JPS6122386Y2 - - Google Patents

Description

[Detailed description of the invention] (a) Industrial application field The present invention relates to a television audio multiplex signal receiver, and particularly relates to preventing malfunction thereof.

(b) Prior Art FIG. 1 shows the structure of a television audio multiplex signal, and this multiplex signal is composed of a main channel signal A, a sub-channel signal B, and a control channel signal C. The control channel signal uses the control signal subcarrier fco (approximately 55.125KHz) as two types of control signals.
Either f ₁ or f ₂ is AM modulated,
Control signal f ₁ (922.5Hz) in case of dual audio broadcasting
But also, for stereo audio broadcasting, the control signal f ₂
(982.5Hz) are used.

By the way, since a television receiver that receives this television audio multiplexed signal usually employs an intercarrier system, amplitude modulation by the video signal appears as noise in the audio detection output due to intercarrier detection. This noise occurs irregularly over almost the entire transmission band of the TV audio multiplexed signal, and when it matches the frequency of the previous control signal f ₁ or f ₂ , this noise causes the receiver to switch modes. The circuit operates (that is, malfunctions). For example, a receiver may malfunction in switching to stereo playback mode or dual audio playback mode even though it is receiving monaural broadcasting.

Therefore, in conventional receivers, the reproduction mode is switched using a detection output indicating the presence or absence of a sub-channel signal as well as a discrimination output of the control signal. This point will be explained below with reference to FIG. In the figure, 1 is an input terminal into which the aforementioned TV audio multiplexed signal is introduced, and after the multiplexed signal is amplified by a preamplifier 2, it is passed through a low-pass filter 3 for extracting the main channel signal and a sub-channel signal and control terminal. The first one for channel signal extraction.
The signal is applied to the bandpass filter 4. The sub-channel signal is demodulated by an FM demodulation circuit 5, passed through a low-pass filter 6, and given as one input to an audio signal processing circuit 7 including a stereo matrix circuit. Further, the main channel signal from the low-pass filter 3 is introduced as the other input of the processing circuit 7.

On the other hand, the control channel signal extracted by the second bandpass filter 8 from the output signal of the first bandpass filter 4 is detected by the AM detection circuit 9, and the control channel signal extracted by the above-mentioned control signal discriminating circuit 10 is detected by the AM detection circuit 9.
f ₁ and f ₂ are determined. This discrimination output is given as one input of the logic circuit 11, and
The output of a sub-channel signal detection circuit 12 for detecting the presence or absence of the sub-channel signal is applied to this circuit 11 as the other input.

When the output of the detection circuit 12 indicates the absence of the sub-channel signal and the output of the discrimination circuit 10 also indicates the absence of the control signals f ₁ and f ₂ , the logic circuit 11 operates as a signal processing circuit. 7 to switch to monaural playback mode. Further, when the output of the detection circuit 12 indicates the presence of a sub-channel signal, if the output of the discrimination circuit 10 indicates the first control signal _f1 , switching to the dual audio reproduction mode; If the signal indicates the second control signal _f2 , the signal processing circuit 7 is controlled to switch to the stereo reproduction mode.

However, in the configuration shown in FIG. 2, even if the above-described malfunction due to noise can be eliminated, malfunction due to harmonics of the horizontal frequency h (approximately 15.75 KHz) of the television video signal remains a problem. This is because when the third and fourth harmonics of the horizontal frequency h and the audio signal of a specific frequency interfere with each other,
Two frequency components fa and fb near the carrier frequency of 3.5h of the control channel signal are generated, and
The beats of fa and fb generate a frequency signal that is substantially the same as the control signal f ₁ or f ₂ and is detected by the discrimination circuit 10, and also corresponds to the carrier frequency of the sub-channel signal. This is because the second harmonic 2fh of the horizontal frequency may be detected by the detection circuit 12.

In order to eliminate malfunctions caused by harmonics of the horizontal frequency h, the detection sensitivity of the sub-channel signal detection circuit 12 is set relatively low, and
This means that it is sufficient to prevent harmonics from being detected by this detection circuit. However, if the sensitivity of this detection circuit is always set low, it will not be possible to reliably detect the sub-channel signal when the sub-channel signal is not large enough, such as during the transition period immediately after the power is turned on to the receiver. Failure to detect this may result in malfunctions, or it may take a relatively long time to reach normal playback mode.

(c) Purpose of the invention This invention was created in view of the above points,
It is an object of the present invention to provide a television audio multiplex signal receiver that can prevent both malfunctions caused by harmonics of horizontal frequencies and malfunctions immediately after power is turned on, and can shift to a normal playback mode immediately after power is turned on. do.

(d) Structure of the invention The television audio multiplex receiver of the invention has a sufficiently high detection sensitivity of the sub-channel signal detection circuit 12 shown in FIG. 1 in a transient state immediately after power-on, and
This configuration is characterized in that in a steady state, the switching is made to be lower than that.

(e) Embodiment Figure 3 shows an embodiment of the sub-channel signal detection circuit used in the television audio multiplex receiver of the present invention. The detection circuit 14 is composed of an envelope detection circuit 14 consisting of a detector or the like, and a level comparison circuit 15 such as a Schmitt circuit.

The first input to the terminal T ₁ of the amplifier circuit 13
The sub-channel signal from the bandpass filter (4 in Figure 2) is guided to the emitter of an emitter follower transistor _Q1 , while one transistor of a differential pair has its common emitter connected to a constant current source S.
It is applied to the base of Q ₂ through a resistor R ₂ and on the other hand through a resistor R ₃ to the base of the other transistor Q ₃ . A capacitor is connected between the base of the above transistor _Q3 and the ground point.
C ₁ and resistor R ₅ are connected in series, and the resistance
A switching transistor _Q4 is connected in parallel with _R5 . The base of this transistor _Q4 is connected via a resistor _R7 to the midpoint between the capacitor _C2 and the resistor _R6 , which are connected between the power supply (+Vcc) and the ground point.

In such a detection circuit, if we consider a transient state immediately after turning on the power switch of the receiver (not shown), in this state, a resistor is connected through capacitor _C2 .
Current flows through _R6 and the positive voltage _generated across this resistor is applied to the base of switching transistor _Q4 , which turns on and shorts across resistor _R5 . Therefore, from the emitter of the emitter follower transistor _Q1 to the resistor
The sub-channel signal derived through _R3 is connected to capacitor _C1 and the above switching transistor.
A ground fault will occur through Q ₄ . therefore,
The above-mentioned sub-channel signal is input to only one of the differential pair transistors, _Q2 , and only the DC bias voltage is applied to the base of the other transistor, _Q3 . Therefore, at this time, the apparent amplification degree of the amplifier circuit 13 is increased, and therefore, the weak signal sub-signal is caused by the multiplex signal receiving circuit in the receiver not operating sufficiently. Even if a channel signal is input, a large sub-channel signal will be obtained from the collector of transistor _Q3 . Since the DC voltage obtained by detecting this signal by the envelope detection circuit 14 sufficiently exceeds the detection level of the level comparison circuit 15, the detection output of the sub-channel signal immediately appears at the terminal _T2 .

On the other hand, in a steady state after a certain period of time after the power switch is turned on, no current flows through the resistor _R6 and _the switching transistor _Q4 is turned off. The sub-channel signal is voltage-divided by a resistor _R3 and input to the base of a transistor _Q3 together with the aforementioned DC bias voltage. At this time, the above sub-channel signal is input to the base of transistor _Q2 without being divided and in the same phase.
This means that the apparent amplification degree of the amplifier circuit 13 has become smaller. However, at this time, the aforementioned multiplex signal receiving circuit etc. are operating sufficiently, so a sufficiently large sub-channel signal is obtained from the collector of transistor _Q3 , and therefore this signal is used in the same way as immediately after the power is turned on. This can be reliably detected by the level comparison circuit 15. Furthermore, since the differential pair operates as described above even for the second harmonic signal of a weak horizontal frequency input to the terminal _T1 , this signal is not amplified significantly, and therefore its detection Output is terminal T ₂
It will not appear on .

(f) Effects of the invention According to the TV audio multiplex signal receiver of the invention,
It is possible to prevent both malfunctions caused by harmonic signals of the horizontal frequency of a television video signal and malfunctions in a transient state immediately after the power is turned on, and it is also possible to shorten the time required to reach a normal operating state.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the structure of a television audio multiplex signal.
FIG. 2 is a block diagram showing a schematic configuration of the main parts of the receiver of the present invention, and FIG. 3 is a circuit block diagram showing an embodiment of a sub-channel signal detection circuit used in the receiver. 4...First band pass filter, 8...Second band pass filter, 9...AM detection circuit, 10...
...Control signal discrimination circuit, 11...Logic circuit, 7...
Audio signal processing circuit, 12...Sub channel signal detection circuit, 14...Envelope detection circuit, 15...Level comparison circuit.

Claims (1)

[Scope of Claim for Utility Model Registration] (1) The above-mentioned method according to the output of a detection circuit that detects the presence or absence of a sub-channel signal of a television audio multiplex signal and the output of a discrimination circuit that discriminates a control signal indicating the content of the multiplex signal. An audio multiplex receiver for switching the reproduction mode of multiplexed signals includes means for detecting a transient state immediately after turning on a power switch, and means for switching the detection sensitivity of the detection circuit between two modes, and detecting the detection sensitivity according to the output of the detection means. A television audio multiplex signal receiver, characterized in that the switching means is controlled to switch the detection sensitivity of the detection circuit so that it is high immediately after power is turned on and becomes low in a steady operating state. (2) The detection circuit includes a differential pair of transistors for amplifying the subchannel signal, and inputs the subchannel signal to the base of each of the differential pair transistors, and also inputs the subchannel signal to the base of one of the differential pair transistors. Claim 1: The detection sensitivity is switched by switching the signal magnitude between immediately after the power is turned on and during steady operation.
The television audio multiplex signal receiver described in Section 1. (3) The differential pair transistor has a first resistor and a capacitor connected in series between one base and a reference potential point, and a second resistor and a capacitor connected to the base.
A sub-channel signal is input through the second resistor, and a switching transistor that is turned on only in a transient state immediately after power-on is connected in parallel to the second resistor. A television audio multiplex signal receiver according to scope 2.