JPS6124841B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a demodulation method for angle-modulated wave signals,
To provide a demodulation method that can arbitrarily select and control the frequency characteristics of a demodulated signal according to the input level of an angle-modulated wave signal to be demodulated, and can perform demodulation while keeping the output level of the demodulated signal substantially constant in the demodulation frequency band. The purpose is to

FIG. 1 shows a block system diagram of an example of a demodulation system for angle-modulated wave signals, which was previously proposed by the applicant in Japanese Patent Application No. 50-98499. In the figure, the angle modulated wave signal input from the input terminal 1 is transmitted to the amplifier 9, which will be described later.
The output is supplied as a control error signal to a variable band filter 2 whose center frequency is controlled. The angle-modulated wave signal output from the variable band filter 2 is applied to a first-order differentiator 3, where it is first-order differentiated. The phase characteristics between the input and output of the first-order differentiating circuit 3 maintain a constant phase difference of approximately 90° within the frequency band of the angle-modulated wave signal. The output signal of the first-order differentiating circuit 3 is waveform-shaped by an amplitude limiting amplifier 4 and then applied to a phase comparator 7.

On the other hand, the angle-modulated wave signal from the input terminal 1 is also supplied to a second-order differentiating circuit 5, where it is second-order differentiated and then supplied to an amplitude limiting amplifier 6. Here, the phase characteristics between the input and output of the second-order differentiator circuit 5 are approximately
A constant phase difference of 180° is maintained. The signal whose waveform is shaped and taken out by the amplitude limiting amplifier 6 is applied to the phase comparator 7, where the phase is compared with the output signal of the amplitude limiting amplifier 4, and error signals with different levels are generated according to the respective phase differences. It is said that

The error signal extracted from the phase comparator 7 is passed through a loop filter 8 that removes the carrier component.
Feedback is applied to the variable band filter 2 through the amplifiers 9, and the center frequency of its pass frequency band is variably controlled. The circuit consisting of the variable band filter 2, the first-order differentiator 3, the amplitude limiting amplifier 4, the phase comparator 7, the loop filter 8, and the amplifier 9 constitutes a single tracking loop, and during tracking, the center of the variable band filter 2 is The frequency automatically tracks the input angle modulated wave signal frequency. As a result, a control error signal (demodulated signal) is obtained at the output of the amplifier 9, and this signal is removed from a minute carrier component by the low-pass filter 10, and the terminal 1
1 is output as a more complete demodulated signal.

The frequency characteristics or tracking range or demodulation sensitivity of the demodulated signal depends on the loop gain. If an analog multiplier is used in the phase comparator 7,
The conversion gain changes depending on the level of the angle-modulated wave signal applied to the phase comparator 7, and the loop gain of the above-mentioned tracking loop changes.

By the way, when playing a multi-channel record in which the multi-channel audio signal is composed of a direct wave signal and an angle-modulated wave signal, and these are multiplexed and recorded in one sound groove, the signal is transmitted from the record sound groove. When picking up, due to the influence of the radius of curvature of the tip of the pick-up cartridge, tracing distortion increases in the high frequency range of the direct wave signal, and as a result, the harmonic components of the direct wave signal are interference occurred,
A phenomenon occurs in which the loop becomes locked to the harmonic components of the direct wave signal.

That is, when playing a multi-channel record, there is interference due to the above-mentioned interference, a decrease in the angle-modulated wave signal level due to interference, a decrease in the angle-modulated wave signal level due to deterioration of the tracing condition, and a decrease in the angle-modulated wave signal level due to record wear. This can frequently occur.

Therefore, the demodulation method proposed above uses the first-order differentiator circuit 3, the second-order differentiator circuit 5, etc.
It has a great suppression effect on noise components (interference waves) existing in the lower sideband of the angle-modulated wave signal and noise components (interference waves) close to the lower sideband. By using a multiplier, the tracking (locking range in the demodulation method using PLL) is automatically controlled to become narrower in response to the decrease in the level of the angle-modulated wave signal, thereby reducing the deterioration of the signal-to-noise ratio of the demodulated signal. It has a number of features, such as being able to significantly improve the occurrence of abnormal noise and eliminating the loss of lock phenomenon for angle modulated wave signals with excessive frequency deviations, which is seen in conventional PLL demodulation systems. In this way, the passing frequency band of the tracking loop is determined according to the level of the angle-modulated wave signal, and it goes without saying that the wider the band width is selected, the more it is affected by noise, and the narrower the band width is selected, the less it is affected.

However, in the above proposed method, the output terminal 11
The relationship between the demodulated signal output level outputted to the amplifier 9 and the loop gain is as shown in Figure 2. When the amplification degree of the amplifier 9 increases, the loop gain increases and the demodulated signal level increases, and when the amplification degree decreases, the demodulated signal output Power level also decreases. In addition, when the loop gain of the above one-round tracking loop becomes large, the passing frequency band (tracking range) of the input angle-modulated wave signal increases.
Since it has the characteristic of expanding, the frequency band of the demodulated signal is expanded as shown in FIG.

Therefore, in the demodulation method proposed above, when a low-level angle-modulated wave signal comes in, the loop gain is lowered and the tracking range is narrowed to reduce and suppress the interference noise, and the demodulated signal level simultaneously decreases. However, if this angle-modulated wave signal level frequently decreases, there is a problem in that cross-modulation distortion occurs. Further, in general, when a demodulated signal is subjected to arithmetic processing and separated into each channel, fluctuations in the level of the demodulated signal within the demodulated frequency band lead to deterioration of separation, which is a problem.

The present invention solves the above problems, and one embodiment thereof will be described below with reference to FIGS. 4 to 7.

FIG. 4 shows a specific circuit diagram of an embodiment of the method of the present invention, and FIG. 5 shows a specific circuit diagram of an example of the main part of the method of the present invention. In FIG. 4, the same parts as in FIG. 1 are given the same reference numerals, and their explanations will be omitted. In FIG. 4, the error signal voltage E ₁ output from the amplifier 9
is divided into two, one is applied to a loop gain adjuster (level setter) 12 constituted by a variable resistor, and the other is applied to an arithmetic circuit 13. The error signal voltage set to an appropriate level by the loop gain adjuster 12 is amplified by the amplifier 14, and then
It is supplied to the variable band filter 2 as a control error signal, and on the other hand, it is supplied to the arithmetic circuit 13 as an error voltage _E2 .

In the arithmetic circuit 13, the error signal voltage _E1 outputted from the amplifier 9 and the error signal voltage _E2 outputted from the amplifier 14 are each subjected to arithmetic processing.

This calculation process will be explained below.

Set the loop gain of amplifier 9 to A, and the gain (feedback coefficient) of loop gain adjuster (level setter) 12.
β, angle modulated input signal ei, amplifiers 9, 14
When the output signals of are e1 and e2, respectively, A(ei-β×e1)=e1, that is, e1/ei=A/(1+β×A).

From the above equation, it can be seen that if the gain E2 (corresponding to βe1) by the loop gain adjuster 12 corresponding to β increases, E1 (corresponding to e1) decreases.

This will be explained next using specific numbers.

When the loop gain A is A=1 and the gain β of the loop gain adjuster 12 is β=0.5, e1=0.666ei, e2=0.333ei Therefore, e1+e2=0.999ei (corresponding to E3) β= When 0.1, e1=0.90909e1, e2=0.0909e1 Therefore, e1+e2=0.9999e1 (corresponds to E3) When β=0.01, e1=0.99009e1, e2=0.0099e1 Therefore, e1+e2=0.99999e1 (E3 Therefore, it can be seen from the above explanation that (e1+e2), that is, E3, is constant regardless of β. Therefore, regardless of the change in the loop gain of the tracking loop that goes around, it is supplied as a constant error signal voltage to the low-pass filter 10, where the carrier component is removed and a demodulated signal with a high signal-to-noise ratio is obtained. can get. That is, the voltages E ₁ and E ₂ change with the same polarity depending on the input angle modulated wave signal frequency of the input terminal 1, and the output voltage E ₃ of the arithmetic circuit (consisting of an adder circuit) 13 also changes according to the input angle modulated wave signal frequency. The level changes according to the modulated wave signal frequency, and a demodulated output is obtained at the output terminal 11.

Therefore, a loop gain adjuster (level setter) 12 composed of a variable resistor, an arithmetic circuit 1
3 and amplifier 14 each constitute an important part of the system of the invention.

The operation of the variable band filter 2 will be explained with reference to FIG. 5. In the figure, A ₁ is an amplifier, its non-inverting input terminal is grounded, and its output terminal has a resistance value R ₃
is connected to its inverting input terminal via a resistor 17. 16 is a field effect transistor (hereinafter abbreviated as FET) as a control element, its source is grounded, and its drain is connected to a resistor 1 with a resistance value R ₁ .
5 to input terminal 1, while the capacitance value
It is connected to the inverting input terminal of amplifier A ₁ via capacitor 19 of C ₂ . In addition, the connection point between the resistor 15 and the capacitor 19 is the capacitor 1 with a capacitance value of C ₁ .
8 to the output terminal 21.

In the above configuration circuit, when a control error signal voltage is applied from the amplifier 14 to the gate and source of the FET 16 via the input terminal 20, the drain-source resistance _R2 of the FET 16 is variably controlled.
The center frequency F ₀ of this variable band filter 2 is expressed by the following equation.

Here, if we select R ₁ ≫ R ₂ , becomes. Therefore, it can be seen from equation (2) that by variably controlling the drain-source resistance R ₂ of the FET 16, the center frequency F ₀ of the variable band filter 2 can be variably controlled. Therefore, if the error signal voltage E ₂ applied to the gate of the FET 16 increases, the center frequency control range of the variable band filter 2 will inevitably be expanded, and the tracking range will be expanded. In other words, the demodulation frequency band is expanded.

Next, the operation of the main parts of the system of the present invention will be explained. However, it is assumed that the input angle modulated wave signal frequency is an instantaneous constant frequency. While the variable band filter 2 is tracking, this tracking loop that goes around is in a kind of negative feedback state, so even if a level fluctuation occurs due to the loop gain adjuster 12, the tracking loop will not change. Maintain a certain relationship. In other words, when the error signal voltage E ₂ increases, the input voltage E ₁ of the loop gain regulator 12 decreases, and the voltages E ₁ and E ₂ increase as the input voltage E 1 of the loop gain regulator 12 increases.
The sixth
As shown in the figure, they are inversely proportional and directly proportional, respectively.
However, this naturally occurs when the loop is in a tracking state and operates within the demodulation frequency band. Therefore, by arithmetic processing of the error signal voltages E ₁ and E ₂ by the arithmetic circuit 13, when the angle-modulated wave signals have the same frequency, a substantially constant voltage value is obtained regardless of the change in the loop gain as shown in FIG. An error signal voltage _E3 can be obtained.

As a result, the demodulated signal level within the demodulated frequency band exhibits a substantially constant characteristic regardless of the change in the loop gain, as shown in FIG.
Fluctuations in the demodulated signal level due to loop gain adjustment are prevented, and an arbitrary demodulated frequency band can be set by the loop gain adjuster 12.

As described above, the demodulation method of the angle-modulated wave signal according to the present invention includes a variable band filter that receives the angle-modulated wave signal as an input signal and whose center frequency of the pass frequency band is variably controlled by a control error signal; a first signal transmission means for second-order differentiation and amplitude-limiting amplification of the angle-modulated wave signal; a second signal transmission means for first-order differentiation and amplitude-limiting amplification of the output of the variable band filter; and a phase comparator that is supplied with the output of the second signal transmission means and outputs a signal at a level corresponding to the phase difference between them, and a level setter that variably sets the output signal of the phase comparator to an arbitrary level. , means for automatically causing the center frequency of the variable band filter to follow the frequency of the angle modulated wave signal by applying the output signal of the level setter to the variable band filter as the control error signal; It consists of an arithmetic circuit that calculates the output signal of the comparator and the output signal of the tracking means, respectively, and a means for obtaining a demodulated signal from the output signal of this arithmetic circuit. Since the structure is configured to prevent fluctuations in the demodulated signal level due to changes in the loop gain due to variable level settings, the tracking band of the variable band filter can be narrowed when the angle-modulated wave signal level is decreasing, and therefore the demodulated output signal can be narrowed. It is possible to improve the signal-to-noise ratio, suppress error signals caused by interference waves by narrowing the tracking range, and keep the demodulated signal level constant in the demodulated frequency band within this tracking band. This can cause harmonic components of the direct wave signal to jump into the frequency band of the angle modulated wave signal, which occurs when playing a multi-channel record in which the angle modulated wave signal is multiplexed with the direct wave signal, and record wear. It is possible to significantly improve the deterioration of the signal-to-noise ratio and the generation of abnormal noise due to a decrease in the level of the angle-modulated wave signal, and also to improve the separation characteristics, which conventionally deteriorated due to fluctuations in the level of the demodulated signal. It is possible to perform accurate music playback, and since the tracking range can be set arbitrarily according to the occurrence of noise components, noise components can be suppressed as necessary. The generation of abnormal noise due to the loss of lock phenomenon for the angle modulated wave signal with excessive frequency deviation is eliminated because the error signal level saturates and becomes a constant level with respect to the input signal frequency that cannot be tracked. By selecting the frequency characteristics of the filter, appropriate gain of the first or second signal transmission means, differentiating circuit, etc., it is possible to minimize the white noise level when the angle-modulated wave signal is not input. This system has a number of features, such as eliminating the need for a muting circuit to remove the excessive level of white noise that occurs during demodulation in the PLL demodulation method and other conventional demodulation methods when no signal is input.

[Brief explanation of the drawing]

Figure 1 is a block system diagram of an example of the demodulation method for angle-modulated wave signals proposed earlier by the applicant, Figure 2 is a characteristic diagram of the loop gain versus demodulated signal level in Figure 1, and Figure 3 is Fig. 1 is a frequency characteristic diagram of the demodulated signal when the loop gain is varied, Fig. 4 is a block system diagram of an embodiment of the method of the present invention, and Fig. 5 is a specific example of a variable band filter used in the method of the present invention. Figure 6 is a diagram showing the relationship between the loop gain during tracking in Figure 4 and the error signal voltage of each part, and Figure 7 is the frequency characteristic of the demodulated signal when the loop gain is varied in Figure 4. It is a diagram. 2... Variable band filter, 3... First order differentiator, 4, 6... Amplitude limiting amplifier, 5... Second order differentiator, 7... Phase comparator, 12... Loop gain adjuster (level setter) ), 13... Arithmetic circuit.

Claims (1)

[Claims] 1. A variable band filter that receives the angle modulated wave signal as an input signal and whose center frequency of the pass frequency band is variably controlled by a control error signal, and a first variable band filter that second-order differentiates the angle modulated wave signal and amplifies it with limited amplitude. The output of the signal transmission means and the variable band filter is 1
a second signal transmission means that performs step-differentiation and amplitude-limited amplification; a phase comparator that is supplied with the outputs of the first and second signal transmission means and outputs a signal at a level corresponding to the phase difference between them; A level setter that variably sets the output signal of the phase comparator to an arbitrary level; and the center frequency of the variable band filter is set by adding the output signal of the level setter to the variable band filter as the control error signal. means for automatically following the frequency of the angle-modulated wave signal; an arithmetic circuit that calculates the output signal of the phase comparator and the output signal of the level setter; and a demodulated signal from the output signal of the arithmetic circuit. It becomes a means to obtain,
A method for demodulating an angle-modulated wave signal, characterized in that fluctuations in the demodulated signal level due to changes in loop gain due to variable setting of the level of the output signal of the phase comparator of the level setter are prevented.