JPH0724369B2 - FM demodulation circuit - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an FM demodulation circuit in a magnetic recording / reproducing apparatus, and in particular, prevents an inversion phenomenon that tends to occur when reproducing an FM modulated wave signal having a high FM modulated wave. To do.

2. Description of the Related Art In a magnetic recording / reproducing apparatus for low-carrier FM recording / reproducing, such as a consumer VTR, which performs FM single-sideband reproduction, when demodulating an FM-modulated wave signal with a large FM modulation index, the FM demodulator has zero The cross points cannot be faithfully reproduced, and the inversion phenomenon easily occurs. This occurs when the level of the lower sideband component J ₁ becomes higher than the level of the FM fundamental wave component J ₀ , and when reproduction noise is further superimposed, it is due to the noise,
The inversion phenomenon occurs even when J ₄ <J ₀ .

Heretofore, various demodulation methods for reducing this inversion phenomenon have been proposed. For example, as disclosed in Japanese Patent Laid-Open No. 57-189311, a fundamental wave component of a reproduced FM signal is converted into a square wave by a limiter circuit, a pulse is generated by this square wave, and this pulse is reproduced FM signal. , And is demodulated by the second limiter circuit. This will be described with reference to the block diagram of FIG. 12 and the waveform diagrams of FIGS. 13 (A) to 13 (J).

The reproduced FM signal h is passed through the bandpass filter 20 to obtain the basic component i
Is taken out and delayed for a predetermined time to obtain a signal j. The signal j is subjected to a limiter to obtain the signal k, and the pulse signal 1 is obtained from the signal k. Here, the signal 1 is a pulse indicating the information of the zero crossing point of the fundamental wave signal j, which is a point that is essentially different from the present invention. Then, a signal m is obtained by superimposing a pulse 1 on the reproduced FM signal h, and there is a zero cross point even at a point where the degree of modulation is high, and the inversion phenomenon does not occur.

However, although the signal 1 which is the time information of the zero crossing point of the signal j obtained by delaying the FM fundamental wave for a predetermined time and the time information of the peak point of the reproduced FM signal waveform are essentially different time information. However, superimposing it distorts the information contained in the FM signal, causing the following problems.

Now, assuming that FM allocation in VTR is 5 to 7 MHz, dark clip is 100%, and white clip is 200%,
Dark clip frequency is 3MHz, white clip frequency is
It becomes 9MHz, and the range where the fundamental wave exists becomes 3-9MHz.

In other words, the inversion period of the fundamental wave changes between about 333 / 2nsec and 111 / 2nsec.

FIG. 13h shows the reproduced FM signal and the fundamental wave signal i, in which the superimposed pulse waveform when the delay time is 100 nsec is 1 and the superimposed waveform is 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 with each other, but when the fundamental wave signal i has a high frequency, they shift. Therefore, the zero cross cannot be restored at x ₁ point, and so-called black and blurring occur,
It can be seen that at x ₂ points, an extra zero cross occurs, so-called white or blurring occurs.

Next, the delay time is shortened to 50 nsec and the pulse waveform is shown in 12 and the superimposed waveform is shown in m2. Looking at m2, x ₃
You can see white and blurring at the points. If the delay time is set shorter, the pulse waveform 12 will be superimposed near the zero crossing of the signal h, and the waveform of the zero crossing point of the reproduced FM signal h will change, adversely affecting the frequency characteristics after demodulation. However, it is not preferable to simply set the delay time.

Furthermore, superimposing the pulse waveform on the reproduced FM signal means that the maximum amplitude of the reproduced FM signal increases, and the maximum amplitude of the FM signal greatly increases in a system with a limited amplitude value such as IC. It was a problem.

As described above, in the conventional example, the time information that the zero crossing point of the fundamental wave of the reproduced FM-modulated wave has is another essential point of the FM information.
It is impossible to superimpose it on the peak point of the playback signal, and the effect of preventing the inversion phenomenon does not hold over the entire frequency range that exists with the FM fundamental wave, and depending on the FM playback signal, black, blur, white, blurring, etc. It was not possible to prevent the inversion phenomenon. Furthermore, the increase of the maximum amplitude of the FM signal has been a big problem in the system such as IC where the amplitude value is limited.

Means for Solving the Problems In order to solve the above problems, the demodulation circuit of the present invention, a peak detection circuit for detecting the peak point of the FM modulated wave, depending on the rising or falling edge of the output signal of the peak detection circuit. A pulse generation circuit for generating a pulse of a minute width,
It comprises a limiter circuit for limiting the FM modulated wave, an arithmetic circuit for performing arithmetic operation using the output of the limiter circuit and the output of the pulse generating circuit as inputs, and a pulse count circuit for pulse counting the output of the arithmetic circuit.

Effect The present invention can prevent the inversion phenomenon over the entire frequency range in which the FM fundamental wave exists, and does not increase the maximum amplitude value of the reproduced FM signal, so that the amplitude of the IC or the like can be prevented. It is extremely effective in a system with limited values.

Embodiment An embodiment of the present invention will be described below 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 the respective portions a to f are shown in FIGS. 2 (A) to (F). The reproduced FM signal a input from the terminal I is input to the fundamental wave peak detection circuit 2. The fundamental wave peak detection circuit 2
Information b indicating the peak point of the fundamental wave component of the reproduced FM signal is output. The pulse generation circuit 3 generates a pulse on the signal c at the rising edge of the signal b and a pulse on the signal d at the falling edge of the signal b, as shown by the signals c and d. Here, the pulse generation points of the signals c and d coincide with the peak points of the signal a.

The signal e is obtained by inputting the signal a to the limiter circuit 4. In this signal e, since the lower sideband is emphasized too much in the sections Z1 and Z2, the normal zero cross is not performed and the inversion phenomenon occurs.

The signal f is obtained by calculating the signal e with the information of the signals c and d in the arithmetic circuit 5. Here, since the signal f restores the zero cross point in the sections Z1 and Z2, the inversion phenomenon does not occur. By performing FM demodulation of the signal f by the pulse counting circuit 6, a demodulation signal without inversion phenomenon can be obtained.

Here, the pulse counting circuit 6 outputs pulses from both edges of the input signal for a certain period of time and is well known, so the description thereof is omitted.

Next, a first embodiment of the fundamental wave peak detection circuit 2 for detecting the peak point of the fundamental wave is shown in FIG. 3, and waveforms of respective parts are shown in FIGS. 4 (A) to (D). The input FM reproduction signal a is passed through the bandpass filter BPF7 to extract the FM fundamental wave g. Here, the pass band of BPF7 is set approximately to the deviation of the FM signal. Next, in order to detect the peak value of the signal g, the signal g is differentiated by the differentiating circuit 8 to obtain the signal h. Here, the zero crossing point of the signal h indicates the peak point of the signal g. Therefore, the signal b can be obtained by limiting the signal h by the limiter circuit 9. Here, the signal b is the same as the signal b in FIGS. 1 and 2. Here, the order of the BPF 7 and the differentiating circuit 8 may be interchanged.

Next, a first circuit example of the differentiating circuit 8 is shown in FIG. FIG. 5 (A) shows a differentiating circuit including a resistor R and a capacitor C.

Next, a second circuit example of the differentiating circuit is shown in FIG. This is composed of a delay circuit 10 and a comparator 11 that delay by a minute time t ₁ . The operation of FIG. 5B will be described with reference to the waveform chart of FIG. Here, the signals g, b, and i shown in FIGS. 5B and 6 correspond to each other. When the FM fundamental wave g and the signal i delayed by a minute time t ₁ are compared, the signal b
Is obtained. Here, b is displaced from the peak point of the signal g by a minute interval t _{1 at the} maximum, but it is possible to make t ₁ as small as possible (for example, 20 nsec).
Further, since the output signal of the comparator 10 is already a square wave, there is an advantage that the limiter circuit 9 of FIG. 3 can be omitted when this configuration is used for the differentiating circuit 8 of FIG.

Next, a second embodiment of the fundamental wave peak detection circuit 2 will be shown. An FM fundamental wave g is extracted by the BPF 7 from the reproduced FM signal a whose block diagram is shown in FIG. 7 and waveform diagrams are shown in FIGS. In the 90 ° shift circuit 12, the phase of the signal g is changed to the FM fundamental wave g.
90 ° shift in the band in which This gives the signal j. The zero crossing point of the signal j coincides with the peak point of the signal g. That is, the signal b can be obtained by limiting the signal j in the limiter circuit 9. Here, the 90 ° shift circuit 12 can be composed of a capacitor and a resistor, and can also be composed of a delay element. An example of shifting the phase by 90 ° using a delay element is shown in FIG. 7 (B).

The 90 ° shift circuit 12 shown in FIG. 7 is a kind of comb filter, and the signal j1 delayed by time t ₂ has a phase difference between j2 and the signal obtained by combining the input signal and the signal delayed by 2t ₂ It is always 90 °. Therefore, when the signal j1 is sent to the limiter circuit 4, the signal j2 is sent to the BPF 7, and the fundamental wave j3 is taken out, the zero cross point of this fundamental wave j3 coincides with the peak point of the signal j1. Therefore, the signal b can be obtained by limiting the signal j3 by the limiter circuit 9. Here, the polarity of the signal b is adjusted by the limiter circuit 9. In the configuration of FIG. 7 (B), the playback FM
j1 since also t ₂ late, have become those block diagram slightly different as shown in Figure 1. This is due to the construction of the 90 ° shift circuit and is not an essential problem of the present invention.

Next, a circuit example of the pulse generating circuit 3 and the arithmetic circuit 5 is shown in FIG. 9, and waveforms of respective parts are shown in FIGS. 10 (A) to 10 (H).

The pulse generation circuit 3 is composed of an inverter, an AND gate and a NOR gate, and the arithmetic circuit 5 is composed of two NOR gates. Here, the signals a to f are the signals a in FIG. 1 and FIG.
Corresponds to ~ f. Since the operation of each gate is clear, its explanation is omitted. Further, FIG. 9 shows one embodiment, and the circuits of the pulse generating circuit 3 and the arithmetic circuit 5 can be realized by combining other gates.

As described above, it is extremely advantageous for IC implementation that the pulse generation circuit 3 and the arithmetic circuit 5 can be realized by digital processing. Further, as seen in the conventional example, since no other signal is added to the FM modulated wave before the limiter, the C / N of the FM modulated wave is not deteriorated and the demodulated waveform is not distorted.

Here, in the above-described first embodiment, BPF7, which is a constituent element of the fundamental wave peak detection circuit 2, has a smaller C / N because the noise amount in the band is reduced as the band is narrower. The improvement effect is great. However, if it is made too narrow, the FM fundamental wave cannot pass and the peak detection of the fundamental wave becomes incomplete. Therefore, the carrier frequency of the reproduced FM signal and C /
According to the N state, the BPF band may be adaptively controlled so as to have an optimum value. As the information for controlling, the output level of the reproduction FM signal, the recording mode of the recording / reproducing apparatus (for example, standard recording, long-time recording in VTR, or standard recording, high-band recording) and the like are considered.

Next, a second embodiment of the present invention will be shown. It is conceivable that a minute delay may occur in the fundamental wave peak detection circuit 2 and the pulse generation circuit 2 due to the calculation of the circuit and the filter. Therefore, in order to compensate for this minute delay, it is desirable to insert an equalizer circuit 13, which is a kind of delay circuit, before the limiter circuit 4, as shown in FIG. The equalizer circuit 13
It is desirable that the group delay characteristic is constant and the frequency characteristic is flat. However, with the intention of further improving the S / N of the demodulated signal, playback F
It may be the characteristic of a low-pass filter that attenuates the high frequency component of the M signal, which has a poor C / N. That is, the conventional demodulated signal
As a means for improving S / N, it is known that the high frequency component of the reproduced FM signal is attenuated before the demodulator 5, but if the amount of attenuation is made large, the inversion phenomenon occurs, so the amount of attenuation cannot be made very large. It was However, by using the present invention, it is possible to increase the size of the high-frequency attenuator of the equalizer circuit 13 and increase the amount of S / N improvement after demodulation while preventing the inversion phenomenon from occurring. That is, as an effect of the present invention, it also has an effect of improving S / N.

In addition, in the detection circuit 2, the band of BPF7 can be made wider as the FM sideband enters, and the BPF7 can be omitted, and the peak point of the FM signal including the sideband can be directly detected by the differentiating circuit 8. Is. That is, the fundamental wave peak detection circuit 2 is considered as a peak detection circuit for the FM modulated wave. In this case, the waveform deterioration of the demodulated signal is further improved, but since there is no band limitation in BPF7, there is no improvement effect of C / N, and the reliability of the output signal b of the limiter circuit 9 is lower than that in the above-mentioned embodiments. Will be.

Further, although the VTR demodulation circuit has been described in the embodiments, the present invention is not limited to this, and it goes without saying that the present invention is effective for all FM demodulation circuits for discs, FM broadcasting and the like.

As described above, the present invention is a fundamental wave peak detection circuit for detecting the peak point of the fundamental wave of the FM modulated wave, and a minute width corresponding to the rising or falling edge of the output signal of the fundamental wave peak detection circuit. A circuit for generating a pulse, a limiter circuit for limiting the FM modulated wave, an arithmetic circuit for performing an arithmetic operation with the output of the limiter circuit and the output of the pulse generating circuit as an input, and a pulse for pulse counting the output of the arithmetic circuit By being composed of a count circuit, the FM inversion phenomenon can be prevented without changing the zero crossing point of the normal FM signal, and the maximum amplitude of the FM reproduction signal is not increased, and further, the digital reproduction is possible. It provides an FM demodulation circuit, which is very advantageous in IC implementation because the main circuit can be realized by processing. Result is there are things to be extremely large.

[Brief description of drawings]

FIG. 1 is a block diagram of the 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 a 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 a differentiation circuit in the fundamental wave peak detection circuit of the present invention, and FIG. 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 present invention, and FIG. 8 is a waveform diagram of each part of FIG. FIG. 9 is a circuit diagram of a pulse generating circuit and an arithmetic circuit, FIG. 10 is a waveform diagram of each part of FIG. 9, FIG. 11 is a block diagram of a second embodiment of the present invention, and FIG. 12 is a block diagram of a conventional example. FIG. 13 is a waveform diagram of a conventional example. 2 ... Fundamental wave peak detection circuit, 3 ... Pulse generation circuit,
4 ... Limiter circuit, 5 ... Arithmetic circuit, 6 ... Pulse count circuit, 7 ... Bandpass filter, 8 ... Differentiation circuit, 9 ... Limiter circuit, 10 ... Delay circuit, 11 ... Comparator, 12 ...... 90 ° shift circuit, 13 …… equalizer circuit.

Claims (9)

[Claims] 1. A peak detection circuit for detecting a peak point of an FM modulated wave, a pulse generation circuit for generating a pulse having a minute width according to a rising edge or a falling edge of an output signal of the peak detection circuit, and the FM modulation. It is characterized by comprising a limiter circuit for limiting a wave, an arithmetic circuit for performing arithmetic with the output of the limiter circuit and the output of the pulse generating circuit as inputs, and a pulse count circuit for pulse counting the output of the arithmetic circuit. FM demodulation circuit. 2. The peak detection circuit comprises a differentiating circuit for differentiating the FM modulated wave and a limiter circuit for limiting the output signal of the differentiating circuit to detect a zero-cross point. The FM demodulation circuit described in (1). 3. A fundamental wave peak detection circuit for detecting a peak point of a fundamental wave of an FM modulated wave, and a pulse for generating a pulse having a minute width according to a rising edge or a falling edge of an output signal of the fundamental wave peak detection circuit. A generation circuit, a limiter circuit that limits the FM modulated wave, an arithmetic circuit that operates by using the output of the limiter circuit and the output of the pulse generation circuit as input, and a pulse count circuit that performs pulse count on the output of the arithmetic circuit. FM demodulation circuit characterized by being. 4. A fundamental wave peak detection circuit includes a bandpass filter for passing a fundamental wave of an FM modulated wave, a 90 ° shift circuit for shifting the phase of an output signal of the bandpass filter by 90 °, and the 90 ° shift circuit. 4. The FM demodulation circuit according to claim 3, wherein the FM demodulation circuit comprises a limiter circuit that limits the output signal of the circuit to detect a zero-cross point. 5. A fundamental wave peak detection circuit comprises a bandpass filter for passing a fundamental wave of an FM modulated wave, a differentiation circuit for differentiating the output of the bandpass filter, and a limiter for the output signal of the differentiation circuit to zero. The FM demodulation circuit according to claim 3, wherein the FM demodulation circuit comprises a limiter circuit for detecting a cross point. 6. The FM modulated wave is passed through a peak detection circuit or a fundamental wave peak detection circuit and an equalizer circuit having a delay time corresponding to the calculation time in the pulse generation circuit,
The FM demodulator circuit according to claim 1, wherein the FM demodulator circuit is input to a limiter circuit. 7. The FM demodulator circuit according to claim 6, wherein the equalizer circuit has a characteristic of attenuating a high frequency component of the frequency of the FM modulated wave. 8. The FM demodulation circuit according to claim 1, wherein the arithmetic circuit is composed of two NOR gates. 9. A differentiating circuit is composed of a delay circuit for delaying an input signal by a minute time and a comparator, and the input signal and the output signal of the delay circuit are input to the two inputs of the comparator to obtain a differential signal. The FM demodulation circuit according to any one of claims (2) and (5).