Pioneer tuning fork Service Manual page 40

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 the C-carrier is down-converted are:
a) Tape recording system is liable to make 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 (LPF).
4 . 1 . 9 Mixing
In this process the low frequency C-carrier is mixed with
the FMed Y-carrier. The 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.
The amplitude fluctuation of the signal is suppressed by
the Limitter. The upper sideband of the signal is re­
produced. The Demodulator 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 Automatic Phase Control (APC) 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. The 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.
5. A u d i o Signal P r o c e s s i n g
In the 1/2" VCRs, audio signal is recorded on the audio
tracks with the help of a bias current in the same manner
as ACRs. Audio dubbing is possible by this method be­
cause the Audio-head is independent from Video-head.
-CH-2-
Audio tracks
- C H - r
Control track
Fig. 18 Audio and Control Head
38
6. H i g h D e n s i t y Recording
1/2" and 8mm VCRs employ guard-bandless system to
save the tape. Fig. 21 shows video track patterns. In (a)
fast-speed m o d e , the tape allows guard bands or vacant
spaces between tracks because the head-gap length is short­
er than the track pitch. In (b) medium-speed m o d e , the
tracks are contiguous to each other leaving no space for
guard bands.
Further, in (c) slow-speed mode, tracks are overlapped each
other. The width of tracks recorded by the overlapping be­
comes narrower than 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.
W i t h the help of guard bands cross talk can be avoided even if the
azimuth of the Video-head is 0 ° .
Slant head is insensitive to the signal recorded on the adjacent track
even w h e n it deviates.
Audio head
Overlapped recording makes the head always pickup the signal on
adjacent track(s) in playback.
Control head
Fig. 19 Video Track Pattern in Various Speed
Audio track
Control track
(a) Fast-speed mode
(b) Medium-speed mode
(c) Slow-speed mode
Head
Gap