JPH0771253B2 - FM demodulator - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an FM demodulator for preventing an inversion phenomenon in a demodulated signal of a narrow band FM signal such as a VTR (Video Tape Recorder).

2. Description of the Related Art In a VTR, when a video signal rising stepwise from a black level to a white level is FM recorded, an inversion phenomenon may occur in which the rising part of the white level of the demodulated signal falls to the black level. This is because the lower sideband is emphasized and the upper sideband is suppressed when the FM signal is recorded and reproduced, and as a result, the crossing point with the zero level of the FM signal disappears.

Conventionally, there is a method of suppressing the lower sideband by HPF (high-pass filter) etc. as one of the inversion prevention measures in the reproducing system, but the lower sidewave with good S / N is suppressed and the upper side with poor S / N is used. There is a problem that the S / N of the demodulated signal deteriorates to emphasize the wave and the frequency characteristic deteriorates. As a conventional method for solving these problems, for example, the 1977 National Conference of the Television Society of Japan 5-11
It is shown in "Prevention of reversal by double limiter and improvement of S / N".

FIG. 4 is a block diagram of this conventional FM demodulator with an inversion prevention function, where 401 is an input terminal of a reproduced FM signal, 402 is a carrier component of the input FM signal, and a lower sideband component HPF (High Pass Filter), which greatly attenuates
03 is a limiter that eliminates the amplitude difference of the input signal to obtain a sinusoidal output signal of constant amplitude, 404 is a phase corrector that corrects the phase, and 405 is a L that sufficiently attenuates the carrier component of the input FM signal.
PF (Low Pass Filter), 406 is phase corrector 404 and LPF405
A combiner that combines the outputs of and, 407 is a limiter that keeps the amplitude constant so that the FM demodulator that follows is not distorted in the demodulated output by the AM component, 408 is the FM demodulator, and 409 is the FM signal It is an output terminal.

In the conventional inversion prevention device configured as described above,
In the portion where the carrier component of the input FM signal is small and inversion is likely to occur, the carrier component is amplified by the limiter 403 until it has a predetermined amplitude (however, the S / N is deteriorated).
At the output of, only the carrier component is amplified and an FM signal that is difficult to invert is obtained. In the part where the carrier wave component of the input FM signal does not largely invert, the carrier wave component is almost not amplified by the limiter 603 (therefore S / N is not deteriorated) and has a predetermined amplitude. An FM signal approximately equal to the signal is obtained. Therefore, a demodulated signal that does not always cause inversion is output.

Problems to be Solved by the Invention However, in the above-mentioned configuration, the carrier component of the input FM signal is always amplified by the limiter 403 even in the portion where the carrier component is small and is liable to be inverted, and the carrier component is always carried by the limiter 403. Since the noise component near the frequency is also amplified and the lower side wave component is suppressed relatively, there is a problem that the S / N and frequency characteristics of the demodulated signal deteriorate in the portion where the carrier frequency component is small.

In view of the above point, the present invention has an object to provide an FM demodulation device which prevents the inversion phenomenon of an input FM signal and has extremely small deterioration in S / N and frequency characteristics due to the processing for the inversion prevention.

Means for Solving the Problems The present invention provides a hearing aid that receives an FM signal I and obtains an FM signal E that has been corrected so as not to cause an inversion phenomenon even when demodulated,
When the inversion phenomenon is detected by determining whether the demodulation signal A obtained by demodulating the FM signal I is within the predetermined level range by inputting the FM signals I and E and detecting the inversion phenomenon. Is the demodulated signal B obtained by demodulating the FM signal E or the demodulated signal C obtained from the FM signals I and E as the output signal D. In other cases, the
An FM demodulation device comprising: a demodulation processor that uses a demodulation signal obtained from the FM signal I as an output signal D.

Effect of the Invention With the above-described configuration, the present invention makes it possible to reliably detect a portion where inversion occurs when the input FM signal I is demodulated, and does not cause inversion although only the portion causing inversion causes deterioration of S / N or deterioration of frequency characteristics. The signal obtained by demodulating the FM signal E as a main component becomes the output demodulated signal, and the signal that is obtained by demodulating the input FM signal I becomes the demodulated signal in the part that does not cause inversion. It is possible to obtain a demodulated signal with extremely small deterioration in frequency characteristics.

First Embodiment FIG. 1 is a block diagram of an FM demodulator according to the first embodiment of the present invention. In FIG. 1, 101 is FM
A signal I input terminal, 102 receives the FM signal I as an input,
A compensator for compensating the level imbalance between the upper sideband and the lower sideband of the signal I or for outputting the FM signal E obtained by emphasizing the level of the carrier wave, 103 is a demodulation processor, and 104 is the FM An FM demodulator that receives the signal I as an input and outputs a demodulated signal A, a delay device 105 that receives the demodulated signal A as an input and causes a predetermined time delay, and a reference numeral 106 that receives the demodulated signal A as an input and is within a predetermined level range. Whether the FM signal E is input to the FM demodulator that outputs the demodulation signal B, and 108 is the delay unit 1
The demodulation signal A from 05 and the demodulation signal B are input, and a combiner controlled by the control signal and outputting a demodulation signal D obtained by combining (here, selecting one) 109 is a demodulation signal D It is an output terminal.

The operation of the FM demodulator of this embodiment having the above configuration will be described below. FIG. 2 is a waveform diagram for explaining the operation. In FIG. 2, when the signal shown in a is FM-modulated, an FM signal as shown in b is obtained, and when recording and reproducing, the upper sideband is suppressed and the lower sideband is emphasized. Become. Modulation signal (a in the figure)
In the case of abrupt change of, the intersection of the FM signal I and the zero level may disappear (indicated by an arrow in c).
Therefore, when the FM signal I input to the terminal 101 is demodulated by the FM demodulator 104, a demodulated signal A as shown by e is obtained. A signal that should originally be shown by a broken line is demodulated as a dropped signal as shown by a solid line, and an inversion phenomenon occurs.

Originally, the modulation signal is FM-modulated with the amplitude level limited within a certain level range so as not to cause modulation.
The demodulated signal also does not significantly exceed the predetermined level range. On the other hand, since the amplitude level of the portion where the inversion phenomenon occurs greatly exceeds this level range, the portion where the inversion phenomenon occurs can be easily detected by the level comparison judgment. Since the determination signal (shown in FIG. 5F) as the level determination result is not the entire period in which the inversion phenomenon occurs, the control signal indicating the period T _O including the vicinity of the period indicated by the determination signal is _output to the controller 106.
Is creating and outputting.

On the other hand, the FM signal I input to the corrector 102 suppresses the lower sideband and emphasizes the upper sideband to correct the imbalance, or the FM signal that does not cause the inversion phenomenon by enhancing the carrier component. It is output as E (Fig. 2d). The FM signal E is demodulated by the FM demodulator 107 and becomes a demodulated signal B having no inversion phenomenon. Since the lower sideband of the FM signal E having a good S / N is suppressed by the corrector 102, the S / N and frequency characteristics of the demodulated signal B are deteriorated as compared with the demodulated signal A. It shows in h. Demodulated signal A is delayed by 105
This causes a predetermined time delay and adjusts the timings of the demodulated signal B and the control signal. Demodulated signal A timing is adjusted, the B becomes the input of the combiner 108, the period T _O is controlled by the control signal is selected demodulated signal B, a period other than the period T _O is selected demodulated signal A is Synthesized demodulation signal D
Is obtained by the combiner 108 and output from the terminal 109.

As described above, according to this embodiment, the demodulated signal A is output via the synthesizer 108 during the period in which the signal A obtained by demodulating the input FM signal I does not cause the inversion phenomenon, and the demodulated signal A is inverted. In the period in which the inversion phenomenon occurs, the inversion phenomenon does not occur by outputting the signal B, which is obtained by demodulating the FM signal E in which the inversion does not occur by correcting the imbalance of the sideband to the FM signal I, through the combiner 108. Moreover, it is possible to obtain a demodulated signal with extremely little signal deterioration due to the inversion phenomenon prevention processing.

FIG. 3 is a block diagram of an FM demodulator showing a second embodiment of the present invention. In the figure, 301 is an input terminal of the FM signal I, 302 is the input of the FM signal I, corrects the level imbalance between the upper sideband and the lower sideband of the FM signal I, or A compensator for emphasizing the level and outputting the FM signal E, 303 is a demodulation processor, 304 is a delay device for inputting the FM signal I and generating a predetermined time delay, 305 is an input for the FM signal I, and the demodulation signal A is input. An FM demodulator for output, 306 is a controller for inputting the demodulated signal A, and for outputting a control signal generated by determining whether the demodulated signal A is within a predetermined level range, 30
Reference numeral 7 denotes the FM signal I and the FM signal E from the delay device 304.
A combiner which outputs an FM signal M obtained by combining (here, selecting one) under the control of the control signal as an input,
308 is an FM that receives the FM signal M as an input and outputs a demodulation signal D
A demodulator 309 is an output terminal of the demodulated signal D.

The operation of the FM demodulator of the second embodiment configured as described above will be described below. In FIG. 3, blocks having the same names as the blocks in FIG. 1 have the same functions, and the basic concept of processing is the same. That is, normally, the inversion phenomenon does not occur even if the demodulation signal is obtained by using the FM signal I that can obtain the demodulation signal with good S / N and frequency characteristics, and only when the FM signal I is demodulated and the inversion phenomenon occurs. The FM signal E is used to obtain the demodulated signal. The method of detecting the inversion phenomenon is also the same as that shown in the first embodiment, and the same control signal is obtained from the controller 306. In the first embodiment, the output demodulated signal is obtained by performing the synthesis in the state of the demodulated signal, but in the present embodiment, the FM signal synthesized by controlling the synthesizer 307 by the control signal in the state of the FM signal. Get M, and FM
The demodulator 308 demodulates to obtain a demodulated signal D. The delay device 304 is for matching the timing of the FM signals I and E.

As described above, according to the present embodiment, during the period in which the input FM signal I is demodulated and the inversion phenomenon does not occur, the FM is output via the combiner 307.
The FM signal I is input to the demodulator 308, and the FM signal E that does not cause inversion is input to the FM demodulator 3 through the combiner 307 only during the period when the inversion occurs.
By inputting to 08, the FM demodulator 308 can obtain a demodulated signal in which the inversion phenomenon does not occur and the signal deterioration due to the inversion phenomenon prevention processing is extremely small.

In the first embodiment, the composition of the synthesizer 108 is a switch circuit for selecting one of the two inputs, but when the two inputs are switched by the control signal, the output demodulation signal D of the output demodulation signal D is changed due to the difference in frequency characteristics of the two inputs. Discontinuity may occur. This discontinuous step is usually small and becomes less noticeable due to the de-emphasis (high frequency suppression) of the demodulation 5. Further, in order to reduce this step, the synthesizer 108 is not a simple switch circuit, but at the time of switching, a signal in which the mixing ratio is changed by using two input signals, an averaged signal, an interpolated signal, etc. are created and output. Conceivable. When the period T _O of the inversion phenomenon of the demodulated signal A is short, the period T _O signal is set to the demodulated signal A other than the period T _O.
A synthesizer configured to interpolate from and output is also conceivable. In this case, the corrector 102 and the FM demodulator 107 can be omitted.

Similarly, in the combiner 307 in the second embodiment, the FM signal M of its output causes discontinuity at the time of switching. Therefore, it is possible to consider a structure in which the mixing ratio of the input FM signals I and E is changed at the time of switching.

The block diagram is shown assuming that the FM demodulators shown in the first and second embodiments do not cause distortion in the demodulation output due to the AM component. However, if distortion occurs, the AM component before each FM demodulation It is obvious that a structure such as a limiter that removes is also acceptable.

Further, as shown in FIG. 2, the two input signals to the combiners 108 and 307 may be configured so that the timings of the control signal and the two input signals coincide with each other.

Although the above embodiments have been described as analog signal processing circuits, it is clear that they can be configured and realized by digital signal processing circuits using sampled and quantized FM signals and demodulated signals as input and output signals, and each block Since it is possible to eliminate the variation in the characteristics of, the processing can be performed with high accuracy.

EFFECTS OF THE INVENTION As described above, according to the present invention, it is possible to obtain a demodulated signal which does not cause inversion and has extremely small S / N deterioration and frequency characteristic deterioration due to inversion prevention processing. Is big.

[Brief description of drawings]

FIG. 1 is a block diagram of an FM demodulator of one embodiment of the invention, FIG. 2 is a waveform diagram for explaining the operation of the embodiment, and FIG. 3 is an FM demodulator of another embodiment of the invention. FIG. 4 is a block diagram of a conventional FM demodulator with an inversion prevention function. 101,301 …… FM signal input terminal, 102,302 …… Compensator, 10
3,303 …… Demodulation processor, 104,107,305,308 …… FM demodulator,
105,304 …… Delay device, 106,306 …… Controller, 108,307 ……
Combiner, 109, 309 ... Output terminal of demodulated signal.

Claims (4)

[Claims] 1. A compensator which receives an FM signal I as an input and obtains an FM signal E which is corrected so as not to cause an inversion phenomenon even when demodulated, and the FM signal I which receives the FM signals I and E as an input. The demodulation signal A obtained by demodulating the signal is determined whether or not it is within a predetermined level range, the inversion phenomenon is detected, and when the inversion phenomenon occurs, the demodulation signal B obtained by demodulating from the FM signal E is obtained. Or a demodulation processor that uses the demodulated signal C obtained from the FM signals I and E as the output signal D, and otherwise uses the demodulated signal obtained from the FM signal I as the output signal D. FM demodulator characterized by. 2. A demodulation processor receives an FM signal I as an input and outputs a demodulation signal A, and an FM demodulator which receives the demodulation signal A as an input and determines whether or not it is within a predetermined level range and inverts it. A controller for detecting a phenomenon, a second FM demodulator for inputting an FM signal and outputting a demodulation signal B, and a signal for receiving the demodulation signal A and the demodulation signal B as input and being controlled by a signal from the controller. The FM demodulator according to claim 1, further comprising a combiner that outputs a signal D obtained by selecting or combining one of the signals. 3. A demodulation processor receives a FM signal I as an input and outputs a demodulation signal A as a third FM demodulator, and receives the demodulation signal A as an input and determines whether or not it is within a predetermined level range. And an FM signal M obtained by inputting the FM signal I and the FM signal E and selecting one of the signals or combining the signals controlled by the signal from the controller. And a fourth FM which inputs the FM signal M and outputs a demodulated signal D
The FM demodulator according to claim 1, further comprising a demodulator. 4. An FM demodulator for demodulating an input FM signal I to obtain a demodulation signal A, and determining whether or not the demodulation signal A is within a predetermined level range to detect and control an inversion phenomenon. A controller that generates a signal, and is controlled by the control signal, outputs the FM signal A during a period when the inversion phenomenon is not occurring, and causes an inversion phenomenon of the FM signal A during the period when the inversion phenomenon is occurring. An FM demodulator, comprising: an interpolator / synthesizer that outputs an interpolated signal obtained by performing an interpolation using a signal of a non-existing portion.