Motion adaptive frequency folding method and circuit

A motion adaptive frequency folding method and circuits thereof for folding high-frequency components over low-frequency components of a brightness signal for enabling recording of a video signal having a full bandwidth onto a recording medium having a restricted bandwidth in order to improve resolution of a video image by preventing deterioration of the video image due to artifacts and noise. The motion adaptive frequency folding method comprises the steps of detecting high-frequency components above a reference frequency band and low-frequency components below the reference frequency band of the brightness signal, detecting a motion coefficient indicative of motion of an image from the brightness signal, controlling a magnitude of the high-frequency components and limiting transmission of the high-frequency components of the brightness signal in dependence upon said motion coefficient, and folding the high-frequency components above said reference frequency band having relative magnitudes controlled and limited by said motion coefficient over the low-frequency components below the reference frequency band of the brightness signal to produce a folded brightness signal.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a motion adaptive frequency folding method 
and circuit suitable to a system having a restricted frequency band, such 
as a television, video tape recorder, laser disc player or the like for 
folding a signal component having a predetermined frequency over a signal 
component below the frequency. 
2. Description of the Prior Art 
In general, it is well-known that a conventional VHS video tape recorder is 
incapable of recording a signal having a high frequency above 2.5 MHz from 
the view point due to the limited characteristics of the magnetic video 
tape which contributes to a deterioration of a picture image upon 
reproduction by the video tape recorder because only 60% of a television 
signal band is utilized in the system. 
Meanwhile, a super-VHS video tape recorder can send a signal beyond a 
television signal band so as to provide an excellent picture image, but it 
is not compatible with the existing VHS video tape recorder. That is, when 
a signal recorded by a standard VHS video tape recorder is reproduced by 
means of the super-VHS video tape recorder system, the resolution of the 
reproduced signal is deteriorated considerably. Further, the super-VHS 
video tape recorder requires a magnetic tape of a high standard as well as 
a recording/replaying apparatus of superior quality. 
To address this deficiency, a video signal recording/replaying apparatus 
which can record a video signal having a bandwidth wider than that of a 
video signal to be recorded by the standard VHS video cassette recorder 
onto the standard magnetic tape and is compatible with the standard-VHS 
video cassette recorder has been proposed in, for example, U.S. Pat. No. 
5,113,202. 
With the patent, a motion signal indicating a movement of the video image 
to be reproduced is appropriately extracted and then the extracted motion 
signal is modified and recorded into a color signal. 
The motion signal is used to control a transmission of a full band-width 
signal arears to be unfolded in an original frequency band during 
reproduction by the system, wherein a high frequency component of a 
brightness signal is folded over a low frequency component thereof. 
According to the construction, however, when the motion signal is not 
correctly detected upon reproduction of a picture image, the picture image 
is deteriorated in quality because of artifacts such as dot crawl patterns 
or the likes occurred owing to frequency interleaving between the to 
motion signal and a folding carrier. 
SUMMARY OF THE INVENTION 
Accordingly, an object of the present invention is to provide a motion 
adaptive frequency folding method and circuit for differently folding a 
signal having a restricted frequency band by using a motion signal 
indicating a motion video image and recording the signal of full bandwidth 
so as to prevent deterioration of the quality of the video image due to 
artifacts upon reproduction the motion. That is, the object of the present 
invention is to provide a method of folding the signal component above a 
predetermined frequency over the signal component below the frequency in 
order to record a video signal of full bandwidth on the recording medium 
having a restricted bandwidth. 
To achieve the above object, a motion adaptive frequency folding method 
comprising the steps of: separately filtering high frequency and low 
frequency brightness signals, each having a bandwidth above and below the 
bandwidth of a recording medium, from a brightness signal with a full 
bandwidth; detecting a motion signal from the brightness signal with a 
full bandwidth; controlling gain of the high-frequency brightness signal; 
limiting an output level of the high-frequency brightness signal 
controlled in the the motion signal; detecting step and folding the 
high-frequency brightness signal at the time of detection of a still image 
over the low-frequency brightness signal. 
Also, the present invention provides a motion adaptive frequency folding 
circuit comprising: a first horizontal low pass filter means for filtering 
a low-frequency brightness signal having a bandwidth below a recording 
medium bandwidth from a brightness signal having a full bandwidth; 
horizontal high pass filter means for separating a high-frequency 
brightness signal having a bandwidth above the recording medium bandwidth 
from the brightness signal; a motion signal detector means for producing a 
factor corresponding to a motion signal from the brightness signal; a gain 
controller means for controlling gain of the high-frequency signal output 
from the horizontal high pass filter; a signal limiter for limiting of the 
high-frequency brightness signal output from the gain controller in 
dependence upon the motion signal factor produced by the motion signal 
detector; an adder for adding the high-frequency brightness signal limited 
by the signal limiter and the low-frequency brightness signal output from 
the first horizontal filter; a subsampling device for folding the 
high-frequency brightness signal above a bandwidth of a transfer medium of 
the brightness signal outputted from the adder over the low-frequency 
brightness signal of the brightness signal; and, a second horizontal low 
pass filter for passing only the signal component below the bandwidth of 
the recording medium in the folded brightness signal. 
The above and other objects, features and advantages will be apparent from 
the following description taken with reference to the accompanying 
drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Thereinafter, a preferred embodiment of the present invention will be 
described with reference to the drawings. 
Referring to FIG. 1, there is shown a block diagram of a motion adaptive 
frequency folding circuit according to the preferred embodiment of the 
present invention. In the drawing, a first horizontal low pass filter 10 
receives a brightness signal Y and passes a signal component below a 
predetermined cutoff frequency, for example, 2.5 to 3 MHz. A horizontal 
high pass filter 20 also receives the brightness signal Y and passes a 
signal component above the cutoff frequency. 
A motion signal detector 30, detects a motion signal from the brightness 
signal Y and produces a motion factor K corresponding to the motion 
signal. Further, an adaptive emphasis/de-emphasis portion 40 which is 
referred as a gain controller, serves to increase or decrease of the 
signal level from the horizontal high pass filter 20. A signal limiting 
portion 50 limits the signal supplied from the adaptive 
emphasis/de-emphasis portion 40 on the basis of the motion factor K 
produced by the motion signal detector 30. An adder 60 adds the 
low-frequency brightness component signal outputted from the horizontal 
low pass filter 10 and the high-frequency brightness component signal from 
the signal limiting portion 50. The added signal is input to a subsampling 
portion 70 which executes a field offset subsampling so as to fold the 
signal component above a predetermined reference frequency band over the 
signal component below the reference frequency band of the adder signal. 
In the drawing, 80 denotes a second horizontal low pass filter which 
extracts the signal below the reference frequency in the folded signal 
from the subsampling portion 70. 
FIG. 2 shows a detailed block diagram of the motion signal detector in FIG. 
1. As shown in FIG. 2, the motion signal detector 30 comprises a temporal 
high pass filter 31 for detecting the motion signal from the brightness 
signal, a rectifier/absolute value producer portion 32 for rectifying the 
motion signal from the temporal high pass filter 31 and obtaining an 
absolute value of the rectified signal, a signal spreader 33 for spreading 
the absolute-valued motion signal in horizontal, vertical and time, and 
softening an abrupt change of the motion signal, and a motion signal 
factor producer 34 for producing a motion factor K corresponding to the 
motion signal output from the signal spreader 33. 
Now, the construction of the present invention will be described in detail 
with reference to FIGS. 3 and 4, in which FIG. 3 shows a gain 
characteristic view of the adaptive emphasis/de-emphasis and FIG. 4 shows 
a field offset subsample. 
In accordance with the conventional VHS video tape recorder as previously 
described, a signal having a high frequency above 2.5 to 5.0 MHz or 3.0 to 
5.0 MHz cannot be recorded on the recording medium on the view point of 
the property of the medium. For this reason, a cut-off frequency is 
preferably set to be 2.5 to 3.0 MHz. 
When the cut-off frequency is set to be 2.5 MHz, the first horizontal low 
pass filter 10 extracts the brightness component having a frequency below 
2.5 MHz, while the horizontal high pass filter 20 extracts the brightness 
signal component having a frequency above 2.5 MHz. Further, the motion 
signal detector 30 detects the motion signal from the brightness signal Y 
so as to produce the motion factor K corresponding to the motion signal. 
Consequently, in the motion signal detector 30 shown in FIG. 2, the 
temporal high pass filter 31 detects the motion signal of the brightness 
signal Y and the rectifier/absolute value producer portion 32 rectifies 
the motion signal applied from the temporal high pass filter 31 and 
produces the absolute value of the rectified motion signal. 
The motion signal thus substituted by the absolute value is spreaded by 
means of the signal spreader 33 in horizontal, vertical and time. Upon 
spreading the motion signal, artifacts such as blurring to be occurred in 
switching an abruptly changed portion of the motion signal by the signal 
limiting portion 50 shown in FIG. 1 can be effectively prevented. More 
particularly, the picture may be deteriorated at an edge due to the 
transition from a still video image to a motion video image. In this 
connection, the spreading is executed to extend the range of the motion 
signal so as to gradually change the motion signal. Next, the motion 
signal factor producer 34 produces the motion signal factor K on the basis 
of the output value of the signal spreader 33 and inputs the motion factor 
K to the signal limiting portion 50 shown in FIG. 1 so that the 
high-frequency signal component to be folded is limited effectively. 
Meanwhile, the brightness signal with a frequency above 2.5 MHz passed 
through the horizontal high pass filter 20 is provided to the adaptive 
emphasis/de-emphasis portion 40 having a gain characteristic as shown in 
FIG. 3. Accordingly, the brightness signal is limited in level by the 
adaptive emphasis/de-emphasis portion 40. For example, assuming that the 
gain characteristic of the emphasis/de-emphasis portion 40 is set to be 
6*A (where, A is an absolute value of the horizontal high pass filter 20, 
when the output value of the horizontal high pass filter 20 is set to be 
4, the output value of the adaptive emphasis/de-emphasis portion 40 
becomes .sqroot.4. As a result, the adaptive emphasis/de-emphasis portion 
40 produces the output value larger than that of the horizontal high pass 
filter 20. 
Alternatively, when the output value of the horizontal high pass filter 20 
is set to be -9 or +9, the output level of the adaptive 
emphasis/de-emphasis portion 40 becomes .sqroot.-54 or .degree.+54 lower 
than the input value of the horizontal high pass filter 20. Accordingly, 
as the output value of the horizontal high pass filter 20 is increased, 
the output value of the adaptive emphasis/de-emphasis portion 40 is 
decreased to 1/4 from the original value depending upon the gain 
characteristic shown in FIG. 3. Hence, it should be noticed that when the 
output value of the adaptive emphasis/de-emphasis portion 40 is decreased 
to 1/4, even if the folded component is unpleasantly displayed on a screen 
when the signal recorded on the magnetic video tape by means of the video 
tape recorder employed with the present invention is replayed by the 
existing video tape recorder, a magnitude of the high frequency signal 
component is controlled to thereby prevent a folding of the signal. As a 
result, it improves the compatibility between the recorders. 
Moreover, when the gain characteristic of the adaptive emphasis/de-emphasis 
portion 40 is set to be 1 or more, if a minute signal in level is recorded 
onto or reproduced from the recording medium such as the video tape, it is 
possible to prevent the signal from being deteriorated and to improve a 
signal resolution even if noise is induced thereto upon reproduction of 
the signal. 
The output signal of the adaptive emphasis/de-emphasis portion 40 thus 
controlled in gain is transferred to the signal limiting portion 50 which 
limits the output signal on the basis of the motion factor K output from 
the motion signal detector 30. More particularly, when the brightness 
signal Y is detected as a signal of a still picture image, the motion 
factor K of the motion signal detector 30 is set to be "zero". In this 
case, the signal limiting portion 50 passes the output value of the 
adaptive emphasis/de-emphasis portion 40 toward the adder 60. 
Alternatively, when the input composite video signal is detected as a 
signal of a semi-motion picture image, the motion factor K of the motion 
detector 30 is set to be 0.125, for example. Therefore, the output, 1-K of 
the signal limiting portion 50 is set to 0.875 in rate so that the 0.875 
in rate of the output value of the adaptive emphasis/de-emphasis 40 is 
folded by passing through the signal limiting portion 50. 
When the brightness signal Y indicates the motion picture image, the motion 
factor K of the motion signal detector 30 becomes 1 and, hence, the signal 
limiting portion 50 cuts off the output value of the adaptive 
emphasis/de-emphasis 40. In other words, when the motion factor K from the 
motion signal detector 30 is set under a predeterminded reference level, 
the signal limiting portion 50 outputs the high-frequency brightness 
signal controlled in gain by the adaptive emphasis/de-emphasis portion 40. 
That is, when a magnitude of the motion factor K is under a predetermined 
level which indicates a still picture image signal, the signal limiting 
portion 50 outputs the gain-controlled high frequency brightness 
component. Further, the reference level can be arbitrarly set to determine 
the still picture image. 
Accordingly, when the still picture image is determined, the adder 60 adds 
the high-frequency component passed through the signal limiting portion 50 
and the low-frequency component passed through the first horizontal low 
pass filter 10. Alternatively, when the motion picture is determined, the 
adder 60 outputs only the low-frequency component passed through the first 
horizontal low pass filter 10. Further, when the semi-motion picture is 
determined, the adder 60 adds the high-passed signal component according 
to the presently set value of the motion factor K and the output signal of 
the first horizontal low pass filter 10. Consequently, the subsampling 
portion 70 performs a field offset subsampling with respect to output 
signal of the adder 60 to fold the high-frequency signal component over 
the low-frequency component. The folded signal is then entered to the 
second horizontal low pass filter 80 which cuts off the signal having a 
frequency above 2.5 MHz. Thereafter, the output signal of the second 
horizontal low pass filter 80 is supplied to an output terminal OUT 
through which the signal is recorded on a predetermined recording medium. 
As described above, even if the folded signal component above 2.5 MHz in 
frequency, it has no effect on the video image to be reproduced. 
Furthermore, under consideration of the compatibility with the existing 
VHS video tape recorder, limitation of the signal band is carried out to 
prevent the deterioration of the picture due to the folded signal 
contained in the component above 2.5 MHz. 
Additionally, a detailed description will be made with relation to the 
frequency filding performed by the subsampling portion 70. 
Firstly, the composite video signal having a bandwidth of 5.0 MHz is 
sampled with sampling rate of 10 MHz satisfying a Nyquist theory by an 
analog to digital (A/D) converter (not shown). The sampled composite video 
signal is passed through a brightness/color signal separating circuit (not 
shown) and the low-frequency component of the video signal is passed 
through the first low pass filter 10 while high-frequency component is 
passed through the horizontal high pass filter 20, adaptive 
emphasis/de-emphasis portion 40 and signal limiting portion 50. The low 
and high frequency components are added by the added 60 and then entered 
to the subsampling portion 70. Accordingly, the subsampling portion 70 
subsamples the video signal having a frequency of 5.0 MHz to fold the high 
frequency component above 2.5 MHz over the low frequency component under 
2.5 MHz on the basis of a reference frequency of 2.5 MHz. In the sampling 
method performed by the subsampling portion 70, the signal sampled with 
the sampling frequency of 10 MHz is further sampled on alternative samples 
in a horizontal direction. More particularly, the samples on the first 
line are alternatively sampled and the sampling between the lines is 
executed in a manner opposed to the upper line, as shown in FIG. 4. In 
addition, each of the first to fourth fields, for example, is sampled in a 
manner shown in FIG. 4 so that the high frequency is folded over the low 
frequency band as described above, and a spectrum of the folded signal is 
positioned in the vicinity of a Funkinuki hole. The folded signal from the 
subsampling portion 70 is entered to the horizontal low pass filter 80 
which cuts off the signal above 2.5 MHz, and then output outwardly. 
As described above, according to the motion adaptive frequency folding 
method and circuit, the high-frequency component of the brightness signal 
is increased or decreased on the basis of the gain to be controlled 
according to the magnitude thereof and is folded depending upon an amount 
of the motion signal so that the brightness signal is recorded in full 
band on the recording medium having a restricted bandwidth, whereby the 
video signal is successfully reproduced in good quality.