Frequency demodulation apparatus

Abstract: A FM signal obtained by modulating a carrier f c with a signal f s is demodulated by a frequency demodulator to obtain a demodulated signals which contains a lower sideband component as an undesired moire component. A moire component generator generates a mf c - nf s component (m, n: natural number) from the demodulated signal or generates a nf₁ - (n-m)f c component f₁: first lower sideband component of the FM signal) from the FM signal. A subtractor subtracts an output of the moire component generator from the demodulated signal thereby to eliminate the undesired moire component. Further, to eliminate a cross-­ modulation moire due to crosstalk of a low-frequency signal f L , the moire component generator generates a 2f L component from the FM signal.

Description: This invention relates to a frequency demodulation circuit for demodulating a frequency modulated (FM) signal. In video tape recorders (VTRs), a FM signal reproduced from a magnetic tape is demodulated by a pulse counting type frequency demodulator which detects zero-crossing points of the FM signal. The demodulated signal obtained by such frequency demodulator contain the lower sideband components of the FM signal. Among the lower sideband components, components appearing in the base band badly affect the desired signal to cause deterioration of signal quality. Such undesired components are called "moire" components. As an example, Fig.1 shows a spectrum diagram of a FM signal obtained by modulating a carrier frequency f c with a signal frequency f s and a spectrum diagram of a demodulated signal obtained by demodulating such FM signal by a double-multiplier pulse counting type demodulator. The lower sideband components J₋₁, J₋₂, J₋₃..., J -n appearing in the demodulated signal are expressed as: J₋₁ = 2f c - f s , J₋₂ = 2f c - 2f s , J₋₃ = 2f c - 3f s , ..., J -n = 2f c - nf s The signal fs in the base band is obtained by passing the demodulated signal through a low-pass filter (LPF). However, there exist some lower sideband components in the base band as undesired moire components, i.e., the moire components, which degrade the quality of the obtained desired signal. In the case of Fig. 1, the third-order lower sideband components J₋₃ becomes the undesired moire component. To reduce such undesired moire effect, the carrier frequency is generally selected to be more than 1.5 times the maximum frequency of the modulation signal. The moire effect can be reduced more by using higher carrier frequency. However, the higher carrier frequency necessarily results in lower signal to noise ratio (C/N). the carrier frequency cannot be made high enough to reduce the moire effect adequately. United States patent US-A-4,561,113 discloses a distortion canceller which removes distortion of a demodulated signal in an FM receiver by producing the n-th power of the demodulated signal and differentiating it to derive an n-th order distortion signal. The derived n-th order distortion signal is adjusted in its amplitude level and then combined with the demodulated FM signal in such a way that the n-th order distortion signal can be cancelled from the demodulated signal. This apparatus is not applicable to the removal of moire interference. United States patent US-A-4,052,740 discloses an apparatus for suppressing spurious interference components that arise in video recording apparatus of the type which utilizes frequency modulation of the carrier signal with a video signal. The unwanted interference signal component is suppressed by generating the second harmonic of a fundamental signal in the video information modulating signal and adding the second harmonic to the fundamental modulating signal at a predetermined phase and amplitude prior to it being modulated upon the carrier signal, the phase being that which heightens the upward deviating peaks and flattens the lower peaks of the fundamental modulating signal, and the amplitude being that which minimizes the interference component. It is possible to reduce the level of the unwanted signal substantially to zero. This apparatus is intended to suppress moire interference, but only that due to harmonics of the main signal. In the home VTRs, the colour-under system is used which converts the frequency of the chrominance signal of the video signal into a lower frequency and mixes the low-converted chrominance signal with the frequency-modulated luminance signal for recording. Supposing that the carrier frequency is designated as f c and the down-converted chrominance signal frequency as F L , a cross-modulation component is generated at the frequency of f c -2f L during playback due to the frequency characteristic of the electromagnetic conversion system, and a cross-modulation moire component of 2f L is generated in the reproduced luminance signal after demodulation, causing deterioration of reproduced picture quality. Conventionally, recording current of the chrominance signal is restricted to a low level to reduce the level of the cross modulation moire component below the allowable limit, but this causes deterioration of S/N of the chrominance signal. Embodiments of the present invention provide a frequency demodulation circuit which is capable of cancelling moire components generated from the lower sideband components of a FM signal. Embodiments of the present invention also provide a frequency demodulation circuit which is capable of cancelling an undesired cross-modulation moire component generated from a cross=modulation signal contained in a FM signal. According to the present invention there is provided a frequency demodulation apparatus comprising frequency demodulating means for demodulating a frequency modulated signal which has a carrier frequency f c and contains a low-frequency signal having a frequency f L and a cross-modulation signal component expressed as f c - 2f L to obtain a demodulated signal which contains a frequency component 2f L as an undesired cross-modulation moire component: characterized by further comprising: moire component generating means for extracting a low-frequency component f L from said frequency-modulated signal and generating a 2f L component as a moire component corresponding to said undesired cross-modulation moire component from the extracted low-frequency signal; and subtracting means for subtracting said moire component generated by said moire component generating means from said demodulated signal thereby to obtain a demodulated signal from which said undesired cross-modulation moire component is eliminated. There is also provided a signal recording and reproducing apparatus comprising: means for extracting a first signal from an input signal and for frequency-modulating a carrier frequency f c with the first signal to obtain a frequency-modulated signal; means for extracting a second signal from the input signal and generating from the second signal a low-frequency signal having a frequency f L ; means for adding the frequency-modulated signal and the low frequency signal to obtain a recording signal; recording and reproducing means for recording the recording signal on a recording medium and for reproducing the recorded signal from the recording medium to obtain a signal containing a reproduced low-frequency signal having the frequency f L and a reproduced frequency-modulated signal which contains a cross-modulation component f c - 2f L ; means for extracting the reproduced frequency-modulated signal from the signal reproduced by the recording and reproducing means and the demodulating the extracted signal to obtain reproduced first signal which contains a component 2f L as an undesired cross-modulation moire component; means for generating a reproduced second signal from the reproduced low-frequency signal; and means for adding the reproduced first and second signals to obtain a reproduced signal, characterized by further comprising: moire component generating means for extracting a low-frequency component f L from the signal reproduced by the recording and reproducing means and for generating a moire component 2f L corresponding to said undesired cross-modulation moire component; and subtracting means for subtracting said moire component generated by said moire component generating means from either said reproduced first signal or said reproduced signal thereby to obtain reproduced signal from which said undesired cross-modulation moire component is eliminated. Generally, a FM signal obtained by modulating a carrier frequency f c with a signal frequency f s contains a nth-order lower sideband component f n = f c - nf s , where n is a natural number. A demodulated signal obtained by demodulating such FM signal by a m-times-multiplier pulse counting demodulator contains a m-multiplied carrier frequency mf c , where m is a natural number, the signal frequency fs and a nth-order lower sideband component j -n - mf c - nf s . This means that the nth-order lower sideband component J -n of the demodulated signal can be calculated as J -n = mf c - nf s by using the m-multiplied carrier frequency mf c and the signal frequency f s which can be obtained from the demodulated signal. Alternatively, since the signal frequency f s is expressed as f s = f c - f n , the component J -n can be expressed as J -n - mf c - n(f c -f n ) = nf n - (n-m)f c . This means that the nth-order lower sideband component J -n of the demodulated signal can be calculated as J -n - nf n - (n-m)f c by using the carrier frequency f c and the nth-order lower sideband component f n of the FM signal. Therefore, for cancelling an nth-order lower sideband component J -n of the demodulated signal as an undesired moire component, the moire component generator may generate a component of mf c - nf s from the demodulated signal or a component of nf n - (n-m)f c from the input FM signal. When a low frequency signal f L is mixed into the FM signal due to crosstalk, the cross-modulation signal appears at f c - 2f L in the FM signal. When such FM signal is demodulated the cross-modulation signal appears at 2f L in the demodulated signal as a cross-modulation moire component. To cancel such cross-modulation more component, the moire component, the moire component generator may generate the 2f L frequency component from the input FM signal. The above and other advantages and features of the invention will be more apparent from the following detailed description of preferred embodiments taken in connection with the accompanying drawings in which: Figure 1 is a frequency spectrum diagram of a conventional example; Figure 2 is a block diagram of a frequency demodulation apparatus according to the present invention; Figure 3 is a block diagram of a further embodiment of a frequency demodulation apparatus according to the present invention; and Figure 4 shows a frequency spectrum for explaining the principle of the Figs. 2 and 3 embodiments. A frequency demodulation apparatus for cancelling cross-modulation moire according to the present invention will be described. Fig. 2 is a block diagram of a preferred embodiment of the present invention which is a colour-under magnetic recording and reproducing system of a video signal. A video signal inputted via a terminal 61 is fed to both a luminance signal recording system 62 and a chrominance signal recording system 63. The luminance signal recording system 62 extracts a luminance signal from the video signal and frequency-modulates the luminance signal to obtain a FM luminance signal in the known manner. The chrominance signal recording system 63 extracts a chrominance signal from the video signal and converts the frequency of the chrominance signal to a low frequency to obtain a down-converted chrominance signal in the known manner. An adder 64 adds the FM luminance and down-converted chrominance signals to obtain a recordable video signal. This recordable video signal is amplified by an amplifier 65 and recorded by a magnetic head 66 on a magnetic recording medium RM such as a magnetic tape. The recorded video signal is reproduced by the head 66, amplified by an amplifier 67 and fed to a luminance signal reproducing system 68, a chrominance signal reproducing system 69 and a moire cancelling circuit 71. The luminance signal reproducing system 68 extracts the FM luminance signal from the reproduced video signal and demodulates the FM luminance signal to obtain a reproduced luminance signal. The chrominance signal reproducing system 69 extracts the down-converted chrominance signal from the reproduced video signal and converts the frequency of the down-converted chrominance signal to the normal high frequency to obtain a reproduced chrominance signal. An adder 70 adds the reproduced luminance and chrominance signals to obtain a reproduced composite video signal, which usually contains, as an undesired cross-modulation moire component, the component of 2f L , where f L is the frequency of the down-converted chrominance signal. This cross-modulation moire is generated in the luminance signal reproducing system 68. That is, the down-converted chrominance signal f L is mixed, as a cross-modulation component, into the FM luminance signal when the video signal is reproduced from the recording medium due to the frequency characteristics of the electromagnetic conversion system comprised by the recording medium RM and the magnetic head 66. This cross-modulation component becomes the cross-modulation moire component when the FM luminance signal is demodulated. The moire cancelling circuit 71 cancels the cross-modulation moire 2f L . The reproduced signal from the amplifier 67 is inputted to a filter 72 which separates the down-converted chrominance signal f L . Either a low-pass filter or a band-pass filter may be used for the filter 72. The output of the filter 72 is sent to a double-multiplier (x2) 73. The double-multiplier 73 doubles the frequency f L to obtain the 2f L component. The 2f L component has its phase and gain adjusted by a phase adjuster 74 and a gain adjuster 75, and then is sent to a subtractor 76. The subtractor subtracts the 2f L component from the reproduced composite video signal outputted from the adder 70 thereby to obtain a video signal from which the cross-modulation moire 2f L is eliminated and which is outputted from an output terminal 77. In this embodiment, the subtractor 76 may not be necessarily positioned after the adder 70, but it may be located at any place after a frequency demodulator located in the luminance signal reproducing system 68. Fig. 3 shows a block diagram of another preferred embodiment of the invention which is such a modification of the Fig. 2 embodiment that the subtractor 76 is positioned in the luminance signal reproducing system 68. The luminance signal reproducing system 68 separates the FM luminance signal from the reproduced video signal, and comprises an equalizer 78 that compensates for the frequency characteristics, a demodulator 79 that demodulates the equalizer output, a low-pass filter 80 that separates signal components from the demodulator output, a de-emphasis circuit 81, and a nonlinear de-emphasis circuit 82. The subtractor 76 is inserted between the low-pass filter 80 and the de-emphasis circuit 81. This configuration provides a benefit of reducing noise components leaking from the moire cancelling circuit 71 by the de-emphasis circuit 81 and the nonlinear de-emphasis circuit 82. Fig. 4 shows a frequency spectrum diagram for explaining the operation of the Fig. 2 or Fig. 3 embodiments. The signal containing the carrier frequency f c of the FM luminance signal and the down-converted chrominance signal frequency f L as shown in Fig. 4 (A) are recorded. Passing such signal through the electromagnetic conversion system (i.e., the head and recording medium) causes f m = f c - 2f L to be generated as shown in Fig. 4 (B). Demodulating in the luminance signal reproducing system causes the f m component to be the cross modulation moire of 2f L as shown in Fig. 4 (C). On the other hand, the 2f L component is produced by the moire cancelling circuit as shown in Fig. 4 (D). Subtracting the spectrum of Fig. 4 (D) from that of Fig. 4 (C) cancels the moire as shown in Fig. 4 (E). In the above embodiments, the frequency f L component is the down-converted chrominance signal, but the present invention is not limited only to such signal. For example, signals obtained by line-sequentially frequency-modulated chrominance signals, such a R-Y, B-Y, or I and Q signals, or signals obtained by time-compressing and frequency-modulating chrominance difference signals may be used. The double multiplier circuit 73 can be easily constructed by a known balanced demodulator and a filter that separates the doubled components. The phase adjuster 74 and gain adjuster 75 can be omitted.

Claims: A frequency demodulation apparatus comprising frequency demodulating means (68-70) for demodulating a frequency modulated signal which has a carrier frequency f c and contains a low-frequency signal having a frequency f L and a cross-modulation signal component expressed as f c - 2f L to obtain a demodulated signal which contains a frequency component 2f L as an undesired cross-modulation moire component: characterized by further comprising: moire component generating means (72-75) for extracting a low-frequency component f L from said frequency-modulated signal and generating a 2f L component as a moire component corresponding to said undesired cross-modulation moire component from the extracted low-frequency signal; and subtracting means (76) for subtracting said moire component generated by said moire component generating means from said demodulated signal thereby to obtain a demodulated signal from which said undesired cross-modulation moire component is eliminated. The apparatus according to claim 1, wherein said moire-component generating means comprises filter means (72) for extracting said low-freqency component from said frequency-modulated signal, and multiplying means (73) for multiplying the frequency f L of the extracted low-frequency component by 2 thereby to obtain said 2f L component. The apparatus according to claim 1 or 2, wherein said moire component generating means includes means (74,75) for adjusting phase and gain of the generated moire component. A signal recording and reproducing apparatus comprising: means (62) for extracting a first signal from an input signal and for frequency-modulating a carrier frequency f c with the first signal to obtain a frequency-modulated signal; means (63) for extracting a second signal from the input signal and generating from the second signal a low-frequency signal having a frequency f L ; means (64) for adding the frequency-modulated signal and the low frequency signal to obtain a recording signal; recording and reproducing means (66) for recording the recording signal on a recording medium and for reproducing the recorded signal from the recording medium to obtain a signal containing a reproduced low-frequency signal having the frequency f L and a reproduced frequency-modulated signal which contains a cross-modulation component f c - 2f L ; means (68) for extracting the reproduced frequency-modulated signal from the signal reproduced by the recording and reproducing means and the demodulating the extracted signal to obtain reproduced first signal which contains a component 2f L as an undesired cross-modulation moire component; means (69) for generating a reproduced second signal from the reproduced low-frequency signal; and means (70) for adding the reproduced first and second signals to obtain a reproduced signal, characterized by further comprising: moire component generating means (72-75) for extracting a low-frequency component f L from the signal reproduced by the recording and reproducing means (66) and for generating a moire component 2f L corresponding to said undesired cross-modulation moire component; and subtracting means (76) for subtracting said moire component generated by said moire component generating means from either said reproduced first signal or said reproduced signal thereby to obtain reproduced signal from which said undesired cross-modulation moire component is eliminated. The apparatus according to claim 4, wherein said input signal is a video signal, and said first and second signals are a luminance signal and a chrominance signal, respectively. The apparatus according to claim 4 or 5, wherein said moire component generating means comprises filter means (72) for extracting said low-frequency components from said reproduced frequency-modulated signal, and multiplying means (73) for multiplying the frequency f L of the extracted low-frequency component by 2 thereby to obtain said 2f L component. The apparatus according to any of claims 4 to 6, wherein said moire component generating means includes means (74,75) for adjusting phase and gain of the generated moire component.

Inventors: HONJO MASAHIRO, HONJO, MASAHIRO
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1988
Main classification: H03D3
IPC codes: H03D3