JPH04192979A - Stereo/sap detecting circuit - Google Patents

Abstract

PURPOSE:To prevent malfunction for stereo discrimination in the case of an idle channel by ANDing or reversely ANDing the output of a stereo detection comparator with hysteresis and the output of a noise detection comparator with hysteresis. CONSTITUTION:The output of a stereo detection comparator 7 with hysteresis and an output applied from a noise detection circuit 6 through a noise detection comparator 9 with hysteresis are respectively inputted to the two inputs of an AND circuit 11. Thus, even when the same horizontal synchronizing signal as the pilot signal of stereo jumps in like the case of the idle channel, the output of the noise detection comparator 9 is set at a low level since a noise level is increased, and the malfunction of a stereo detection circuit 4 can be prevented.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to an American television audio multiplexing demodulator, and particularly to stereo detection and S A P (Second Audio P).
The present invention relates to a detection circuit that is effective in preventing malfunction of detection (abbreviation of ``program'').

[Conventional technology 1] The audio signal of American television audio multiplex broadcasting (composite signal of S IF output) is converted into 2f++ (
Multiplexes AM modulation signals with a carrier frequency of 31.5 times the frequency of the horizontal synchronization signal (H2), and during bilingual broadcasting, SAP is sent to an even higher frequency as the second audio.
Signal (Second Audi.

Program) is an FM with a center frequency of 5fN.
It is multiplexed as a modulation signal. Furthermore, during stereo broadcasting, fH is multiplexed as a pilot signal in order to determine the presence or absence of stereo broadcasting.

Therefore, conventionally, in order to discriminate a stereo signal, the American TV audio multiplexing demodulator uses fH(15,
734k) lz) is detected as a stereo detection output signal, and the SAP signal is determined by detecting the level of the 5APF'M modulation signal (midpoint frequency = 5 f o ) included in the composite signal and determining the SAP signal. It was used as a detection output signal.

A conventional example will be explained using FIG.

Composite signal input terminal 1 is stereo detection circuit 4, SA
It is connected to the inputs of the P detection circuit 5 and the noise detection circuit 6, and the output of the stereo detection circuit 4 is connected to the stereo detection output terminal 2 via a stereo detection comparator 7 with hysteresis. Output of SAP detection circuit 6 and noise detection circuit 5
The outputs of each SAP detection comparator 8 with hysteresis
and are connected to each input of an AND circuit 12 via a noise detection comparator 9 with hysteresis, and the output of the AND circuit 12 is connected to the SAP detection output terminal 3. In addition, in FIG. 3, 24, 25, and 26 are reference bias circuits.

In the above configuration, the SAP detection incorrectly determines that the SAP signal is present due to increased noise even if there is no SAP signal during the weak electric field of the TV radio wave because the SAP FM modulation signal included in the composite signal is smaller than the main sub signal. SA is used to prevent malfunctions due to increased noise due to weak electric fields, etc.
The output of the P detection comparator 9 and the output outputted from the noise detection circuit 6 which detects only the noise component via the noise detection comparator 9 (low level output when the noise level is large) are respectively input to the two AND circuits 12. in,
When the electric field is weak, even if the output of the SAP detection comparator 8 outputs a high level signal for misjudgment, the output of the noise detection comparator 9 outputs a low level signal with noise.
The output of the AND circuit 12 becomes low level, and it is determined that there is no SAP signal. SAP with its noise malfunction prevention function
The detection output (output of the AND circuit 12) was obtained and used as an SAP display or a mono/stereo/SAP switching signal.

[Problems to be Solved by the Invention] The conventional stereo/SAP discrimination circuit described above is useful for preventing malfunction of the SAP detection circuit when the electric field of television waves is weak. The problem is that the stereo display may turn on when used as a TV or VTR set.

An object of the present invention is to provide a stereo/SAP discrimination circuit that does not malfunction in stereo discrimination when there is an empty channel.

[Means to solve the problem]

The stereo/SAP detection circuit of the present invention includes a stereo detection circuit that detects a pilot signal included in a composite signal and outputs a stereo detection output signal, and a stereo detection circuit that detects the level of an SAPFM modulation signal included in the composite signal and outputs an SAP detection output signal. A SAP detection circuit that outputs a signal, a noise detection circuit that detects only the noise component included in the composite signal, and a hysteresis system in which the output of the stereo detection circuit is connected to the non-inverting input terminal, and the reference bias circuit is connected to the inverting input terminal. A stereo detection comparator, an SAP detection comparator with hysteresis in which the output of the SAP detection circuit is connected to the non-inverting input terminal, a reference bias circuit is connected to the inverting input terminal, the output of the noise detection circuit is connected to the non-inverting input terminal, and the reference bias circuit is connected to the non-inverting input terminal. a noise detection comparator with hysteresis connected to the inverting input terminal; a first gate circuit that ANDs or negates the AND of the output of the stereo detection comparator and the output of the noise detection comparator; and the output of the SAP detection comparator and noise. and a second gate circuit that performs logical product or negation of the logical product of the outputs of the detection converters.

[For production]

By taking the AND (or inverted AND) of the output signal of the noise detection comparator and the output signal of the stereo detection comparator, the noise level will increase even if the same horizontal sync signal as the stereo pilot signal jumps in, as in the case of an empty channel. Therefore, the output of the noise detection comparator is at a low level, and malfunction of the stereo detection circuit is prevented.

[Example] Next, an example of the present invention will be described with reference to the drawings.

FIG. 1 is a block diagram of a stereo/SAP discrimination circuit according to an embodiment of the present invention.

The composite signal enters from the composite signal input terminal 1 and passes through the stereo filter 13 and the noise BPF 20 and 5APB.
Enter PF22. The output of the stereo filter 13 is connected to one input of the phase comparator 14, and the output of the phase comparator 14 is connected to the other input of the phase comparator 14 via a loop filter (hereinafter abbreviated as LP) 15 and a VCO 16. ing. PL with these phase comparator 12, LP15, and VCO16
An L circuit is configured. The output of VCOl 6 is 90°
It is connected to one input of a phase comparator 18 via a phase shifter 17, the output of the stereo filter 13 is connected to the other input of the phase comparator 18, and the output of this phase comparator 18 is connected to a low-pass filter ( It is connected to one input of an AND circuit 11 via an LPF (LPF) 19 and a stereo detection comparator 7 with hysteresis. These phase comparators 18 and L
The PF19 constitutes a synchronous detection circuit for stereo pilot signals.

The output of the SAP bandpass filter 22 is connected to one input of the AND circuit 12 via the level detection circuit 23 and the SAP detection comparator 8 with hysteresis. Similarly, the output of the noise BPF (approximately 11.1f) 20 (a band near which high frequency components of various signals are not present) is sent via the level detection circuit 21 and the upper 2 noise detection comparators (inverting comparators) 8 with steresis. Each AND circuit 1
1.12 is connected to the other input.

With the above configuration, in the same way as the malfunction prevention of the conventional SAP detection circuit 5, the stereo detection circuit 4 also performs an AND operation with the output signal of the noise detection circuit 5 and the output signal of the stereo detection circuit 4, so that the stereo detection circuit 4 can be Even if the same horizontal synchronizing signal as the pilot signal is input, the noise level has increased, so the output of the noise detection comparator 8 becomes a low level, thereby preventing the stereo detection circuit 4 from malfunctioning.

FIG. 2 is a circuit diagram of a stereo/SAP discrimination circuit according to a second embodiment of the present invention.

The circuit shown in FIG. 2 has the same function as that shown in FIG.
In the circuit, the output of 5APBPF22 is connected to one input of phase comparator 27, and the output of phase comparator 27 is connected to LP.
28 and the other input of the phase comparator 27 through the VCO 29, the output of the VCO 29 is connected to the 90" phase shifter 30.
The output of the 5AP BPF 22 is connected to the other input of the phase comparator 31, and the output of the phase comparator 31 is connected to the LPF 32 and the SAP detection comparator 8 with hysteresis. The configuration is such that it is connected to one input of the AND circuit 12. Since the band is limited by the PLL circuit rather than the SAP detection circuit 5 of FIG. 1, the level of malfunction due to noise is slightly improved.

[Effects of the Invention] As explained above, the present invention prevents malfunctions in stereo discrimination during empty channels by taking the AND or inverse AND of the output of the stereo detection comparator with hysteresis and the output of the noise detection comparator with hysteresis. It has a preventive effect.

[Brief explanation of the drawing]

1 and 2 are block diagrams of a stereo/SAP discrimination circuit according to an embodiment of the present invention, and FIG. 3 is a block diagram of a conventional stereo/SAP discrimination circuit.
It is a block diagram of a P discrimination circuit. １・・・・・・・・・Composite signal input terminal,
2......Stereo detection output end, 3.
......-3A P detection output terminal, 4...
...... Stereo detection circuit, 5...
...SAP detection circuit, 6...
・Noise detection circuit, 7...Stereo detection comparator with hysteresis, 8...Noise detection comparator with hysteresis, 9... ......SAP detection humpator with hysteresis, 11.12-...AND circuit, 13...Stereo filter, 14
．． 27-... Phase comparator, 15.28... Loop filter (LF), 16
, 29--・-V C01 17, 30...90@phase shifter, 18.31--
...Phase comparator, 19, 32...LPF. 20, 22...BPF, 21.23...Level detection circuit, 24, 25.
26...Reference bias circuit.

Claims (1)

[Claims] 1. A stereo detection circuit that detects a pilot signal included in a composite signal and outputs a stereo detection output signal; and a stereo detection circuit that detects the level of an SAPFM modulation signal included in the composite signal and outputs an SAP detection output signal. A stereo detection circuit with hysteresis in which the output of the SAP detection circuit that outputs, the noise detection circuit that detects only the noise component included in the composite signal, and the stereo detection circuit is connected to the non-inverting input terminal and the reference bias circuit is connected to the inverting input terminal. A comparator, an SAP detection comparator with hysteresis in which the output of the SAP detection circuit is connected to the non-inverting input terminal, the reference bias circuit is connected to the inverting input terminal, the output of the noise detection circuit is connected to the non-inverting input terminal, and the reference bias circuit is connected to the inverting input terminal. a noise detection comparator with hysteresis connected to the input terminal; a first gate circuit that performs the logical product or negation of the logical product of the output of the stereo detection comparator and the output of the noise detection comparator; and the output of the SAP detection comparator and the noise detection comparator. and a second gate circuit that performs an AND or a negation of the outputs of the stereo/SAP detection circuit.