JPH065926B2 - Signal detection circuit - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to SAP (Second Audio) during audio multiplex broadcasting in a television receiver capable of receiving US multiplex audio broadcasting.
The present invention relates to a signal detection circuit that detects the presence or absence of broadcasting.

2. Description of the Related Art Conventionally, in US TV audio multiplex broadcasting, an audio signal up to 15 KHz is used as an L + R signal, and additionally, a stereo pilot signal is used at f _H = 15.734 KHz and 2f _H is used as a carrier 2 (L-
The R) signal is used for stereo broadcasting, and is composed of a signal having 5f _H as an FM carrier as a second audio program (SAP). So SAP
The presence / absence of the broadcasting of 5 f _H carrier (hereinafter referred to as SAP carrier) must be determined.

The SAP carrier (f ₀ = 78.67 KHz) detection circuit will be described with reference to FIG. In FIG. 1, the input signal to the terminal 1 is an FM wave whose carrier is f ₀ , the bandpass filter 2 removes unnecessary signals, and the amplifier 3 amplifies it to a certain level. The signal is first subjected to peak detection by the signal level detection circuit 4, and the voltage comparator 5 judges whether the detected voltage is equal to or higher than or equal to the reference voltage V ₁ applied to the terminal 11, and is applied to one input terminal of the AND gate 6. Apply. On the other hand, the output of the amplifier 3 is also applied to the noise detection circuit 7 to detect noise when the broadcast wave becomes weak.
The comparator 8 determines whether the detected noise voltage is higher or lower than the reference voltage V ₀ applied to the terminal 9, and inputs the result to the other input terminal of the AND gate 6.

Next, the relationship between the detected voltage and the electric field strength of broadcast radio waves will be described with reference to FIG. With a strong electric field strength, the presence or absence of the SAP carrier signal is detected by the signal level detection circuit 4 and the comparator 5, and the difference between the signal level detection voltage A with SAP carrier and the signal level detection voltage B without carrier shown in FIG. 4 is clear. Thus, the presence / absence of the SAP carrier can be determined by the level of the reference voltage V ₁ of the comparator 5 set between the detection voltages A and B. However, when the electric field is weaker on the left side than the level E in FIG. 4 and the noise becomes large, the signal level detection voltage B becomes larger than the reference voltage V _{1 at the} terminal 11 even when there is no SAP carrier. This is a detection error. Therefore, the noise detection circuit 7 is activated. That is, when the noise level becomes high (when the noise detection voltage C exceeds the reference voltage V ₀ ), the output of the comparator 8 is set to the low level and the output of the AND gate 6 is set to the low level to prevent the malfunction of the carrier discrimination output. Here, if the reference voltages V ₁ and V ₀ of the comparators 5 and 8 are increased, even weaker electric field strength can be detected.

Problems to be Solved by the Invention However, in the conventional circuit, the reference voltages V ₁ and V ₀ of the comparators 5 and 8 are used to obtain the detection capability up to a weak electric field.
If is set to a high value, the difference between the carrier signal level detection voltage A and the reference voltage V ₁ becomes small, and when the SAP carrier becomes small on the broadcast station side, detection is not possible, and the carrier reduction margin becomes small. Had.

An object of the present invention is to solve the problem that a small SAP signal cannot be detected, which is intended to compensate for the fact that the detection of the SAP carrier is likely to malfunction due to noise in a weak electric field of television radio waves.

Means for Solving the Problems In order to solve the above problems, the signal detection circuit of the present invention is
The SAP carrier signal is supplied to the limiter amplifier, the output of the limiter amplifier is input to the first differential amplifier and the second differential amplifier, and the output of the first differential amplifier causes the output of the second differential amplifier. A configuration in which a current is controlled, an integrator is provided at the output stage of the second differential amplifier, and a signal is detected by determining whether or not the output of the integrator has reached a specified level by a comparator. Is.

Effect The present invention can accurately detect the presence or absence of the SAP carrier signal without being influenced by noise and less affected by the carrier level.

Embodiments Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a signal detection circuit diagram in an embodiment of the present invention.

In FIG. 1, an input signal to the input terminal 1 is applied to a differential amplifier composed of transistors 13 and 14, a resistor 15 and a constant current source 16 via a bandpass filter 2 and a limiter amplifier 21. The outputs of this differential amplifier are transistors 17, 18 and resistors 19, 20, 29, a first differential amplifier composed of a constant current source 23, transistors 32, 33, 31, 34, and resistors 37, 38, Three
And a second differential amplifier composed of 6, 39 and 30. The output of the first differential amplifier is inverted by the inversion transistor 22 and switches the current source transistor 31 forming the second differential amplifier. Further, the output of the second differential amplifier is added to a resistor 27 and a capacitor 26 which form an integrator with a transistor 34, the integrated output thereof is added to a comparator 24, and the output of the comparator 24 is taken out to an output end 28. ing.

Here, when a regular SAP signal is applied to the input terminal 1, the limiter amplifier 21 passes through the bandpass filter 2.
Then, the waveform is shaped by the differential amplifier composed of the transistors 13 and 14. The output V ₂ is the part (a) of FIG. The output V _{2 of the} differential amplifier is the transistor 17,
The first differential amplifier composed of 18 compares the voltage with the comparison voltage V ₃ and shapes the waveform. The comparison voltage V ₃ is the output V of the differential amplifier.
The voltage is lower than the center voltage of ₂ . The base of one of the transistors 32 forming the second differential amplifier is the power supply voltage V ₁ , the output V _{2 of the} differential amplifier is connected to the base of the other transistor 33, and the second differential amplifier is further formed. A signal obtained by inverting the output of the first differential amplifier by the transistor 22 and the resistors 30 and 35 is added to the base of the constant current source transistor 31. Therefore, when the regular SAP carrier signal is applied to the input terminal 1, the output of the second differential amplifier is Vcc in the signal (a) of FIG. Next, when the electric wave becomes weak and the noise becomes larger than that of the SAP carrier, the SAP carrier is lost due to the noise. Then, the output V _{2 of the} differential amplifier composed of the transistors 13 and 14 is as shown in FIGS.
In this state, the peak of the signal falls below the voltage V ₁ . Therefore, the differential amplifier output V ₂ is applied to the base of the transistor 33 which is one input of the _second differential amplifier.
And the voltage V ₁ is input to the base of the other transistor 32, the resistance 3 which is the second differential amplifier output is added.
The voltage of 6 becomes (V ₁ −V ₂ ) × (resistor 36 ÷ resistor 37), and a signal proportional to the lack of the SAP carrier signal is obtained. However, from the state of (a) and (c) of FIG.
There is a first differential amplifier for extracting only the missing signal of the carrier signal, and a voltage V ₃ which is equal to or less than the average value of the differential amplifier output V ₂ is added to the base of the transistor 17 of the first differential amplifier to output a signal on the positive side of the SAP carrier signal (FIG. 2b) The voltage V ₄ is taken out. By applying this V ₄ voltage to the base of the transistor 31 which forms the current source of the second differential amplifier, the SAP is accurately
The signal of the missing part of the carrier signal can be taken out.
The output of the second differential amplifier is inverted by the transistor 34 and integrated by the integrators 26 and 27 to obtain a positive voltage including the ripple shown in FIG. 2c. This voltage is 0 when the SAP signal is normal, and the second voltage is set by the comparator 24.
SAP is performed by shaping the waveform of the positive voltage including the ripple in Figure c.
The presence or absence of a carrier signal can be detected. Limiter amplifier 2
By using 1, regardless of the size of the SAP carrier,
The presence or absence thereof can be reliably detected.

EFFECTS OF THE INVENTION As described above, according to the present invention, it is possible to realize a signal detection circuit capable of detecting the presence or absence of a SPA carrier signal up to a level at which a limiter amplifier cannot operate, and stably detecting the amount of noise. be able to.

[Brief description of drawings]

FIG. 1 is a circuit diagram of a signal detection circuit in one embodiment of the present invention, FIG. 2 is a waveform diagram for explaining the operation thereof, FIG. 3 is a circuit diagram of a conventional signal detection circuit, and FIG. 4 is its operation. It is a wave form chart for explanation. 13, 14, 17, 18, 22, 32, 33, 34 ...
Transistors, 15, 19, 20, 27, 29, 30,
35, 36, 37, 38, 39 ... Resistor, 26 ... Capacitor, 24 ... Comparator, 16, 23 ... Constant current source.

Claims (1)

[Claims] 1. A carrier signal is input to one input terminal of each of a first differential amplifier and a second differential amplifier through a limiter amplifier, and a second difference is output by the output of the first differential amplifier. Second, the current of the current source of the dynamic amplifier is controlled
A signal detection circuit characterized in that an integrator is provided at the output stage of the differential amplifier and the signal is detected by determining whether or not the output of the integrator has reached a specified level by a comparator. .