JPH0787557B2 - Signal detection circuit - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to SAP (Second Audio Pr) during audio multiplex broadcasting in a television receiver capable of receiving US multiplex audio broadcasting.
It relates to a signal detection circuit used when detecting the presence or absence of broadcasting.

Conventional techniques U.S. television sound multiplex broadcasting the audio signal to a conventional 15kHz and (L + R) signals, added a stereo pilot signal f _H (15.734 kHz) and presented signal 2 to the 2f _H and carrier, (LR) Perform stereo broadcasting using signals. Further, the second voice program (SAP) is composed of a signal having 5f _H as an FM carrier.

Therefore, it is necessary to determine the presence / absence of the SAP broadcast by the presence / absence of the 5f _H carrier, and a conventional example of the SAP carrier (f _O = 78.67 kHz) detection circuit will be described with reference to FIG. The input signal at the terminal 1 is an FM wave whose carrier is f _O , and the bandpass filter 2 removes unnecessary signals and the amplifier 3 widens 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 _O applied to the terminal 9, and inputs the result to the other input terminal of the AND gate 6.

Next, the relation between the detected voltage and the electric field strength of the broadcast wave is
Shown in the figure and described.

When the electric field strength is high, the presence or absence of the SAP carrier signal is detected by the signal level detection circuit 4 and the comparator 5 shown in FIG. 3 depending on the signal level detection voltage A (with carrier) and the signal level detection voltage B without carrier shown in FIG. And the presence or absence of the SAP carrier can be discriminated by the level of the comparator 5 set between the detection voltages A and B with respect to the reference voltage V ₁ .

However, the electric field is weaker on the left side than the level E in FIG. 4, and when the noise increases, the signal level detection voltage B becomes higher 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 of FIG.
To work. That is, when the noise level becomes high (when the noise detection voltage C exceeds the reference voltage V _O ), the output of the comparator 8 is set to low level and the output of the AND gate 6 is set to low level to prevent malfunction of the carrier discrimination output.

Here, if the reference voltages V ₁ and V _O of the comparators 5 and 8 are increased, even weaker electric field strength can be detected, but the difference between the carrier signal level detection voltage A and the reference voltage V ₁ becomes small, so that the broadcasting station etc. When the SAP carrier becomes small, it cannot be detected and the carrier reduction margin is lost.

Problems to be Solved by the Invention As described above, conventionally, SAP detection is apt to malfunction due to noise in a weak electric field of TV radio waves. If you try to make up for it, a small SAP signal cannot be detected. The present invention solves the above.

Means for Solving the Problems In order to solve the above problems, the signal detection circuit of the present invention is an SA
The output of the circuit that amplifies the P carrier signal is multiplied by the multiplication circuit, the integrated output that integrates the multiplication calculation force is compared with the reference level, and the comparator that determines whether this integrated output has reached the specified level Signal detection is performed.

Effect According to the present invention, the presence or absence of the SAP carrier signal can be accurately detected without being affected by noise, and unlikely to be influenced by the magnitude of the carrier level.

Embodiment An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 is a signal detection circuit diagram according to an embodiment of the present invention. The signal input terminal 1, the bandpass filter 2, the amplifier 3, the level shift 11 for shifting the DC level of the signal, the multipliers 12, 13,
14, 17, 18, 19, 20 to 24, a comparator 5 for comparing the output signal levels thereof, integrators 26, 27 for integrating the outputs thereof, and a comparator 8 for comparing the output levels thereof.

Here, the output of the amplifier 3 when a regular SAP signal is applied to the input terminal 1 is as shown in (a) of FIG. However, when the signal weakens, noise increases and (a) in FIG. 2 (a),
The waveform of the SAP carrier signal is missing as shown in (c). The above signals are applied to the bases of the multiplier transistors 12 and the bases of the multiplier transistors 17 and 20 through the level shift 11. At this time, a DC bias near the center of the carrier signal is applied to the transistors 14 and 18 and 19 which form a differential with the above transistor.

In the multiplier shown in FIG. 2 (a), the transistor 12 is turned on and the transistors 17 and 20 are turned on in the positive half cycle of the signal, but the transistor 14 is turned off. Current flows through the transistor 12 and the transistor 17 to the resistor 22, and in the negative cycle, the transistor 12 is off and the transistor 14 is on. The transistors 17 and 20 are off and the transistors 18 and 19 are on. The current of 13 flows through the transistors 14 and 19 to the resistor 22, and in both cases, the current of the constant current source 13 entirely flows to the resistor 22 and the terminal 21
Is a positive voltage as shown in (a) of FIG. 2 (b).
However, in the case of noise or no signal, (a) in Fig. 2 (a).
It looks like (c). That is, since almost the same voltage is applied to the transistors 12 and 14, half of the current of the current source 13 flows to the transistors 12 and 14, and the transistors 17, 18, 1
Current flows evenly through 9 and 20. As a result, 1 / of the constant current source 13
Two electric currents flow, and the voltage of the calculation force 21 drops, and (a) in FIG. 2 (b). It looks like (c). Further, when the carrier signal is at a level slightly higher than the bias (2) level in FIG. 2A (when the radio wave is weak), for example, when a current of 3/5 of the current source 13 flows through the transistor 12, the transistor 17 receives 3
/ 5 squared 9/25 current flows. That is, when the carrier signal is normal and noise increases and the carrier signal is distorted and becomes small, the difference becomes a squared difference in the multiplication circuit, and the carrier signal discrimination accuracy is improved. Also, one of the carrier signals
Since one waveform is discriminated as noise, it is easy to detect noise, and the carrier signal discrimination speed is increased.

Therefore, when the applied voltage is calculated by the comparator 5 at the level of the bias 3 and its output is integrated by the integrators 26 and 27, a positive voltage is obtained when the carrier signal disappears as shown in FIG. 2 (c). By further shaping the waveform of this voltage by the comparator 8, the comparator output terminal 10 becomes zero if the SAP carrier signal is present correctly, and becomes a positive voltage if there is no carrier signal or the carrier signal is incorrect, and the presence or absence of the carrier signal can be detected.

In this circuit, if the amplification of amplifier 3 is set high, SAP
Whether or not there is a carrier can be reliably detected.

Effects of the Invention As described above, according to the present invention, the presence or absence of the SAP carrier signal can be detected up to a level at which the amplifier cannot operate,
It is possible to provide a signal detection circuit that can stably detect the amount of noise.

[Brief description of drawings]

FIG. 1 is a circuit diagram of a signal detection circuit in 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 its operation. is there. 12,14,17,18,19,20,23,24 …… Transistor, 22,26 ……
Resistance, 27 …… Condenser, 5,8 …… Comparator, 1
3 ... Constant current source.

Claims (1)

[Claims] An amplifier for amplifying a carrier signal, a multiplication circuit having a two-stage differential amplifier for multiplying a signal passed through the amplifier, a comparison circuit for comparing the output of the multiplication circuit with a reference level, and A signal detection circuit comprising: an integrator that integrates the output of the comparison circuit, and performing signal detection by determining whether or not the output of the integrator has reached a specified level.