JPH0727704Y2 - Audio signal detection circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] The present invention relates to an audio circuit for outputting a received audio signal in a receiver capable of receiving FM broadcasting such as a television receiver. More specifically, the present invention relates to an audio signal detection circuit including an audio muting circuit that mutes a sound output from a speaker when there is no signal.

[Prior Art] As is conventionally known, a television receiver is provided with an audio circuit for outputting a received audio signal.

This audio circuit 20 is an intermediate frequency amplifier as shown in FIG.
11, limiter amplifier 12, FM detection circuit 2, detection coil 3,
It is composed of an attenuator 14, an audio output circuit 15 and a speaker 16.

With this configuration, it is not possible to eliminate noise when there is no signal (state where no audio signal is received).
There is a drawback that noise is output from the speaker 16 even when there is no signal.

Therefore, in order to eliminate this drawback, conventionally, the synchronization signal detection circuit 30 is provided separately from the audio circuit.

As shown in FIG. 3, the sync detecting circuit 30 is composed of a sync separating circuit 31 and a signal detecting circuit 32.

The sync separation circuit 31 separates and extracts the horizontal sync signal, and the horizontal sync signal s is supplied to the signal detection circuit 32.

The signal detection circuit 32 uses the flyback transformer pulse p to determine whether the horizontal synchronizing signal s is a received signal or noise.

The detection signal t of the signal detection circuit 32 is supplied to the attenuator 14,
The mute is applied according to the detection signal t.

[Problems to be Solved by the Invention] By the way, in the case of controlling the muting by the above-mentioned configuration, a circuit for detecting an audio signal, that is, a synchronization signal detecting circuit 30 is required in addition to the audio circuit 20, which increases the cost. There was a problem that became a factor.

Further, when the voice circuit 20 and the synchronization signal detection circuit 30 are provided on separate substrates, the flyback transformer pulse signal line may be laid around for a long time.

As is well known, since the flyback transformer pulse p is a high-frequency signal, spurious tends to occur when the above-mentioned wiring of the signal line becomes long, and this spurious affects other circuits. In particular, there is a problem that causes arcing (black or white vertical lines) and raster stripes (vertical stripes) on the screen.

Therefore, in the present invention, such a conventional problem is simply solved, and with simplification of the circuit structure,
It proposes an audio signal detection circuit that eliminates the influence on other parts.

[Technical Means for Solving the Problems] In order to solve the above problems, in the present invention, the FM having a S-shaped characteristic in the relationship between the frequency deviation and the detected output voltage.
The detection circuit, the FM detection coil set to shift the center frequency of the audio signal detected by the FM detection circuit from the center frequency of the S-shaped curve by a predetermined amount, the detection output voltage when noise is detected, and the audio output An offset detection circuit that detects whether it is a voice signal or noise (when there is no signal) based on the difference from the detection output voltage when the signal is detected, and the output signal from the offset detection circuit is when there is no signal (noise). At this time, the output sound from the speaker is muted.

[Operation] In this configuration, the audio signal a supplied to the audio circuit 10 including the audio signal detection circuit 1 is supplied to the audio signal detection circuit 1 via the intermediate frequency amplifier 11 and the limiter amplifier 12.

The signal a supplied to the audio signal detection circuit 1 is first detected by the FM detection circuit 2.

The FM detection coil 3 is set with its center position shifted so that the center frequency f0 of the audio signal detected by the FM detection circuit 2 deviates from the center frequency f1 of the S-shaped curve by a predetermined amount f '.

Therefore, the offset detection circuit 4 detects the detection output voltage V of each signal.

The detection output voltage V detected by the offset detection circuit 4 makes it possible to easily detect whether the signal a supplied to the audio circuit 10 is an audio signal or noise.

[Embodiment] Next, an example of an audio signal detection circuit according to the present invention will be described in detail with reference to FIG. 1 and subsequent drawings, in the case of being applied to an audio circuit of a television receiver.

FIG. 1 is a diagram showing an audio circuit 10 to which an example of the audio signal detecting circuit 1 according to the present invention is applied.

First, the audio circuit 10 will be described in detail with reference to FIG.

The audio circuit 10 includes an intermediate frequency amplifier 11 and a limiter amplifier 1.
2, audio signal detection circuit 1, drive circuit 13, attenuator 14,
It is composed of an audio output circuit 15 and a speaker 16.

The intermediate frequency amplifier 11 is a circuit that amplifies the audio signal a to a signal level required to drive the FM detection circuit 2.

Since the limiter amplifier 12 may change the amplitude of the frequency-modulated audio signal a during transmission, the audio signal a is made into a frequency-modulated wave having a constant amplitude except for the amplitude.

The drive circuit 13 is a switch circuit, and is a circuit for applying a voltage necessary for eliminating noise in the attenuator 14 according to the detection signal c from the audio signal detection circuit 1.

The muting state (on / off state) of the attenuator 14 is controlled according to the drive signal d from the drive circuit 13.

The audio signal detection circuit 1 provided in the audio circuit 10 having such a configuration will be described in detail below.

The audio signal detection circuit 1 is composed of an FM detection circuit 2, an FM detection coil 3 and an offset detection circuit 4.

In the FM detection circuit 2, the relationship between the frequency deviation Δf and the detection output voltage V has an S-shaped characteristic as shown in FIG.

The FM detection coil 3 is set so that the center frequency f0 of the audio signal detected by the FM detection circuit 2 deviates from the center frequency f1 of the S-shaped curve by a predetermined amount f '.

The offset detection circuit 4 extracts only the direct current component of the detection output voltage from the FM detection circuit 2, detects noise and a voice signal, and outputs the detection signal c.

A case will be described in which the audio signal a is supplied to the audio circuit 10 including the audio detection circuit 1 having such a configuration.

First, the audio signal a is amplified to the signal level necessary for driving the FM detection circuit 2 via the intermediate frequency amplifier 11 described above.

Next, the audio signal a supplied to the limiter amplifier 12 is converted into a frequency-modulated wave having a constant amplitude and output to the FM detection circuit 2.

In the FM detection circuit 2, the supplied signal a is amplitude-detected.

As a result, the FM detection coil 3 outputs a signal b whose center frequency f0 is deviated from the center frequency f1 of the S-shaped curve by f ', as shown by the solid line in FIG.

This output signal b is supplied to the offset detection circuit 4 and the attenuator 14.

Since the signal b supplied to the offset detection circuit 4 is modulated, the difference between the signal state and the non-signal state cannot be discriminated.

Therefore, when the offset detection circuit 4 extracts only the DC component of the modulated signal b, the voltage V0 shown in FIG. 2 is detected.

Here, as described above, as the FM detection coil 3, the center frequency f0 of the audio signal detected in the FM detection circuit 2 is S
Since chosen to be shifted from the center frequency f1 of-curve by a predetermined amount f', the average voltage of the detection output voltage when the signal is V _0. On the other hand, when noise is input to the FM detection circuit 2, the frequency distribution of the noise is very wide and the FM
Since it can be considered that the noise is distributed at a uniform level within the bandwidth of the detection circuit 2, the average voltage of the detection output voltage when such a noise component is detected is V1 (= 0).

Therefore, it is possible to determine whether there is a signal or no signal based on the detected voltage V0.

As a result, the detection signal c is output to the drive circuit 13.

The drive circuit 13 outputs the drive signal d to the attenuator 14 in response to the detection signal c.

In this case, it is not necessary to drive the attenuator 14 at the time of signal input, that is, it is not necessary to perform muting,
The drive signal d is turned off.

Therefore, the output signal b from the FM detection circuit 2 is attenuator 14
The sound is output from the speaker 16 via the sound output circuit 15 without being affected by.

Next, a case where noise is supplied to the audio circuit 10 will be described.

First, noise is supplied to the FM detection circuit 2 via the intermediate frequency amplifier 11 and the limiter amplifier 12 as in the case of the audio signal a.

In the FM detection circuit 2, the supplied noise is detected.

As a result, the FM detection coil 3 outputs the signal B that matches the center frequency f1 of the S-shaped curve as shown by the broken line in FIG.

The output signal B is supplied to the offset detection circuit 4 and the attenuator 14.

In the offset detection circuit 4, since the AC component which is a noise component is removed, the detection output voltage becomes V1 = 0 [V], and the detection output voltage V0 at the time of signal has a difference of ΔV.

Therefore, from the offset detection circuit 4, the drive circuit 13
The detection signal c is output to.

As a result, the drive circuit 13 outputs the drive signal d to the attenuator 14 in response to the detection signal c. That is, this causes the muting state, and the output signal B from the FM detection circuit 2 is muted in the attenuator 14.

As a result, no audio signal is transmitted to the audio output circuit 15,
No sound (noise) is output.

Although noise is muted by the attenuator 14 in the embodiment, a squelch circuit may be used instead of the attenuator 14.

[Advantage of Device] As described above, the device of the present invention has an FM detection circuit in which the relationship between the frequency deviation and the detected output voltage has an S-shaped characteristic, and the center frequency of the audio signal in the FM detection circuit is greater than the specified center frequency. An FM detection coil set to shift by a predetermined amount, an offset detection circuit that detects the audio signal from the difference between the detection output voltage when the noise is detected and the detection output voltage when the audio signal is detected. It is characterized in that the audio signal is detected from the input signal by the output signal from the offset detection circuit.

Therefore, it is possible to accurately detect whether the signal supplied to the audio signal detection circuit is the audio signal or the noise signal only by the detection output voltage detected by the offset detection circuit.

Therefore, it is not necessary to separately provide a circuit for detecting an audio signal other than the audio circuit, and the cost of the audio circuit itself can be reduced.

In addition, since a circuit for detecting an audio signal such as the synchronization detection circuit 30 is not used, other circuits are not adversely affected and interference such as arcing can be prevented even on the screen.

Therefore, the audio signal detection circuit according to the present invention is extremely suitable when applied to an audio circuit of a television receiver.

[Brief description of drawings]

FIG. 1 is a system diagram showing an example of an audio circuit equipped with an audio signal detection circuit according to the present invention, and FIG. 2 shows a relationship between a detection output voltage and a frequency deviation of a signal detected by an FM detection circuit of the present invention. FIG. 3 and FIG. 3 are system diagrams showing connections between a conventional audio circuit and an audio signal detecting circuit. 1 …… Voice signal detection circuit 2 …… FM detection circuit 3 …… FM detection coil 4 …… Offset detection circuit 10 …… Voice circuit 13 …… Drive circuit 14 …… Attenuator 1 a …… Voice signal b …… Detection Signal c …… Detection signal V …… Detection output voltage Δf …… Frequency deviation

Claims (1)

[Scope of utility model registration request] 1. An FM detection circuit in which the relationship between frequency deviation and detected output voltage has an S-shaped characteristic, and a center frequency of an audio signal detected in the FM detection circuit is shifted from a specified center frequency by a predetermined amount. Offset detection circuit that detects audio signal or no signal (noise) from the difference between the set FM detection coil, the detection output voltage when noise is detected, and the detection output voltage when audio signal is detected And when there is no signal, by the output signal from the offset detection circuit,
An audio signal detection circuit comprising an audio noise muting circuit for muting output audio.