Motion video signal processing for recording or reproducing – Local trick play processing – With randomly accessible medium
Reexamination Certificate
1998-12-03
2002-06-11
Christensen, Andrew B. (Department: 2615)
Motion video signal processing for recording or reproducing
Local trick play processing
With randomly accessible medium
C360S061000
Reexamination Certificate
active
06404976
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to VISS (VHS Index Search System) signal detection circuits, and more particularly, to a VISS signal detection circuit detecting a VISS signal recorded on a magnetic tape in a VTR (Video Tape Recorder).
2. Description of the Background Art
In a VHS type VTR, there is provided a feature for a user to write desired data on a magnetic tape to facilitate retrieval of data in a recorded tape. More specifically, a VISS signal having two types of duties corresponding to 0 and 1 is recorded on a magnetic tape taking advantage of the control track used in servo control. This VISS signal indicates the index code to be used mainly for head position detection.
A signal recorded on a magnetic tape is read out by a control head. The level of the read out signal varies according to the moving speed of the magnetic tape. When the speed of the magnetic tape is low as at the time of initiating reproduction, the amplitude of the signal read out by the control head is small. In contrast, when the speed of the magnetic tape is high as in fast-forwarding, rewinding, fast-forward reproduction, and rewind reproduction, the amplitude of the read out signal is great. Therefore, the amplitude of the VISS signal read out by the control head depends upon the tape speed.
FIG. 9
is a circuit diagram showing a structure of a conventional VISS signal detection circuit that can detect a VISS signal for both the cases of a high speed and a low speed tape. Referring to
FIG. 9
, a signal recorded on a magnetic tape is read out by a control head
1
. A control head signal CH from control head
1
is applied to a VISS signal detection circuit
2
. Control head signal CH is biased by a voltage Vbias by a bias power supply
3
. Control head signal CH is amplified by a differential amplifier
4
. The gain (amplification factor) of differential amplifier
4
is determined by resistors
5
and
6
and a resistor array
7
. Resistor array
7
is formed of a plurality of resistors having various resistances. A switch circuit
8
switches the resistor in resistor array
7
according to the tape speed. More specifically, switch circuit
8
functions to switch the resistor in resistor array
7
so that the gain of differential amplifier
4
becomes greater when the tape speed is low and smaller when the tape speed is high. Therefore, the amplitude of a signal APout output from differential amplifier
4
is constant as shown in
FIG. 10
regardless of the tape speed. Signal APout output from differential amplifier
4
includes erase noise
10
in addition to a signal VISS
9
of a large amplitude. Erase noise
10
is the noise that remains when the VISS signal is rewritten to 0 from 1 or vice versa. A VISS signal
9
higher than bias voltage Vbias is detected by a comparator
11
shown in
FIG. 9. A
VISS signal lower than bias voltage Vbias is detected by a comparator
12
. Therefore, comparator
11
is supplied with a reference voltage Vrefh lower than the peak value of VISS signal
9
and higher than bias voltage Vbias. Similarly, comparator
12
is supplied with a reference voltage Vrefl. An output signal CPouth from comparator
11
is applied to the set terminal of a RS flip-flop circuit
13
. An output signal CPout
1
of comparator
12
is applied to the reset terminal of RS flip-flop circuit
13
. Therefore, detection signal Dout from RS flip-flop
13
attains an H level (logical high) when output signal APout exceeds reference voltage Vrefh and an L level (logical low) when output signal APout exceeds reference voltage Vref
1
. This signal Dout is the detection signal of a VISS signal. When the ratio of a term t
1
to term t
2
(duty ratio) is approximately 60%, recognition is made of 0. When the duty ratio is approximately 25%, recognition is made of 1. By recording such a VISS signal by a predetermined data pattern (for example, 011 . . . 110) on a magnetic tape, head position detection can be facilitated by detecting this VISS signal.
By switching the gain of differential amplifier
4
according to the tape speed, the amplitude of the signal read out by control head
1
can be made constant substantially. However, the amplitude of this signal varies slightly as shown in
FIG. 11
even when the tape speed is constant. Although all the peaks of VISS signal
9
can be detected by setting reference voltage Vrefh of comparator
11
low and reference voltage Vrefl of comparator
12
high, there is a possibility of erroneous detection of erase noise
10
as a peak of VISS signal
9
. In contrast, if reference voltage Vrefh of comparator
11
is set high and reference voltage Vrefl of comparator
12
is set low, there is a possibility of not detecting all the peaks of VISS signal
9
although no erase noise
10
will be detected erroneously. There will be no problem if reference voltages Vrefh and Vref
1
are set intermediate the peak of VISS signal
9
and the peak of erase noise
10
. However, this setting is not so easy since the amplitude of erase noise
10
differs depending upon the tape speed and the like.
When VISS signal detection circuit
2
of
FIG. 9
is employed, resistor array
7
must be externally controlled by a microcomputer and the like. However, it is difficult to implement task sharing with another real time task since the microcomputer will exclusively carry out this detection task during the VISS signal detection.
SUMMARY OF THE INVENTION
In view of the foregoing, an object of the present invention is to provide a VISS signal detection circuit that can properly detect a VISS signal.
Another object of the present invention is to provide a VISS signal detection circuit that can always detect a VISS signal properly even when the tape speed changes.
A further object of the present invention is to provide a VISS signal detection circuit that can detect a VISS signal properly even when the tape speed is constant.
Still another object of the present invention is to provide a VISS signal detection circuit that does not erroneously detect erase noise as a VISS signal.
A still further object of the present invention is to provide a VISS signal detection circuit that does not require external control.
According to an aspect of the present invention, a VISS signal detection circuit detecting a VISS signal recorded on a magnetic recording medium includes a plurality of first comparators, a plurality of second comparators, a select circuit, and an output circuit. The plurality of first comparators have a threshold value differing from each other. Each first comparator compares the level of the VISS signal with its threshold value to supply an output signal when the level of the VISS signal is higher than the threshold value. The plurality of second comparators have a threshold value differing from each other. Each second comparator compares the level of the VISS signal with its threshold value to supply an output signal when the level of the VISS signal is lower than the threshold value. The select circuit selects the output signal from the comparator that has the second highest threshold value out of the first comparators supplying an output signal at the prior comparison, and the output signal from the comparator having the second lowest threshold value out of the second comparators supplying an output signal at the prior comparison. The output circuit provides a detection signal that is driven to a first level in response to one output signal from the select circuit and that is driven to a second level in response to the other output signal from the select circuit.
When the peak level of the VISS signal is detected in the above VISS signal detection circuit, the level of the threshold value to detect the next VISS signal is set slightly lower or higher than that detected peak level. Since the threshold value is reset to an optimum level every time a VISS signal is detected, a VISS signal can be detected reliably without erroneous detection of the erase noise.
Preferably, the VISS signal detection circuit further includes an a
Chieu Po-lin
Christensen Andrew B.
McDermott & Will & Emery
Mitsubishi Denki & Kabushiki Kaisha
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