JPS60169285A - Luminance fm signal reproducing circuit - Google Patents

Abstract

PURPOSE:To obtain a picture with high quality by applyig amplitude limit to a luminance FM signal after amplitude modulation is applied to the said signal with a distortion component produced by the luminance FM signal and a sound FM signal so as to eliminate distortion produced at the magnetic recording and reproducing process. CONSTITUTION:A signal reproduced by a video head 1 is fed to an HPF5 via a preamplifier 2 and a signal eliminated with a low frequency conversion chrominance signal is eliminated at the HPF5. This signal is fed to an adder 9 and a BPF10, and a sound signal passing through the BPF10 is amplified by an amplifier 11, the phase is shifted by a phase shift circuit 12 and the level is adjusted by an anttenuator 13 and the result is inputted to an adder 9. As a result, the state that the luminance FM signal fC is subject to amplitude modulation by a component (fC-fS) being the difference between the luminance FM signal fC and the sound FM signal fS, that is, the distortion component is generated at the adder 9, the signal is subject to amplitude limit by a limiter 7 and the limited signal is fed to an FM detector 8.

Description

[Detailed Description of the Invention] [Technical Field of the Invention] The present invention is used in a video tape recorder (hereinafter referred to as VTR) that records a video FM signal with an audio FM signal superimposed thereon. The present invention relates to a luminance FM signal reproducing circuit that demodulates a luminance FM signal after removing both cross-modulation distortion components and an audio FM signal from a signal containing the same.

[Technical background of the invention and its problems]

Generally, in home VTRs, audio recording is performed using an AC bias system, but the current situation is that sufficient sound quality cannot be obtained due to the slow tape speed. For this reason, some Beta system VTRs do not allow audio signals to be
By taking the M-modulated audio FM signal as - and superimposing it on the video FM signal and recording it on the video track,
There are some that offer high quality sound. In such a VTR, as shown in FIG. 1, the audio FM signal AF is recorded in the gap between the low frequency converted color signal C and the luminance FM signal Y by frequency separation.

FIG. 2 is a block diagram showing an example of a luminance FM signal reproducing circuit used in the above-described high quality VTR. The signal reproduced by the video radar 1 is amplified by a preamplifier 2 and input to a bandpass filter (BPF) 3.4 and a bypass filter αIPF) 5. As a result, the υ audio FM signal AF is removed from the BPF3 and sent to the audio circuit, and the low-range converted color signal C is removed from the BPI"4 and guided to the color circuit. Furthermore, the low-range converted color signal C is removed by the HPF5. The signal is input to the trap 6, where the audio FM signal is removed, leaving only the luminance FM signal.This luminance FM signal is suppressed by the AM acid component by the limiter 7, and then converted into the luminance FM signal by the FM detector 8. The luminance signal is demodulated by the luminance FM signal reproducing circuit as described above, but unlike the VTR described above, which records audio using the AC bias method, the audio FM signal is directly recorded on the video signal. Since a method of recording on tracks has been adopted, there arises the problem of distortion caused by the magnetic recording and reproducing process.

Here, if the carrier wave frequency of the luminance FM signal is fe, and the carrier wave frequency of a signal placed lower than this is f@, then these two signals are mixed and subjected to distortion when passing through the tape head system. Among these distortions, the third-order distortion component is particularly remarkable.
Various unnecessary components such as Zfc±fs) are produced. Among these distortions, the distortion component of 3fc) (2fe+f') is outside the band and therefore does not provide damage protection, but when the other distortion components are demodulated, they become (fc - 3f,), 2fl, (fe
- It becomes a f-like beat. Among these, the beat component (fc-3fg) generally has a small level and does not pose a problem.

Also, 2fi tr; the beat component is the carrier wave of f.
(horizontal synchronization frequency) and can be made relatively unnoticeable by frequency interleaving. Furthermore, when fs is a low-frequency conversion color signal, the fII value of the (fc-fl) beat component is low, so the frequency is relatively high and the level is low.
is rarely a problem.

However, in the VTR which performs FM modulation of the audio signal and performs frequency separation and recording in the gap between the low frequency conversion color signal and the luminance FM signal, cross-modulation distortion between the audio FM signal and the luminance FM signal also occurs. At this time, f is the carrier frequency of the audio FM signal, and most of the beat components are not a problem as mentioned above, but the beat component of (fe-fa) is f
If - is an audio FM signal, its value is higher than that of a low-frequency conversion color signal, so the level increases so much that this beat component cannot be ignored.

Next, the generation process of the beat component (f, -f,) above is explained in the third section.
This will be explained using the spectrum diagram shown in the figure. For simplicity, in Fig. 3, the luminance FM signal and the audio FM signal are unmodulated and have only the υ carrier wave, and the cross-modulation distortion component is (2f-
fm) component only.

Figure 3 (4) is the HPF for brightness signal removal shown in Figure 2.
It is a spectrum diagram of a partial person who has passed 5. Brightness FM
At a distance of (fe-fs) from the carrier frequency f, the audio FM carrier frequency fs exists below and the frequency of the cross-modulation distortion component (2fe-fa) exists above. In part B, where this signal passes through the audio FM signal removal filter, the third
The spectrum diagram becomes as shown in FIG. 3), and the spectrum diagram at the portion C that passes through the limiter 7 becomes as shown in FIG. 30. Compared to the state in Fig. 3 (the state of Q is shown in Fig. 3), the level of the 2fe-f component is only half, and a distortion component a with the same level as this component is generated. This is because third-order cross-modulation distortion of fC and (2fc-fm) is generated due to limit and tough nonlinearity, and a new distortion element is generated. Good.Furthermore, in the state shown in FIG. 3(C), the luminance FM of fe
The state is the same as when the signal is angularly modulated by the signal (f, -f,). Therefore, this is detected by the FM detector 8.
In the M-detected portion, a spectrum diagram as shown by ◎ in FIG. 3 is obtained, and the beat component of (f-fs) is reproduced.

Therefore, in order to remove the beat component of (fC-fa),
It is also conceivable to insert a trap for the distortion component (2fCf-). However, in reality, the luminance FM signal fc is not unmodulated as shown in FIG. 3, but changes from moment to moment depending on the luminance signal. Therefore, the cross-modulation distortion component (2fe-fs
) is also changing accordingly, so it is impossible to trap it. Even if it were possible to insert this trap, the distortion component (2f-fi) is within the band of the luminance FM signal and would cause image quality deterioration. Therefore,
Currently, measures are being taken to make the beat component (fc-fa) less noticeable. This measure is to record while suppressing the recording level of the audio FM signal to some extent so as to reduce the cross-modulation distortion component (2fC-f,) which is the basis of this beat component. However, the reality is that this cannot be suppressed to a very low level due to the relationship with the S/N of the audio signal, and when high-quality images are desired, the existence of the beat component becomes a problem. come.

[Purpose of the invention]

SUMMARY OF THE INVENTION In view of the above-mentioned drawbacks, an object of the present invention is to provide a luminance FM signal reproducing circuit that can remove distortion caused in the magnetic recording and reproducing process and obtain high-quality images.

[Summary of the invention]

The present invention manually generates a signal containing a luminance FM signal and an audio FM signal, first limits the amplitude of the signal containing both the luminance FM signal and the audio FM signal, and converts the luminance FM signal into a luminance FM signal and an audio FM signal. The angle is modulated by cross-modulation distortion components with the signal, and then this angle-modulated signal and the audio FM signal are added to obtain the brightness F.
By amplitude-modulating the M signal with the distortion component, limiting the amplitude of this amplitude-modulated signal to '1fill' again, and removing the audio FM signal and the cross-modulation distortion component, a signal component multiplied by a degree is obtained. , which achieves the above objectives.

Next, the principle of the present invention will be explained. FIG. 4 is a block diagram of a luminance FM signal reproducing circuit showing the principle of the present invention.

A signal reproduced by the video head 1 is amplified by a preamplifier 2, and a bypass filter (HPF) 5 produces a signal from which the low frequency conversion color signal is removed. This signal is sent to adder 9
is input to bypass filter 0. The audio FM signal that has passed through the bandpass filter 10 is amplified by an amplifier 11, then phase-shifted by a phase shift circuit 12, further level-adjusted by an attenuator 13, and then added to an adder 9.
is input. After the amplitude component of the output of the adder 9 is suppressed by the limiter 7, the output from the adder 9 is converted into an FM signal by the FM detector 8.
It is detected and becomes a luminance signal.

5 and 6 are diagrams showing spectra and vectors of each part of the circuit shown in FIG. 4.

Again, for simplicity, the carrier frequency f of the luminance FM signal (sometimes referred to as the luminance FM signal fe) and the carrier frequency f of the audio FM signal in FIGS. 5 and 6.

(9, which is sometimes referred to as the audio FM signal f1, is unmodulated, and only the (2fc-fs) component that causes cross-modulation distortion is shown.) First, the spectrum of the signal output from the HPF 5 is shown in ), and f,, fc.

(2fc-fs), and the vector diagram of these components is shown in FIG. The spectrum of the portion E exiting the BPFIO is as shown in FIG. This vector diagram is as shown in FIG. 6(G). The audio FM signal f@ is phase-shifted by the phase shift circuit 12, and
The level is adjusted by the attenuator 13. This part F
The spectrum of is as shown in FIG. 5(g), and its vector diagram is as shown in FIG. Since the signal of part F of C is added to the signal of part A of 1b by adder 9, the vector of figure 6 is added to the vector of figure 6. It becomes a diagram. This corresponds to the signal vector of the output section G of the adder 9, and the spectrum at this time is as shown in FIG. 5G). In this spectrum diagram,
)'AI degree FM signal f, the lower wave in the amplitude modulation state corresponds to the f bow S, and the upper side wave corresponds to (2fc-f, ). Therefore, this spectrum shows a state in which the luminance FM signal f is increased in amplitude by the distortion component (f, -f,). When such a signal is input to the limiter 7, fs and (2f, -fa) are canceled due to the nonlinearity of the limiter 7, and only the luminance FM signal f is obtained as the output of the limiter 7. The spectrum of the output section H of this limiter 7 is as shown in Fig. 5 σ, and the luminance FM signal f
, only the ingredients.

Looking at the vector diagram of FIG. 60 showing this vector, it can be seen that the components of fm and (2fC-f,) are canceled.

That is, the principle of the present invention is to extract only the audio FM @ f5,
By adjusting the level and phase of this f, it becomes fs,
By adding fc to a signal containing (2fC-f-), a state is created in which the luminance FM signal fC is amplitude-modulated by the component of (f-fi), and by adding an amplitude limit to this, p, (2fC-fs) and f- are canceled and removed. This state is represented by the vector of the prisoner in Figure 6.
To realize this by adding the vectors in Figure 6,
The phase and level of the audio FM signal fa are adjusted to realize the vector state of . Therefore, the amplitude (level) adjustment may be performed by adjusting the gain of the amplifier 11 instead of using the attenuator 3. However, in the circuit shown in Figure 4 above, BP
It is assumed that the FIO is an ideal filter with only a band limiting function and no delay time.

Using the principle of the present invention as described above, the cross-modulation distortion component (
2f, -f, ) and the audio FM signal f, can be removed at the output part H of the limiter 7. Furthermore, in the circuit shown in FIG. 4, the trap for removing the audio FM signal can be omitted compared to the conventional example.

Generally, in a trap for removing an audio FM signal, the phase around the trap rotates greatly and tends to impair the side wave balance of the luminance FM signal, so this circuit also has the advantage of being able to remove this effect.

However, the circuit shown in FIG. 4 has poor practicality in the following points. First, the first
Generally, in a tape head system, the balance of sideband waves of a luminance FM signal is lost, and the lower sideband wave is more emphasized than the upper sideband wave. Therefore, in order to maintain this balance in the playback system, a playback equivalent circuit that emphasizes the high frequencies is required.In many home VTRs, G-induced resonance is caused by head inductance, transmission line, and preamplifier input resistance. In addition, countermeasures have been taken to enhance the high frequency range after the preamplifier. Therefore, the level and phase of the cross-modulation distortion component (2fC-f,) change with respect to the audio FM signal f- due to the frequency change of the luminance FM signal fe. Therefore, in the circuit of Fig. 4, the limiter 7
There is only one point where the amplitude modulation state is completely achieved before the input of .

Second, in the above explanation, it is assumed that BPFlo is ideal with no delay time, but in reality it has a certain delay time. Through the meridian of BPFlo, amplifier 11, phase shift circuit 12, and attenuator 13, audio FM
The phase rotates around the signal fm. When the audio FM signal has a 1h2 transmission frequency, the fact that the audio signal has a relatively narrow band of 7h does not pose a problem. However, when converting to stereo, the carrier frequency is changed to two frequencies, and the video head carrier frequency is changed to four frequencies, etc., the audio signal spans a wide band, and the phase relationship is maintained Countermeasures will have to be taken. In the embodiment described below, a luminance FM signal reproducing circuit which eliminates the above-mentioned drawbacks is realized.

[Embodiments of the invention]

An embodiment of the present invention will be described below with reference to the drawings, in which the same parts as those of the conventional example are denoted by the same reference numerals. Figure 7 shows the brightness F of the present invention.
FIG. 2 is a block diagram showing an example of an M signal reproducing circuit.

The signal reproduced by the video head 1 is sent to the preamplifier 2
After being amplified by a bypass filter (HPF) 5, the low frequency converted color signal is removed and input to a limiter 4.

The signal whose amplitude component has been suppressed by the IJ transmitter 14 is input to the adder 9 after being delayed by a time τ by the delay element 15. The output signal from the limiter 14 is also input to a bandpass filter (BPF) 1o, where it is delayed by a time τ and becomes only the audio FM signal, which is inverted and amplified by the inverting amplifier 16 at the next stage. Thereafter, the phase corrector 17 performs some phase correction, and the attenuator 13 further adjusts the level before inputting the signal to the adder 9. The amplitude component of the signal output from the adder 9 is suppressed by the limiter 7, and further detected by the FM detector 8 to become a luminance FM signal.

Next, the operation of this embodiment will be explained with reference to the spectrum diagram in FIG. 8 and the vector diagram in FIG. 9. HPF5
The spectrum of part A on the output side is as shown in Fig. 8 (4), excluding the low-frequency conversion color signal and including the audio FM signal fs, luminance FM signal fC, and distortion components (2f e-fs). . Vectors of these signals appear as shown in FIG. 9 (4). The output of the HPF 5 is amplitude limited by a limiter 14, and the spectrum of the output side I of this limiter 14 is as shown in FIG. 8(I), and the vector is
The result will be as shown in Figure (I). That is, this is equivalent to a state in which the luminance FM signal fc is angularly modulated by the components (f, -f,). This state is exactly the same as the spectrum shown in FIG. signal fm
and the distortion component (2f-fa) have the same level, and this positional relationship is symmetrical with respect to the luminance FM signal fc. Therefore, in the circuit of FIG. 7, even if the luminance FM signal fc changes, the audio FM signal f and the distortion component (2f) change.

-f*) level and phase relationship remain unchanged. The output signal of the limiter 14 is transmitted to the BPFIO as the audio FM signal f.
Only the m component is extracted, further phase-inverted by an inverting amplifier, then subjected to a slight phase correction by a phase corrector 12, and level-adjusted by an attenuator 13. This portion J has a spectrum as shown in FIG. 8 (, J). The vector of the audio FM signal fs in this state is shown in Figure 9 (, J
), and the audio FM signal f1 at this time is
Compared to the audio FM signal f-, which includes the signal after the output signal of the limiter 14 has passed through the delay element 15, its level is twice that and the phase is reversed by 180 degrees. Therefore, if these two signals are added by the adder 9, the luminance FM signal fO becomes the distortion component (f
C-fs), and the spectrum in this part becomes as shown in FIG. 8 [share], and the vectors of these components become as shown in FIG. 9 [share]. Therefore, this state is the same as the state shown in FIG. 5 and FIG. Figure 8 υ
A spectrum as shown in FIG. 9 and a vector as shown in FIG. 9 can be obtained. Also in this circuit,
The signal is delayed in BPFIO, but since the delay element 15 having the same delay time is placed in the path that directly inputs the signal from limiter 14 to adder 9, BPF
lo, inverting amplifier 15, phase corrector 12, attenuator 1
Compensate the phase rotating in the path 3 over the entire band,
The phase rotation problem that occurred in the circuit of FIG. 4 has also been solved.

According to this embodiment, the luminance FM signal fc is converted into a distortion component (fC-
An adder 9 creates a state in which the amplitude is modulated by f,), and the limiter 7 limits the amplitude of this to remove the audio FM signal fs and distortion components (2f, -f,) at once. Additionally, since beats caused by cross-modulation of the luminance FM signal fC and the audio FM signal f can be eliminated from the playback screen, a high-quality image can be obtained. Furthermore, since distortion components are removed, the recording level of the audio FM signal fs can be increased, and higher quality sound can be obtained. Further, since a trap for removing an audio FM signal with a large phase rotation is not required, it is possible to obtain a high-quality image from this point as well.

〔Effect of the invention〕

As described above, according to the luminance FM signal reproducing circuit of the present invention, after creating a state in which the luminance FM signal is amplitude-modulated with distortion components generated by the luminance FM signal and the audio FM signal, amplitude limitation is applied to the luminance FM signal. This has the effect of removing distortion that occurs during the magnetic recording and reproducing process to obtain high-quality images.

[Brief explanation of drawings]

Fig. 1 is a diagram showing the frequency relationship of a low-frequency color signal, audio FM signal, and luminance FM signal, Fig. 2 is a block diagram showing an example of a conventional luminance FM signal reproducing circuit, and Fig. 3 is a diagram 4 is a block diagram showing an example of a luminance FM signal reproducing circuit illustrating the principle of the present invention; FIG. 5 is a diagram showing spectra in each part of the circuit of FIG. 4; Figure 6 is a vector diagram corresponding to each spectrum shown in Figure 5, and Figure 7 is an uninvented luminance FM.
A block diagram showing one embodiment of the signal regeneration circuit, FIG. 8 is a diagram showing the spectrum of each part of the circuit in FIG. 7,
FIG. 9 is a vector diagram corresponding to each spectrum shown in FIG. 8. 1...Video head 5...Bypass filter 7
．． 14...-Limiter 8--FM detector 9-...
Adder 10... Bandpass filter 13... Attenuator 15... Delay element 16... Inverting amplifier 1
7... Phase corrector agent Patent attorney Noriyuki Chika (and 1 other person)

Claims (1)

[Claims] Input a signal containing a luminance FM signal and an audio FM signal,
In a luminance FM signal reproducing circuit that demodulates the luminance FM signal after removing the audio FM signal, the luminance FM signal and the audio FM signal are
a first amplitude limiting means for amplitude limiting a signal including the signal to create a state in which the luminance signal is angularly modulated with a distortion component caused by cross-modulation of the luminance FM signal and the audio FM signal; and the first amplitude limiting means. an audio FM signal processing means for extracting an audio FM signal from a signal output from the means, inverting and amplifying it, and then subjecting it to predetermined phase correction and level adjustment; A state in which the luminance FM signal is amplitude-modulated by the distortion component by adding the signal output from the delay means and the signal output from the audio FM signal processing means, and adding the signal output from the delay means to the signal output from the audio FM signal processing means. , a second amplitude limiting means for amplitude limiting the signal outputted by the adding means, and a detection means for detecting the luminance FM signal outputted by the second amplitude limiting means. Features a luminance FM signal regeneration circuit.