JPH027203B2 - - Google Patents

Description

[Detailed Description of the Invention] (Technical Field) This invention relates to a demodulation device for low carrier FM recording and playback used in a video magnetic recording and playback device, which removes unnecessary waves generated by a pulse count type demodulation mechanism. By doing so, the carrier frequency is set closer to or equal to the highest frequency of the baseband of the signal, thereby reducing the recording band.

(Prior Art) FIG. 2 shows the configuration of the main parts of a conventional pulse count type demodulator for a video magnetic recording and reproducing apparatus. In the case of magnetic recording and reproduction, especially recording, the signal is frequency modulated (FM), as is well known, and a zero cross method is employed. To achieve this scheme, an input, for example a sine wave (FIG. 3a), is placed in a limiter (FIG. 3b) and recorded in the form of a square wave. + and - in FIG. 3b indicate that the directions of magnetization on the tape recording are opposite to each other.

When a signal is reproduced from a medium recorded in this manner by a magnetic head, the reproduced input signal is rounded and becomes close to a sine wave. To obtain an FM demodulated signal from this, a pulse count demodulator (see FIG. 2), for example, is used to faithfully reproduce the zero-crossing points. The reproduced input signal entering the demodulator is again shown in Fig. 3b.
The signal is applied to the limiter 11 as shown in FIG. Here, the signal is divided into two parts, and one remains as is.
The other signal is passed through a delay device 12 having a minute delay time (τ) (FIG. 3c), and then passed through an exclusive OR circuit 13 to which both signals are input. Then, the output is shown in Figure 3 d.
become that way.

Now, during magnetic recording, the carrier wave frequency f _c is modulated as a signal
When FM modulated with f _p , the frequency spectrum of the reproduced input signal is as shown in Figure 4, while the frequency spectrum of the output of the pulse counter, that is, the output of the exclusive OR circuit in Figure 2, is as shown in Figure 5. The demodulated signal f _p and unnecessary waves 2f _c and their sidebands (2f _c −2f _p , 2f _c −f _p , 2 _fc +f _p , etc.) are generated.

The highest baseband frequency f _n or higher can be removed by the sharp cut-off low-pass filter 14 using the characteristics shown by the broken line in Figure 5, but the unnecessary waves 2f _c −2f _p generated within the low-pass filter within f _n are removed. This caused deterioration in the image quality of the demodulated signal.
Therefore, in order to keep the image quality deterioration of the demodulated signal below the allowable value, that is, to keep the unnecessary wave spectrum 2f _c −f _p always higher than f _p and remove it by the low-pass filter 14, the carrier frequency f _c is set to the highest of the baseband. The frequency had to be increased to approximately 1.5 times or more than f _n .

Here, we only need to consider the spectrum 2f _c −2f _p as unnecessary waves. It is spectrum 2f _c from f _c > f _n
−f _p will never be less than f _n , and the spectrum
This is because the amplitudes of 2f _c −3f _p , 2f _c −4f _p , ... are considerably smaller than f _p .

(Summary of the Invention) An object of the present invention is to eliminate the above-mentioned conventional drawbacks, and to eliminate unnecessary waves generated within the highest frequency of the baseband of a modulated signal using a pulse count demodulator with a simple configuration. The frequency is brought closer to the highest frequency of the signal baseband than before,
or set equal.

That is, the low carrier wave FM modulation device of the present invention comprises a counting type demodulator for demodulating a low carrier wave FM modulation signal, a doubler, an amplitude adjuster, a phase adjuster, and a subtracting means are connected in parallel, and the counting type demodulator demodulates the low carrier wave FM modulation signal. By adjusting the amplitude and phase of the output signal of the doubler and subtracting it from the output signal of the multiplier, unnecessary wave components generated in the counting demodulator are canceled out and removed.

(Embodiment) FIG. 1 shows the configuration of an embodiment of an unnecessary wave removal circuit (hereinafter referred to as a Moire canceller) according to the present invention. When the frequency spectrum of the reproduced input signal is shown in FIG. 4, the pulse counter output generates a demodulated signal f _p , an unnecessary wave 2f _c , and its sideband as shown in FIG. On the other hand, 2f _c and its sideband are generated from the frequency spectrum of the output of the doubler 2 connected in parallel with the pulse counter. This has the same frequency as the unnecessary wave generated by the pulse counter, and the amplitude ratio of 2f _c and the sideband is also the same. Therefore, by appropriately adjusting the amplitude and phase of the output of the doubling circuit using the amplitude compensator 3 and delay compensator 4 and subtracting it from the pulse counter output, unnecessary waves including 2f _c −2f _p in f _n can be removed. can do.

The low-pass filter 6 in FIG. 1 is a filter for removing odd harmonic spectra 3×2f _c , 5×2f _c , . . . related to 2f _c included in the pulse counter output. Further, the characteristics as a doubler are required to be able to sufficiently suppress the fundamental wave f _c and to have flat frequency characteristics from reduction of the lower side wave of the fundamental wave to a high frequency range of about four times the fundamental wave. There are diode multipliers, double-balanced differential amplifiers, logarithmic multipliers, and the like as circuits that implement double-multiplier circuits.

(Effects of the Invention) By implementing the present invention, unnecessary waves generated by a pulse counter demodulator can be effectively removed, so that the carrier frequency f _c for recording can be changed from the video signal without deteriorating the demodulated image quality. It can be set as low as the highest baseband frequency f _n , and the recording band for magnetic recording and reproduction can be reduced. For example, the conventional FM recording transmission band of _3fn as shown in Figure 6,
As shown in Fig. 7, it has the effect of reducing it to 2.5f _n .
Here, the modulation index is set to 0.25.

Furthermore, in the present invention _{, unnecessary waves generated by the pulse counter type demodulator are removed, so the low-pass filter after the Moire canceller removes the odd harmonic components 3×2f c ,} 5×2f _c , etc. of 2f c A low-pass filter with good phase characteristics can be used. This also improves the high frequency characteristics of the demodulated signal.

[Brief explanation of drawings]

FIG. 1 is a block diagram of a configuration example of an unnecessary wave removal circuit including a pulse counter type demodulator according to the present invention.
Figure 2 is a block diagram showing the configuration of the main parts of a conventional pulse counter type demodulator, and Figure 3 is a waveform diagram passing through each part of the pulse counter type demodulator, where a is a reproduced input sine waveform diagram, and b is the waveform diagram after limiter,
c is a waveform diagram after the delay line (τ), d is a pulse waveform diagram after the exclusive OR circuit, and Figures 4 and 5 are the reproduced input signal of the pulse counter demodulator and after the exclusive OR circuit, respectively. frequency spectrum of the signal,
FIGS. 6 and 7 are diagrams showing FM transmission bands according to the prior art and the present invention, respectively. 1...Pulse counter, 2...2 multiplier, 3...
...Amplitude compensator, 4...Delay compensator, 5...Subtractor, 6, 14...Low pass filter, 11...Limiter, 12...Delay line, 13...Exclusive OR circuit.

Claims (1)

[Claims] 1 A multiplier, an amplitude adjuster, a phase adjuster, and a subtraction means are connected in parallel to a counting demodulator that demodulates a low carrier FM modulation signal, and the output of the counting demodulator is A low carrier wave FM demodulation device characterized in that unnecessary wave components generated in the counting type demodulator are canceled out and removed by adjusting and subtracting the amplitude and phase of the output signal of the doubler from the signal. 2. The low carrier FM demodulator according to claim 1, wherein the carrier frequency of the low carrier FM modulation signal is set to the highest frequency of the baseband signal.