JP2529887B2 - FM audio signal presence / absence detection circuit for video device - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an FM audio signal presence / absence detection circuit for a video device, and more particularly, when switching between FM audio / normal audio, it is affected by switching noise of a head switching pulse. The present invention relates to an improvement of an automatic FM detection circuit which can generate an FM non-detection signal at high speed and improve its responsiveness.

[Prior Art] In a hi-fi type video device, a plurality of audio heads are arranged corresponding to the video heads, and each audio head rotates in the same manner as the video heads, and FM audio recorded in the deep portion of the video track is recorded. Scan the recording track and switch the head according to the timing of the scan,
A continuous playback FM audio signal is obtained from the recorded FM audio signal.

On the other hand, in a normal type video device, an audio track is provided separately from the video track, for example, on a straight line above the audio track, and a stationary audio head scans the audio track to reproduce an audio signal. There is.

Therefore, when a video tape recorded by a normal video device is attached to a hi-fi video device, it is detected by the rotary audio head that there is no audio recording in the hi-fi system, and it is detected by the FM audio signal. The process of switching the state from the normal audio to the normal audio is performed. On the contrary, in a high-fidelity video device, when a high-fidelity video tape is loaded, the state of the signal obtained from the rotary audio head is detected and the state is switched from normal audio to FM audio. There is.

[Problems to be Solved] Conventionally, in a high-fidelity video apparatus of this type, for example, a method for automatically switching between FM audio and normal audio is performed by extracting a noise component of an FM audio signal obtained from a rotating audio head. Are known. This is to obtain an FM non-detection signal (normal audio detection signal) when the noise level is above a predetermined value, and when the noise level is below a certain value, it is switched to the FM audio state.

However, among such switching processes, the responsiveness is particularly poor during the switching from the FM audio state to the normal audio state, and if the switching does not occur instantaneously, FM demodulation due to the loss of the FM audio signal from the rotary audio head etc. White noise from the device is output to the speaker, which appears as an instantaneous noise sound. Although muting is performed to avoid such a situation, noise sound is likely to be output because muting cannot be performed at a sufficient timing. In addition, if the detection response of the FM non-detection signal is made faster (or the sensitivity is made higher), it is more susceptible to noise from the outside, which increases the possibility of false detection. In particular, there is a high possibility that a response may occur due to pulse noise generated during head switching.

The present invention solves the above problems of the prior art, and provides an FM audio signal presence / absence detection circuit of a video device capable of obtaining an FM non-detection signal which is fast in detection response and hardly affected by noise. The purpose is to provide.

[Means for Solving the Problems] The features of the FM voice signal presence / absence detection circuit of the present invention for achieving such an object are a circuit for receiving the output of a demodulator and extracting a noise component, and an extracted noise component. Detection circuit for generating a first detection signal by detecting that the level is above a certain level, a switching signal for switching the first detection signal and the rotary voice head, or a signal corresponding to this switching signal. In response to this changeover signal or a signal corresponding to the changeover signal, the detection operation is suppressed so that the output is not affected by the noise generated by the changeover of the head, and the first detection is performed according to the first detection signal. When the signal is continuously generated for the first predetermined time or longer
Generates a second detection signal indicating the absence of an FM voice signal and a first
When the second detection signal is not generated for a second predetermined time longer than the first predetermined time or longer than the first predetermined time, the second detection signal is in the stopped state or the third detection signal And a second detection circuit that is generated.

Another feature of the present invention is that the first detection signal is generated when the level of the noise component extracted by the first detection circuit is below a certain level. Since the content is the opposite of the above example,
The second detection circuit generates a second detection signal when the first detection signal has not been generated for a first predetermined time or longer, and the first detection signal has a second predetermined time or longer. If the second detection signal is in the stopped state or the third
The detection signal of is generated.

[Operation] As described above, when the generation time of the first detection signal for detecting the presence / absence of a noise component based on a threshold value of a constant level is generated in the presence of noise for a first predetermined time or more, FM
FM audio signal with no audio signal, FM audio signal when it has been generated for a second predetermined time or more without noise, and with a first predetermined time set shorter than a second predetermined time A signal presence / absence detection circuit is provided, and the output of the FM audio signal presence / absence detection circuit at the time of occurrence of switching noise that occurs when the head switching signal or a signal corresponding thereto is generated in synchronization with the head switching timing Since the detection operation is suppressed so as not to respond to the detection output of the first detection signal corresponding to noise above a certain level during the generation period, for example,
In a hi-fi system video device, when there is no FM audio signal, the detection signal can be generated early, and a fast responsive FM audio signal presence / absence detection circuit can be realized. Further, even if the responsiveness is increased, malfunction due to noise of the head switching pulse can be prevented, and judgment malfunction for head switching noise can be reduced.

As a result, it is possible to almost eliminate the generation of noise sound from the speaker due to the slow switching, and when switching from the FM audio state to the normal audio state, the first predetermined time is shortened, so that switching with high sensitivity is performed. be able to.

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

FIG. 1 is a block diagram centering on an FM audio signal non-detection circuit of a video device of one embodiment to which the invention is applied, and FIG. 2 is an explanatory diagram of another concrete example of a head switching noise suppression gate. , FIG. 3 is an explanatory diagram showing the relationship between the head switching pulse noise and the FM detection state.

In FIG. 1, reference numeral 1 is an FM demodulator for demodulating an FM audio signal obtained by a preamplifier for amplifying a reproduction signal obtained from a rotary audio head, a head switch for connecting the signals of the rotary head, and a bandpass filter for extracting a required band. Device (DEM), which is shown here for one channel. Reference numeral 2 denotes an FM voice signal presence / absence detection circuit, which obtains a signal from the DEM 1 and detects the presence / absence of an FM voice signal from the noise component thereof, thereby generating an FM detection signal / FM non-detection signal. The FM audio signal presence / absence detection circuit 2 may further receive a noise component signal from a DEM of another channel.

3 is a low-pass filter (LP
F), and 4 sample-holds the audio signal in order to correct head switching noise generated at the connection point of the reproduction signal of the rotary head with respect to the demodulated audio signal obtained through the low-pass filter 3. A switching noise correction circuit (S / H) including a sample hold circuit.

Reference numeral 5 denotes noise reduction, which performs noise reduction on the signal whose noise has been corrected by the switching noise correction circuit 4 and sends the reproduced audio signal from which noise has been removed to the audio amplification circuit 6.

Here, the FM audio signal presence / absence detection circuit 2 includes a bandpass filter (BPF) 21, an amplifier 22, and a comparator (COM) 2
3, output time response circuit 24, a comparator (COM) 25, and a head switching noise suppression gate 26 (hereinafter simply noise suppression gate 26), of the demodulated audio signal obtained from the DEM1, For example, 100kHz
Use the BPF21 to remove noise components in the range of ~ 400kHz.
In other words, the demodulated voice signal is a voice signal (generally 20Hz to 20kHz) that has a predetermined band as its spectrum, FM
Carrier and its harmonic components (distributed over 1 MHz as a band), interference components with other channels, and C / N of reproduced signal
There is a noise component depending on the signal state, and due to the property of FM modulation, the noise distribution becomes higher as the frequency becomes higher within the range where the filter of DEM1 does not affect. Therefore, by measuring this noise component, it is possible to make a determination according to the state of C / N. Therefore, this noise component is amplified by the amplifier 22 and detected as necessary, and the comparator 23 compares the noise level with a reference voltage (VHT ₁ ) 23a to determine whether the noise level is equal to or higher than a certain value. It is detected whether or not the peak value or the envelope exceeds a predetermined value, and the detection signal is added to the output time response circuit 24. The detection circuit and the comparator 23 are an example of the first detection circuit of the present invention.

As a result, when the FM audio signal is not recorded on the video track, that is, in the non-FM state, the noise level (its instantaneous peak value or envelope) exceeds the fixed value determined by the reference voltage 23a. For,
In response to this, a detection output corresponding to the change in the envelope of the noise component is obtained in the comparator 23, which is input to the output time response circuit 24.
And the comparator 25 are an example of the second detection circuit of the present invention.

The output time response circuit 24 is a so-called integrating circuit,
It has two types of time constant circuits consisting of a capacitor and a resistor inside, and responds to the detection output of the comparator 23 when the output continues to be generated for a predetermined period (response of voltage that sequentially increases). Signal). On the contrary, when the state in which the detection output is not generated for a certain period of time continues for the other period, the output in the opposite direction (a response signal having a voltage that sequentially drops) is generated in response to the detection output.

This is done by receiving the output of the comparator 23 and integrating the output signal to raise the output to a predetermined value or to lower the output, and the setting determined by the time constant with respect to the detection output of the comparator 23. A time response circuit that responds to the continuous time of the presence and absence of the detection output for a predetermined duration of time. Then, the output of this time response circuit is next input to the comparator 25.

The comparator 25 receives the output of the output time response circuit 24 at the (−) phase input, compares it with the reference voltage 25a inserted on the (+) phase side, and outputs the output time response circuit 24 to the reference voltage. When the voltage drops below 25a, an FM non-detection signal (HIGH level (hereinafter "H")) is generated, which is the reference voltage 25
When it exceeds a, the FM detection signal (LOW level (hereinafter "L")) is output.

Here, the output time response circuit 24 has a transistor Q, the base of which is connected to the input terminal 24a, and a noise suppression gate is provided between the input terminal 24a and the ground (GND).
26 (or the switch circuit 26a shown in FIG. 2) is inserted. The collector of the transistor Q is connected to the power source + Vcc via the resistor R ₁ and its emitter is connected to the ground. Further, the collector resistance R ₂ of the comparator 25 via (-) connected to the phase input terminal, the capacitor C is inserted between the input side and the ground of the comparator 25. The noise suppression gate 26 is supplied with a hold pulse synchronized with the head switching pulse obtained from the hold pulse generation circuit 27 that receives the switching pulse and generates the hold pulse, and suppresses noise during the period in which this pulse is generated. The gate 26 is closed, and is open otherwise (in the switch circuit 26a, it is in the "ON" state during the period when this pulse is generated, and is in the "OFF" state otherwise). Therefore, the output of the first detection circuit is the noise suppression gate 26 (or the switch circuit 26 shown in FIG. 2).
Due to a), the input to the output time response circuit 24 is stopped during the period in which pulse noise is generated, and as a result, the response operation of the output time response circuit 24 is stopped and the detection operation of the second detection circuit is suppressed. Will be.

Now, the above-mentioned resistor R ₂ and the capacitor C constitute a first time constant circuit, and the resistance value of the resistor R ₂ is kept small. As a result, the discharge time constant r ₂ × c of the charge accumulated in the capacitor C (where r ₂ is the resistance value of the resistor R ₂ ,
Is the capacitance of the capacitor C. ) Is a small value. Therefore, the charge of the capacitor C is discharged quickly.
On the other hand, the resistor R ₁ and the capacitor C has a large value constitutes a time constant circuit of the second, the resistance value of the resistor R ₁ is compared with the resistance R _2. The charging time constant of the electric charge charged in the capacitor C is
Mainly determined by the value of this resistor R ₁ , its time constant r ₁ × c
(However, r ₁ is the resistance value of the resistor R _1. ) is larger than the discharge time constant. Therefore, the capacitor C
Charging to is slow and slow.

Now, when the first detection output is output from the comparator 23, the transistor Q is turned "ON", and when the discharge time constant determined by the first time constant circuit is small as described above, the voltage of the capacitor C rapidly increases. To VTH
₂ (25a) or less, early "H" from comparator 25
FM non-detection signal of can be obtained. On the other hand, when no detection output is generated in the comparator 23, the transistor Q is in the “OFF” state, so if the charging time constant is large,
The capacitor C is charged relatively slowly from the power source + Vcc via the resistor R ₁ , and the voltage thereof rises slowly. As a result, when the threshold value V TH ₂ (25a) of the comparator 25 is exceeded, the FM detection output from the comparator 25 becomes "L", and the FM
The output is a judgment output. This is the second detection output. That is, an FM detection signal is obtained in which the time from the presence of FM input to the absence (second predetermined time) is fast, and the time from the absence of FM input to the presence (first predetermined time) is slow.

The FM non-detection signal (“H”) and the FM detection signal (“L”) are related to the output duration when the output of the comparator 23 is generated and the duration when it is not generated. To do. The duration thereof is determined by the first time constant circuit and the second time constant circuit, and the FM non-detection signal is
In response to it in a short time, "H" (no FM audio signal) is generated, and this output becomes a control signal for the switch circuit, suppressing the tracking noise sound from the speaker. Therefore, switching from FM audio to normal audio is performed immediately. On the other hand, when the noise at a level below a predetermined level continues for a certain period over a longer period than when FM is not detected, or when the noise-free state also continues for a certain period, In response “L”
Generate (with FM audio signal). Therefore, switching from normal audio to FM audio is performed slowly.

In this case, the FM voice signal presence / absence detection circuit 2 quickly responds to noise to perform the FM non-detection operation, so that a non-detection signal is generated in response to the noise of the head switching pulse, which is a problem. Generation timing of the head switching pulse when the first detection output to the output time response circuit 24 is input by the noise suppression gate 26 that operates "ON" in response to the hold pulse generated by the hold pulse generating circuit 27 from the switching pulse. It is solved by being suppressed according to.

The relationship between the operation and the noise of the head switching pulse will be described with reference to FIG. The figure shown on the left side of FIG. 3 is the FM audio signal detection operation, and the right side is the FM non-detection operation.

FIG. 3A shows a head switch pulse.
(B) shows the state of the FM voice signal, and the shaded portion is the portion where the FM voice signal is present, and this is input to the DEM 1. (C) is a demodulated output signal of DEM1, in which a voice signal and a noise component are mixed. (D) is a signal in which only a noise component is extracted by a bandpass filter.
(E) is an output signal of the comparator 23, and (f) is
The hold pulse is a timing at which the noise suppression gate 26 is closed (or the switch circuit 26a of FIG. 2 is turned "ON") and is synchronized with the head switch pulse. As a result, the portion of the first detection output that is generated as head switch noise is suppressed, and the input waveform shown in (g) is obtained. (H) is a voltage waveform of the capacitor C. And
When the threshold value V TH _{2 of the} comparator 25 is exceeded, an output of “H” is obtained, and the FM / non-FM detection signal is output from the comparator 25 to the output terminal 30 as shown in (i). When it is "H", it becomes a non-detection signal which does not detect FM, and an FM non-detection signal which is a detection signal of normal audio is generated. When it is "L", FM is detected and an FM detection signal which is a detection signal of FM audio is generated.

As shown on the left side of the drawing of FIG. 9B, when the rotary audio head scans the audio FM track and the transition from the state without FM audio signal to the state with FM audio signal 28 occurs,
As shown on the left side of the drawing in (c) of the figure, noise N appears in the output of the FM demodulation circuit 1 in response to the absence of the FM audio signal.
Occurs, and the noise N disappears from a certain position of the FM audio signal (hatched portion in (b)). As a result, as shown in the left side of the figure in (e) of the figure, even if the noise N first occurs, if it enters the range (b) of the FM audio signal being recorded), the comparator 23 detects it. The output does not occur when the noise N disappears, and as shown in FIG. 6 (h), the transistor Q is in the "OFF" state at that time,
The capacitor C is slowly charged with the charging time constant determined by the resistor R ₁ , and the output of the output time response circuit 24 is the second of r ₁ × c.
It gradually rises according to the time constant of. As a result, as shown in FIG. 6F, when the voltage of the capacitor C becomes higher than the voltage VTH ₂ of the reference voltage 25a, the detection output of the comparator 25 changes from "H" to "L".

In addition, P shown by a dotted line in (e) and (f) of the figure is noise due to the head switching pulse, and the hold pulse (f) corresponding to this noise P causes the noise suppression gate 26 to be closed (switch circuit). In 26a, it is in the "ON" state. In this case, the input side of the output time response circuit 24 becomes the ground potential, momentarily transistor Q is "OFF" Suruga, r ₁ × c
Since the charging time constant due to is large, the voltage of the capacitor C is hardly affected. Further, in the portion where the FM audio signal is not recorded, the transistor Q is in the “ON” state due to the output of the comparator 23, so the noise suppression gate 2
Even if 6 is momentarily closed (in the switch circuit 26a, it is in the "ON" state), it is not affected in the same manner as described above. Also,
The part where the FM audio signal is recorded (the shaded part in (b))
Since there is no output from the comparator 23, the transistor Q
Becomes the "OFF" state. Therefore, the opening operation of the noise suppression gate 26 (the "ON" operation of the switch circuit 26a) has nothing to do with the detection operation.

On the other hand, as shown in the right side of FIG. 7B, when the state where the FM audio signal is recorded (hatched portion) is changed to the state where the FM audio signal is not recorded, As shown on the right side of the drawing, the noise N is not generated at first, and the noise N is generated when the FM audio signal is not recorded. Therefore, noise N from that position
Becomes higher than a certain value, an output is generated in the comparator 23, and the transistor Q is turned on. Therefore, the capacitor C is immediately discharged with the second time constant (discharge time constant) determined by the resistance R ₂ . Even if noise P due to the head switching pulse is generated in this switching process, the noise suppression gate 26 is momentarily closed (in the switch circuit 26a, the "ON" state) in response to this noise P, and the transistor Q is turned on. Although it is turned “OFF”, it is ignored because the charging time constant is large at this time as well. Therefore, the response operation of the output time response circuit 24 is not affected by the noise P of the head switching pulse.

In this case, in this embodiment, when the noise P generated corresponding to the head switching pulse appears by utilizing the fact that the charging time constant of the output time response circuit 24 is large, the output time response circuit 24 is charged on the charging operation side. By switching to, the detection operation of the comparator 25 is suppressed so that the head switching noise P is not affected by the output of the comparator 25. However, in the present invention, the operation of the output time response circuit 24 or the operation of the comparator 25 may be stopped only in response to head switching noise, and is limited to the operation of switching to the charging side as in the embodiment. Not a thing. That is, the present invention provides a logic circuit such as a switch circuit or an AND circuit which operates in response to a head switching pulse or a signal corresponding to the head switching pulse so as not to affect the detection output of the presence or absence of an FM audio signal. The detection operation may be suppressed according to the timing.

As described above, the output time response circuit 24 and the comparator 25 in the embodiment constitute a specific example of the second detection circuit of the present invention. However, the second detection circuit
The configuration is not limited to this.

The output time response circuit in the embodiment includes a transistor Q
The resistance R ₁ having a large resistance value is inserted as the load resistance of, so that the charging time becomes slower. For example, “ON / OFF” of the operation of the transistor Q of the output time response circuit is the first. If it is opposite to the detection signal, the charging / discharging relationship is opposite to that of the example given here, and conversely the charging operation is made faster,
It is necessary to discharge slowly. In such a case, the load resistance of the transistor Q is connected in series from the power source + Vcc to a small resistance value R ₂ and a large resistance value R ₁ in order to connect the connection point of these resistances R ₂ and R ₁ to the ground. It can be easily realized by inserting a capacitor C between the two. Therefore,
In the present invention, the time constant for the charging / discharging operation itself may be either longer or shorter.

In addition, the operation of the comparator 23 in the embodiment may be such as to detect noise below a certain level, and the signal levels of the comparators 23 and 24 may be inverted. Further, the continuous period in which the detection signal is generated is not limited to the case where the “H” or “L” signal is “H” or “L” for a certain period, and the detection signal is a pulse. In the case of occurrence as above, the case where the frequency of occurrence during a certain period is a certain value or more is also included. This is because a signal having a predetermined constant voltage can be obtained by simply passing the signal through an integrating circuit or the like.

[Effects of the Invention] As can be understood from the above description, according to the present invention, the generation time of the first detection signal for detecting the presence or absence of a noise component with reference to a threshold value of a constant level is FM audio signal is absent when it is generated for a predetermined time of 1 or more, FM audio signal is present when it is generated for a second predetermined time or more in the state of no noise, and the first predetermined time is An FM voice signal presence / absence detection circuit set to be shorter than the second predetermined time is provided, and head switching signal generated in response to a head switching signal or a signal corresponding thereto is generated at the head switching timing. The detection operation of the FM audio signal presence / absence detection circuit is suppressed so that it does not respond to the detection output of the first detection signal corresponding to noise above a certain level during the switching noise generation period. Since the way, when there is no FM audio, can generate a detection signal at an early stage, it is possible to realize a fast response FM audio signal presence detecting circuit. Also,
Even if the response is fast, malfunction due to noise in the head switching pulse can be prevented, and judgment malfunction for head switching noise can be reduced.

As a result, it is possible to almost eliminate the generation of noise sound from the speaker due to the slow switching, and when switching from the FM audio state to the normal audio state, the first predetermined time is shortened, so that switching with high sensitivity is performed. be able to.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows a video device of an embodiment to which the present invention is applied.
Block diagram centering on FM voice signal presence / absence detection circuit, 2
The figure is an illustration of another specific example of the head switching noise suppression gate. Figure 3 shows the head switching pulse noise and FM.
It is explanatory drawing which shows the relationship with a detection state. 1 …… FM demodulator (DEM), 2 …… FM signal presence / absence detection circuit, 3 …… Low pass filter (LPF), 4 …… Switching noise correction circuit (S / H), 5 …… Noise reduction, 6 …… Audio amplifier circuit, 21 …… Bandpass filter (BPF), 22 …… Amplifier, 23 …… Comparator, 24 …… Output time response circuit, 25 …… Comparator, 26 …… Head switching noise suppression gate.

Claims (2)

(57) [Claims] 1. A video apparatus having a plurality of rotary audio heads, switching the rotary audio heads to read an FM audio signal from a track, and demodulating the read FM audio signal with a demodulator to reproduce an audio signal. A circuit that receives the output of the demodulator and extracts a noise component; a first detection circuit that detects that the level of the extracted noise component is above a certain level and generates a first detection signal; 1 detection signal and a switching signal for switching the rotary voice head or a signal corresponding to this switching signal, so that the output is not affected by noise generated by head switching by this switching signal or the signal corresponding to this switching signal. When the detection operation is suppressed and the first detection signal is continuously generated for a first predetermined time or longer in response to the first detection signal. A FM audio signal when a second detection signal indicating that there is no FM audio signal is generated at the same time and the first detection signal has not been generated for a second predetermined time longer than the first predetermined time. FM audio signal presence / absence detection circuit for a video device, comprising: a stop state of the second detection signal or a second detection circuit for generating a third detection signal as a signal indicating 2. The first detection circuit generates the first detection signal when the level of the extracted noise component is below a certain level, and the second detection circuit includes the first detection signal. When the second detection signal is generated when the detection signal has not been generated for the first predetermined time or longer and the first detection signal has been continuously generated for the second predetermined time or longer. 2. The FM audio signal presence / absence detection circuit for a video device according to claim 1, wherein the second detection signal is in a stopped state or the third detection signal is generated.