JPS61184064A - Signal detecting circuit - Google Patents

Abstract

PURPOSE:To detect accurately by presence of a second audio program SAP carrier without being affected by noise by discriminating whether or not an output of an integration device reaches a specified level to detect the signal. CONSTITUTION:An output of an amplifier 3 when a normal SAP signal is fed to an input terminal 1 is as shown in a figure (a) A, and when the signal becomes weak, the noise is increased and the waveform of the SAP carrier signal is missing as shown in a figure (a) B, C. An output 21 of a multiplier is compared with a bias 25 by a comparator 25 and its output is integrated by integration devices 26, 27, a positive voltage is obtained when no carrier signal exists as shown in a figure (c). The voltage is subjected to waveform shaping by a comparator 8, then the level of an output terminal 10 of the comparator is zero when the SAP carrier signal exists correctly and the level goes to a positive voltage when no carrier signal exists or incorrect carrier exists to discriminate the presence of the carrier signal.

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, which is used for 5AP (5econd) during audio multiplex broadcasts.
Audio Program) This relates to a signal detection circuit used for detecting the presence or absence of broadcasting.

Conventional technology In US television audio multiplex broadcasting, the conventional audio signals for up to 15 songs are (L-)-R) signals, and in addition fH (16,73
4k) lI) stereo pilot signal, and 2f
Stereo broadcasting is performed using a signal 2 (LR) signal with H as a carrier. Furthermore, the second audio program (SA
P ) is composed of a signal using 5 fH as an FM carrier.

Therefore, the presence or absence of SAP broadcasting must be determined by the presence or absence of the 5fH carrier, and the SAP carrier (fo = 78
．． A conventional example of the erh) detection circuit is shown in FIG. 1 and will be described.

The terminal 10 input signal is f above. The bandpass filter 2 removes unnecessary signals, and the amplifier 3 amplifies the signal to a certain level.

The signal is first subjected to peak detection in the signal level detection circuit 4, and the voltage comparator 6 determines whether the detected voltage is above or below the reference voltage 71 applied to the terminal 11.
It is applied to one input terminal of the ND gate 6. On the other hand, the output of the amplifier 3 is also applied to a noise detection circuit 7 to detect noise when the broadcast radio waves become weak. The detected noise voltage is converted into a reference voltage v applied to the terminal 9 in the comparator 8.
Determine whether it is higher or lower than 0 and apply the result to AND gate 6.
input to the other input terminal.

Next, the relationship between the detected voltage and the electric field strength of the broadcast radio wave is determined by the second
It is shown and explained in the figure.

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 shown in FIG.
The difference between the signal level detection voltage A (with carrier) in Figure 2 and the signal level detection voltage B without carrier is clear.
The presence or absence of the SAP carrier can be determined based on the level of the reference voltage v1 of the comparator 5 set between the rain detection voltages A and B.

However, the electric field is weak on the left side of level E in FIG. 2, 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. 1 is activated. That is, when the noise level increases (noise detection).

When the 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.

If the reference voltages v1 and vo of the comparator 6.8 are increased, even weaker electric field strengths can be detected, but the difference between the carrier signal level detection voltage A and the reference voltage v1 becomes smaller, and the SAP carrier is If it becomes small, it cannot be detected and a problem arises in which carrier reduction margin is lost.

Problems to be Solved by the Invention As mentioned above, conventional SAP detection tends to malfunction due to noise in the weak electric field of television waves. If an attempt is made to compensate for this, small SAP signals cannot be detected. The present invention solves the above problems.

Means for Solving the Problems In order to solve the above problems, the signal detection circuit of the present invention uses S
A comparator that multiplies the output of the circuit that amplifies the AP carrier signal using a multiplication circuit, integrates the multiplication output, compares the integral output with a reference level, and determines whether or not this integral output has reached a specified level. It performs signal detection.

According to the present invention, it is possible to accurately detect the presence or absence of an SAP carrier signal without being affected by noise and without being influenced by the magnitude of the carrier level.

EXAMPLE Hereinafter, an example of the present invention will be described based on the drawings.

FIG. 1 is a signal detection circuit diagram according to an embodiment of the present invention, which includes a signal input terminal 1, a bandpass filter 2, an amplifier 3, a level shifter 11 for shifting the DC level of the signal, and a multiplier 1.
2, 13, 14, 17°18, 19, 20-24,
It consists of a comparator 5 for comparing the output signal levels, integrators 26 and 27 for integrating the output signal levels, and a comparator 8 for comparing the output signal levels.

Here, when a regular SAP signal is applied to the terminal 1, the output of the amplifier 3 is as shown in FIG.

However, as the signal weakens, the noise increases (in Figure 2 (, )).
The waveform of the SAP carrier signal is missing as shown in () and (tsu). The above signals are applied to the bases of multiplier transistors 17 and 20 via the pace and level shift 11 of multiplier transistor 12. At this time, a DC bias near the center of the carrier signal is applied to the transistors 14, 18, and 19 that are differential with the above transistors.

Here, when the multiplier is shown in FIG. 2 (, ), the signal current flows through the resistor 22 and the current from the constant current source 13 flows through the terminal 21 both during the positive half cycle and during the negative half cycle. The voltage becomes a positive voltage as shown in FIG. 2(b). However, in the case of noise or no signal, (a) in Figure 2 (,).

It becomes like (tsu). At this time, a current of % of the constant current source 13 flows through the transistor 24 and the resistor 22 of the output of the multiplier, and the voltage of the multiplier output 21 decreases, as shown in (A) and (T) of FIG. 2(b). become.

Therefore, this multiplied output voltage is compared by a comparator 5 at the level of bias 3, and the output is sent to an integrator 26.27.
When the carrier signal disappears, a positive voltage is obtained as shown in FIG. 2(c). By further shaping the waveform of this voltage using a comparator, the comparator output terminal 1o becomes zero if the carrier signal of SAP is present correctly, and becomes a positive voltage if the carrier signal is absent or incorrect, making it possible to detect the presence or absence of the carrier signal.

In this circuit, if the increase degree of amplifier 3 is set high, the SA
The presence or absence of the P carrier can be reliably detected regardless of its size.

As described in detail, according to the present invention, the presence or absence of an SAP carrier signal can be detected to a level where the amplifier cannot function.
It is possible to provide a signal detection circuit that can stably detect noise regardless of its magnitude.

[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 a waveform diagram for explaining its operation, Fig. 3 is a circuit diagram of a conventional example, and Fig. 4 is a waveform diagram for explaining its operation. be. 12.14, 17.1B, 19, 20, 23°24...
...Transistor, 22,26...Resistor,
27... Capacitor, 6,8... Comparator, 13... Constant current source. Figure 1
15---ha1as(2)l
ff---Hegoas fl1 2f----Hato Figure 2

Claims (1)

[Claims] An amplifier that amplifies the carrier signal, a multiplication circuit that multiplies the signal passed through this amplifier, a comparison circuit that compares the multiplication output with a reference level, and an integrator that integrates this comparison output are provided. 1. A signal detection circuit that detects a signal by determining whether or not the signal has reached a specified level.