JPS6316027Y2 - - Google Patents

Description

[Detailed Description of the Invention] The present invention relates to an automatic frequency control circuit (AFC circuit) in a signal detection circuit (hereinafter referred to as a pickup circuit) of a capacitance detection type high-density recording video disc player.

As shown in the block diagram in Figure 1, the pickup circuit of a conventional capacitance detection video disc player includes a high-frequency oscillation circuit, a tuning circuit, an envelope detection circuit, and an AFC circuit to keep the resonance frequency of the tuning circuit constant. It consisted of In this circuit, the AFC circuit compares the high-frequency signal average level detected by the envelope detection circuit with a reference level, and controls the tuning circuit by changing its operating point in its resonance characteristics so that the high-frequency signal average level remains constant. was. However, the resonance characteristics of this tuned circuit vary due to temperature changes. At this time, if the high frequency average level is controlled to be constant, the differential slope of the resonance characteristic at the operating point determined by the reference voltage changes, causing problems such as a decrease in the high frequency signal level and a deterioration of the C/N of the high frequency signal. Conventionally, no countermeasures have been taken against this.

An object of the present invention is to eliminate the above-mentioned drawbacks of the prior art and to provide an AFC circuit for a pickup circuit of a capacitive disc player that always detects a reproduced signal from a video disc at the best point of the signal C/N. .

In order to achieve the above object, the present invention is equipped with an information signal amplitude level detector in addition to the conventional AFC circuit, and detects fluctuations in resonance characteristics due to temperature changes in the tuned circuit. Detected as a level change,
This amplitude detection level was added to the reference voltage, and the AFC circuit was operated as a new reference voltage at each temperature. Furthermore, if the detection level is not stable, such as immediately after the power is turned on, the feedback loop from the detector is turned off, and the AFC
It is operated at the reference voltage, and after stabilization is operated as if the loop is closed.

The present invention will be described in detail below with reference to a preferred embodiment. An embodiment of the present invention is shown in FIG. 2, in which 1 is a high-frequency oscillation circuit, 2 is a tuning circuit consisting of a capacitance consisting of a disk and a cartridge, a tuning coil, a coupling capacitor, and a tuning diode, 3 is the envelope detection section, 4 is the AFC device,
5 is a differential amplifier comparison circuit, 6 is an AFC reference voltage generation circuit A, 7 is a preamplifier section, 8 is a level detection circuit for detecting the signal level amplified by the preamplifier section 7, 9 is a reference voltage generation section 6 and an addition circuit that adds both signals from the level detection circuit of 8;
10 is a switch circuit, and 14 is an FM signal output.

In the figure, the output from the high frequency oscillation circuit 1 changes in amplitude according to the change in disk capacitance in the tuning circuit, is detected by the detection section 3, and the detected voltage is
V _DC (high frequency signal amplitude average level). This detected voltage V _DC has the characteristics shown in FIG. 3 depending on the voltage applied to the tuning diode D ₁ . On the other hand, the signal component exhibits a characteristic (V _SD characteristic in Figure 3) that corresponds to the degree of change in the slope of the detected voltage characteristic, but this reproduced signal output is amplified by the preamplifier 7 and is subjected to subsequent signal processing (not shown). It is output to the circuit as well as to the level detection circuit No. 8. The level detection circuit 8 detects the level of the reproduced signal using means such as diode detection and outputs it to the addition circuit 9. Note that the detection time constant of the level detection circuit 8 is made sufficiently long so that it does not respond to sudden level fluctuations.

The adder circuit 9 adds the signal level detection signal from the level detection circuit 8 and the reference signal from the reference voltage generator 6, and applies the result to the comparison circuit 5 as a new reference voltage. The comparator circuit 5 uses the signal from the adder circuit 9 as a reference voltage to convert the DC detection voltage of the detector 3 into
V _DC is compared, and the voltage of the tuning diode of the tuning circuit 2 is controlled until the detected voltage V _DC is balanced with the output voltage of the adder circuit 9.

At this time, the optimum operating point of the AFC circuit is point a in FIG. 3, that is, the point where the reproduced signal level is maximum (under the condition of best C/N). As shown in Figure 4, when the ambient temperature rises, the detection voltage V _DC decreases, and the sharpness Q of the tuning characteristic decreases.
will also decrease. That is, since the optimum operating point of the AFC circuit changes depending on the temperature (it tends to decrease as the temperature rises), it is necessary to change the reference voltage of the comparator circuit 5 along with this. On the other hand, since the output signal level also decreases as the temperature rises, the configuration is such that this output signal level is detected, this detected level is added to the reference voltage, and the AFC control is performed using the new reference voltage. In other words, the reference voltage
When V _ref is the detection level and V _s is the detection level, this AFC circuit operates at an operating point that satisfies V _ref + V _s = V _DC (1).

In this embodiment, the level detection circuit 8 is provided as an independent circuit, but it is obvious that the level detection section of the AGC circuit, which is generally provided for absorbing reproduction signal level fluctuations, can also be used. Note that A in FIG. 4 shows the movement of the optimum operating point depending on the temperature.

In addition, the output of the level detection circuit 8 is added by the reference voltage generation circuit 6 and the addition circuit 9, but when the signal level is unstable such as when the power is turned on, or even if the tuning characteristics do not change, due to deterioration of the signal detector etc. A switch circuit 10 is provided to prevent the AFC circuit from malfunctioning when the signal level becomes extremely low. As shown in FIG. 5, the switch circuit 10 is a control circuit 1 that does not output a switch-on signal unless the signal level exceeds a certain level.
1. It is composed of a reference voltage generating circuit B12 and a switch 13, which generate a voltage corresponding to the normal signal level detection voltage _Vs when the signal level is low and the control circuit 11 generates an off signal. When the signal level is low in this switch circuit 10, the control circuit 11 generates an off signal, and outputs the reference voltage circuit B12 and the output voltage E _s to the adder circuit 9. On the other hand, when the signal level becomes sufficiently large, the control circuit 11 operates to generate an on signal and output the output level of the signal level detector 8 to the adder circuit 9. In FIG. 5, B indicates the adder circuit, and C indicates the preamplifier.

As explained above, according to the present invention, it is possible to always detect signals at the optimum operating point by following changes in the resonance characteristics of the tuning circuit due to temperature changes, thereby preventing C/N deterioration of the reproduced signal and achieving good image quality. can get. Furthermore, since the ON/OFF control signal of the SW circuit operates depending on the signal level, this signal can be used as a mute signal for images and audio, or as a means of detecting deterioration of the playback needle (dust adhesion is the end of its life). It can also be used as. Also, this AFC
Since the level detection circuit of the circuit can be substituted for the AGC circuit built into the conventional video disk player, the above-mentioned effects can be obtained with almost no increase in cost.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing the configuration of a conventional pickup circuit, FIG. 2 is a block diagram of a pickup circuit according to an embodiment of the present invention, and FIG. 3 is a conceptual diagram showing the resonance characteristics of a tuned circuit in the pickup circuit.
Figure 4 shows the change in tuning circuit resonance characteristics with respect to temperature, and Figure 5 shows switch circuit 1 of the AFC circuit.
0 is a detailed block diagram of 0. 5: comparison circuit, 6: reference voltage generation circuit, 7: preamplifier, 8: level detection circuit, 9: addition circuit,
10: Switch circuit.

Claims (1)

[Scope of utility model registration request] It is formed to transmit a high-frequency signal via a tuned circuit, and as a result of minute changes in capacitance between the disk and the signal detection section, the resonant frequency of the tuned circuit changes, and the amplitude of the transmitted high-frequency signal changes. In a pickup circuit for a disk player that detects information using changes in the information signal, the resonance of the tuning circuit is detected by comparing the sum signal of the information signal amplitude level and the reference voltage with the average level of the high frequency signal amplitude. An automatic frequency control circuit for a pickup circuit for a video disc player, characterized in that the circuit is configured to control the frequency.