JPH0724368B2 - Inversion prevention device - Google Patents

Description

The present invention relates to an inversion prevention device for preventing an inversion phenomenon in a narrow band FM signal.

[Conventional technology]

FIG. 6 is a block diagram of a conventional inversion prevention device disclosed in, for example, Japanese Patent Application Laid-Open No. 62-219372, in which 601 is an input terminal for a reproduced FM signal and 602 is an input FM signal. HPF (High Pass Filter) that passes the carrier wave component and greatly attenuates the lower sideband component, 603 is a limiter that eliminates the amplitude difference of the input signal and obtains a sinusoidal output signal of constant amplitude,
604 is a phase corrector that corrects the phase, 605 is an LPF (Low Pass Filter) that sufficiently attenuates the carrier component of the input FM signal, 60
6 is a combiner for combining the outputs of the phase corrector 604 and the LPF 605,
Reference numeral 607 is a limiter for making the amplitude constant so that the FM demodulator following the latter stage does not cause distortion in the demodulation output due to the AM component, and 608 is an output terminal for the FM signal.

In the conventional inversion prevention device configured as described above,
In the part 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 603 until it has a predetermined amplitude (however, S / N is deteriorated).
Only the carrier component is amplified in the output of the FM, which is difficult to reverse
The signal is obtained. In the part where the carrier component of the input FM signal does not largely invert, the carrier component is hardly amplified by the limiter 603 (thus the S / N does not deteriorate).
Since it has a predetermined amplitude, the combiner 606 can obtain an FM signal that is substantially equal to the input FM signal. Therefore, no inversion always occurs
The FM signal is output.

By the way, performing signal processing by digital technology has advantages such as circuit miniaturization, no adjustment, and high accuracy, and it is necessary to study a method of realizing a conventional analog part by a digital circuit even in an inversion prevention device. There is. FIG. 7 is a block diagram for directly implementing the inversion prevention device disclosed in Japanese Patent Laid-Open No. 62-219372 by a digital processing circuit.

This device uses digital FM signals, which are sampled and quantized FM signals, as input and output signals. In FIG. 7, 70
1 is input terminal of digital FM signal, 702 is HPF, 703 is HPF7
A limiter that makes the output amplitude of 02 a constant value, 704 is an amplitude detector that obtains an amplitude signal that is the amplitude of the output signal of the HPF 702, 705
Is a divider that divides the output signal of the HPF704 by the amplitude signal from the amplitude detector 704 to obtain a carrier component of constant amplitude.
F and 707 are the output of the limiter 703, that is, the output of the divider 705
708 is a digital FM synthesizer that synthesizes with the output of LPF706
This is a signal output terminal. Since the HPF702 and LPF706 can easily have a linear phase characteristic, a phase corrector is not necessary, and the limiter 607 in FIG. 6 is omitted because it is a circuit that is not directly related to inversion prevention.

The basic operation of the conventional digital inversion prevention device configured as described above is the same as in the case of FIG. 6, and therefore its explanation is omitted.

[Problems to be Solved by the Invention]

However, the above-mentioned configuration requires a divider having a circuit scale larger than that of the multiplier. Moreover, while the output signal of the LPF706 is a sideband component with a small amplitude, the HPF7
Since the output signal of 02 is a carrier component having a large amplitude, it is necessary to increase its data word length. Therefore, the divider 705,
There is a problem in that the circuit scale of the amplitude detector 704 becomes larger and it becomes difficult to realize the digital inversion prevention device.

In view of this point, the present invention has an object to provide an inversion prevention device having a small circuit scale, a low cost, and an easy realization.

[Means for Solving the Problems]

An inversion prevention device according to the present invention has a first discriminator which receives a quantized digital FM signal as an input and discriminates whether or not its absolute value is equal to or less than a first predetermined value, and the digital FM signal.
A sideband suppression filter that receives the signal as input and suppresses the upper and lower sidebands or the lower sideband, a subtracter that outputs the difference between the digital FM signal and the output of the sideband suppression filter, and a subtractor of this subtractor. A second discriminator for discriminating whether or not the absolute value of the output is a second predetermined value or more;
And a selector for selecting and outputting either the digital FM signal or the output of the sideband suppression filter according to the output result of the second discriminator.

[Action]

In the present invention, the first discriminator determines whether or not the absolute value of the digital FM signal is less than or equal to the first predetermined value,
A subtractor takes the difference between the digital FM signal and the output passed through the sideband suppression filter, and a second discriminator determines whether the absolute value of this difference is greater than or equal to a second predetermined value. , The FM of which the upper sideband and the lower sideband are unbalanced by selecting and outputting either the input digital FM signal or the sideband suppression filter output according to the output results of the first and second discriminators. The signal is output as it is as a digital FM signal in the part where the amplitude of the carrier component is not largely inverted, but the carrier component is relatively output in the part where the discriminator determines that the amplitude of the carrier component is small and causes inversion. It is possible to output a digital FM signal that has passed through a sideband suppression filter corresponding to the increase, and obtain a digital FM signal that does not cause inversion.

(Embodiment) FIG. 1 is a block diagram of an inversion prevention device according to an embodiment of the present invention. In FIG. 1, 1 is an input terminal of a digital FM signal, 3 is a sideband suppression filter that suppresses the upper and lower sidebands or the lower sideband of the digital FM signal,
Reference numeral 2 denotes a delay compensator that delays the digital signal by the same delay time as that of the signal output from the sideband suppression filter 3. The outputs of the delay compensator 2 and the sideband suppression filter 3 are quantized signals at the same time. Can be regarded as 4 is a subtracter for outputting the difference between the output of the delay compensator 2 and the output of the sideband suppression filter 3, and 5 is 1 if the absolute value of the digital FM signal output from the delay compensator 2 is less than a predetermined value D, so Otherwise, a discriminator that outputs 0, 6 is a discriminator that outputs 1 if the absolute value of the output of the subtractor 4 is greater than or equal to α, and 7 that outputs 0 if it is not AND gate, 8
Is a selector that selects the output of the delay compensator 2 when the output of the AND gate 7 is 0, and selects and outputs the output of the sideband suppression filter 3 when it is 1, and 9 is an output terminal of the digital FM signal is there.

Next, the operation of the inversion prevention device according to this embodiment will be described. Now, the output of the delay corrector 2 x (h), when the output of the sideband suppression filter 3 was x _F (h), may be set in the discriminator 5 and 6 in advance D, alpha becomes the value, | Only when x (h) | ≦ D and | x (h) −x _F (h) | ≧ α ..., the output of the sideband suppression filter 3 is selected and output. In other cases, the delay corrector 2 The output of is selected and outputted. If x (h) is the quantized waveform shown by the solid line in FIG.
In the vicinity shown by (n), the intersection with the zero level of the FM signal disappears, and if demodulated as it is, an inversion phenomenon occurs. The output obtained by passing x (h) through a bandpass filter that suppresses the upper and lower sidebands or a highpass filter that suppresses the lower sideband is the waveform shown by the broken line in FIG. In the vicinity of x _F (n) corresponding to x (n), the intersection with the zero level is restored and the waveform does not cause the inversion phenomenon even if demodulated. However, since the lower sideband with good S / N is suppressed, the inversion phenomenon does not occur when demodulated, but S / N
The demodulation output is bad for N.

Therefore, normally, the waveform of the solid line in FIG. 4 is used, but x
Only the portion where the intersection with the zero level near (n) disappears is used the waveform of the broken line near x _F (n). In other words, by demodulating with the FM signal shown in FIG. 5 (which is a diagram in which the quantized signal is smoothly connected by the solid line), if S / N is not extremely deteriorated,
Moreover, since the intersection with the zero level is restored, it is possible to obtain a demodulation waveform that does not cause the inversion phenomenon. We use conditions to achieve that. In other words, first of all, it is noted that there is no intersection at the part where it is determined that the intersection with the zero level has disappeared, but it has reached quite near the zero level. Therefore, a value ± D, which is relatively close to the zero level, is set, and a value close to the zero level that satisfies | x (h) | ≦ D is the first condition for the disappearance of the intersection with the zero level. And Next, in x _F (h) obtained by passing x (h) through the sideband suppression filter, the carrier wave component is emphasized and the sideband component is suppressed, so that the amplitude fluctuation of the FM wave is reduced, and as shown in FIG. As shown by the broken line, the amplitudes become uniform. Therefore, since the amplitude near x (n) where the intersection with the zero level disappears is originally small, the difference with the corresponding x _F (n), that is,
It is known that | x (n) −x _F (n) | has a relatively large value. Therefore, | x (h) −x _F (h) | is observed in advance, and the fact that it is equal to or greater than the predetermined value α is the second condition for the disappearance of the intersection with the zero level. As described above, when the first condition is satisfied and the second condition is satisfied, it is determined that the intersection with the zero level has disappeared, and x _F (h) is selected and output.

In FIG. 1, the quantized data input to the input terminal 1
The FM signal is input to the sideband suppression filter 3. Further, the FM signal is also input to the delay compensator 2, where the output x _F (h) of the sideband suppression filter 3 and the delay time are output together (x (h)). That is, x (h) and x _F (h) can be regarded as signals quantized at the same time. Therefore, x (h) is first input to the discriminator 5 and | x (h) | ≦
It is determined whether or not D, and the discriminator 5 determines that | x (h) | ≦ D
If so, 1 is output, and if not, 0 is output and input to one input of the AND gate 7. Also, x (h) and x
_F (h) is input to the subtractor 4, and the output of the subtractor 4 is input to the discriminator 6 to discriminate whether or not | x (h) −x _F (h) | ≧ α. The device 6 is | x (h) −x _F (h) | ≧ α
If so, 1 is output, otherwise 0 is output and input to the other input of the AND gate 7. The AND gate 7 outputs 1 to the selector 8 only when both inputs are 1, and in that case, the selector 8 selects the output x _F (h) of the sideband suppression filter 3 and outputs it to the output terminal 9. When the output of the AND gate 7 is 0, the selector 8 selects the output x (h) of the delay corrector 2 and outputs it to the output terminal 9. With the above operation, | x
If (h) ≦ D and | x (h) −x _F (h) | ≧ α, then x
This means that the operation of outputting _F (h) and x (h) otherwise is realized.

Further, the discriminator 5 can be realized by the configuration shown in FIG. 2, for example. First, input x (h) to the absolute value calculator 10 and input | x
(H) | is output. | x (h) | and the value D held in the ROM 12 are input to the subtractor 11 to calculate (| x (h) | -D) and the resulting code pit is output. That is,
The discriminator 5 can be realized by outputting 1 for negative and 0 for positive. Similarly, the discriminator 6 inputs (x (h) −x _F (h) to the absolute value calculator 13 as shown in FIG.
(H) -x _F (h) | is output. | x (h) −x _F (h) |
When ROM15 a value held α are input to the subtracter 14 to the (α- | x (h) -x F (h) |) calculated by the an output sign bit of the result. In other words, it can be realized by outputting 1 for negative and 0 for positive.

The values of D and α are values to be determined by the frequency allocation of the FM wave determined by the VTR method, the transmission characteristics of the FM transmission system and the frequency characteristics of the sideband suppression filter.

Further, in the above embodiment, the ROMs 12 and 15 are used for holding the data, but this does not need to be RO, other digital memories may be used, and the data may be held by a switch.

〔The invention's effect〕

As described above, according to the present invention, the first discriminator for discriminating whether or not the absolute value of the digital FM signal is less than or equal to the first predetermined value, and the upper and lower sidebands or A sideband suppression filter that suppresses the lower sideband, a subtracter that outputs the difference between the digital FM signal and the output of the sideband suppression filter as an input, and the absolute value of the output of this subtractor is the second predetermined value. A second discriminator for discriminating whether or not it is a value or more, and either the digital FM signal or the output of the sideband suppression filter is selected according to the output results of the first and second discriminators. Since it is provided with a selector for outputting, a digital FM signal which does not cause inversion can be obtained without using a divider, and an inversion prevention device having a small circuit scale and being inexpensive can be obtained.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram showing an inversion prevention device according to an embodiment of the present invention, and FIGS. 2 and 3 are block diagrams showing an example of a discriminator thereof, FIG. 4 and FIG. FIG. 5 is a waveform diagram of an FM wave for explaining the operation of the inversion prevention device according to one embodiment of the present invention, FIG. 6 is a block diagram showing a conventional inversion prevention device, and FIG. 7 is a diagram of the device of FIG. It is a block diagram which shows the inversion prevention apparatus which digitized the principal part. 2 ... Delay compensator, 3 ... Sideband suppression filter, 4 ...
Subtractor, 5,6 ... Discriminator, 7 ... AND gate, 8 ... Selector. In the drawings, the same reference numerals indicate the same or corresponding parts.

Claims (1)

[Claims] 1. A first discriminator for inputting a quantized digital FM signal and discriminating whether or not an absolute value of the signal is less than or equal to a first predetermined value, and the digital FM signal as an input. , A sideband suppression filter that suppresses the upper and lower sidebands or the lower sideband, a subtracter that outputs the difference between the digital FM signal and the output of the sideband suppression filter, and the output of the subtractor. A second discriminator for discriminating whether or not the absolute value is equal to or greater than a second predetermined value, and according to the output results of the first discriminator and the second discriminator,
An inversion prevention device, comprising: a selector that selects and outputs either the digital FM signal or the output of the sideband suppression filter.