JPS61247109A - Low carrier fm demodulator - Google Patents

Abstract

PURPOSE:To eliminate a spurious wave component by adjusting and subtracting amplitude and phase of output signals of a 2-multiplier multiplying an input signal from an output signal of a digital demodulator. CONSTITUTION:A reproduced input signal is fed to a pulse counter 1 and a 2-multiplier 2. A demodulation signal fP, a spurious wave 2fC and its side bands are generated at the counter output. On the other hand, 2fC and its side bands are generated at the output of the 2-multiplier 2. Thus, the output of the 2-multiplier 2 is subjected to amplitude and phase adjustment by an amplitude compensation device 3 and a delay compensation device 4 and the result is subtracted from the output of the pulse counter 1 so as to eliminate the spurious wave.

Description

[Detailed Description of the Invention] (Technical Field) This invention relates to a demodulation device for low carrier FM recording and reproduction used in a video magnetic recording and reproduction device, and eliminates unnecessary waves generated in 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, particularly recording, the signal is frequency modulated (FM) as is well known, and moreover, a zero cross method is employed. To achieve this method, a sine wave input (FIG. 8(a)), for example, is limited (FIG. 8(b)) and recorded in the form of a square wave.

In FIG. 8(b), + and - indicate that the directions of magnetization on the tape recording are opposite to each other.

When a signal is reproduced by a magnetic head from a medium recorded in this manner, the reproduced input signal is distorted and becomes close to a sine wave. In order to obtain an FM demodulated signal from this, in order to faithfully reproduce the zero crossing point, a pulse count demodulator (
Figure 2) is used. The reproduced input signal entering the demodulator is again applied to the limiter 11 as shown in FIG. 8(b) and is made into a substantially square wave. Here, the signal is divided into two parts, one of which is passed through a delay device 12 having a minute delay time (τ) (Fig. 8 (0)), and an exclusive OR circuit 18 with both signals as input. Pass. Then, the output becomes as shown in FIG. 8(d).

Now, when the carrier wave frequency f0 is FM modulated by the modulation signal fp during magnetic recording, the frequency spectrum of the reproduced input signal becomes as shown in Figure 4, while the frequency of the output of the pulse counter, that is, the output of the exclusive OR circuit in Figure 2, is As shown in Figure 6, the spectrum consists of the demodulated signal f and two unnecessary waves.
f0 and its sidebands (zf −2f, z
f - f, zfo+fp c p
cp etc.) occurs.

Waves above the highest frequency ffn of the baseband can be removed by the sharp cutoff low-pass filter 4 using the characteristics shown by the broken line in Figure 6, but the unnecessary waves 2fo-2fp generated in the low-pass filter within fm cannot be 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 2 fo - 2fp higher than fp and remove it with the low-pass filter 14, the carrier frequency f0 is set to approximately the highest frequency fIn of the baseband. I had no choice but to increase it by more than 1.5 times.

Here, it is sufficient to consider only the spectrum 2 fo - 2 fp as unnecessary waves. That's f. > Spectrum 2 from fIn
fo-fp never becomes less than fm, and the spectrum 2f −+1f , 2f −4f ,...
This is because the amplitude of cp cp fp is considerably smaller than that of cp cp fp.

(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 carrier frequency is set closer to or equal to the highest frequency of the signal's baseband than before.

That is, the low carrier FM demodulator of the present invention
A multiplier, an amplitude adjuster, a phase adjuster, and a subtraction means are connected in parallel to a counting demodulator that demodulates a modulated signal,
By adjusting and subtracting the amplitude and phase of the output signal of the doubler from the output signal of the counting demodulator,
The present invention is characterized in that 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, an unnecessary wave 2f0, and its sideband as shown in FIG. On the other hand, 2f0 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 also has the same amplitude ratio between 2fo and the sideband.

Therefore, by appropriately adjusting the amplitude and phase of the output of the doubling circuit using the amplitude compensator 8 and the delay compensator 4, and subtracting it from the pulse counter output, gf-2fp in fIn is calculated.
It is possible to remove unnecessary waves including.

The low-pass filter 6 in FIG. 1 has a high-order spectrum 8X2f0,
5 X2f0. This is a filter for removing things such as... Further, the characteristics of the doubler are required to be able to sufficiently suppress the fundamental wave f0 and to have flat frequency characteristics from a low frequency range of about the lower side wave of the fundamental wave to a high frequency range of about 4 times the fundamental wave. 2
Circuits that implement multiplier circuits include diode multipliers, double-balanced differential amplifiers, and logarithmic multipliers.

(Effects of the Invention) By implementing the present invention, unnecessary waves generated by a pulse counter demodulator can be effectively removed. It is possible to set the frequency down to the highest frequency fm of the band, thereby reducing the recording band for magnetic recording and reproduction. For example, F which used to be affll as shown in Figure 6
This has the effect of reducing the M recording transmission band to 2.5 fm as shown in FIG. Here, the modulation index is set to 0.25.

Furthermore, in the present invention, since unnecessary waves generated by the pulse counter type demodulator are removed, the low-pass filter after the Moire canceller has the harmonic components 8X2f, 5X of 2f0.
A low-pass filter with good CC and phase characteristics can be used as long as it can remove 2f, . . . . This also improves the high frequency characteristics of the demodulated signal.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of a configuration example of an unnecessary wave removal circuit including a pulse counter demodulator according to the present invention, and FIG. 2 is a block diagram showing the configuration of the main parts of a conventional pulse counter demodulator. Figure 8 is a diagram of waveforms passing through each part of the pulse counter type demodulator, (a) is a reproduction input sine waveform diagram, (b) is a waveform diagram after the limiter, and (0) is a waveform diagram after the delay line (τ). Figure, (d)
are the pulse waveforms after the exclusive OR circuit Figure 1 Figures 1 and 5 are the frequency spectra of the reproduction input signal of the pulse counter demodulator and the signal after the exclusive OR circuit, respectively; Figures 6 and 7 are the frequency spectra of the signals after the exclusive OR circuit. 7M according to the conventional and the present invention, respectively.
FIG. 3 is a diagram showing transmission bands. 1...Pulse counter 2...2 multiplier 8...
Amplitude compensator 4...Delay compensator 6...Subtractor 6, 14-...Low pass filter 11...
...Limiter 12...Delay line 18...Exclusive OR circuit

Claims (1)

[Claims] 1. A counting demodulator that demodulates a low carrier FM modulation signal,
A doubler, an amplitude adjuster, a phase adjuster, and a subtraction means are connected in parallel, and from the output signal of the counting demodulator,
A low-carrier FM demodulator, characterized in that unnecessary wave components generated in the counting demodulator are canceled out by adjusting and subtracting the amplitude and phase of the output signal of the doubler. 2. The low carrier FM demodulation device 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.