JPS648513B2 - - Google Patents

Description

[Detailed Description of the Invention] Industrial Application Field The present invention relates to a video signal recording and reproducing device, and in particular to a signal obtained by sampling an input video signal with a sampling pulse having a frequency slightly higher than the upper limit frequency of its required frequency band. The present invention relates to a recording and reproducing apparatus that records and reproduces a resampled signal, adds and mixes a resampled signal to the low frequency component of the reproduced sampled signal, and outputs the resultant mixture.

Conventional technology and its problems In general, helical scanning VTRs:
A rotating head records a video signal on a running magnetic tape, and the already recorded video signal is reproduced by the rotating head. The above video signal has a wide band with an upper limit frequency of, for example, about 4.2 MHz, and in order to frequency modulate this wide band video signal, record it on a magnetic tape, and play it back, the relative speed between the head and tape must be adjusted. It is well known that it is necessary to increase the speed above a predetermined value and to use a high-performance head with high sensitivity in a high frequency range.

However, in the case of home-use VCRs, the relative speed between the tape and the head has to be much lower than the above prescribed value due to demands for lower prices, smaller devices, and lighter weights. The recording and reproducing band becomes narrower than the original band of the video signal, which poses a problem in reproducing higher quality video signals.

Therefore, the present applicant previously proposed in Japanese Patent Application No. 107379/1983 that the input video signal is sampled and recorded at a frequency slightly higher than the upper limit frequency of the required frequency band of the input video signal, and when played back, the above sampling is performed. We have proposed a video signal recording and reproducing device that alternately samples signals with approximately the same frequency and 180° phase difference from each other.

The present invention is a recording and reproducing device proposed by the present applicant that can be applied to the recording and reproducing system of the recording and reproducing device, and in particular, a clip is applied to the output stage of a low-pass filter that waves the low frequency components of the reproduced sampled signal. It is an object of the present invention to provide a video signal recording and reproducing device that can improve vertical resolution by providing a circuit.

Means for Solving the Problems The present invention provides sampling at a frequency less than twice the upper limit frequency of an input video signal such as a luminance signal, and at a frequency higher than the upper limit frequency, as shown by the following formula. The input video signal is sampled by a signal with the frequency f _S f _S ≒ (1/2)・(2n+1)・f _H (where, in the above formula, n is a natural number and f _H is the horizontal scanning frequency of the input video signal) a means for recording the sampled signal on a recording medium and reproducing it; and a means for recording the sampled signal on a recording medium and reproducing it; means for obtaining a reproduced sampled signal;
The two signals obtained by alternately sampling the first and second reproduced sampling signals with the signal of the sampling frequency f _S are further added and combined to obtain a resampled signal that is substantially resampled at the frequency of 2f _S. means for obtaining a sampled signal; a low-pass filter for frequency-selecting and outputting a low-frequency component of the first or second reproduced sampled signal; and an intermediate portion of the waveform to which the output signal of the low-pass filter is supplied. and a mixing means for mixing at least the high frequency component of the resampled signal with the output signal of the clipping circuit and outputting the mixture as a reproduced video signal. Each of the embodiments will be described below with reference to the drawings.

Embodiment FIG. 1 shows a block system diagram of a first embodiment of the apparatus of the present invention. In the figure, an input terminal 1 receives a video signal to be recorded. This input video signal is a signal consisting of frequency components that are integral multiples of the horizontal scanning frequency _fH , and is, for example, a luminance signal. As shown in FIG. 2A, this input video signal (luminance signal) is a wideband video signal with an upper limit frequency fa of about 4 MHz, for example, and is supplied to the sampler 2 and the sampling pulse generator 3, respectively. Sampler 2
samples and holds the input video signal using a sampling pulse having a repetition frequency f _S from the sampling pulse generator 3. This results in
A sampled signal obtained by sampling the input video signal at a sampling frequency f _S is obtained from the sampler 2 .

Here, the sampling pulse generator 3 has a configuration as shown in FIG. 3, as an example. In the figure, a video signal from the input terminal 1 is input to the input terminal 14 during recording, and a reproduced sampled signal reproduced by the recording/reproducing device 4 is input to the input terminal 14 during reproduction. The horizontal synchronizing signal is separated and extracted here. After the frequency of this horizontal synchronization signal is divided by half by the flip-flop 16, it is supplied to the phase comparator 17, where the phase is compared with the signal of the repetition frequency f _H /2 from the frequency divider 19.
It is converted into an error voltage according to their phase difference.
The output error voltage of the phase comparator 17 is applied as a control voltage to a voltage controlled oscillator (VCO) 18 to variably control its output oscillation frequency. The output pulse of the VCO 18 is divided by the frequency divider 19 to 1/(2n+1) (where n is a natural number) and the repetition frequency is
After being made into a signal of f _H /2, it is supplied to the phase comparator 17.

Therefore, the loop consisting of the phase comparator 17, VCO 18, and frequency divider 19 constitutes a well-known phase locked loop (PLL).
From the VCO 18, a pulse expressed by the repetition frequency f _S of the following equation is taken out, which is phase synchronized with the horizontal synchronizing signal of the input video signal.

f _S ≒ (1/2)・(2n+1)・f _H (1) This repetition frequency f _S is determined based on the upper limit frequency fa of the input video signal, considering that the recording/playback band of the recording/playback device 4, which will be described later, is narrow. The frequency is selected to be less than twice the frequency and higher than the upper limit frequency fa, and as an example, 5.011 MHz is selected (this is when n is 318 and f _H is 15.734 kHz). Note that the highest frequency of the recording/reproduction band is a frequency that is 1/2 or more of this repetition frequency f _S and less than the upper limit frequency fa, and here, as an example,
It is set to 2.5MHz. Furthermore, the recording/reproducing band referred to in this specification refers to a baseband signal band in which recording/reproducing is possible. VCO of this repetition frequency f _S
The output pulse φ ₁ of 18 is supplied to the frequency divider 19 described above, and is also supplied from the output terminal 20 to the sampler 2 shown in FIG. 1 as a sampling pulse. Note that a pulse φ 2 having the same repetition frequency f _S as the output pulse φ ₁ and having a phase different from φ ₁ by 180 _° is outputted from the VCO 18 to the output terminal 21 .

Therefore, the input video signal is sent to the sampler 2 as follows:
Since sampling is performed (sampled and held) at the sampling frequency f _S shown in equation (1), which is less than twice the upper limit frequency fa, the frequency spectrum of the output sampling signal of sampler 2 is shown in Figure 2B. Be like this,
From the upper limit frequency fa to the sampling frequency f _S and the upper limit frequency fa
The aliasing component is included in the band shown by diagonal lines up to the frequency f _S −fa of the difference between the two frequencies. However, as can be seen from the frequency spectrum of FIG. 2C, which is an enlarged view of the circled part in FIG. 2B, the frequency spectrum of the aliasing component indicated by the broken line in FIG. Since it is inserted into the gap part of the frequency spectrum arranged at intervals of horizontal scanning frequency _fH (because of the frequency interleaving relationship), it is band-sharing multiplexed to the input video signal.

The sampled signal taken out from this sampler 2 is supplied to the recording system of the recording/reproducing device 4 shown in FIG.
For example, frequency modulated (FM) data is recorded on a magnetic tape (not shown) by a rotating head, then reproduced from the magnetic tape by a rotating head, and then taken out through an FM demodulator (not shown). Here, the recording/reproducing device 4 is for home use.
It is assumed that the device has only a narrow recording and reproducing band like a VTR, and for example, the recording and reproducing band is about 2.5 MHz, which is much narrower than the input video signal band. Therefore, as shown in FIG. 2D, the upper limit frequency is output from the reproduction signal output terminal of the recording/reproducing device 4.
At about 2.5 MHz, a reproduced sampled signal is extracted in which the aliasing component is band-sharing multiplexed into a video signal band indicated by diagonal lines from f _S -fb to about 2.5 MHz.

This reproduced sampling signal is generated by the sampling pulse generator 3, sampler 5, and 1H delay circuit 6 shown in FIG.
and the subtracter 9, respectively. 1H delay circuit 6
The reproduced sampled signal delayed by one horizontal scanning period (1H) is supplied to a sampler 7 and a subtracter 9, respectively. The samplers 5 and 7 each have the sampling frequency f _S taken out from the sampling pulse generator 3.
Sampling pulses φ ₁ and φ ₂ that are phase-synchronized with the reproduced horizontal synchronizing signal are supplied, whereby the input reproduced sampled signal is sampled and held (resampled) and output. Here, the sampling pulses φ ₁ and φ ₂ are pulses taken out from the output terminals 20 and 21 shown in FIG. 3, respectively, and have a phase difference of 180° from each other as shown in FIG. 4. Therefore, in the samplers 5 and 7, the reproduced sampled signal and the 1H delayed reproduced sampled signal are alternately sampled (sampled and held) at the sampling frequency f _S and extracted. Both signals taken out from the samplers 5 and 7 are supplied to an adder 8, where they are added and combined to produce a reproduced video signal as if it had been sampled at a frequency of _2fS .

That is, to explain this in more detail, the reproduced sampling signal reproduced and output by the recording/reproducing device 4 changes the sampling frequency f _S to the horizontal scanning frequency f _H
Information about a number of sample points approximately equal to the value divided by is displayed on the screen in time series per scanning line, and the number of sample points is a natural number. However, since the above sampling frequency f _S is selected to be an odd multiple of 1/2 of the horizontal scanning frequency f _H as shown in equation (1) above, 1
The number of sample points per scanning line is a natural number plus a fraction of 0.5 (incidentally, if the sampling frequency f _S is 5.011 MHz as described above, the number of sample points per scanning line is 318.5). Therefore, on the playback screen of the same field, the above-mentioned reproduced sampling signal displays information on, for example, 318 sample points in one scanning line, and 319 samples in the next scanning line. Point information will be displayed, and the distance between two adjacent scanning lines is about 1/1 in the horizontal scanning direction.
(2f _S ) are displayed at different positions for a period of time.

That is, as shown in FIG. 5, if four arbitrary scanning lines are indicated by l ₁ , l ₂ , l ₃ and l ₄ on the playback screen 23 of one field, then 1H
Information on each sampling point (only a part of which is shown in FIG. 5) of the reproduced sampling signal supplied to the delay circuit 6 is arranged at the position of a diagonally shaded circle on each scanning line. The display positions are 1/1/1 in the horizontal scanning direction between adjacent scanning lines.
The positions differ by a time interval of (2f _S ).

On the other hand, the information of each sampling point of the reproduced sampled signal delayed by 1H by the 1H delay circuit 6 is obtained from the position indicated by the hatched circle in FIG. 5 of the input reproduced sampled signal of the 1H delay circuit 6. The information of the sample point is moved by one scanning line in the vertical direction as shown by the arrow in the same figure.
It will be displayed at the position indicated by the solid circle on the next scanning line.

Therefore, since the information of each sampling point of the output reproduced sampling signal of the adder 8 is a composite output of the input and output reproduction sampled signals of the 1H delay circuit 6, there are diagonal lines on the scanning lines _l1 to _l4 . It will be displayed both at the position indicated by the circle mark and at the position indicated by the solid circle mark. That is, this means that a signal obtained by sampling the reproduced sampled signal at a frequency substantially twice the sampling frequency f _S is displayed. As a result, adder 8
As shown in FIG. 2E, the frequency spectrum of the reproduced video signal extracted from the oscilloscope is such that the aliasing component is superimposed on the frequency spectrum of the original video signal of 2.5 MHz or less at frequencies above 2.5 MHz, as shown in FIG. Therefore, a reproduced video signal having a wider band than the recording/reproducing band of the recording/reproducing device 4 is taken out from the adder 8, and this reproduced video signal is supplied to the mixer 12.

Here, the reproduced video signal from the adder 8 is a resampled signal obtained by adding and combining the reproduced sampled signals having a relative time difference of 1H using the 1H delay circuit 6 after resampling, Consists of frequency components that are a natural number multiple of the horizontal scanning frequency _fH , and the vertical resolution is 1/2
It has deteriorated to Therefore, regarding the low frequency component that mainly determines this vertical resolution, in order to compensate for the above-mentioned deterioration of the vertical resolution, the frequency component of a natural number multiple of 1/2 of the horizontal scanning frequency _fH is also added to the output terminal 13. It is configured to output to. Therefore, the subtracter 9 subtracts the two reproduced sampled signals having a relative time difference of 1H, and consists of a frequency component that is a natural number multiple of 1/2 of the horizontal scanning frequency _fH in the reproduced sampled signal. The signal is output and supplied to the low pass filter 10. The low frequency component extracted from the low pass filter 10 is supplied to the mixer 12 after the intermediate portion of the waveform is removed by a clip circuit 11 (described later) which is a main part of this embodiment. is mixed with the resampled signal of The mixer 12 mixes the two input signals and outputs the resulting signal to the output terminal 13 as a reproduced video signal.

However, in designing a low-pass filter, it is generally difficult to design a low-pass filter that has a steeply sloped amplitude-frequency characteristic and also has an excellent phase-frequency characteristic. Therefore, it is difficult to select the cutoff frequency of the low-pass filter 10 due to the balance between the above characteristics. That is, if the reproduction system is configured without the clip circuit 11, the frequency characteristics (amplitude-frequency characteristics) of the low-pass filter 10 are as shown in FIG. 6A, and the cutoff frequency _f1 is
If the value is selected to be slightly higher than f _S -fa, the overall frequency characteristics of the reproduction system will be as shown in Figure 6B, and will be flat in the low frequency region below frequency f _{1 .} In the high frequency range above, f _H /2
The characteristics of a comb filter are shown in which the passband is a frequency that is an even multiple of f _H /2 and the attenuation band is a frequency that is an odd multiple of f H /2. However, as explained in conjunction with _FIG . This causes aliasing distortion and deteriorates the reproduced image quality.

On the other hand, if the frequency characteristics of the low-pass filter 10 are selected such that the cutoff frequency f ₂ is slightly lower than the frequency f _S −fa as shown in FIG. 7A, the overall frequency characteristics of the reproduction system are as shown in FIG. 7B. As shown, the frequency f _S −
In a high frequency region of frequency f ₂ or higher including fa, the filter has the same comb filter characteristics as in FIG. 6B, so the aliasing distortion described above is significantly reduced. However, in this case, because the vertical resolution is mainly determined by a part of the low frequency region or by the shape filter characteristics, the deterioration of the vertical resolution in small image parts is noticeable, and small characters etc. appear blurry. .

Therefore, this embodiment is characterized in that a clip circuit 11 is provided between the low-pass filter 10 and the mixer 12. As a result, even if the frequency characteristics of the low-pass filter 10 are selected to have a cutoff frequency _f3 slightly higher than the frequency fs- _fa as shown in FIG.
The clip circuit 11 is a circuit that removes low-level signals below level L (the small amplitude part near the center level of the input signal) shown by the broken line in the figure and outputs the high-level signal as it is. The aliasing components below the level L existing in the high frequency region indicated by F are removed. Therefore, according to this embodiment, aliasing distortion can be significantly reduced. According to experiments conducted by the present inventors, it has been confirmed that the clip level may be extremely small, and therefore, the deterioration in vertical resolution at low frequencies is negligible.

Next, a second embodiment of the apparatus of the present invention will be described. FIG. 9 shows a block system diagram of the second embodiment of the apparatus of the present invention. In the figure, the same components as in FIG. 1 are denoted by the same reference numerals, and their explanations will be omitted. In FIG. 9, the reproduced sampled signal taken out from the recording/reproducing device 4 is supplied to the clipping circuit 25 through the low-pass filter 24, where the small amplitude portion including the center level is removed, and the other signal is removed. The amplitude portion of the waveform is passed through the mixer 26 as it is.
supplied to On the other hand, the output resampled signal of the adder 8 is supplied to a high-pass filter 27, where the frequency
High-frequency components of approximately f _S −fa or higher are frequency-selected and supplied to the mixer 26 . The mixer 26 adds and mixes both the output signals of the clip circuit 25 and the high-pass filter 27, and outputs a wideband signal obtained to the output terminal 13 as a reproduced video signal.

In this embodiment, as in the first embodiment, even if the cut-off frequency of the low-pass filter 24 is slightly higher than the frequency f _S -fa, the clip circuit 25 allows the low-pass filter 24 to
Since high frequency components and low level signals in the pass frequency band are blocked, aliasing distortion can be reduced without deteriorating vertical resolution. It should be noted that the overall frequency characteristic of the reproduction system of this embodiment, as in the first embodiment, exhibits the above-mentioned comb filter characteristic in the high frequency region above the cut-off frequency of the low-pass filter 24, and in the low frequency region below that It is clear that it exhibits flat characteristics in the frequency domain.

Application Example Note that the present invention is not limited to the above embodiment, and the two input signals to be added and combined by the adder 8 may be reproduced sampled signals having a time difference of 1H relative to each other. , a first delay circuit having a delay time kH (k is any natural number) is provided on the input side of the sampler 5, and a delay time of (k+1)H is provided on the input side of the sampler 7 instead of the 1H delay circuit 6. Of course, it is also possible to provide a second delay circuit having .

Furthermore, since recording and reproduction are forms of transmission, the recording system is equivalent to the transmission system (transmission system), and the reproduction system is equivalent to the reception system, and the present invention can also be applied to these transmission and reception systems.

Effects As described above, according to the present invention, at a frequency slightly higher than the upper limit frequency of the input video signal, and at a frequency slightly higher than the upper limit frequency of the input video signal,
Record and reproduce the sampled signal obtained by sampling the input video signal with a sampling pulse whose phase is synchronized with the horizontal synchronizing signal at a frequency f _S that satisfies equation (1), and record and reproduce the sampled signal obtained by sampling the input video signal with a sampling pulse having a frequency f _S that satisfies the equation (1). Since the two signals obtained by alternately resampling with two sampling pulses with a phase difference of 180° are added together and output, it is possible to obtain a reproduced video signal with a wider band than the recording and reproduction band (transmission band). In addition, the intermediate part of the waveform is removed from the transmission path of the low frequency signal from the low pass filter to improve the vertical resolution, which is to be mixed with the resampled signal obtained by the above additive synthesis. Since we have provided a clip circuit that outputs the waveform part of the output signal, the high-frequency components of the output signal of the low-pass filter and low-level signals are blocked, so we selected a relatively high cut-off frequency for the low-pass filter. The aliasing distortion that sometimes occurs can be reduced, and the pass band of the low-pass filter can be widened.
In addition, since the clip level of the clip circuit only needs to be low, the vertical resolution does not deteriorate in the necessary low frequency range, and the design of the low-pass filter is easy, as there is plenty of leeway. It is something that you have.

[Brief explanation of drawings]

1 and 9 are block system diagrams showing respective embodiments of the device of the present invention, and FIGS.
FIG. 3 is a block system diagram showing an example of the main part of the block system in FIG. 1, and FIG. 4 is a diagram showing the sampling pulse waveform. , FIG. 5 is a diagram for explaining the operating principle of the device of the present invention, FIGS. 6A and 7A are diagrams showing examples of frequency characteristics of the low-pass filter, and FIGS. 6B and 7 are diagrams, respectively. B is a diagram showing the overall frequency characteristics of the reproduction system when the low-pass filters shown in FIGS. 6A and 7A are used, respectively, and FIG. 8 is the frequency characteristics of the low-pass filter in the device of the present invention and the waveform removal operation by the clip circuit. FIG. 1, 14...Video signal input terminal, 2, 5, 7...
...Sampler, 3...Sampling pulse, 4...
Recording and reproducing machine, 6...1H delay circuit, 8...adder,
9...Subtractor, 10, 24...Low pass filter, 1
1, 25... Clip circuit, 12, 26... Mixer, 13... Playback video signal output terminal, 17... Phase comparator, 20, 21... Sampling pulse output terminal, 27... High frequency filter.

Claims (1)

[Claims] 1. 2 of the upper limit frequency fa of the luminance signal of the input video signal
Sampling frequency f _S f _S ≒ (1/2)・(2n+1)・f _H (However, the above equation (where n is a natural number and f _H is the horizontal scanning frequency of the input video signal); a sampling means for sampling the input video signal using a signal with a maximum frequency of f The upper limit frequency fa when _S /2 or more
A recording means for recording on a recording medium with a recording/reproducing band of less than means for obtaining first and second regenerated sampled signals having a relative time _difference ; The two signals obtained by separately resampling with the first resampling pulse and the second resampling pulse with a phase difference of 180° are further summed and combined to produce a resampled signal that is substantially sampled at a frequency of 2f _S. means for obtaining a sampled signal; and a low-pass filter that has a cut-off frequency slightly higher than the frequency f _S -fa and selects and outputs a low-frequency component of the first or second reproduced sampled signal. , a clip circuit to which the output signal of the low-pass filter is supplied, removes a small amplitude portion of the waveform and outputs another waveform portion; and at least a high frequency component of the resampled signal and the output signal of the clip circuit. What is claimed is: 1. A video signal recording and reproducing device comprising: mixing means for mixing the respective signals and outputting the mixture as a reproduced video signal.