JPH053953B2 - - 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 15 kHz as L+R signals, stereo pilot signals on _H (15.734 kHz), and signal 2 (L-R) with 2 _H as the carrier. Broadcast. Furthermore, as a second audio program (SAP)
It consists of a signal with _5H as the FM carrier. Therefore, it is necessary to determine whether SAP is broadcasting or not based on the presence or absence of the _5H carrier. The conventional SAP carrier ( _O = 78.67kHz) detection circuit described above is shown in FIG. 3 and will be explained. The input signal inputted from the input terminal 1 is an FM wave using the above _O as a carrier, unnecessary signals are removed by the bandpass 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,
It is applied to one input terminal of AND 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 transferred to the comparator 8
, it is determined whether it is higher or lower than the reference voltage V _O applied to the terminal 9 , and the result is input to the other input terminal of the AND 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. That is, when the noise level becomes high (when the noise detection voltage C exceeds the reference voltage _VO ), 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 the voltage and V _O 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 V ₁ becomes smaller, making it difficult for broadcasting stations, etc.
This has the problem that if the SAP carrier becomes small, it cannot be detected and there is no margin for carrier reduction.

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 extracts and amplifies the SAP signal from the input American audio multiplex signal using a filter, and one of the amplified signals is used together with the clock signal. Pass it through the gate, count the output of the gate with a counter,
The other of the amplified signals is used as a reset pulse for the counter, and the output of the counter is decoded.
The present invention is characterized in that an integrator is provided to integrate the decoded output, and a detector is provided to detect whether or not the integrator output reaches a specified level.

Therefore, according to the present invention, it is possible to provide a signal detection circuit that can accurately detect the presence or absence of an SAP carrier signal without being affected by noise and not easily influenced by the magnitude of the carrier level.

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

FIG. 1 is a diagram of an SAP signal detection circuit in an embodiment of the present invention, in which a bandpass filter 2 extracts an SAP signal from an audio multiplexed signal applied to a terminal 1.
, amplifiers 3 and 17, the output of amplifier 3 and terminal 1
AND with the clock signal added to 2 as input
a counter 15 that counts the output of the AND gate 13 and to which the output of the amplifier 3 is added as a reset pulse; a decoder 16 that decodes the output of the counter 15; a resistor 19 and a capacitor 20 forming an integrating circuit; It consists of a comparator 21 to which a predetermined bias potential is applied to the − input terminal and the output of the amplifier 17 is applied to the + input terminal.

Here, when an audio multiplexed signal containing a regular SAP signal is applied to input terminal 1, bandpass filter 2 first separates only the SAP signal, and amplifier 3 performs limiter amplification, resulting in a waveform as shown in A in Figure 2. Become.

However, as the radio waves become weaker, noise increases in the signal, and the output of the amplifier 3 has a signal duty that deviates significantly from 50%, as shown in part A in Figure 2a. This signal and an external clock signal (signal with a sufficiently higher frequency than the SAP carrier) are connected to AND gate 1.
3, a signal with a different number of pulses is obtained depending on the pulse width of the output signal of the amplifier 3, as shown in FIG. 2b. This signal is counted by a counter 15. The counter 15 is reset using the output of the amplifier 3. Then, the count output of the counter 15 is decoded by the decoder 16. The function of this decoder 16 is to output one pulse when the counter 15 counts a certain value or more.

Here, since the count number in the part A of FIG. 2 is a certain constant value, no decode output is output. However, in the noisy part of FIG. This state is the second
Shown in Figure c. Therefore, the output of this decoder 16 is amplified by an amplifier 17, and the output is integrated by an integrator consisting of a resistor 19 and a capacitor 20 (the integrated output is shown in FIG. 2d). When compared with the applied bias voltage, the comparative output will be at Low level when only the SAP signal is input, as shown in Figure 2e, and on the other hand, when noise increases, the output will be at Hi level.
The presence or absence of SAP signals can be detected. Even when there is no SAP signal, it can be detected as noise in the same way as above.

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 an SAP carrier signal to a level where the amplifier cannot function, and can stably detect the presence or absence of an SAP carrier signal regardless of the amount of noise.

[Brief explanation of drawings]

FIG. 1 is a block diagram of a signal detection circuit according to an embodiment of the present invention, FIGS. 2 a to e are waveform diagrams for explaining its operation, FIG. 3 is a block diagram of a conventional signal detection circuit, and FIG. The figure is a waveform diagram for explaining the operation. 1...Input terminal, 2...Band pass filter,
3, 17...Amplifier, 15...Counter, 16...
...Decoder, 19...Resistor, 20...Capacitor, 21...Comparator.

Claims (1)

[Claims] 1 A filter that extracts the SAP carrier signal from the audio multiplexed signal, an AND gate that receives the output of this filter and a clock signal whose frequency is sufficiently higher than the SAP carrier signal, and the output of this AND gate is counted and the output of the above filter is reset. A counter that is added as a pulse, a decoder that decodes the output of the counter, an integrating circuit that integrates the output of the decoder, and means for detecting whether the output of the integrating circuit reaches a predetermined level. Signal detection circuit.