal. These are suppressed by azimuth effect
discussed later.
Audio track
(a) Fast-speed mode
Slant head is insensitive t o t h e signal recorded on the adjacent track
even w h e n it deviates.
(b) Medium-speed mode
5.
A u d i o Signal P r o c e s s i n g
In the 1/2" VCRs, a u d i o signal is recorded on the audio
tracks with the help of a bias current in the same manner
as A C R s . A u d i o dubbing is possible by this method be
cause the Audio-head is independent from Video-head.
Audio tracks
Control track
-CH-2-
- C H - r
Audio head
Control head
Fig. 18 Audio and Control Head
Overlapped recording makes the head always pickup the signal on
adjacent track(s) in playback.
(c) Slow-speed mode
Fig. 19 Video Track Pattern in Various Speed
38
Control track
W i t h t h e help of guard bands cross talk can be avoided even if the
azimuth of the Video-head is 0 ° .
Head
Gap
