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7-6.. ATF SYSTEM
In 8 mm video, an ATF (Automatic Track Finding) system is
incorporated for tracking.
In this method, four types: of pilot signals are recorded on
the tape and these signals are processed to detect the shift
amount of the trace position of the head during playback
for tracking.
7-6-1,
Principles
[Recording]
As shown in Fig. 7-11, the pilot signals called f1, f2, #3, and
4 overlapped on the video signal are switched for each
track
and
recorded
on
the
tape with the
sequence
repeating every 4 tracks.
The frequencies are shown in Table 7-2.
fi and 8 are recorded by + azimuth head (CHA) and f2
and {4 by — azimuth head (CHB).
Frequency (kHz)
REC
Frequency
NTSC
head | Mray*
—
ae
Pepe el er
[4 [148.89 | eae [ CHE |
Pm freess [0r.02 | crar |
Table 7-2.
Pilot Sci
ae
Rating
Pilot
(fi)
* fi <Fo/N
Fo =378 fH=5.95 MHz (NTSC) (f4=15.734 kHz)
=375 f4=5.86 MHz (PAL) (fH=15.625 KHz)
7-10
i
ma
AN
as ai, ae
NENT
REC
HEAD:
CHB
CHA
CHB
CHA
CHB
Fig. 7-11.
Pilot signal recording pattern
3
Frequency [Hz]
Fig. 7-12.
Pilot signal recording spectrum
[Tracking Error Detection]
In the playback mode, trace position of the video head is
detected by the circuit shown in Fig. 7-13.
LPF
(f¢=200KHz)
PB PILOT
VIDEO HEAD
DIVIDER
Y#C
REC
Fig. 7-13.
A pilot signal with the same frequency as recording is used
as the reference signal (REF).
Assuming that the REF is f1 and that the head is in the A
position (recording f1) in Fig. 7-14.
REF R11
-2P-P
0 +P 42P
Fig. 7-14.
At this time, the playback pilot signal (PB) will be the mixed
signal
of f4 (left side)
and
f2 (right side)
from
the
neighboring tracks in addition to fi. The two difference
frequency components are extracted by converting the PB
and REF into a spectrum with mixer circuit.
f1-f4=46 kHz
ft-f2=16 kHz
The level of 46 kHz and 16 kHz are proportional to the f4
and
{2 levels,
respectively.
The
46 kHz
and
16 kHz
components are extracted by the BPF at the next stage
and the level is detected.
46 kHz increases when the video head is shifted in the 14
(left) direction and 16 kHz increases when shifted in the f2
{right) direction.
An amount of shift can be detected by comparing the level
difference of 46 kHz and 16 kHz.
When REF is f1 and the trace position of the head shifts
LEVEL
DETECTOR
for376fn
(NTSC}=
5.95 MHz
375th (PAL) = 5.86MHz
LEVEL
DETECTOR
VOLTAGE
COMPARATOR
ATF
ERROR SIGNAL
16K/46K
ATF processing
from the A position (origin) in the following sequence;
=>B (+p)=>C (+-2p)=>D (+3p=-p),
the level of 46 kHz and 16 kHz components change as
shown in Fig, 7-15.
When
REF is changed, the reference point will change.
When REF is f2, the B position is the origin for obtaining
the detection characters, C for £3, and D for f4. (Table 7-3.)
Thus, REF cycles between;
fi=>f2=>f3=>f4=>,
for each track (field), and the detection
reference point
(origin) moves to;
A=>B=>C=>D=>,
at the switching point.
Mistracking
Reference (REF)
x
aa
oe
®
#8,
ie
fl
Cc
D
A
B
Cc
2
D
A
B
Cc
D
£3
A
B
Cc
D
A
£4
B
Cc
D
A
B
Table 7-3.
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