JPH0317433B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention is directed to a television receiver capable of receiving American audio multiplex broadcasts.
(Second Audio Program) relates to a signal detection circuit that detects the presence or absence of broadcasting.

Conventional technology US audio multiplex broadcasting uses conventional audio signals up to 15KHz as L+R signals, a pilot signal for stereo on _H (15.734kHz), and signal 2 (L-R) with _2H as the carrier. It also consists of a signal using _5H as the FM carrier for broadcasting and as a second audio program (SAP). Therefore, it is necessary to determine whether SAP is broadcasting or not based on the presence or absence of the _5H carrier. The above conventional
The third SAP carrier ( _O = 78.67kHz) detection circuit
It is shown and explained in the figure. The input signal input from the input terminal 1 is an FM wave using the above _O as a carrier, unnecessary signals are removed by the band pass filter 2, and the signal is amplified to a certain level by the amplifier 3. The amplified signal is first subjected to peak detection in the signal level detection circuit 4, and the voltage comparator 5 determines whether the detected voltage is higher than or lower than the reference voltage _V1 applied to the terminal 11, and one of the AND gates 6 is detected. Applied to the input terminal. On the other hand, the output of the amplifier 3 detects noise when the broadcast radio waves applied to the noise detection circuit 7 become weak. The detected noise voltage is converted into a reference voltage applied to terminal 9 by comparator 8.
Determine whether V is higher or lower than ₀ and AND the results
It is input to the other input terminal of gate 6.

Next, the relationship between the detected voltage and the electric field strength of the broadcast radio wave is shown in FIG. 4 and will be explained.

At strong electric field strength, the presence or absence of the SAP carrier signal is determined by the signal level detection circuit 4 and comparator 5 in Fig. 3 using the signal level detection voltage shown in Fig. 4 when there is a carrier, and the signal level detection voltage when there is no carrier shown in Fig. 4. The difference from the level detection voltage is clear, and the presence or absence of the SAP carrier can be determined based on the level relative to the reference voltage _V1 of the comparator 5 set between both detection voltages A and B.

Problems to be Solved by the Invention However, the electric field is weak on the left side of level E in FIG. 4, and when the noise becomes large, the signal level detection voltage B becomes larger than the reference voltage _V1 of the terminal 11 even when there is no SAP carrier. This results in a detection error.
Therefore, the noise detection circuit 7 shown in FIG. 3 is activated. In other words, the noise level becomes higher. That is, when the noise detection voltage C exceeds the reference voltage _V0 , the output of the comparator 8 is set to a low level, and the output of the AND gate 6 is set to a low level to prevent malfunction of the carrier discrimination output.

Here, the reference voltage V ₁ of comparators 5 and 18
As V ₀ is increased, it is possible to detect even weaker electric field strengths, but the difference between the carrier signal level detection voltage A and the reference voltage V ₁ becomes smaller, and SAP at broadcasting stations, etc.
There is a problem in that when the carrier becomes small, it cannot be detected and there is no margin for reducing the carrier.

The present invention solves the above conventional problems,
It is an object of the present invention to provide a signal detection circuit that can accurately detect the presence or absence of a carrier signal without being affected by noise and not easily influenced by the magnitude of the carrier level.

Means for Solving the Problems In order to achieve the above object, the signal detection circuit of the present invention has a first circuit that integrates the output of a circuit that limits an SAP carrier signal with a capacitor and a resistor.
an integrator circuit, a comparator circuit that compares this integral output with a reference level, and a second integrator that integrates this comparison output, and determines whether or not the output of this second integrator has reached a specified level. The configuration is such that signal detection is performed with a comparator.

Effects According to the present invention, the SAP carrier signal is passed through the limiter amplifier, and when the noise is large or the SAP carrier signal itself is small, the present invention integrates the output of the limiter amplifier and compares the integrated output with a reference voltage. By doing so, the presence or absence of the SAP carrier signal is detected.

Embodiment Hereinafter, an embodiment of the present invention will be described based on the drawings.

FIG. 1 is a circuit diagram of a signal detection circuit in one embodiment of the present invention. In FIG. 1, 1 is a signal input terminal, 2 is a bandpass filter, 11 is a limiter amplifier, 12 is a transistor, 13 and 14 are a capacitor and a resistor, one end of which is connected to the emitter of transistor 12, and the other end of which is grounded.
and the resistor 14 constitute a first integrator 14a.
15 and 16 are transistors constituting a waveform slicing amplifier; 17 is an input terminal to which a reference voltage V ₂ is applied; the input terminal is connected to the base of the transistor 16; A constant current source 18 has one end connected to the emitters of the transistors 15 and 16. 19 is a transistor whose base and collector are connected, and one end of a resistor 20 is connected to its emitter; the other end of the resistor 20 is a terminal to which voltage Vcc is applied; 22 is a transistor whose anode is connected to the terminal 21, and whose cathode is connected to a transistor 23;
The transistors 19 and 23, the resistor 20, the terminal 21, and the diode 22 constitute a charging circuit.

24 is a comparator, 26, 27 having one end connected to the terminal 25 to which the reference voltage _V3 is applied and the other end connected to the collector of the transistor 23;
connects one end to the collector of transistor 23,
The capacitor 26 and the resistor 27 constitute a second integrator 27a, which is a capacitor and a resistor whose other end is grounded. 28 is an output terminal of the comparator 24.

The operation of this embodiment configured as above will be described below.

First, when an audio multiplex signal in the United States is applied from the signal input terminal 1, the bandpass filter 2
The SAP carrier signal is selected, and then sufficiently amplified by the limiter amplifier 11. This output waveform is shown in FIG. 2a. When this waveform is supplied to a first integrator constituted by a capacitor 13 and a resistor 14 through the emitter follower transistor 12, the waveform shown in FIG. 2b is obtained. Here, the normal signal corresponds to the part shown by arrow A in FIG. 2a. However, as the signal weakens, noise increases and limiter amplification is no longer sufficient as shown by arrows A, U, and E in Figure 2a. Capacitor 13 and resistor 1 at this time
The integral waveform according to 4 is the reference voltage V ₂ as shown in Figure 2b.
will be lower than Therefore, transistors 15 and 16
The integrated waveform is sliced by the reference voltage _V2 using a differential amplifier consisting of a transistor 19 and a resistor 2.
0, the slice signal is converted into a current through a diode 22 and a transistor 23, and when it is integrated by a capacitor 26 and a resistor 27, the voltage shown in Fig. 2c is obtained.
_V5 is obtained. When this voltage is compared with the voltage _V3 at the terminal 25 of the comparator 24, the output terminal 28 is at a low level, and it is detected that there is no SAP carrier signal and that it is minimal. In addition, the output of the limiter amplifier 11 is constant for the carrier signal of a large level without noise, and there is no shortage of wave height as shown in the parts indicated by arrows A, U, and E in Fig. 2a, and as a result, as shown in Fig. 2b. The integral output V ₄ voltage is the reference voltage V ₂
Everything will be higher than. As a result, capacitor 2
No charging current flows to V 6 and the voltage V ₅ is zero.
Therefore, the output terminal 28 of the comparator 24 is at a high level, and it is detected that the SAP carrier signal is normally present.

As described above, according to this embodiment, when the SAP carrier signal is passed through the limiter amplifier 11 and the noise is large or the SAP carrier signal itself is small, the output from the limiter amplifier 11 stops or the level becomes extremely low. Based on this, by integrating this and comparing the integrated output with a reference voltage, the presence or absence of the SAP carrier signal can be reliably detected.

Effects of the Invention As explained above, according to the present invention, it is possible to provide a signal detection circuit that can detect the presence or absence of a carrier signal to the level where the limiter amplifier does not operate normally, and can also stably detect the magnitude of noise. Can be done.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram of a signal detection circuit according to an embodiment of the present invention, FIG. 2 is an operation waveform diagram of each part of the same, and FIG.
The figure is a circuit diagram of a conventional signal detection circuit, and FIG. 4 is an explanatory diagram of the same operation. 11... Limiter amplifier, 12, 15, 16, 1
9, 23... Transistor, 13, 26... Capacitor, 14, 20, 27... Resistor, 14a... First integrator, 18... Constant current source, 22... Diode, 24
...Comparator, 27a...Second integrator.

Claims (1)

[Claims] 1. A first integrating circuit that supplies the SAP carrier signal to the limiter amplifier and integrates the signal that has passed through the limiter amplifier, a comparison circuit that compares the integrated output with a reference level, and a second integrating circuit that integrates this comparative output. 1. A signal detection circuit comprising a second integrator and detecting a signal by determining whether the output of the second integrator has reached a specified level.