JPS6161205A - Detecting circuit for fm audio signal - Google Patents

Abstract

PURPOSE:To eliminate malfunction at the time of AFM recording mode of low recording level and normal recording mode by enabling surely the titled circuit to detect the presence of record of FM aural signals. CONSTITUTION:Envelope detection output SER including drop-out etc. is rectified by a low pass filter 38, and at the same time, this envelope detection output SER is shifted down by the first DC level Valpha set to specified level and used as a reference level VREF. When recording level of FM aural signals is low, the second DC level Vgamma that changes according to noise level including unmodulated noise is superposed on the reference valve VREF to raise the level. Thus, by comparing envelope detection output SER by the reference level VREF shifted down by Valpha, presence of FM aural signals recorded at ordinary recording level can be detected surely. When recording level is low, the reference level VREF becomes high correspondingly, accordingly, FM aural signals can be detected.

Description

[Detailed Description of the Invention] (Industrial Application Field) This invention records an FM audio signal on a video signal using a rotary magnetic head different from a rotary magnetic head for recording and reproducing video signals, and also reproduces the same. FM suitable for application to FM audio signal discrimination system of VTR
The present invention relates to an audio signal detection circuit.

[Conventional technology]

2. In order to improve the quality of the audio signal and obtain high-fidelity sound quality in a VTR using the 7D heli scan method,
Audio signals such as left and right audio signals are FM converted and then supplied to a pair of rotating magnetic heads to be recorded together with color video signals.

In this case, a VTR has been proposed in which audio signals are recorded and played back using a pair of rotating magnetic heads HAA and HAS, which are different from the pair of rotating magnetic heads HVA and HVB used to record and play back video signals, as shown in Figure 6. (For example, Japanese Patent Application Laid-Open No. 103478/1983).

The audio head H, ^, HAB is installed in front of the video head HV^ + HvB, and as shown in Fig. 7, the video head HVA,
The azimuth angle θV^, θVB of HVB, the azimuth angle θA^ of audio head H^^, HAB.

θAB is selected to be large, and the gap width CV^~GA
B also goes to Hefd H for audio ^+HA8 is selected as a larger one.

The video signal and the FM audio signal are recorded overlappingly on the same track by these HERADOs HV~HAB. FIG. 8 shows an example of this. Audio signals are recorded in a deep layer by the preceding heads H^^ and HAS, and video signals are recorded in a shallow layer thereon by the following heads HV^+)(vs.

Figure 9 shows the frequency characteristics when such a recording method is adopted, and as is well known, the frequency characteristics of the FM-modulated luminance signal YFM and the low-frequency converted carrier color signal Co are as shown in the figure. become. On the other hand, FM audio signal s
The occupied bandwidth of A is selected so that it exists on the low frequency side of the FM luminance signal YFM, which partially includes the high frequency side of the low frequency carrier color signal CD, as shown in FIG.

In this example, the FM carrier frequency f1 of the left audio signal is selected to be 1.35 MHz, and the FM carrier frequency f2 of the right audio signal is selected to be 1.50 MHz, and the frequency deviation width Δf of each is set to about ±200 kHz. .

If the audio signal recording and playback method is selected in this way, the video signal will be compatible with the normal method, and the audio and video signals will be recorded overlappingly at different recording azimuths. Even if the occupied bandwidth of the audio signal is wide and its level is high, it hardly affects the recording and reproduction of the video signal.

Consequently, the dynamic range is as wide as 80dB, for example.
Effects such as being able to record and reproduce audio signals whose frequency characteristics are extended to high frequencies (20 Hz to 20 k) lx) can be obtained.

Now, F in a VTR that uses such a recording/playback method.
The M audio signal reproducing circuit can be configured as shown in FIG. 10, for example. FIG. 10 shows only the reproduction circuit (1) for the right audio signal.

First, an FM audio signal LP obtained by FM modulating the carrier frequency f using the left audio signal and an FM audio signal R obtained by FM modulating the carrier frequency 12 using the right audio signal R.
Let the audio signal be sA, when it is recorded by head HAA, it is 5AA (=L, ^+R do^), and when it is recorded by the other head HAB, it is SAD (=L
pa+Rpa).

The FM audio signals SAA and SAB reproduced by the pair of heads H^^ and HAB are respectively preamplified (31, (4
1 to bandpass filters (51, t6T) with center frequencies fl, respectively, to output the FM audio signals SAA,
The right audio signal R, ^, RFB is extracted from SAB,
The extracted right audio signals RF^ and RFB are supplied to the first and second switching circuits +7) and (8). These are supplied with switching pulses PGl r PG2 having different switching phases as shown in FIG. Right audio signal R
Fl + RF2 is formed.

The first and second right audio signals R, R, 2 are sent to an FM demodulator (11), via a limiter (91, (lQl), respectively).
(12) and are FM demodulated, and the FM demodulated first and second right audio signals R1 and R2 are further passed through low pass filters (13) and (14) and DC cut capacitors C1 and C2. is supplied to the third switching circuit (15), and the first and second switching circuits +71. Switching noise generated by the switching operation of C81 is removed.

That is, for convenience of explanation, the operation will be explained using an FM demodulated audio signal.The first and second switching circuits (
7) and (8) output the first and second right audio signals R1 and R2 shown in the 11th diagram E, and at this time, the switching noise Ns is superimposed.

By supplying the switching pulse pG of FIG. 11A to the third switching circuit (15), the first and second right audio signals R1 without switching noise NS are supplied.
, 'R2 are extracted, and therefore the right audio signal R output from this is as shown in FIG. 11F.

The right audio signal R is a sampling hold circuit (16) whose operation is prohibited when it drops out.
The audio signal is supplied to the audio selector switch (18) via the amplifier (17) and recorded as FM by a pair of rotating magnetic heads H^^, HAB, and the audio signal is recorded by a fixed head exclusively for audio. The audio signal can be switched.

That is, in the former recording mode (referred to as AFM recording mode), the audio selection switch (18) is switched as shown in the figure, and in the latter recording mode (referred to as normal recording mode), it is switched to the opposite side from that shown in the figure. Then, the audio signal reproduced by the fixed head and supplied to the terminal (20) is output. Terminal (19) represents the R channel line out terminal.

The event holding circuit (16) and the audio changeover switch (1 day) are controlled by the output of the FM audio signal detection circuit (30).

The detection circuit (30) includes an envelope detection circuit (21) that envelope-detects the second right audio signal (FM signal) RF2 output from the IJ and the transmitter αψ, and a comparator (22).
A predetermined reference voltage (reference level) VREF is supplied to the comparator (22) from a reference voltage source (23).

The reference level VREF is set as follows.

In Fig. 12, level L1 corresponds to the Luengherov detection output level when the FM carrier level is low when the audio signal is recorded by AFM i-mode, and sound breakage occurs, and level L2 corresponds to the unmodulated reproduction output level. Time (AF
This corresponds to the total noise level including unmodulated noise obtained when no audio signal is recorded in the M recording mode, and noise from the head and amplifier systems. The reference level VR1EF is set to an arbitrary level equal to or higher than the level L2.

When the envelope detection output is equal to or higher than the reference level VRgp, the sampling and holding circuit (16) executes sampling based on the comparison output at that time, and
Depending on the outcome of the comparison, the audio selection switch (18) is switched as shown.

When a dropout occurs, the envelope detection output will be below 2 in level, so the comparison pulse obtained at this time is supplied to the sampling and hold circuit (16), and the sampling operation is prohibited at the time of a dropout.
Ru. The comparison output is provided by a time constant circuit with a larger time constant (24
). Since the time constant is large, there is almost no level fluctuation in the time constant circuit (24) in the comparison output at the time of dropout. Therefore, the audio changeover switch (18) controlled by the output of the time constant circuit (24) does not malfunction due to dropout.

[Problem that the invention seeks to solve]

Incidentally, the carrier level of the FM audio signal recorded by the pair of heads HAA and HAS may become lower than the reference level VRIKF due to fluctuations in the recording level or the like. In such a case, if the level of the envelope detection output is above L1 and below VREEF, the detection circuit (3
0), the audio changeover switch (18) is switched to the normal recording mode, assuming that no audio signal is recorded in the AFM recording mode.

In order to avoid such malfunction, the reference level VRg
If F is set to a level lower than 2, the mode will be switched to the AFM recording mode during non-modulation playback, resulting in the same malfunction.

Therefore, if the reference level VRIEF is fixed in this manner, malfunctions cannot be avoided when the recording level of the FM audio signal is low or in the normal recording mode.

This invention solves these problems,
By making it possible to reliably detect the presence or absence of recording of an FM audio signal, malfunctions in the AFM recording mode and the extreme recording mode in which the recording level is low are eliminated.

[Means for solving problems]

In order to solve the above problems, in the present invention, as shown in FIG. 1, the envelope detection output SER including dropout etc. is converted to DC by a low pass filter (38), and the envelope detection output SER is set to a predetermined level. The set first DC level ■α shifted down is used as the reference level VR1EF, and when the recording level of the FM audio signal is low, the second DC level that changes according to the noise level including unmodulated noise is used as the reference level VR1EF. D.C.
Level Vγ is superimposed on this reference level VRKT' to raise the level.

[Effect]

In this way, the reference level VR shifted down by Vα
By comparing the envelope detection output SER'tr with HF', it is possible to reliably detect the presence or absence of an FM audio signal recorded at a normal recording level, and when the recording level is low, the reference level VR1EF increases accordingly. 12th
An FM audio signal can be detected even at an FM carrier level below L2 in FIG.

Also, since non-modulated noise increases when not in AFM recording mode, the second DC level Vr increases and the reference level VR1F becomes higher.At this time, if the reference level VR1CF is raised to level 2 or higher, the AFM recording mode It can be detected that this is not the case.

〔Example〕

FIG. 1 shows an example in which the FM audio signal detection circuit according to the present invention is applied to the above-mentioned VTR playback system, and the R channel playback system (1) is the same as the configuration shown in FIG. Therefore, its explanation will be omitted.

The detection circuit (30) according to the present invention is configured as follows.

The second right audio signal RF2 (
2B) is supplied to the envelope detection circuit (31), and an envelope detection output 5ER (FIG. 2C) corresponding to the FM carrier level is obtained. When dropout occurs during playback, the FM carrier level decreases to the noise level VN (B in Figure Ii) during that period TD, so the envelope detection output sy becomes as shown in C in the same figure.

The envelope detection output SER is supplied via an amplifier (32) to a low-pass filter (33) with a small time constant τ1, and its output SR (D in the figure) is supplied to one terminal of a comparator (35).

The envelope detection output 5ifR is further subjected to the first addition 5(
36) and the negative polarity first DC voltage (first D
C level) Vα is added, and as a result, the envelope detection output sy is DC-level down by Vα, and this is applied to a low-pass filter with time constant τ2 (τ2)τ1) (
38) and converted to direct current. This DC voltage is supplied as a reference signal VRIEF (FIG. 2D) to the child terminal of the comparator (35). The first DC level Vα is set to approximately 1/2 of the FM carrier level obtained when recording at a normal recording level.

Therefore, in the case of an FM audio signal recorded at a normal recording level, since SR>VRIEF, a low level comparison output Sc (Fig. 2E) is obtained from the comparator (35), and this causes the sampling The hold circuit (16) performs a normal sampling operation, and the comparison output Sc switches the audio selector switch (18) to the AFM recording mode side, and the high quality right audio signal R is sent to the line out terminal (19). ) is output.

When a dropout occurs during playback, the dropout pulse DO is included in the output sR as shown in the second figure, so at this time, a high level comparison output SO (E in the figure) is obtained only during the period TD. Therefore, sampling operation is prohibited during period TO. Note that even if dropout occurs, the audio changeover switch (18) remains unchanged.

Now, the FM audio signal S^^ played from tape (2)
, SAB's FM carrier level changes due to recording level fluctuations,
Due to fluctuations due to playback tracking errors, etc.
The output level of the signal sR supplied to the comparator (35) may vary as shown in FIG.

When the detection level is as shown in FIG. 5A or FIG. Since it is higher than the low level, dropouts can be detected correctly.

However, as shown in C in the same figure, the detection level drops further and v
If the signal S is such that R, , =vP-vα<VH, the reference level VR1EP becomes even lower than the lower limit of the dropout pulse DO3, so in such a case, dropout is no longer possible. It becomes undetectable.

Of course, when not in the AFM recording mode, the peak level Vp of the signal sR based on the envelope detection output SEH of unmodulated noise including tape and amplifier system noise is lower than in the case of C in the same figure, so Reference level VRI at this time! F further decreases, making it impossible to detect that it is not in AFM recording mode. Similarly, since the carrier level of the FM audio signal is low, the peak level Vp of the signal sR based on its envelope detection output SEH becomes extremely low. In this case, this AFM recording mode cannot be reliably detected.

Therefore, in the present invention, the following configuration is further added to the above-described configuration.

That is, the right audio signal R output from the demodulator (12)
2 (FIG. 4A) is supplied to the bypass fill body (41). 20KH when no questions are changed, that is, in normal recording mode.
Unmodulated noise and white noise above z increase, and A
Even in the FM recording mode, when the FM carrier level is low, white noise increases as the FM carrier level decreases, so such a state is detected by the bypass filter (41). In the above case, the filter output 31 of the bypass filter (41) whose cutoff frequency is 20 KHz according to the amount of noise (Fig. 4B)
level increases. The filter output S1 is output from a detector (42
), the level is detected and the detection output S2 (C
) is further smoothed by a low-pass filter (43).

This smoothed output S3 (E in the figure) is supplied to a comparator (44) and compared with a reference level vR. In this example, the reference level vR is set to level L1 in FIG. 12.

The comparison output is sent to the second adder (
37) and added to the envelope detection output SIH. As mentioned above, the second DC level ■γ increases as the FM carrier level decreases, so if we assume that the second DC level Vγ changes as shown by the broken line in FIG. The second DC level Vγ is output from around the FM carrier level at point a, and this is added to the DC level of the reference level VR1CP (=Vp-Vα), so as the FM carrier level decreases, the comparator (
35) rises as shown by curve 1a.

Therefore, even if the envelope detection output SER as shown in FIG. The range of SR can be further expanded than before. Conventionally, a dropout below the FM carrier level at point b could not be detected, but according to the present invention, the detection range can be expanded to signal sR at point C.

Also, FM audio signals S^^ whose FM recording level is low or are reproduced due to mistracking.

Even when the FM carrier level of the SAH decreases and the level of the signal sR at that time is, for example, between points b and c in FIG. 5, the reference level VRIEF does not exceed the peak level Vp of the signal sR. The presence of the FM audio signal SA^l5AH can be detected even at the FM carrier level, and A
Malfunctions such as erroneous switching from FM recording mode to normal recording mode can be reliably avoided.

Furthermore, the reference level VR with the addition of the second DC level ■γ
IEF is the noise level at the time of dropout (this is the noise level of a pair of heads H^^ during normal recording) as shown in Figure 5.
r ) (equal to the unmodulated noise level including the tape reproduced by As and the amplifier system noise), so the normal recording mode can be reliably detected.

In the embodiment shown in FIG. 1, the present invention is applied to the R channel reproduction system (1), but it can also be applied to the L channel reproduction system.

Furthermore, the embodiment shown in FIG. 1 is applied to a playback system of a VTR that records and plays back audio signals such as voice signals using a pair or more of rotating magnetic heads different from the rotating magnetic head for recording and playing back video signals. The present invention can also be used in a VTR playback system that obtains high sound quality by superimposing and recording audio signals using a rotating magnetic head that records and plays back video signals.

The television standard system may be applicable to PAL system, NTSC system, or other television standard system.

〔Effect of the invention〕

As explained above, according to the present invention, if the FMM recording level is low or the FMM recorded due to mistracking is
Signal sR based on M audio signal S AA I S AB
Even if the peak level Vp is very low, the AFM recording mode can be reliably detected, and even in the normal recording mode, the reproduced unchanging noise can be detected, so it can be determined that this is the AFM recording mode. There is no, both A
It has a feature that allows accurate discrimination between FM recording mode and normal recording mode.

Furthermore, even when the peak level V of the reproduced FM audio signal S^^ and SAH-based signal sR is very low, dropouts that occur at that time can be detected correctly, so the dropout detection range is expanded and , which has the effect of constantly reproducing high-quality audio signals.

[Brief explanation of the drawing]

FIG. 1 is a system diagram showing an example of an FM audio signal detection circuit according to the present invention, FIGS. 2 to 5 are waveform diagrams for explaining its operation, and FIGS. 6 to 9 are for explaining the present invention. FIG. 10 is a system diagram of a conventional FM audio signal detection circuit, and FIGS. 11 and 12 are diagrams for explaining its operation. (30) is a detection circuit, (31) is an envelope detection circuit, (35) is a comparator, (16) is a sampling hold circuit, (18) is an audio selection switch, ■α. (2) γ is the first and second DC level, VRIEP is the reference level, and (12) is the FM demodulator. Figure 8 Figure 9

Claims (1)

[Claims] The FM audio signals reproduced by the pair of rotating magnetic heads are supplied to first and second switching circuits, respectively, and are switched in synchronization with the rotation of the rotating magnetic heads to generate continuous FM audio signals. In the VTR, the serialized FM audio signals are respectively supplied to a demodulator for FM demodulation, and the FM signal supplied to the input stage of the demodulator is supplied to an envelope detection circuit to convert the FM carrier into an FM carrier. An envelope output corresponding to the level of is formed, and this envelope output is supplied to one input terminal of the comparator, and
The D depending on the noise level included in the above FM signal.
Detecting an FM audio signal, wherein the envelope output whose C shift level has been changed is supplied to the other input terminal of the comparator, so that a detection output corresponding to the presence or absence of the FM audio signal is obtained from the comparator. circuit.