JPH0242605A - System for recording three-level fm signal - Google Patents

Abstract

PURPOSE:To improve a resolution by recording an FM signal with a waveform, in which a positive polarity pulse and a negative polarity pulse are alternately arranged, on a recording medium. CONSTITUTION:When an FM signal B modulated by a video signal A is counted to the frequency of a half by a counter driven with the fall edge, an FM signal C is obtained. In the same way, when the signal B is counted to the frequency of the half by the counter driven with a rise edge, an FM signal D is obtained, the FM signal with the waveform of E is obtained by superposing both waves of C and D, and it is recorded through a recording head onto a magnetic tape. Namely, since the FM signals C and D maintain a phase difference at 90 deg. mutually and they compensate the lacking of the opponent mutually, a beat disturbance can be improved by the waveform indicated by the E in which the waveforms of C and D are superposed. Thus, the resolution can be improved.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention is mainly applied to devices that record low-carrier FM signals, such as video tape recorders and video discs, to improve resolution, image quality, etc.

BACKGROUND TECHNOLOGY AND PROBLEMS> In conventional video tape recorders and the like, beat interference occurs when the high frequency of a video signal is raised to approach the FM signal frequency. This development has a particularly large effect on color video signals, and is a well-known phenomenon called moire fringes or color beats. For example, in home video tape recorders with low FM signal frequencies, color signals are recorded with low frequency conversion in order to avoid this beat disturbance. In order to avoid this beat disturbance, it is necessary to limit the high frequency components of the video signal to be recorded, and this is an obstacle to increasing the resolution of the video tape recorder.

It is generally explained that this beat disturbance is caused by non-linear distortion of the FM signal in the recording/reproducing system. This is certainly true, but here I will focus on and explain three characteristics of this beat disturbance that are necessary for the following explanation.

(1) The biggest cause of beat interference is that the FM signal is a square wave.

(2) The essence of beat interference is the high frequency of the video signal and the
When the phase relationship with the M signal frequency becomes a phase difference of 90 degrees, the shading of the video signal is caused by the loss of video signal information from the FM signal.

(3) The frequency band of the video signal in which beat interference does not pose a practical problem is two-thirds or less of the FM signal frequency.

First, (1) and (2) will be explained using FIG.

Let A in FIG. 1 be a high frequency component included in a video signal. An example of a square wave FM signal of the same frequency as A, modulated by A, is shown in C and D. The FM signal of C is
It is in phase with the video signal of A, and the wavelength is short in the part corresponding to the positive half cycle of the video signal of A (and the wavelength is long in the part corresponding to the negative half cycle, i.e. FM). Modulation is taking place.F shown on one side
Each half cycle of the M signal corresponds to half the positive and negative parts of the video signal A. Therefore, within a half cycle of the square wave FM signal, the video signals are averaged and cancel each other out, and no FM modulation is performed.

In other words, when the video signal A and the FM signal have a phase relationship of 90 degrees, the information of the video signal is lost from the FM signal, resulting in no modulation. In reality, the video signal frequency and the FM frequency do not continue to match, and usually modulation and modulation reduction are repeated at a frequency twice the frequency difference between the two.
The resulting shading of the video signal is the true cause of beat disturbance that occurs in video tape recorders.

Next, (3) will be explained using FIG.

In pulse count method FM demodulation, two pulses are generated per cycle of FM (K), so FM
The signal frequency f disappears and an FM frequency 2f, which is twice as high, appears in its place. Along with the FM signal of twice the frequency, secondary sideband waves are also generated at a large level. This situation is shown in FIG. In the figure, the video signal is represented by a solid line, and the FM signal is represented by a dotted line. In Figure 2, the high frequency 2 of the video signal,
The second FM lower sideband waves 5 generated due to the video frequency 2 overlap when the video frequency 2 is two-thirds of the FM frequency. In other words, the video signal band is
When the frequency increases beyond two-thirds of the FM signal frequency, the second FM lower sideband indicated by 5 enters the video band and causes beat disturbance.

Finally, consider the circuit that generates beat disturbance.

Since the FM signal is a square wave, which is the cause of beat disturbance, multi-node ibrator type FM whose output is a square wave is used.
In the modulator, beat disturbance has already occurred at the modulator stage. When the FM modulator output is a sine wave, the nonlinearity of the magnetic tape causes considerable beat disturbance. However, the decisive factor is that a square wave is created in the demodulator's amplifier, and the beat disturbance that occurs here has been considered to be unavoidable in principle.

<Object of the Invention> The present invention improves the resolution of a video tape recorder by improving the square wave FM signal waveform that causes beat disturbance and devising an FM signal waveform that does not cause beat disturbance.

<Structure of the Invention> If the phase relationship between the high frequency of the video signal and the square wave FM signal is the cause of beat disturbance, then the FM signals of C and D in Figure 1
Beat jamming can be improved by recording the signals simultaneously. Because these two FM signals are 90 on t7.
Maintain a phase difference and compensate for each other's deficiencies. Here it is! - Based on this idea, a three-level FM signal system with a waveform shown as E was invented by superimposing the C and D waveforms. Its characteristic lies in the shape of the waveform, in which the polarity of the signal is reversed every other cycle.

Below, the entire process from modulation to demodulation according to the present invention will be explained in the first part.
Follow along using the diagram.

It is assumed that the FM signal modulated by the video signal shown in A has twice the frequency of the video signal in A, and this is shown as B. Here, since the frequency of the FM signal indicated by B is less than two-thirds of the frequency of the video signal, no beat disturbance occurs. B
When the FM signal of C is counted to half the frequency by a counter driven at its falling edge, an FM signal indicated by C is obtained. Similarly, if B is counted to half the frequency by a counter driven by the rising edge, F
M signal is obtained. The C and D waves are superimposed to obtain an E waveform FM signal, which is recorded on a magnetic tape through a recording head.

New ideas are also needed on the playback and demodulation sides. because,
The FM signal shown at Fl in FIG. 1 obtained from the reproducing head has the same frequency as the video signal shown at A, so if it is converted into a square wave using a limiter as is in the past, beat interference will occur. Therefore, we decided to use a method in which the signal is converted into an FM signal (2) with a frequency twice that of F and then demodulated, thereby satisfying the condition that the video signal band is two-thirds or less of the FM signal frequency.

The method of doubling the frequency uses two systems of voltage comparators having reference voltages indicated by two horizontal lines that alternate with the reproduced FM signal F. That is, a G waveform is obtained at the voltage comparator output on the positive half cycle side of the F waveform, and an H waveform is obtained at the voltage comparator output on the negative half cycle side. The G and H waveforms are superimposed to obtain the I FM signal, which is pulse count demodulated. Pulse count demodulation generates a pulse with a constant width at each rise and fall of the FM signal r to obtain the pulse train shown in J. If J is guided to a low-pass filter, the video shown in K is generated depending on the density of the pulses. The signal is demodulated.

Another example of the processing of reproduced waves will be shown.

By double-wave rectifying the reproduced FM signal in Fig. 1F, 2
Obtain the FM signal (i) of twice the carrier frequency, apply it to a limiter, make it an FM signal of 1, and demodulate it.

The reason why the recorded E 3-level FM signal is obtained as an F waveform close to a sine wave from the playback node is because the harmonic components of the E carrier wave are lost due to various high frequency losses during recording and playback. It is from.

However, as long as the sideband component of E is not lost, the information of E is conveyed to F in the form that the wavelength of F changes every quarter cycle. That is, F does not take advantage of the characteristics of a 3-level FM signal, and no beat disturbance occurs. When a three-level FM signal is magnetically recorded, the residual magnetism of the magnetic recording medium-L is actually recorded in the F waveform.

In magnetic recording, the recording magnetization process is non-linear due to a hysteresis curve, which causes deterioration of the characteristics of the FM signal. For example, a sine wave FM signal is an ideal signal that does not have unnecessary frequencies that cause beat disturbance in its sidebands, but when human or magnetic recording is performed, it is subject to waveform distortion and becomes a square wave. Generates beat interference that is not much different from FM signals.

Since a sinusoidal FM signal is recorded continuously following a magnetic hysteresis curve, it is natural that it is subject to waveform distortion. In the case of a three-level FM signal, recording is performed discontinuously, skipping over the curved portion of magnetic hysteresis. In other words, the positive polarity pulse of the 3-level FM signal is recorded at the maximum residual magnetic flux density, the negative polarity pulse is recorded at the maximum residual magnetic flux density in the opposite direction to the positive polarity pulse, and the residual magnetic flux density in the signal portion where no pulse exists is recorded at zero. . There is no signal portion in the three-level FM signal that is recorded following the curved portion of the magnetic hysteresis. 3 level F, with the condition that it is limited to relatively low recording densities where the effects of recording demagnetization can be ignored.
The M signal is a signal form that is not subject to distortion from the magnetic hysteresis curve.

Recording demagnetization of a 3-level FM signal proceeds in such a way that the magnetic field of the pulse being recorded demagnetizes the residual magnetism of the opposite polarity pulse recorded immediately before. This demagnetization naturally acts more strongly on the latter half of the residual magnetism of the pulse than on the first half, distorting the waveform of the three-level FM symbol. At first glance, it seems impossible to magnetically record three-level FM signals at high density. However, although this is also one of the excellent characteristics of the 3-level FM signal, the waveform distortion caused by recording demagnetization can be compensated for by a simple method.

This compensation method for recording demagnetization is a separate invention from the present application, so a redundant explanation will be avoided, but the waveform of an FM wave that has only three levels is better than that of an FM wave that changes continuously like a sine wave. It can also be said that compensation for distortion is easy.

High-density magnetic recording is possible with a sine wave FM signal and 3-level FM (Nυ) without interference.

To explain the present invention from another perspective, it can be said that it is a technique for recording an FM signal at one half of its frequency. The interesting phenomenon that if the polarity of a square wave FM signal is reversed every two cycles, the frequency is halved, seems to have been overlooked by conventional techniques.

Using this technology, it is possible to record FM signals without changing their frequency.
It can record twice as much information, dramatically improving the image quality and resolution of video tape recorders, etc.

<Embodiment> FIG. 3 is a block diagram when magnetic recording is performed using a 3-level FM signal. The output of the FM modulator 7, which has twice the frequency of the recording FM signal, is divided into a counter 8 that is driven at the falling edge of the output waveform, and a counter 9 that is driven at the rising edge of the output waveform, each having a frequency that is half that of the recording FM signal. The frequency is divided into FM signals. Both signals are superimposed by an adder IO to form a recording FM signal according to the present invention, which is sent to the RAD 11 for recording and recorded.

Even when recording a 3-level FM signal on a video disc, the block diagram in FIG. 3 can be used as is by changing "Record" and "C" accordingly. However, the shape of the bits recorded on the disk must be designed to accommodate the three-level FM signal. Assuming that the fourth diagram is a conventional bit, M changes the bit width, and N changes the bit prominence to two stages to correspond to the waveform of a three-level FM signal.

<Performance of the invention> Here, the bandwidth of the demodulated signal of the 3-level FM signal, which can be obtained without beat interference, will be described.

3 level F shown in the 1st UXJE without recording and reproducing
If the M signal is demodulated, a demodulated signal with a bandwidth 4/3 times the E FM signal frequency is obtained.

The fact that the bandwidth of the demodulated signal causes the FM signal frequency seems strange from the perspective of conventional technology, but this is because when the FM signal frequency is doubled before demodulation, the folded wave of the lower sideband and
This is because the corresponding upper sideband waves are combined to generate 111 normal lower sideband waves.

In the demodulation of only the lower sideband wave, the folded wave described in -h directly becomes an interfering beat, so the bandwidth of the demodulated signal must be limited to a value equal to or less than the FM signal frequency.

Video tape recorders and the like use vestigial sideband recording because of the need for high-density recording. This can be considered to be the same as recording only the lower sideband, and the bandwidth of the demodulated signal is directly below the FM signal frequency.

This is the performance when a 3-level FM signal is used for magnetic recording.

<Effects of the Invention> Some specific examples of the effects of applying the present invention will be listed.

(1) Even when applied to a home video tape recorder with an FM signal frequency of about 4MH2, no improvement in performance can be expected.

This is because the frequency band in which beat disturbance occurs is used for low-frequency conversion color signals. However, 7 to 9
In a high-band home video tape recorder in which the FM signal frequency is raised to MH2, beat interference in the luminance signal can be removed, so the reproduced image quality is improved.

(2) Broadcasting video tape recorders use a video frequency bandwidth of approximately one-half of the FM signal frequency.
The video signal frequency bandwidth can be expanded to twice the frequency of the conventional one, and the resolution is improved.

(3) It can be used for FM signals with a low frequency of about IOMH2, and it is sufficient to record and play video signals with a bandwidth of about 8MI (Z), so it can be applied to MUSE system high-definition video tape recorders, etc.

When applying the present invention, it is easy to implement because the main focus is on improving the recording/reproducing circuit.

When a recording medium capable of higher density recording is developed in the future, the performance of the present invention can be added to that performance.

<Additional notes> For the purpose of simplifying the explanation, each waveform in FIG. 1 is drawn ignoring the delay time and phase shift of the reproduced output.

4. Brief explanation of page 121 Fig. 1 is a time chart of a video tape recorder using the present invention, Fig. 2 is a frequency spectrum diagram illustrating the occurrence of beat disturbance, Fig. 3 is a block diagram of the embodiment, FIG. 4 shows the complete shape of the bits of a video disc.

A is a video signal waveform, E is a 3-level FM signal waveform according to the present invention, B-D is an FM signal waveform in the process of creating E, F~■
(indicates each waveform from the reproduced signal to the demodulated output, L is a bit of a conventional video disc, and M and N are bits for recording a 3-level FM signal.

1 is the video signal band, 2 is the high frequency of the video signal, 3 is an FM signal with twice the frequency of the above 9 FM signal, 4 is its first lower sideband, and 5 is its second lower sideband. .

6 is a video input terminal; 7 is an FM modulator with twice the frequency of the recording FM signal; 8 is a counter driven by the falling edge of the output of 7; 9 is a counter driven by the rising edge of the output of 7; 10 is an analog adder, and 11 is a recording head.

Frequency → Figure 2 Figure 3 Figure 1 Figure 4

Claims (1)

[Claims] The square wave FM signal shown in Figure 1B is regarded as a pulse train of positive polarity, and every other positive pulse is inverted to a negative pulse as shown in Figure 1E. A three-level FM signal recording method characterized by recording a waveform FM signal in which pulses are alternately arranged on a recording medium.