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Description  |
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BACKGROUND OF THE INVENTION
The invention relates to magnetic recording and reproducing apparatuses
such as home VTRs in which a luminance signal of a video signal to be
recorded is subjected to frequency modulation and then to single-sideband
recording.
In home VTRs the frequency band of signals which can be recorded and
reproduced is generally narrower than the frequency band of a
frequency-modulated (FM) luminance signal of a video signal to be recorded
and, as a result, high-frequency components of the upper sideband of this
FM signal are neither recorded nor reproduced. It is for this reason that
the FM, signal takes the form of a single-sideband wave when recorded and
involves an amplitude variation when reproduced. If this FM signal is
demodulated with its amplitude variation removed by a limiter, the
aforesaid high frequencies of its upper sideband can be recovered. This is
how the single-sideband recording is performed.
By the way, in the FM signal that has been through with the limiter, the
modulation indices of the high-frequency components of its recovered upper
sideband and the low-frequency components of its lower sideband are
decreased, and this acts to decrease the level of the FM signal. When this
FM signal is demodulated, the energy of the high-frequency components of
its demodulated luminance signal becomes smaller than that of its
low-frequency components, thereby decreasing the signal level as a whole.
To compensate for this decrease of the signal level, the home VTR employs
a peaking circuit to boost the high-frequency components of the
demodulated luminance signal. This peaking circuit serves to keep the
level of the high-frequency components of the demodulated luminance signal
as high as that of its low-frequency components.
Further, video tapes for the home VTRs include normal tapes and so-called
"high-grade tapes" that exhibit higher performance than the normal tapes.
To make the most of the performance of high-grade tapes, the
characteristics of a recording or reproducing system of a VTR must be
differentiated between high-grade tapes and normal tapes. For example, the
following means could be proposed to implement such differentiation.
(1) Controlling the level of emphasis at a detail emphasis circuit in a
recording system in which a luminance signal of a video signal is
frequency-modulated and then magnetically recorded. The detail emphasis
circuit serves to emphasize small-level signal components in the
high-frequency components of the luminance signal before frequency
modulation;
(2) Controlling the amount of equalization at an FM equalizer which serves
to boost the low-frequency components of an FM luminance signal in the
recording system in which the luminance signal of a video signal is
frequency-modulated and then magnetically recorded;
(3) Controlling the amount of peaking at an FM peaking circuit which serves
to boost the high-frequency components of an FM luminance signal before
demodulation in a reproducing system in which a reproduced luminance
signal is frequency-demodulated; and
(4) Controlling the amount of cancellation at a noise canceller which
serves to cancel noise of a demodulated luminance signal in the
reproducing system in which a reproduced luminance signal is
frequency-demodulated.
The recording and reproducing systems of a home VTR will be outlined next
with reference to FIG. 3.
The recording system will be described first. In FIG. 3, an input terminal
1 receives a video signal from a TV receiver or the like (not shown). This
video signal is applied to a LPF 2, where a luminance signal is extracted,
and the extracted luminance signal is then applied to an AGC 3, which
controls the applied signal in such a manner that the level of its
synchronizing signal can be kept constant. The video signal fed to the
input terminal 1 is also applied to a BPF (not shown), where a chroma
signal is extracted, and the extracted chroma signal is then converted
into a low-frequency signal at a chroma signal processing circuit (not
shown).
The luminance signal which has been through with the AGC 3 is then fed to a
detail emphasis circuit 4, where small-level signal components in
high-frequency components of the luminance signal are emphasized, and the
emphasized signal is then applied to a luminance signal processing circuit
5. In the luminance signal processing circuit 5, the high frequencies are
subjected to emphasis and a like process and then applied to a frequency
modulator 6. The luminance signal frequency-modulated at the frequency
modulator 6 has its low frequencies boosted by an FM equalizer 7, and low
frequencies that correspond to the band of the low-frequency converted
chroma signal are cut off at a HPF 8. The FM luminance signal which has
been through with the HPF 8 is then applied to a recording amplifier 9,
where it is amplified while mixed with the low-frequency converted chroma
signal from the input terminal 10, and the thus processed signal is then
supplied to a video head 11 to be magnetically recorded on a video tape
12.
The reproducing system will be described next. A video head 13, though
shown separately from the video head 11 for purposes of convenience, is
the same as the video head 11. A video signal reproduced from the video
head 13 is amplified by a preamplifier 14, and not only a chroma signal
that is subjected to low-frequency conversion by a LPF 15 is extracted but
also a luminance signal that is frequency-modulated by a HPF 16 is
extracted. The chroma signal which has been through with the LPF 15 is
converted into a chroma signal of a subcarrier band by a chroma signal
processing circuit 17 and the converted chroma signal is then supplied to
a mixing circuit 18.
The luminance signal which has been through with the HPF 16 has its
high-frequency components (around 5 MHz) boosted by an FM equalizer 19, is
kept at a constant level by an FM AGC 20 thereafter, and is then
demodulated when applied to a frequency demodulator 21. The luminance
signal delivered from the FM demodulator 21 is appropriately processed by
a luminance signal processing circuit 22, and has its high-frequency
components (2 MHz or more) boosted by a subsequent peaking circuit 23. The
luminance signal which has been through with the peaking circuit 23 is
applied to a line noise canceller 24.
As shown in FIG. 4 in detail, the line noise canceller 24 delays a
luminance signal fed from an input terminal 27 via the peaking circuit 23
for a single horizontal scanning period (hereinafter referred to simply as
"1H") by a 1H delay element 28. The thus delayed luminance signal and the
original luminance signal that has not been delayed are subjected to
subtraction at a subtractor (which is shown as an adder 29, but acts as a
subtractor because its polarity is inverted by the 1H delay element 28).
Only a small-level signal which can be deemed as a noise component is
extracted from the subtracted signal by a limiter 30, and the level of
this small-level signal is attenuated by 1/2 at an attenuator 31. The
attenuated signal is then subtracted by a subtractor shown as an adder 32,
so that the noise component can be canceled. The luminance signal whose
noise component has been canceled is then fed to a noise canceller 25 from
an output terminal 33.
The noise canceller 25 extracts high-frequency components containing much
noise from the luminance signal that has been through with the line noise
canceller 24, and components whose levels are greater than a predetermined
level are removed from the high-frequency components by a slice circuit
(not shown). The output of the slice circuit is then subtracted from the
luminance signal that has been through with the line noise canceller 24 to
cancel the noise.
The luminance signal which has been through with the noise canceller 25 is
applied to the mixing circuit 18 so as to be mixed with the chroma signal,
the mixed signal is then delivered to an output terminal 26 and fed to a
not shown well known CRT or the like, so that a reproduced image can be
obtained.
The so-called high-grade tapes are commercially available as video tapes
for home VTRs. The high-grade tape can provide not only a high reproducing
output from the tape but also a remarkably improved signal-to-noise ratio
by its extremely high degree of granulation and loading of magnetic
powder.
That is, as shown in a characteristic diagram in FIG. 5, a high-grade tape
al exhibits an output level higher than a normal tape b1, with the
characteristic that its output level becomes higher especially in a
high-frequency range (4 to 6 MHz) than in a low-frequency range (1 to 2
MHz).
When a video signal is recorded/reproduced using the high-grade tape, the
levels of components such as the carrier wave, the low-frequency
components of the upper sideband, and the high-frequency components of the
lower sideband in the reproduced FM signal are boosted, making the levels
of components such as the carrier wave, the low-frequency components of
the upper sideband, and the high-frequency components of the lower
sideband in the reproduced FM signal that has been through with the
limiter 30 higher than in the normal tape. Also, the levels of components
such as the high-frequency components of the upper sideband and the
low-frequency components of the lower sideband in the reproduced FM signal
that has been through with the limiter 30 become relatively lower than the
levels of the low-frequency components of the upper sideband and the
high-frequency components of the lower sideband as described before. And
this makes the high-frequency signal level of the demodulated luminance
signal low with respect to its low-frequency signal level that is
maintained at a predetermined level, with this high-frequency signal level
being lower than that in the normal tape. When the high-frequency signal
level is so low as this, it cannot be adequately compensated for by the
peaking circuit 23 if the circuit 23 is set to the normal tape mode.
FIG. 6 shows levels of demodulated video signals for both a high-grade tape
a2 and a normal tape b2, indicating that the high-grade tape outputs a
lower signal level at high frequencies than the normal tape. It is well
known that high-grade tapes produce low reproduced outputs at high
frequencies, and this results in blurring reproduced images.
To compensate for the impaired reproduction due to blurred images which are
caused by deterioration in the high frequencies of a luminance signal of a
recorded/reproduced video signal when operating a home VTR, the detail
emphasis circuit 4, which serves to emphasize low-level signal components
in high frequencies of a luminance signal to be recorded is provided as
described above.
As shown in FIG. 13, the aforesaid detail emphasis circuit 4 is made up of
a HPF 78, an amplifier 79, a limiter 80, an attenuator 81, an adder 82,
and the like. A luminance signal received at an input terminal 70 from the
AGC 3 is subjected to a process so that its high-frequency components are
extracted by the HPF 78, and the extracted components are then amplified
by the amplifier 79. The amplified high-frequency components are then
applied to the limiter 80 so that the small-level signal components are
extracted. The high-frequency small-level signal components which have
been through with the limiter 80 are subjected to level adjustment by the
attenuator 81 and to addition thereafter by the adder 82 so as to be added
to the luminance signal from the input terminal 70. Then, the thus
processed signal is applied to the luminance signal processing circuit 5
via an output terminal 84.
In a VTR having this detail emphasis circuit 4, if it assumed that a
high-frequency small-level signal component incorporated in the luminance
signal of a video signal to be recorded, i.e., an input detail signal, is
such as one shown by b3 in FIG. 15A, then this input detail signal b3 is
subjected to emphasis by the detail emphasis circuit 4 in the normal tape
mode so that it will become a signal such as shown by b4 in FIG. 15A. When
the thus processed input detail signal b4 is recorded/reproduced by a
normal tape, and further demodulated, a signal similar to the input detail
signal b3 is reproduced as shown by b5 in FIG. 15A.
By the way, if an input detail signal a3 shown in FIG. 15 which is similar
to the input detail signal b3 is subjected to emphasis by the detail
emphasis circuit 4 in the normal tape mode so that it will be reformed
into a signal a4 that is similar to the signal b4, and if this signal a4
is recorded/reproduced using a high-grade tape and further demodulated,
then the level of this signal a4 becomes lower than that of the input
detail signal a3 as shown by a signal a5 in FIG. 15A. Such decrease in the
level of the detail signal results in blurs on reproduced images.
As described above, the amount of emphasis in the conventional detail
emphasis circuit is not sufficient to compensate for the blurs on the
reproduced images when high-grade tapes are used.
Further, to automatically provide the optimal performance of a video tape
in accordance with its type as well as to automatically compensate for
deterioration in the characteristic of a video tape in accordance with its
type, the following means have heretofore been known.
(1) To identify the type of a video tape by the level of a reproduced
signal, the frequency characteristic of a recording system is adjusted in
such a manner that a reproduced level is detected and stored and the
frequency characteristic of an FM luminance signal is adjusted in
accordance with the stored reproduced level (Japanese Patent Unexamined
Publication No. 146674/1988);
(2) The amount of compensation for a frequency characteristic is kept
unchanged by changing the peaking amount at an FM equalizer in the
reproducing system while detecting the envelope of a reproduced FM signal
and using this detected output as a control signal, even if the frequency
characteristic of a video tape has been changed (Japanese Patent
Unexamined Publication No. 59287/1988); and
(3) A noise component from an original signal is subtracted by separating
high-frequency components in the reproduced luminance signal at which
noise tends to concentrate by a filter, i.e., the noise canceller 25
described as being included in the reproducing system, and by further
filtering the low-level signal components out by the slice circuit, so
that the high-frequency low-level signal is Considered as the noise and
subtracted from the original signal; that is,
an FM luminance signal reproduced from a video tape is detected using an FM
reproducing level detector and the slice level of the slice circuit is
decreased with increasing reproduced levels (e.g., in the case of a
high-grade tape) (Japanese Patent Unexamined Publication No. 14178/1989).
Incidentally, since the noise canceller 25 at which the slice level remains
constant regularly cancels any high-frequency low-level signal components,
the original signal components other than the noise may also be canceled.
For this reason, it is preferable to set the amount of cancellation to a
small value for high-grade tapes in which the signal-to-noise ratio is
significantly improved but the high-frequency level of their demodulated
luminance signal is decreased.
The line noise canceller shown in FIG. 4 does cancel not only noise
components but also small-level signal components (detail signals) to a
certain degree, thus causing the level of small-level signals to be
decreased.
Further, while there is no attenuation in the level of a luminance signal
that is in identical correlation with a luminance signal of 1H before,
once there is any deviation from the correlation, the level of the
deviating signal is decreased as much as such deviation.
The cases where the correlation is identical and where there is a deviation
in the correlation will be described with reference to FIGS. 7A-7E and
8A-8E. Signals X1 to X5 in FIGS. 7A-7E show a case of the identical
correlation, where X1 is an original signal; and X2, a signal of 1H
before. These signals are in phase. Signal X3 is an output of the adder
29, which is set to level 0; X4, an output of the attenuator 31, which is
likewise set to level 0; and X5, an output of the adder 32, producing a
signal identical to the original signal shown by X1. Signals Y1 to Y5 show
a case where a signal of 1H before is 90.degree. ahead of an original
signal, with the original signal and the signal of 1H before being assumed
to be such small-level signals as not to be limited by the limiter 30.
Signal Y1 is an original signal, expressed by sin.theta.; Y2, a signal of
1H before, expressed by sin(.theta.+.pi./2), or
sin(.theta.+.pi./2)=cos.theta.. Signal Y3 is an output of the adder 29,
which is expressed by Y1-Y2=sin.theta.-cos.theta., or
sin.theta.-cos.theta.=.sqroot.2 sin(.theta.-.pi./4) in a composite
equation. Signal Y4 is an output of the attenuator 31, which is 1/2 the
level shown by signal Y3, or .sqroot.2/2 sin(.theta.-.pi./4). Signal Y5 is
an output of the adder 32, which is sin.theta.-1/2
{sin.theta.-cos.theta.}=1/2 sin.theta.+1/2 cos.theta., or 1/.sqroot.2
sin(.theta.+.pi./4) in a composite equation.
As described above, the level of the original signal Y1 is decreased by 30%
at the stage of the output signal Y5 and the phase of signal Y5 is
45.degree. ahead.
Small-level signals are generally less correlative in terms of line
compared with large-amplitude signals. While no images can be recognized
with the large amplitude signals having no line-based correlation, the
small-level signals can produce recognizable images with no such
correlation. For example, in the case of irregularities of human skin,
there is no correlation. Signals Z1 to Z5 shown in FIGS. 8A-8E present a
case where there is no correlation between an original signal and a signal
of 1H before. In FIGS. 8A-8E, signal Z1 is a projecting original signal;
Z2, a flat signal of 1H before, these signals having such small-amplitudes
as not to be limited by the limiter 30; Z3, an output of the adder 29,
which has a projecting waveform identical with the original signal; Z4, an
output of the attenuator 31, whose amplitude is 1/2 that of the original
signal Z1; and Z5, an output of the adder 32, whose amplitude is 1/2 that
of the original signal Z1.
Accordingly, while serving to reduce noise, the line noise canceller 24
also decreases the level of the detail signal to some extent, and if there
is a deviation from the correlation between the detail signal and the
luminance signal of 1H before, the line noise canceller decreases the
level of the detail signal as much as that deviation.
By the way, the aforesaid means have heretofore been known to automatically
provide an optimal performance of a video tape in accordance with its type
such as a normal tape or a high-grade tape, as well as to automatically
compensate for deterioration in the characteristic of a video tape in
accordance with its type. However, these means are not successful in
effectively providing the optimal performance and compensating for the
deterioration in the characteristic in accordance with the type of a video
tape.
Let us now take a look at a case where a noise canceller disclosed in
Japanese Patent Unexamined Publication No. 14178/1989 is used.
It is generally known that, when a color video signal is reproduced from a
VTR and a luminance signal thereof is demodulated thereafter, an unwanted
component of about 1.2 MHz is present in the demodulated luminance signal.
The level of this unwanted component is increased with higher level of a
reproduced chroma signal, i.e., higher degree of color saturation.
Some reasons why this unwanted component is present will be described. In
the case of magnetic recording on a VTR, a recording current is subjected
to third order distortion by hysteresis and magnetized as such on the
tape.
It is assumed that:
FM luminance signal=A sin.alpha.
Low-frequency converted chroma signal=B sin.beta.,
then, the above phenomenon on the tape can be expressed as follows.
##EQU1##
With FM luminance signal component=A.sup.3 sin.sup.3 .alpha.
Low-frequency converted chroma signal component=B.sup.3 sin.sup.3 .beta.
3A.sup.2 Bsin.sup.2 .alpha..multidot.sin.beta.; 3AB.sup.2
sin.alpha..multidot.sin.sup.2 .beta. will be developed as follows
##EQU2##
Similarly,
##EQU3##
FIG. 9 is a spectrum obtained when a signal magnetized on a video tape has
been reproduced. Reference character a designates a low-frequency
converted chroma signal sin.sup.3 .beta.; b, an unwanted component
sin(.alpha.-2.beta.); c, an FM luminance signal sin.sup.3 .alpha.; d, an
unwanted component sin(.alpha.+2.beta.); and e, f, unwanted components
sin(2.alpha.-.beta.), sin(2.alpha.+.beta.), respectively. Among these
unwanted components, the high-frequency components e, f produce so small
output levels that they can be considered negligible.
In the case where noise of a luminance signal reproduced from a normal tape
is to be canceled by the noise canceller whose amount of cancellation is
varied in accordance with the level of the reproduced FM luminance signal
as described above, the slice level is increased, so that the unwanted
components b, d will also be canceled.
However, in the case where noise of a luminance signal reproduced from a
high-grade tape is to be canceled and where an image such as an animation
whose color saturation is high is to be reproduced, the unwanted
components b, d will not be canceled despite the fact that the slice level
is small, because these components b, d are comparatively large.
The presence of the unwanted components b, d in a luminance signal causes
mesh-like noise to appear over the entire part of a reproduced image,
making the image indistinct.
In the case of an image with a low color saturation, the levels of the
unwanted components b, d are low even if a
Luminance signal is derived from a high-grade tape, thereby causing no mesh
noise to appear.
SUMMARY OF THE INVENTION
A first object of the invention is to provide a magnetic
recording/reproducing apparatus which is capable not only of automatically
providing a better performance of a video tape or the like in accordance
with the type thereof than conventional apparatuses, but also of
automatically compensating for deterioration in the characteristic of a
video tape or the like in accordance with the type thereof.
A second object of the invention is to provide a magnetic recording
apparatus or a magnetic recording/reproducing apparatus capable of
producing a single-wavelength reference signal with using no independent
oscillator in a magnetic recording/reproducing apparatus, in which, to
automatically control the characteristics of a recording or reproducing
system in accordance with the type of a video tape, a single-wavelength
reference signal is recorded on and reproduced from the video tape, and
the level of the reproduced reference signal is detected and stored.
A third object of the invention is to provide a magnetic
recording/reproducing apparatus which is capable of preventing the mesh
noise from appearing.
To achieve the above objects, the invention is applied to a magnetic
recording/reproducing apparatus, which includes:
a reproduced level detector for detecting the level of a reproduced signal
and a memory for storing the reproduced level detected by the detector;
and
when recording a video signal, the signal is recorded and reproduced in
advance, and this reproduced level is stored in the memory, so that an
amount of emphasis of a detail emphasis circuit in a reproducing system
and an amount of equalization of an FM equalizer in the recording system
can be controlled in accordance with the stored reproduced level; while
when reproducing the video signal, the level of a reproduced FM luminance
signal is detected by the reproduced level detector, so that an amount of
equalization of an FM equalizer, an amount of cancellation of a line noise
canceller, and an amount of cancellation of a noise canceller in a
reproducing system can respectively be controlled in accordance with the
detected reproduced level; whereby
the characteristic of a video tape can be controlled in accordance with the
type thereof on an apparatus-wide and integrated basis.
Instead of controlling the amount of equalization of the reproducing system
FM equalizer, a peaking amount of a reproducing system peaking circuit may
be controlled.
The magnetic recording/reproducing apparatus also includes a chroma level
detector which serves to detect the level of a reproduced chroma signal at
the time a low-frequency converted and recorded chroma signal is
reproduced and further converted into a subcarrier band, so that an amount
of cancellation of the noise canceller can be controlled in accordance
with the chroma level detected by the chroma level detector.
The thus implemented magnetic recording/reproducing apparatus records and
reproduces a video signal in advance when recording the video signal, and
the reproduced level of the video signal is stored in the memory. And the
amount of emphasis of the recording system detail emphasis circuit and the
amount of equalization of the recording system FM equalizer are controlled
in accordance with the stored reproduced level.
When reproducing the video signal, the level of the reproduced FM luminance
signal is detected by the reproduced level detector, and the amount of
equalization of the FM equalizer, the amount of cancellation of the line
noise canceller, and the amount of cancellation of the noise canceller in
the reproducing system can respectively be controlled in accordance with
the detected reproduced level.
If the peaking amount of the reproducing system peaking circuit is
controlled instead of the amount of equalization of the reproducing system
FM equalizer, then the peaking amount is controlled.
Further, the level of a reproduced chroma signal at the time the
low-frequency converted and recorded chroma signal is reproduced and
further converted into a subcarrier band is detected by the chroma level
detector, and the amount of cancellation of the noise canceller is
controlled in accordance with the detected chroma level.
Further, the invention is applied to a magnetic recording or magnetic
recording/reproducing apparatus which has a dc voltage source that
selectively applies a predetermined level of dc voltage to a frequency
modulator, and the output of the frequency modulator obtained by applying
the dc voltage to the frequency modulator is used as the reference signal.
The thus implemented magnetic recording or magnetic recording/reproducing
apparatus obtains a single-wavelength reference signal from the frequency
modulator by applying the dc voltage from the dc voltage source to the
frequency modulator.
Further, the invention is applied to a magnetic recording/reproducing
apparatus which includes means for detecting the level of a reproduced
signal and storing the detected level, so that an amount of emphasis of
the detail emphasis circuit can be controlled in accordance with the
stored detected level.
The thus implemented magnetic recording/reproducing apparatus detects the
level of a reproduced signal and stores the detected reproduced level, and
the amount of emphasis of the detail emphasis circuit is controlled in
accordance with the stored detected level.
Further, the invention is applied to a noise canceller used in a magnetic
recording/reproducing apparatus, which noise canceller includes: an FM
reproduced level detector which detects the level of a reproduced FM
luminance signal; means for extracting a noise component from a luminance
signal demodulated from the FM luminance signal; a slice circuit which has
a slice level for removing a component whose level is higher than a
predetermined level from the noise component and controls the slice level
so as to be decreased with increasing reproduced level; and a subtractor
which subtracts the output of the slice circuit from the luminance signal.
In such a noise canceller, a chroma level detector is provided to detect
the level of a reproduced chroma signal, so that a level of the slice
circuit can be controlled so as to be increased with increasing detected
chroma level.
In the thus implemented noise canceller in a magnetic recording/reproducing
apparatus not only improves the resolution of the apparatus with the
amount of cancellation being decreased as the level of the reproduced FM
luminance signal increases, but also removes the unwanted components
contained in the luminance signal with the amount of cancellation being
increased when the level of a reproduced chroma signal is large, thereby
eliminating mesh noise from reproduced images.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram showing an outline of a main portion of a
recording/reproducing system of a home VTR, which is a first embodiment of
the invention;
FIG. 2 is a block diagram showing a denial of the main portion of the
recording/reproducing system shown in FIG. 1;
FIG. 3 is a block diagram showing an outline of a main portion of a
recording/reproducing system of a conventional home VTR;
FIG. 4 is a block diagram showing a line noise canceller in the
recording/reproducing System shown in FIG. 3;
FIG. 5 is a characteristic diagram showing reproduced output levels of a
normal tape and a high-grade tape;
FIG. 6 is a characteristic diagram showing reproduced video levels of the
normal tape and the high-grade tape;
FIGS. 7A-7E and 8A-8E are waveform diagrams illustrative of an operation of
the noise canceller;
FIG. 9 is a spectrum of a signal which is magnetized on a video tape by a
VTR and reproduced by a video head;
FIG. 10 is a characteristic diagram of an FM equalizer of the recording
system shown in FIG. 1;
FIG. 11 is a characteristic diagram of an FM equalizer of the reproducing
system shown in FIG. 1;
FIG. 12 is a characteristic diagram of a peaking circuit of the reproducing
system shown in FIG. 1;
FIG. 13 is a block diagram showing a detail emphasis circuit in the
recording/reproducing system shown in FIG. 3;
FIGS. 14A and 14B are diagrams showing signal levels in a conventional
example;
FIGS. 15A and 15B | | |
