JPS63279464A - Fm demodulation circuit - Google Patents

Abstract

PURPOSE:To prevent the inversion phenomenon of FM without changing the zero cross point of a normal FM signal by generating the odd number-th higher harmonic frequency components of an FM fundamental wave by means of a higher harmonic frequency wave generator circuit, superposing the result on an FM modulated signal in equal phase, then modulating. CONSTITUTION:An FM fundamental wave is detected from an FM modulated signal, the odd number-th higher harmonic components of the FM fundamental signal are generated by the higher harmonic frequency wave generator circuit 13, and thus a high harmonic frequency signal is generated. This signal is super posed on an FM modulated signal in same phase by an adder 4, thereafter is FM modulated. In such a way, a high-frequency pulse wave obtained from the same information as that of the peak point of a reproduced FM signal is superposed on the reproduced FM signal. Therefore, the inversion phenomenon can be prevented without changing the zero cross point of a normal FM signal throughout the entire frequency range where FM fundamental waves are present.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to an FM demodulation circuit in a magnetic recording/reproducing device, and is particularly intended to prevent the inversion phenomenon that tends to occur when reproducing an FM modulated wave signal with a high degree of FM modulation. It is something to do.

Conventional technology: Low carrier FM recording and playback like consumer VTRs, and FM
In a magnetic recording/reproducing device that performs M sideband reproduction, F
When demodulating an FM modulated wave signal with a large M modulation index,
In the FM demodulator, the zero-crossing point cannot be faithfully reproduced, and an inversion phenomenon tends to occur. This occurs when the level of the lower sideband component J, becomes larger than the level of the FM fundamental component J0, and When reproduction noise is superimposed, an inversion phenomenon occurs even when J, , <Jo, due to the influence of the noise.

Conventionally, various demodulation methods have been proposed to reduce this inversion phenomenon. For example, as shown in Japanese Patent Application Laid-Open No. 57-189311, the fundamental wave component of the reproduced FM signal is converted into a square wave by a limiter circuit, this square wave generates a pulse, and this pulse is used as the reproduced FM signal. After the signal is superimposed on the signal, the signal is demodulated by a second amplification circuit. This will be explained using the block diagram in FIG. 11 and the waveform diagrams in FIGS.

A fundamental wave component i is extracted from the reproduced FM signal by a bandpass filter 20 and delayed for a predetermined time to obtain a signal j. A signal j is applied to a limiter to obtain a signal k, and a pulse signal l is obtained from the signal. Here, the signal l is a pulse indicating information on the zero cross point of the fundamental wave signal j, which is essentially different from the present invention. Then, a signal m is obtained by superimposing a pulse l on the reproduced FM signal, and a zero crossing point exists even at a point where the degree of modulation is high, and no inversion phenomenon occurs.

However, even though the time information of the signal l, which is the time information of the zero cross point of the signal j obtained by delaying the FM fundamental wave by a predetermined time, and the time information of the peak point of the reproduced FM signal waveform are essentially different time information. , superimposing them would distort the information contained in the FM signal, resulting in the following problems.

Currently, the FM allocation on the VTR is 5-7Mllz.
If the dark clip is 100% and the white clip is 200%, the dark clip frequency is 3MHz'
, the white clip frequency is 9MHz, and the range where the fundamental wave exists is 3 to 9! It becomes vHIz.

In other words, the inversion period of the fundamental wave is approximately 333/2 nsec ~ 11
It will change within 1/2 nsec.

In Figure 12, the reproduced FM signal and fundamental wave signal are shown as i, the superimposed pulse waveform when the delay time is 100 nsec is shown as l, and the waveform when superimposed is shown as m.

Looking at m, the phase relationship between the superimposed pulse and the reproduced wave shifts depending on the frequency. That is, when the fundamental wave signal i has a low frequency, the peak points of the pulse and the fundamental wave signal coincide, but when the fundamental wave signal i has a high frequency, they deviate. Therefore, it can be seen that zero crosses cannot be restored at the χ point, so-called black blurring occurs, and so-called white blurring occurs at the χ2 point, where an extra zero cross occurs.

Next, the pulse waveform when the delay time is shortened to 5Qnsec is shown in 12, and the waveform when superimposed is shown in m2.0m2
If you look at the image, you can see that white blurring occurs at the χ3 point. If the delay time is set even shorter, the pulse waveform 12 will be superimposed near the zero cross of the signal.
It is highly conceivable that the waveform of the zero crossing point of the reproduced FM signal will change, which will have an adverse effect on the frequency characteristics after demodulation, and therefore it is not preferable to simply set the delay time short.

Problems to be Solved by the Invention As described above, in the conventional example, the time information possessed by the zero cross point of the fundamental wave of the reproduced FM modulated wave is superimposed on the peak point of the FM reproduced signal, which is essentially different time information. is unreasonable,
The effect of preventing the inversion phenomenon does not hold over the entire frequency range in which the FM fundamental wave exists, and depending on the FM reproduction signal, it has not been possible to prevent inversion phenomena such as blurred blacks and blurred whites.

Means for Solving the Problems In order to solve the above problems, the demodulation circuit of the present invention provides an FM
Detecting an FM fundamental wave from a modulated wave signal, generating odd-numbered harmonic components of the FM fundamental wave in a harmonic generation circuit as a harmonic signal, and generating the harmonic signal in the same phase as the FM modulated wave signal. It is configured to perform FM demodulation after superimposition.

Effect of the present invention With the above-described configuration, the harmonic pulse waveform obtained from the same information as the peak point of the reproduced FM signal is reproduced by the reproduced FM signal.
Since it is superimposed on the M signal, the inversion phenomenon can be prevented without changing the zero-crossing point of the normal FM signal over the entire frequency range where the FM fundamental wave exists.

EXAMPLE Hereinafter, an example of the present invention will be described with reference to the drawings.

A block diagram of the first embodiment of the present invention is shown in FIG. 1, and waveform diagrams corresponding to waveforms a''-e of each part are shown in FIG. The harmonic generation circuit 13 inputs the reproduced FM
A harmonic signal d having the same phase as the signal a is output to the adder 4.

An adder 4 adds signals a and d to obtain a signal e. Here, since the harmonic generation point of the signal d completely coincides with the peak point of the signal S, harmonics are always superimposed on the peak point of the waveform in the signal e.

Here, an example of a harmonic generation circuit will be shown.

The harmonic generation circuit is composed of a fundamental wave peak detection circuit 2 and a pulse generation circuit 3. The fundamental wave peak detection circuit 2 is
Information C indicating the peak point of the fundamental wave component of the reproduced FM signal is output. The pulse generating circuit 3 generates a pulse in the positive direction at the rising edge of the signal C, and a pulse in the negative direction at the falling edge of the signal C, as shown in d from the signal C. In other words, the pulse generating circuit 3 may be a differentiating circuit. .

Conventionally, this signal e is generated in a section Zl where an inversion phenomenon occurs.
, Z2 is restored, and no inversion phenomenon occurs even when input to the conventional pulse counter type demodulator 5.
Good FMy! , ll are realized.

Next, the first embodiment of the detection circuit 2 for detecting the peak point of the fundamental wave is shown in FIG. 3, and the waveform diagram is shown in FIGS.
Here, the passband of the BPF is set approximately to the deviation of the FM signal.In order to detect the peak value of the zero-order signal, the signal is differentiated by a differentiating circuit 7. Obtain signal b2. Here, signal b2
The zero cross point of the signal indicates the peak point of the signal. Therefore, the signal b2 is limited by the limiter circuit 8 to obtain the signal C1, and by inverting cl, the signal C1 is obtained.
can be obtained. Here, the polarity of the limiter circuit is adjusted so that it is in phase with the retransmitted FM signal. Further, signal C is the same as signal C in FIGS. 1 and 2.

Next, a first circuit example of the differentiating circuit 7 is shown in FIG. 5, which is a differentiating circuit using a resistor R and a capacitor C.

Here, the order of the BPF 6 and the differentiating circuit 7 may be changed.

Next, a second circuit example of the differential circuit is shown in Figure 5.

This is composed of a delay circuit 9 that delays by a minute time t1 and a comparator 10. The operation shown in FIG. 5 will be explained using the waveform diagram shown in FIG. 6, (2). Here, the signals of FIG. 5B and FIG. 6, b3. c correspond to each other. When the FM fundamental wave is compared with the signal b3 delayed by a minute time t1, a signal C is obtained. Here, C is shifted from the peak point of the signal by a minute interval t1, but tl is made as small as possible (for example, 2
0nsec) is possible. Furthermore, comparator 1
Since the output signal C of 0 is already a square wave, the third
When this configuration is used for the differential circuit 7 shown in the figure, there is an advantage that the limiter circuit 8 shown in FIG. 3 can be omitted.

Next, FIG. 7 shows a second embodiment of the detection circuit 2.

FIG. 8 shows a block diagram, and FIG. 9 shows a waveform diagram. Play FM
Extract the FM fundamental wave from signal a using BPF6, 9
The 0° shift circuit 11 shifts the phase of the signal by 90° in the band where the FM fundamental wave exists. This results in signal b4. The zero cross point of the signal b4 coincides with the peak point of the signal b4. That is, by limiting the signal b4 with the limiter circuit 8, the signal C can be obtained. Here, the 90" shift circuit 11 can be configured with a capacitor and a resistor, or can also be configured with a delay element. Using a delay element, the phase can be changed to 90".
An example of shifting by 0 is shown in FIG.

The 90° shift circuit 11 shown in FIG. 8 is a type of comb filter, and the phase difference between the signal a1 delayed by time t2 and the signal a2 which is a composite of the input signal and the signal delayed by time 2t2 is always 90". Therefore, the signal a
1 is sent to the adder 4, and the signal a2 is sent to the BPF 6 to extract the fundamental wave b4, the zero cross point of this fundamental wave b4 will coincide with the peak point of the signal a1. Therefore,
By limiting the signal b4 with the limiter circuit 8, the signal C can be obtained. In the configuration shown in Figure 8,
Since the reproduced FM signal a is also delayed by t2, the block diagram is slightly different from that shown in FIG. this is,
This is due to the configuration of the 90'' shift circuit and is not an essential problem of the present invention.

In addition, in the embodiment described above, the narrower the band of the BPF 6, which is a component of the fundamental wave peak detection circuit 2, the lower the amount of noise within the band, so the C/N is improved, and the effect of improving the inversion phenomenon is greater. becomes. However, if it is too narrow, on the other hand, the FM fundamental wave will not be able to pass through, resulting in incomplete peak detection of the fundamental wave.

Therefore, it may be configured to adaptively control the BPF band to the optimum value according to the carrier frequency and C/N state of the reproduced FM signal. output level and recording mode of the recording/playback device (for example, standard recording, long-time recording, standard recording, high band recording for a VTR)
etc. are possible.

Further, it is conceivable that a slight delay may occur in the fundamental wave peak detection circuit 2 and the pulse generation circuit 3 due to circuit calculations and filters. Therefore, in order to compensate for this minute delay, it is desirable to insert an equalizer circuit 12, which is a type of delay circuit, before the adder 4, as shown in FIG.
It is desirable for the equalizer circuit 12 to have a constant group delay characteristic and a low frequency characteristic.However, in order to further improve the S/N of the demodulated signal, the equalizer circuit 12 attenuates the high-frequency components with poor C/N of the reproduced FM signal. In other words, it is known that conventionally, as a means of improving the S/N of a demodulated signal, the high-frequency components of the reproduced FM signal are attenuated at the stage before the demodulator 5, but the attenuation If the amount was increased, an inversion phenomenon would occur, so the amount of attenuation could not be increased too much. However, by using the present invention, it is possible to increase the high-frequency attenuator of the equalizer 12 and to increase the amount of S/N improvement after demodulation without causing the inversion phenomenon.

In other words, the present invention also has the effect of improving S/N as a secondary effect.

Furthermore, in the first embodiment of the detection circuit 2, the band of the BPF 6 is made wide enough to include the FM sideband, and furthermore, the BPF 6 is omitted, and the peak point of the FM signal including the sideband is determined by the differentiating circuit 7. Direct detection is also possible. However, in this case, since there is no bandwidth limit with BPF, C
There is no improvement effect on /N, and the reliability of the output signal C of the limiter circuit 8 is also lower than in the embodiment described above.

The band of BPF6 is as narrow as possible, including the high frequency range from the carrier center of the FM signal to the deviation.
/H is preferable.

Further, various configurations of the harmonic generation circuit other than those shown in the embodiments may be considered, but they do not go beyond the scope of the present invention.

Effects of the Invention As described above, the present invention detects an FM fundamental wave from an FM modulated wave signal, generates odd-numbered harmonic components of the FM fundamental wave in a harmonic generation circuit as a harmonic signal, and generates a harmonic signal from the harmonic wave. By superimposing the wave signal on the FM modulated wave signal in the same phase and then FM demodulating it, the FM fundamental wave component is present in all bands.
The harmonic pulse is superimposed on the peak point of the fundamental wave, and the zero cross point of the normal FM signal does not change.
This can prevent the M inversion phenomenon, and this effect is extremely significant for VTRs and the like that perform FM single-band wave reproduction.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of a first embodiment of the present invention, FIG. 2 is a waveform diagram of the first embodiment of the present invention, and FIG. 3 is a first embodiment of the fundamental wave peak detection circuit of the present invention. FIG. 4 is a waveform diagram of the first embodiment of the fundamental wave peak detection circuit of the present invention, FIG. 5 is a circuit diagram of the differential circuit in the fundamental wave peak detection circuit of the present invention, and FIG. A waveform diagram of the differentiating circuit in the fundamental wave peak detection circuit of the invention, FIG. 7 is a block diagram of the second embodiment of the fundamental wave peak detection circuit of the invention, and FIG. 8 is a block diagram of the second embodiment of the invention. 9 is a waveform diagram of the second embodiment of the present invention, FIG. 10 is a block diagram of the third embodiment of the present invention, FIG. 11 is a block diagram of the conventional example, and FIG.
FIG. 2 is a waveform diagram of a conventional example. 2...Fundamental wave peak detection circuit, 3...
Pulse generation circuit, 4... Adder, 5...
・Demodulation circuit, 6...Band pass filter, 7.
... Differential circuit, 8. Old... Limiter circuit, 9.
...Delay circuit, 10. Old... Comparator, 11.
...90' shift circuit, 12.old equalizer circuit, 13.....harmonic generation circuit. Name of agent Patent attorney Toshio Nakao Haka1 Figure 1 Figure 2 Zr Zp Figure 3 Figure 4 (EJ C Figure 5 Figure 6 L8J C- Figure 7 Figure 8 Figure 9 Figure 1O Figure 11 Figure 12

Claims (1)

[Claims] An FM fundamental wave is detected from the FM modulated wave signal, odd-numbered harmonic components of the FM fundamental wave are generated in a harmonic generation circuit as a harmonic signal, and the harmonic signal is in phase with the FM modulated wave signal. An FM characterized by demodulating the FM after superimposing it with
Demodulation circuit.