JPH0673218B2 - Digital inversion prevention device - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an inversion prevention device 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, HPF (High
There is a method to suppress the lower side band by using (Pass Filter) etc., but the lower side wave with good S / N is suppressed and the upper side wave with poor S / N is emphasized, so the S / N of the demodulated signal deteriorates and the frequency characteristics There was a problem that As a conventional method for solving these problems, for example, it is shown in 1977 National Conference of the Television Society of Japan 5-11 "Prevention of reversal by double limiter and improvement of S / N".

FIG. 6 shows a block diagram of this conventional anti-reverse device. 601 is an input terminal of a regenerated FM signal, 602 is a HPF that allows a carrier component of the input FM signal to pass and a lower sideband component to be greatly attenuated. (High Pass Filter), 603 is a limiter that eliminates the amplitude difference between input signals and obtains a sinusoidal output signal with a constant amplitude, 604 is a phase corrector that corrects the phase, and 605 is an input FM.
LPF (Low Pass Filt) that sufficiently attenuates the carrier component of the signal
lter), 606 is a combiner that combines the outputs of the phase corrector 604 and the LPF 605, and 607 is a limiter that keeps the amplitude constant so that the FM demodulator that follows is not distorted by the AM component in the demodulated output, Reference numeral 608 is an FM signal output terminal.

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, the merit of digital signal processing is also noticed in consumer devices as seen in digital TVs, and it is necessary to study a method of realizing a device that has been realized by an analog circuit in the past with a digital circuit.

A configuration shown in the block diagram of FIG. 7 is conceivable for realizing the conventional inversion prevention device by a digital signal processing circuit. This device uses digital FM signals, which are sampled and quantized FM signals, as input and output signals. In FIG. 7, 701 is a digital FM signal input terminal and 702 is an HP
F and 703 are limiters that set the output amplitude of HPF702 to a constant value, 70
4 is an amplitude detector that obtains a vibration signal that is the amplitude of the output signal of the HPF 702, 705 is a divider that divides the output signal of the HPF 704 by the amplitude signal from the amplitude detector 704 to obtain a carrier component of constant amplitude, and 706 is LPF, 707 is the output of limiter 703 or divider
A combiner for combining the output of the 705 and the output of the LPF 706, and 708 is an output terminal of the digital FM signal. 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.

Since the basic operation of the conventional digital inversion prevention device configured as described above is the same as that of the case of FIG. 6, description thereof will be omitted.

Problems to be Solved by the Invention However, the above-mentioned configuration requires a divider having a larger circuit scale than 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 the above point, the present invention has an object to provide a digital inversion prevention device having a small circuit scale and easily realized.

Means for Solving the Problems In the present invention, the digital FM signals I and E are input signals,
A digital inversion prevention device as an output signal, which is a sideband suppression filter that suppresses both sidebands or lower sidebands of a digital FM signal I, and sidebands that emphasizes both sidebands or lower sidebands of the digital FM signal I. An emphasis filter, a carrier separation filter for extracting a carrier component of the digital FM signal I, a control signal generator for detecting an amplitude of an output signal of the carrier separation filter, and outputting a control signal substantially proportional to the amplitude, and the sideband Digital inversion provided with a multiplier that multiplies the output signal of the emphasis filter and the control signal, and a combiner that combines the two output signals of the sideband suppression filter and the multiplier to obtain the digital FM signal E It is a prevention device.

Operation The present invention has an input digital FM signal I
The signal obtained by multiplying the sideband component of the digital FM signal I by the control signal which is almost proportional to the amplitude of the carrier component and the carrier component of the digital FM signal I are combined to obtain the output digital FM signal E. . In the part where the amplitude of the carrier component of the digital FM signal I is large and inversion does not occur, it is digital
The FM signals I and E are almost equal, and the carrier component is relatively increased by attenuating the sideband component in the portion where the carrier component amplitude is small and inversion is likely to occur, or the imbalance between the upper sideband and the lower sideband is improved. Therefore, the digital FM signal E without inversion is always obtained.

Embodiment FIG. 1 is a block diagram of a digital inversion prevention device according to an embodiment of the present invention. In FIG.
101 is a digital FM signal I input terminal, and 102 is a digital
Sideband suppression filter that suppresses both sidebands or lower sidebands of FM signal I, 103 is a sideband enhancement filter that enhances both sidebands or lower sidebands of digital FM signal I, and 104 is a carrier component of digital FM signal I Carrier wave separation filter for extracting the, 105 is a control signal generator that detects the amplitude of the output signal of the carrier wave separation filter 104, and outputs a control signal substantially proportional to the amplitude, 106 is the control signal at the output of the sideband enhancement filter 103 Numeral 107 is a multiplier for multiplying two side output signals of the sideband suppression filter 102 and the multiplier 106 to obtain a digital FM signal E which is an output signal.

The operation of the digital inversion prevention device of the present embodiment configured as described above will be described below.

A control signal (output of the control signal generator 105) substantially proportional to the amplitude of the carrier component of the input digital FM signal I (output of the carrier separation filter 104) is supplied to the digital FM signal I as both sideband components or lower sideband components (sideband). The signal obtained by multiplying the output of the wave enhancement filter 103) (the output of the multiplier 106) and the carrier component of the digital FM signal I or the carrier component and the upper sideband component (the output of the sideband suppression filter 102) are combined. Output digital FM signal E is obtained.

In the part where the carrier component of the digital FM signal I does not largely invert, the control signal also becomes large and the output digital FM
The signal E is almost equal to the input digital FM signal I, and the S / N and frequency characteristics do not deteriorate even if it is inverted. In addition, the control signal is also small in the portion where the carrier component of the digital FM signal I is small and inversion is likely to occur.
Both sidebands or lower sidebands of the FM signal E are attenuated and the carrier component becomes relatively large, or the imbalance between the upper sideband and the lower sideband is improved, so the output digital FM signal E is inverted. Absent. Therefore, the digital FM signal E does not always invert.

In the present embodiment, the divider 705, which is required when the conventional example is digitally processed (FIG. 7), is unnecessary, and the multiplier 106 is required instead. If the same data word length is compared, the multiplier has a smaller circuit scale than the divider, and the sideband has a smaller amplitude than the carrier wave. Therefore, the data word length of the input signal of the multiplier 106 (output signal of the sideband enhancement filter 103). Is smaller than the input signal of the divider 705, so that the circuit scale becomes smaller.

FIG. 2 shows a block diagram of an example of the control signal generator. 201 is a carrier wave component input terminal, 202 is a 90 ° phase shifter that changes the phase of the input signal and outputs two signals X and Y having a phase difference of 90 °, and 203 and 204 are squarers that square the input and output. , 205 is an adder that adds two inputs, 206 is a square rooter that obtains the square root of the input, 207 is a control signal that is approximately proportional to the input, that is, a function value with a differential coefficient of 0 or more in the input / output characteristic (control signal value) Is a signal converter for generating.

The operation of this control signal generator will be described below. Input terminal
If the carrier wave component at 201 is A · cos w · t (A, w, t are amplitude, angular frequency, and time, respectively), the two output signals X and Y of the 90 ° phase shifter are A · cos, respectively. (Wt +
θ), A ・ sin (w ・ t + θ) (where θ is a constant)
The output of Kaihei 206 That is, the amplitude A of the carrier component is obtained. Signal converter 20
7 outputs a control signal C which is almost proportional to the amplitude A which is an input. FIG. 3 shows an example of the input / output characteristics of the signal converter.
(Characteristics are not limited to this, various characteristics can be easily realized because it is a ROM.) If the 90 ° phase shifter is expressed by Z conversion, (However, i, n: integer, bi: coefficient) It is necessary for the digital filter to have a flat frequency-amplitude characteristic within the required band, but the required band is as narrow as about 2 MHz, so it can be realized with a simple configuration. The square devices 203 and 204, the square root detector 206, and the signal converter 207 can be easily realized by a semiconductor memory ROM (Read Only Memory). In this case, the square root detector 206 and the signal converter 207 can be realized by one ROM.

Further, the sampling frequency s in the signal processing is set to 4 of the carrier frequency of the FM signal (in a domestic VTR, it is within the range of about 3.5 MHz to 5.5 MHz, and the frequency deviation is about 1 to 1.2 MHz).
Roughly equal to or slightly higher (18 MHz in this embodiment)
And Since this frequency is an integer multiple of the line frequency of NTSC, PAL, SECAM3 system, it is also a frequency suitable for signal processing after FM demodulation. ) By setting, it is possible to simplify the configuration of various filters 102, 103, 104, 90 ° phaser 202 which are digital filters.

For example, the sideband suppression filter 102, the sideband enhancement filter 1
03 is Z conversion expression Or Such a simple configuration (not limited to this configuration) can also be realized. These frequency characteristics are shown in Fig. 4,
It is shown in FIG. In each figure, a, b, and c respectively represent frequency characteristics of the sideband enhancement filter 103, the sideband suppression filter 102, and the entire digital inversion prevention device.

In addition, the sideband suppression filter 102, the sideband enhancement filter 10
3. The carrier separation filter 104 can also share a part or most of the circuit, which further reduces the circuit scale.

As a configuration example of a 90 ° phase shifter, Can be given. However, in this example, since the gain is not 1, it is necessary to take measures such as correcting the ROM data of the squarer in advance.

As described above, according to the present embodiment, the sideband component is controlled by the multiplier with the control signal that is substantially proportional to the amplitude of the carrier component, and the sample frequency is set to approximately four times the carrier frequency of the FM signal. As a result, the circuit scale of the digital inversion prevention device can be reduced.

As the amplitude detection method in the control signal generator, other than the method shown in FIG. 2, a method in which the input signal is squared or made into an absolute value and the amplitude value is obtained through the LPF is also conceivable. It can also be used as an out detection output.

Further, if the input signal of the digital inversion prevention device has the double sideband component or the lower sideband component suppressed in advance, the sideband suppression filter may be omitted.

As described above, according to the present invention, the circuit scale can be reduced, so that the digital inversion prevention device can be easily realized and its practical effect is large.

[Brief description of drawings]

FIG. 1 is a block diagram of a digital inversion prevention device according to an embodiment of the present invention, FIG. 2 is a block diagram of a control signal generating circuit, FIG. 3 is an input / output characteristic diagram of a signal converter, and FIGS. 6 is a frequency characteristic diagram of each part, FIG. 6 is a block diagram of a conventional inversion prevention device, and FIG. 7 is a block diagram of a digital inversion prevention device obtained by digitalizing the main part of FIG. 102 ... Sideband suppression filter, 103 ... Sideband enhancement filter, 104 ... Carrier separation filter, 105 ... Control signal generator, 106 ... Multiplier, 107 ... Combiner.

Claims (2)

[Claims] 1. A digital inversion prevention device using digital FM signals I and E, which are sampled and quantized FM signals, as an input signal and an output signal, respectively. A sideband suppression filter for suppressing waves, a sideband enhancement filter for enhancing both sidebands or lower sidebands of the digital FM signal I, a carrier separation filter for extracting a carrier component of the digital FM signal I, and the carrier separation filter. A control signal generator that detects the amplitude of the output signal and outputs a control signal that is substantially proportional to the amplitude; a multiplier that multiplies the output signal of the sideband enhancement filter by the control signal; and the sideband suppression filter 2 with the multiplier
A digital inversion prevention device comprising: a synthesizer for synthesizing an output signal to obtain a digital FM signal E which is an output signal. 2. The sampling frequency is 4 of the carrier frequency of the FM signal.
The digital inversion prevention device according to claim 1, which is approximately equal to twice.