JPS5923158B2 - Phase locked loop for multi-channel record demodulation - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a phase locked loop for demodulating multi-channel records.
A voltage controlled oscillator (hereinafter referred to as V
By attenuating or removing frequency components that are substantially the same as the free-running oscillation frequency of the VCO in the input control voltage of the VCO (referred to as CO), the S/N (signal-to-noise ratio required to construct a loop) is improved, and the VCO It is an object of the present invention to provide a PLL that can improve the linearity of the PLL and thereby improve the distortion rate of the demodulated signal.

Figure 1 shows a conventional PLL for demodulating multi-channel records.
An example block system diagram is shown.

A multi-channel audio signal is made into an unmodulated direct wave signal and an angle-modulated wave signal, and from a multi-channel record in which these are multiplexed and recorded, a high-pass filter or a high-pass filter or The angle-modulated wave signal separated and filtered by the bandpass filter is supplied to a phase comparator (hereinafter referred to as PC) 2 via an input terminal indicated by 1 in FIG. The phase is compared with a signal from the VCO 4 which is selected to have the same frequency as the carrier frequency of the modulated wave signal. Phase comparison error voltages with different levels are extracted from this PC 2 according to the phase difference between the above two signals, and after the unnecessary high frequency components are attenuated by the low-pass filter 3, this voltage is applied to the VC04 as a control voltage. While controlling its output oscillation frequency, it is output from the output terminal 5 as a demodulated signal of the angle-modulated wave signal.

In a PLL that performs a demodulating operation of such an angle-modulated wave signal, a multiplier is normally used in the actual circuit configuration of PC2.

Therefore, when the carrier frequency of the input angle-modulated wave signal is, for example, 30kHz, the free-running oscillation frequency of the VCO4 is also 30kHz.
Since the frequency is selected to be kHz2, the output signal frequency of PC2 is 30kHz±30kHz, as will be described later. By the way, multi-channel records are currently cut at one speed, but in that case the crosstalk of the second harmonic of the carrier frequency is bad, and in pick-up cartridges, the crosstalk of the second harmonic of the carrier frequency of 60kHz is bad. In addition, the reproduced signal contains crosstalk from other channels.

In addition, second harmonic distortion of the own channel also occurs.
A high-pass filter or bandpass filter is used on the input side of the PLL to remove or attenuate these unnecessary components and direct wave signals, but it is difficult to realize an ideal circuit.
Not only high-pass filters but also bandpass filters are designed taking into account the influence of phase characteristics on the baseband, so they have a characteristic of poor roll-off as shown in Figure 2A, or as shown in Figure 2B. Therefore, the second harmonic component (here, 6
0kHz) and its neighboring frequency components are not sufficiently attenuated and are applied as they are to the PLL. In this case, the PC configured with the multiplier as described above
The output signal frequency of 2 is the free-running oscillation frequency of VCO4 of 3.
Since it is 0kHz, it becomes 60kHz±30kHz, but in an ideal state, these signals are attenuated by the low-pass filter 3 and only the necessary signals are supplied to the CO4.

Here, the above-mentioned low-pass filter 3 has a resistor R connected between an input terminal 6 and an output terminal 7, as shown in FIG.
1. A lug-lead type low-pass filter consisting of a resistor R2 and a capacitor C1 connected in series between the output terminal 7 and ground was used. The frequency characteristics of this filter 3 are as follows:
As shown in the figure, it is attenuated at 6 dB/0ct only between the frequency F determined by the capacitance value of capacitor C and the resistance value of resistor R, and the frequency F2 determined by the capacitance value of capacitor C1 and the resistance value of resistor R2. , the characteristics are flat at frequencies below f1 and above F2. The low pass filter 3 has a band of 20 to 45 kHz, which is close to the direct signal band (less than 20 kHz).
In order to prevent the influence of phase distortion on the demodulated signal of the Hz angle-modulated wave and to avoid steep roll-off characteristics in consideration of the transient response characteristics of the loop, the frequency f1 is selected to be, for example, about 15 kHz. has been done. Therefore, even if the low-pass filter 3 attenuates the high frequencies above the demodulated signal, the 30kHz component will not be attenuated so much, and the 60kHz component will be doubled.
Relatively good damping properties are only obtained in the case of . Therefore, when an ideal angle-modulated wave signal that does not contain the above harmonic components is input to the PLL, the PC2
There is no problem with the output of
As mentioned above, the output of C2 is 60kHz±30kHz.
z, of which the 30 kHz component is not sufficiently attenuated by the low-pass filter 3 and is applied to the VCO 4.

However, this 30 kHz component is not necessary for the signal to be demodulated, and is also not necessary for applying to the VCO4. This is because only a modulation signal is originally applied to the VCO 4, and it is therefore necessary to change its oscillation frequency. Therefore, in the conventional PLL, the component generated by the bead between the second harmonic component of the angle-modulated wave signal from another channel and the output oscillation frequency of the VCO 4, in other words,
Almost the same frequency component as the free-running oscillation frequency of VCO4 is the VCO
In this case, the ratio of noise to the signal required to operate the loop (this is called the loop S/N) decreases, which worsens the distortion rate of the demodulated signal. There was a drawback of doing so.

The present invention eliminates the above-mentioned drawbacks, and one embodiment thereof will be described below with reference to FIGS. 5 to 8.

FIG. 5 shows a block system diagram of an embodiment of the PLL for multi-channel record demodulation according to the present invention.

In the figure, the same parts as in FIG. 1 are designated by the same reference numerals, and their explanations will be omitted. Reference numeral 8 denotes a trouble circuit, which has the same frequency F as the free-running oscillation frequency of the VCO 4, as shown in FIG.

The frequency F should have at least an attenuation pole of about 20 dB or more compared to the demodulated signal band, and should not affect the phase of the demodulated signal transmission band. It has the property of trapping only nearby areas. The output signal of the trap circuit 8 is applied to the VCO 4 via the low-pass filter 3. A filter block 9, which is a combination of a trap circuit 8 and a low-pass filter 3, has a circuit configuration as shown in FIG. 7A or B. In Figures A and B, the same parts as in Figure 3 are designated by the same reference numerals. A filter block 91 shown in FIG. 7A has a parallel resonant circuit consisting of a capacitor C2 and a coil L1 connected in series between a resistor R1, a connection point of a capacitor C1 and an output terminal 111, and a parallel resonant circuit consisting of a capacitor C2 and a coil L1. It constitutes a trap circuit 81. A filter block 92 shown in FIG. 7B is a parallel connection of a series circuit of a capacitor C1 and a resistor R2 constituting a lag-lead type low-pass filter and a series resonant circuit composed of a capacitor C3 and a coil L2. C3 and coil L2 constitute a trap circuit 82. Note that 112 is an output terminal. Filter block 9 (91 or 92) above
The signal extracted from the trap circuit 8 (81 or 82) and applied to the VCO 4 is a signal component generated by the bead of the second harmonic component of the angle-modulated wave signal from the other channel and the output signal of the VCO 4. In the middle, only the frequencies near the free-running oscillation frequency of the VCO 4 are attenuated or removed, and the low-pass filter 3 attenuates the high-frequency components higher than the demodulated signal, so that only the baseband signal remains, and the VCO output Control the oscillation frequency.

In this case, the influence of the second harmonic component is relatively far away from the demodulated signal and does not have a significant influence. Next, the present invention will be explained in more detail. Now, let VR(t) be the angle modulated wave signal of the left (0 channel) and the angle modulated wave signal of the right (5) channel to be recorded in the multi-channel record, and these signals are calculated by the following formulas. It is expressed as

VL: amplitude ω of the angle-modulated wave signal of the left channel. ; Carrier angular frequency θL(t); Left channel modulation signal VR; Right channel angle modulated wave signal amplitude θR(t
); Right channel modulation signal By the way, (1), (2)
The second harmonic component of each signal VL6O (
t), VR6O(t) can be expressed by equation (3).

Here, (k: constant), ψ1(t), φ1″(t): phase change.

Next, this signal (component of high-order distortion due to crosstalk from R channel to L channel) VR6O(t)
Katsuta and pick-up. The interference signal VnR(t) from the R channel to the L channel resulting from poor cartridge crosstalk has an amplitude multiplied by L and a phase change of φ.
2(t) can be expressed by equation (4). VnR(t)-
LVR6OSin{2ωCt+θn(t)} (4) However, θn(t) 2θR(0+φ1(t)+φ2(t)) Therefore, the input signal of the L channel applied to the PLL, if this is ViL(t), is as follows. It is expressed by the formula.

ViL(t)-VL(t)+VnR(t) (5) However, in this case, only the signals related to the present invention are adopted and the others are omitted.

By the way, the oscillation frequency output from VCO4 is VO(t)
, the input modulation signal of VCO4 is θ.

(t), then Vc(t)=VccOs{ωCt+θ
c(t)} (6).

Normally, PC2 is composed of a multiplier as described above, and if its conversion gain is Kpc, its output signal Vpc(t) is (1), (4), . (5) and (6)
From the formula.

This signal Vpc(t) is passed through the low-pass filter 3 to 2ωc,
Although the signal components of There is no major impact, and the second term is important as a direct impact. By the way, the change in the output frequency of VCO4 from ωc is due to the input error of VCO4 expressed as a time function [
The differential voltage is V. (t), and when the conversion gain of the VCO 4 is Kc, it can be expressed by the following equation. In addition, the above V.

(t) is the impulse response of low-pass filter 3.
(t), the equation (7) is obtained. In normal operation, the PLL operates linearly and can therefore be expressed in Laplace transform form.

That is, if θc(s) is the Laplace transformed form of θc(t), then equations (8) to Qω are obtained.

By the way, in order to solve the aυ equation in the time domain, the specific shape of the low-pass filter must be known. However, in the present invention, rather than increasing the distortion in the specific form of each low-pass filter, a general case will be considered. Convert formula (7) into the following format.

Furthermore, by rewriting equation (9), and applying equation al) to each individual case, the following equation is obtained.

Now, once again, θc(t) is replaced as shown in the following equation.

However, here we find the inverse Laplace transform form of (self) equation,
After converting to the time domain, the (self) equation was obtained.

In this case, consider the case where equation 06) exists. If we rewrite equation (7), we get:

(auto) In the equation, focus on the first term on the right side.

Generally, when θ(t) is small, the following equation holds. Si
n{θ(t)}Zθ(t) 08) Therefore, in this case, the first term on the right side of equation (17) simply becomes the addition of the signal component and the interference wave.

If this output is removed by a low-pass filter connected to the demodulation output, high-order distortion is removed, so almost no demodulation distortion occurs. However, when the phase component of the first term, that is, the signal component, is large, the equation (self) does not hold approximately, and naturally the second term in the brace of the first term has a large effect, and the above (7 ) The original (
7) The phase θL(t)−θc(t
) will be different. That is, this occurs as demodulation distortion. Naturally, this distortion cannot be removed by the low-pass filter connected to the demodulation output. This phenomenon is especially true for θL
This mainly occurs when (t) and θR(t) are large.
This will be explained with reference to FIG. 8. In the figure, θ1 is the (7
) is the phase angle when the second term on the right side of equation (7) is not present (the signal component in this case is o1(t)), and △θ is the phase distortion due to the influence of the second term on the right side of equation (7) (the signal component in this case is o1(t)). is V.l′(t))
and VO, (t) to V. The signal output with respect to the phase change changes up to /(t). Therefore, it is necessary to remove the second term on the right side of equation (7). Therefore, in order to minimize the influence of phase distortion on the demodulated signal, the present invention configures the trap circuit 8 (81 or 82) described above, and attenuates only the frequency band that is approximately the same as the free-running oscillation frequency of the VCO 4. and the level difference with the demodulated signal band is at least 20
The attenuation is about dB or more.

As a result, among the leakage components caused by bad crosstalk from other channels, the VC caused by the second harmonic component of the carrier has the greatest influence.
The CO input having substantially the same frequency as the free-running oscillation frequency of O4 is removed by inserting a trap circuit 8 configured to minimize the influence of phase distortion on the demodulated signal.

In this case, only the crosstalk of second-order harmonic distortion from other channels was considered, but as mentioned above, even if the 60kHz component of second-order harmonic distortion of the own channel is applied to the PLL, the free running of the VCO will not occur. A similar effect can be obtained by trapping at the same frequency as the oscillation frequency.

In this case, similarly, PC2
An unnecessary term of 30kHz component is generated in the output. As described above, this method removes unnecessary mixed harmonic distortion components other than the demodulated signal, and also reduces the so-called S/N of the loop.
This improves the fidelity of the demodulation signal of VCO4 changes, and eliminates the shift in the duty cycle of the VCO4 output related to PLL non-linearity, the phase comparison operation in the limiter operating region of PC2, and the offset of PC2. In this case, the causes of demodulation distortion caused by these factors are removed, and as a result, a reduction in the distortion rate of the demodulated signal is achieved.

In the above explanation, the case of the L channel was explained, but the case of the R channel can be explained in exactly the same manner as above, and requires a PLL having the same configuration as that in FIG. 5.

Furthermore, the present invention is not limited to the above embodiments,
The trap circuit 8 may be inserted and connected between the low-pass filter 3 and the VCO 4, and in short, it may be inserted and connected in the transmission path from the output side of the PC 2 to the input side of the VCO 4.

In addition, the carrier frequency of the angle modulated wave signal is set to 30kHz.
However, other frequencies may also be used, and in this case as well, V
Of course, a similar effect can be obtained by removing from the PC2 output a frequency that is substantially the same as the free-running oscillation frequency of CO4.

As mentioned above, the phase-locked loop for demodulating multi-channel records according to the present invention is capable of producing multi-channel records in which multi-channel audio signals are made into direct wave signals and angle-modulated wave signals, and these are multiplexed and recorded. In the phase-locked loop that demodulates the angle-modulated wave signal regenerated from A trap circuit was inserted into the transmission line leading to the input side of the selected voltage controlled oscillator, which attenuates or removes only the same frequency as the free-running oscillation frequency and its neighboring frequencies by more than a predetermined amount. Unnecessary frequency components generated by the bead between the second harmonic component of the modulated wave signal and the output signal of the voltage controlled oscillator and added to the voltage controlled oscillator can be removed, thereby eliminating noise from the signal necessary to operate the loop. ratio (loop S/N)
Furthermore, the fidelity of the output change of the voltage controlled oscillator with respect to the modulation signal is increased (improved linearity), and as a result, the distortion rate of the modulation signal can be improved. It is.

[Brief explanation of the drawing]

Fig. 1 is a block system diagram of an example of a conventional phase-locked loop for demodulating multi-channel records, Figs. 3 is a specific circuit diagram of an example of the low-pass filter shown in FIG. 1, FIG. 4 is a diagram showing the frequency characteristics of the example of FIG. 3, and FIG.・A block system diagram of one embodiment of the loop, FIG. 6 is a diagram showing the frequency characteristics of the main part of FIG. 5, and FIGS. 7A and B are specific circuits of each embodiment of the main part of FIG. 5. FIG. 8 is a vector diagram showing phase distortion for explaining the present invention. 1... Angle modulated wave signal input terminal, 2...
...Phase comparator, 3...Low pass filter, 4...
. . . Voltage controlled oscillator, 5 . . . Demodulated signal output terminal, 8, 81, 82 . . . Trap circuit.

Claims (1)

[Claims] 1. In a phase-locked loop for demodulating the angle-modulated wave signal reproduced from a multi-channel record in which a multi-channel audio signal is recorded as a direct wave signal and an angle-modulated wave signal, The free-running oscillator is connected to the input side of the voltage-controlled oscillator whose free-running oscillation frequency is selected to be the same as the carrier frequency of the angle-modulated wave signal. 1. A phase-locked loop for demodulating multi-channel records, characterized in that a trap circuit is inserted and connected to attenuate or remove a predetermined amount or more of the same frequency as the oscillation frequency and frequencies in the vicinity thereof.