JPH0319043Y2 - - Google Patents

Description

[Detailed Description of the Invention] (Industrial Application Field) The present invention relates to a signal detection circuit that is most suitable for detecting an interval between songs on a recording medium such as a record.

(Prior Art) In recent years, many automatic music selection type record players and the like have been provided in which the pickup position is controlled by detecting blank areas between songs on a record. The light-emitting element used to detect the blank area is generally one in which a light-emitting diode installed in the pick-up area illuminates the top of the record, and the surface-reflected light is received by a light-receiving element such as a phototransistor. This makes it possible to detect only the blank parts between songs with high light reflectance. However, since the characteristics of the above-mentioned photodetector elements vary widely between parts (about 5 times), it is necessary to adjust the sensitivity for each product, and semi-fixed resistors, etc. are required for sensitivity adjustment, which reduces productivity. There were disadvantages such as high cost and high cost. Moreover, even after adjustment, if dust adheres to the light-emitting element and the light-receiving element or misalignment occurs between the elements, readjustment is required, and there is also the problem of changes in ambient temperature. There was also the problem that the output signal level from the light receiving element fluctuated as a result.

(Purpose of the invention) The present invention was made in view of the above points, and its purpose is to eliminate the need for sensitivity adjustment of the signal detection circuit, and to reduce the output signal level of the light receiving element due to temperature changes or other disturbances. It is an object of the present invention to provide a signal detection circuit that can accurately compensate for fluctuations and can be configured at low cost.

(Structure of the invention) The signal detection circuit according to the invention is a signal detection circuit for detecting a signal recorded on a recording medium such as a record or a non-signal portion on the recording medium, and is a signal detection circuit for detecting a signal recorded on a recording medium such as a record or a non-signal portion on the recording medium. It is equipped with a detection section for detecting only the DC component in the output signal, and a variable gain amplifier whose gain can be varied according to the output voltage level from the detection section. It is characterized in that it is configured so that a constant detection signal can be obtained regardless of the amount of the detection signal.

(Example) An example of the signal detection circuit according to the present invention will be described based on FIG.

In the figure, 1 is a phototransistor which is a light receiving element, 1
Reference numeral 2 denotes a light-receiving element that illuminates the surface of the record, and 14 a detection unit for detecting only the DC component I _F of the output signal If obtained from the phototransistor 1. In the embodiment, the differential amplifier 3 and the It consists of a high-pass filter 2 and a resistor 4 for limiting the amplification band of the amplifier 3, and is provided between the inverting input terminal (-) of the differential amplifier 3 and the output terminal of the differential amplifier 3. The output signal of the phototransistor 1 is averaged by the high-pass filter 2, and the operation of the transistor 6 is stabilized. The transistor 6 is a variable gain amplifier whose gain Av can be varied according to the output voltage level from the detection section 14, and in the embodiment, an NPN transistor is used. The base of the transistor 6 is connected to the output terminal of the detection section 14, and its emitter is connected to the output terminal of the phototransistor 1 via a resistor (R _F ) 4 and to ground via a high-pass filter 5. It is connected to GND, and its collector is connected to the power supply Vcc via a resistor R _L 9, respectively. Voltage Vs in the diagram
is the reference voltage determined by resistors 15 and 16. A high-pass filter 7 is connected to the collector of the transistor 6, and an amplifier 8 is provided downstream of the high-pass filter 7, so that a desired detection signal can be obtained from its output terminal OUT.

An example of the operation of the above configuration will be explained based on FIGS. 1 and 2.

The output signal of the phototransistor 1 that is output when the phototransistor 1 moves while maintaining a predetermined distance from the record includes a DC component and an AC component that is a pulse-like signal that is output when passing between songs. It can be divided into

Now, although the phototransistor 1 normally has a drawback of large variations in characteristics among its components, the magnitude of the peak value of the AC component described above is proportional to the magnitude of the DC component.

Therefore, this invention attempts to stably detect the intervals between songs by utilizing the fact that the ratio between the peak value of the AC component and the DC component (referred to as the discrimination ratio) is approximately constant between each component.

The signal detection circuit shown in Fig. 1 varies the bias current I _E of the transistor 6 depending on the level of the DC component obtained from the phototransistor 1, which is a light receiving element, and changes the gain. It is operated so that the signal level is the same between the two. This operating principle is expressed as follows: The output current of the phototransistor is I _f , its DC component is I _F ,
Let us explain two different phototransistors whose AC component is i _f and whose discrimination ratio D is D=1+if/ _IF . The output currents I _fA and I _fB of phototransistors A and B have their respective DC components as follows. I _FA , I _FB , AC component i _fA ,
If i _fB , then I _fA = I _FA + i _fA I _fB = I _FA + i _fB , and since the discrimination ratios D of phototransistors A and B are equal, the DC component or AC component of the output current of the two phototransistors is Sensitivity ratio S, which is the ratio
is S=I _FA /I _FB =i _fA /i _fB .

Here, in order to make the AC components if of phototransistors A and B equal regardless of the sensitivity ratio S, if the gain Av of transistor 6 is set as Av=K/I _F (K is a constant), the output voltage of transistor 6
V ₀ becomes Vo=Avif=Kif/I _F (∴if/I _F = constant), and if/I _F in the above equation is more constant than the discrimination ratio D, so a constant output voltage V ₀ is always obtained. That will happen. In other words, for the DC component I _F of the output current I _f of the phototransistor, the gain Av of the transistor 6 is
The ideal gain correction is Av=K/I _F (Av: amplification factor, K: constant) (Curve 10 in Figure 2).

In the present invention, control is performed linearly as shown by the correction straight line 11 in FIG. To explain this using Fig. 1, only the DC component I _F of the output current I _f of the phototransistor 1 is amplified by the high-pass filter 2, differential amplifier 3, and resistor 4 in the figure, and the output voltage V ₂ is The corresponding emitter current _IE is determined by the transistor 6 and the high-pass filter 5. Here, when the DC component I _F of the output current I _f of the phototransistor 1 is large, the emitter current I _E of the transistor 6 becomes small, and when the DC component I _F is small, the emitter current I _E becomes large. At this time, by setting the cutoff frequency of the high-pass filters 5 and 2 to be sufficiently lower (approximately 10 Hz) than the frequency component of the signal to be detected (approximately 100 Hz), it is determined that it is a DC component in the output current of the phototransistor.

First, the DC component I _F of the output current I _f of the phototransistor 1 will be explained. Assuming that the impedance of high-pass filter 2 is considered to be very large and can be ignored, the emitter current I _E of transistor 6 is: I _E = V ₂ /Re = Vs - R _F I _F /Re (Vs is the reference voltage, Re is the high-pass filter 5), and the emitter resistance re of the transistor 6 at this time is re=Vt/I _E (Vt is the V _BE of the transistor 6 ≈26 mV). Therefore, the operating point of the transistor 6 is determined by the above equation. Next, the output current I _f AC component if of the phototransistor 1 will be explained. If the impedance of the high-pass filter 5 can be considered zero and can be ignored, the output voltage V ₀ will be V ₀ = R _L /reV ₁ = R _L /Vt・I _E・V ₁ from re=Vt/I _E. . However, the voltage V ₁ is the base potential of the transistor 6.

Therefore, the gain Av of the transistor 6 is Av=V ₀ /V ₁ =R _L /VtI _E =R _L /Vt・Re(Vs−R _F I _F ) =−R _L・R _F /Vt・ReI _F +R _L・Vs/Vt・Re ...(1), and when formula (1) is shown on a graph, the straight line 11 in Figure 2
become that way. If the usage range of the circuit of the present invention is determined as shown in Fig. 2, the correction straight line 11 becomes the ideal correction curve 1.
By using the phototransistor at a point that crosses the center of 0, variations in sensitivity due to phototransistors can be suppressed to a small level. Although the ideal correction curve 10 and the correction straight line 11 in FIG. 2 produce errors such as the shaded portion 13, there is no problem in practical use, and the advantage is that the circuit can be simplified.

Although the embodiment has been described with reference to an application to a music selection device for a record, the present invention is not limited to this example and can of course be applied to a device for detecting the end of a cassette tape for a tape player.

(Effect of the invention) According to the signal detection circuit according to the invention, there is a detection section for detecting only the DC component in the output signal obtained from the light receiving element, and a gain variable according to the output voltage level from the detection section. Because it is equipped with a variable gain amplifier that allows for There is no need to adjust the gain. Therefore, not only productivity can be significantly improved, but also an adjustment circuit for gain adjustment is no longer required, so that a large cost reduction can be achieved. In addition, the sensitivity of the photodetector may be affected by dust, vibration, etc.
Although it fluctuates due to temperature changes, etc., it has excellent features such as being able to automatically respond to sensitivity changes due to such disturbances, so stable operation can always be expected.

[Brief explanation of the drawing]

The drawings show an embodiment of a signal detection circuit according to the present invention, FIG. 1 is a circuit diagram showing an embodiment of the signal detection circuit according to the present invention and its operating principle, and FIG. 3 is a graph showing the gain versus the DC component of the light receiving element output current. 1: Phototransistor as a light receiving element, 14: Detector, 3: Differential amplifier, 6: Transistor as a variable gain amplifier.

Claims (1)

[Claims for Utility Model Registration] A signal detection circuit for detecting a signal recorded on a recording medium such as a record or a non-signal portion on the recording medium, which detects a direct current in an output signal obtained from a light receiving element. It is equipped with a variable gain amplifier whose gain is varied according to the output voltage level of the input component, and is configured so that a constant detection signal can be obtained regardless of the sensitivity of the light receiving element. Characteristic signal detection circuit.