JPH0442749B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention utilizes a magnetic recording and reproducing apparatus that sequentially switches a plurality of heads to reproduce a magnetic recording medium in which frequency-modulated video signals and frequency-modulated audio signals are recorded by frequency multiplexing. From the reproduced signal,
The present invention relates to a noise removal method and circuit for reproduction without noise caused by pulse noise or discontinuity of DC components.

Traditionally, video tape recorders (hereinafter simply referred to as
In a VTR (abbreviated as VTR), the luminance signal is frequency modulated (FM), and the chromaticity signal is frequency-converted and added to the lower side of the FM luminance signal. were recorded sequentially. Furthermore, the audio signal was recorded on a recording track in the longitudinal direction of the magnetic tape using a fixed head. However, the improvement in recording density in recent years has been remarkable, with 17
Achieved a high-density record more than twice as high. For this reason, the running speed of magnetic tape is extremely slow at approximately 10 mm/sec. Therefore, the conventional method of recording audio signals on a recording track in the longitudinal direction of a magnetic tape using a fixed head is insufficient in terms of the reproduction signal band, wow and flutter characteristics, and reproduction level fluctuations of the audio signal. It is becoming difficult to obtain good sound quality.

One example of a method to improve this drawback is to frequency-multiplex an FM-modulated audio signal with the FM-modulated video signal and sequentially record and reproduce it on a magnetic tape using a rotating head (hereinafter abbreviated as audio FM multiplexing method). ) is known.

The features of the audio FM multiplexing system are: (1) The playback signal band does not depend on the tape running speed and is wideband.

(2) It has good wow and flutter characteristics because it is less susceptible to time axis fluctuations due to uneven tape running speed.

(3) There is no playback signal level fluctuation.

(4) Low distortion and high S/N.

etc., and high quality playback audio can be obtained.
Figure 1 shows an example of the configuration of the audio FM multiplexing system, and Figure 2 shows the recording frequency spectrum.

In FIG. 1, an audio signal input from an input terminal 1 is input to an FM modulator 3 through an amplitude compression circuit 2 which pairs with an amplitude expansion circuit 19 to reduce noise. The audio signal FM modulated by the FM modulator 3 is passed through a low pass filter (hereinafter abbreviated as LPF).
After unnecessary band components are removed in step 4, the signal is added to the FM luminance signal and the low frequency converted chromaticity signal input from the video input terminal 5 in an adder 6. The output signal of the adder 6 passes through the recording amplifier 7 and is sent to the magnetic heads 8 and 9.
is recorded on the magnetic recording medium 10. During reproduction, the burst reproduction signal reproduced from the magnetic recording medium 10 by the magnetic heads 8 and 9 is sent to the preamplifiers 11 and 1.
After being amplified at 2, the signals are alternately connected by a switch 13 controlled by a reproduction track switching signal input from an input terminal 21 to form a series of signals. A part of this signal is outputted from the output terminal 14 to a video signal reproducing circuit (not shown in this figure). Further, an FM audio signal is extracted from the output signal of the switch 13 by a band pass filter (hereinafter abbreviated as BPF) 15, and FM demodulated by an FM demodulator 16. this
The FM demodulated audio signal has the FM carrier removed by the LPF 17, and the previous value holding circuit 18 compensates for reproduction track switching noise. This previous value holding circuit is controlled by a reproduction track switching signal inputted from an input terminal 21. The output signal of the previous value holding circuit 18 is sent to the expansion circuit 1 which has the opposite characteristics to that of the amplitude compression circuit 2.
After the dynamic range is returned to its original state at step 9, it is outputted from the output terminal 20 as a reproduced audio signal.

FIG. 2 is an example of a recording signal frequency spectrum, in which an FM-modulated audio signal A is frequency-multiplexed between an FM-modulated luminance signal band Y and a low-frequency conversion chromaticity signal band C.

The audio FM multiplex recording method is an effective audio recording method, but when the signal is played back by multiple heads, the problem is how to connect the playback signals. When audio signals are connected in the FM band as shown in the conventional example, pulse noise occurs when FM demodulation is performed due to discontinuity in the phase of the carrier wave at the time of switching the reproduction track, and in order to remove this pulse noise, must maintain previous values or interpolate intermediate values. However, when these processes are performed, there is a problem in that the waveforms are periodically distorted, leading to deterioration in sound quality.

The present invention has been made in order to solve the above-mentioned drawbacks of the prior art, and an object of the present invention is to reduce the pulse noise generated when switching the reproduction track and the reproduction signal to a level that does not pose a practical problem. It is an object of the present invention to provide a noise removal method and device that can reduce noise caused by DC discontinuity and have small waveform distortion.

The present invention provides a plurality of reproduction heads and first and second frequency demodulation circuits for frequency demodulating the outputs from the plurality of reproduction heads, and a period during which the plurality of reproduction heads simultaneously scan a magnetic tape (so-called overlap period). By switching between the demodulated audio signals, the impulsive noise that inevitably occurs when switching in the FM band is prevented from occurring. However, in that case, discontinuity occurs in the playback signal when connecting the playback signals due to the playback output offset of each channel, so the DC potential of one playback signal is used as a reference to set the DC offset from the other playback signal. By automatically drawing a corresponding potential every cycle of head switching, it is possible to eliminate noise caused by waveform discontinuity during head switching.

Furthermore, after compressing the dynamic range of the audio signal to be recorded using an amplitude compression circuit, it is frequency-modulated and recorded. During playback, after switching the demodulated audio signal, the dynamic range is expanded using an amplitude expansion circuit and then output. do. By companding the dynamic range, the audio signal level can be made to have the same amplitude characteristics as the original signal, while the low-level noise generated by switching is reduced by the signal expansion circuit.
oppressed. This makes it possible to suppress small noises remaining at the time of head switching relative to the audio signal.

Hereinafter, one embodiment of the present invention will be explained with reference to FIG. 3, taking a two-head helical scan VTR as an example.
This embodiment has some parts in common with the block diagram of the VTR shown in FIG. 1, and since the common parts are given the same numbers, detailed explanation thereof will be omitted. 22,
23 is a BPF for extracting the FM audio signal, 24 and 25 are audio signal FM demodulators, 26 and 27 are LPFs, and 28 is a switching circuit for switching the audio signal reproduced by the first head and the second head.

FIG. 4 is a signal waveform diagram of each part of the circuit shown in FIG. 3. In Fig. 4, A is the amplitude compression circuit 2.
B is the signal reproduced by the first magnetic head 8 (hereinafter this system will be referred to as CH-1), C is the signal reproduced by the second magnetic head 9 (hereinafter this system will be referred to as CH-2). D is the waveform reproduced by the first magnetic head 8 and then demodulated by the FM demodulator 24, E is the signal reproduced by the second magnetic head 9.
The waveform reproduced by the FM demodulator 25 is demodulated by the FM demodulator 25, and the audio signal is switched and synthesized by the switching circuit 28.

Next, the operation will be explained with reference to FIGS. 3 and 4. The audio signal input from the recording audio signal input terminal 1 is amplitude-compressed through the amplitude compression circuit 2, which pairs with an amplitude expansion circuit 19 and suppresses noise in the low frequency range.
Figure waveform a) Input to FM modulator 3. center frequency
The audio signal FM modulated by the FM modulator 3 of fo is
The signal is mixed with a video signal input from a video input terminal 5 by a mixer 6, amplified by a recording amplifier 7, and then recorded on a magnetic recording medium 10 by magnetic heads 8 and 9.

On the other hand, during reproduction, signals B and C reproduced from the magnetic recording medium 10 by the magnetic heads 8 and 9 are amplified by preamplifiers 11 and 12. The burst reproduction signal reproduced by the first head 8 and the second head 9 is switched by a switch 13 controlled by a reproduction track switching signal inputted from a terminal 21, and a continuous video reproduction signal is converted into a video signal. It is obtained from the output terminal 14. The modulated audio signal amplified by the preamplifiers 11 and 12 is extracted only in the transmission band by the BPF 22 and 23, and the center frequency corresponding to the FM modulator 3 is extracted.
It is demodulated by audio FM demodulators 24 and 25 of o, and audio signals d and e are obtained. FM demodulator 2 for D and E
Since it contains the carrier fo leaked from 4 and 25, it is necessary to pass it through LPF 26 and 27 to remove the fo.

Next, by switching the audio signals that have overlapping portions on the time axis at the overlapping positions on the time axis using the switching circuit 28, problems such as pulse noise and previous value retention that were problems in the conventional example can be avoided. An audio signal without causing waveform distortion can be obtained.

Furthermore, by passing the signal through an amplitude expansion circuit 19 having characteristics opposite to those of the amplitude compression circuit 2, the dynamic range can be restored to its original value. However, even with this method, as shown in FIG. 4, discontinuity occurs in the DC component when the switching circuit 28 switches. this is
Since the center frequencies of the FM demodulators 24 and 25 differ due to adjustment errors and differences in temperature characteristics, the FM
This is because the DC potentials of the demodulators 24 and 25 are shifted. This discontinuity in the DC component is caused by fixed adjustment.
Considering the temperature characteristics, it cannot be completely removed and is reproduced as an abnormal sound, which is undesirable. However, this discontinuity can be eliminated by replacing the switching circuit 28 with an automatic DC off-set compensation circuit shown in FIG.

FIG. 5 shows the audio signal reproduced by the first head 8 and FM demodulated, and the audio signal reproduced by the second head 9,
FM demodulated audio signal can be converted to DC off-state without noise.
FIG. 2 is a block diagram of an automatic DC off-set compensation circuit that removes and switches sets. In the same figure, 1
00 is an input terminal for the FM demodulated audio signal (hereinafter referred to as CH-1 audio signal) reproduced by the first head 8, and 101 is an input terminal for the audio signal reproduced by the second head 9 (hereinafter referred to as CH-2 audio signal). 102 is a DC off-set compensation circuit that subtracts the difference voltage between the CH-1 audio signal and CH-2 audio signal from the CH-1 audio signal; 103 is a subtraction circuit;
04 is the period T ₁₂ +ΔT ₁₂ , T ₂₁ +ΔT ₂₁ during which the CH-1 audio signal and CH-2 audio signal are output simultaneously
Of these, ₁₀₅ is a switch that is turned on only during the period T 21 of the period T ₂₁ +ΔT ₂₁ shown in FIG. 4 when the CH-2 audio signal is switched to the CH-1 audio signal.
106 is the output terminal for the audio signal obtained by subtracting the DC offset amount of CH-1 and CH-2 from the audio signal of 1.
The audio output terminal of CH-2, 107 is a switch for switching the demodulated audio signal, and 108 is the output terminal for the demodulated audio signal. During the period when the CH-2 audio signal is being played, the switch 107 is connected to the terminal 106, and the CH-2 audio signal is directly output to the output terminal 106.
Obtained from 8. During the period when the audio signal of CH-1 starts to be played and the audio signals of CH-2 and CH-1 are played simultaneously, the switch 104 is turned on.
The DC offset amount of CH-2 and CH-1 is subtracted from the CH-1 audio signal by the noise removal circuit 102. Therefore, during this period, the audio output terminals 105 and 1
There is no DC offset in the audio output from 06, and if switch 107 is switched during this period, no waveform discontinuity will occur due to DC offset. After switching switch 107 to terminal 105
Before the CH-2 audio playback signal disappears,
Open the switch 104, maintain the offset amount of the CH-1 audio signal input to the terminal 100 and the CH-2 audio signal input to the terminal 101, and subtract it from the CH-1 audio signal. . next
When the audio signal of CH-2 starts playing, CH
If the amount of offset between CH-1 and CH-2 can be maintained sufficiently, the DC offset of the audio signals output from terminals 105 and 106 can be ignored, and switch 107
Even if the voltage is switched to the terminal 106 side, waveform discontinuity due to DC offset does not occur. By repeating this operation, a reproduced output with almost no noise due to DC offset can be obtained from the terminal 108.

In this embodiment, the period during which the switch 104 is turned on is
Although the explanation was given as only T ₂₁ , Switch 10
It is possible to operate the direct current off-set automatic compensation circuit even if the periods during which the circuit 4 is on are set to T ₁₂ and T ₂₁ .

Next, a specific embodiment of the automatic DC off-set compensation circuit shown in FIG. 5 is shown in FIG. Note that the parts shown in the block diagram of FIG. 5 are given the same numbers. 109 is a constant current source, Q ₁ and Q ₂ are transistors forming a differential amplifier, and R ₁ is its output resistance. _Q4 is a transistor whose collector is grounded and serves as an output buffer. R ₃ and R ₄ are the emitter resistances that determine the amount of DC feedback. _C1 is a rectifier capacitor, which maintains the amount of DC feedback when the switch 104 is off. Q ₃ is a FET, which together with resistor R ₂ forms a high input impedance current source.

The operation of the circuit of FIG. 6 can be explained as follows.
When the DC potential of the signal input to terminal 100 is higher than the DC potential of the signal input to terminal 101, the current flowing through R ₁ decreases and the base potential of Q ₄ increases. Then, the potential of Q ₃ gate also rises, and the current of the current source composed of Q ₃ and R ₂ increases,
The base potential of Q ₂ decreases. Conversely, terminal 100
When the DC potential of Q2 is lower than the DC potential of terminal 101, the opposite operation occurs, and the base potential of transistor _Q2 rises to become equal to the DC potential of the signal input to terminal 101. At this time switch 107
If you switch , there will be no discontinuity in the DC potential.

In the case of the audio FM multiplex recording system, crosstalk from adjacent tracks becomes a problem. In this case, there is a two-carrier system in which a carrier with a different frequency is set up for each channel for recording. The present invention is also effective for a two-carrier system. In the case of 2 carrier system,
Two modulators with different center frequencies are required,
Compared to the one-carrier method, the number of variation factors increases, and DC offset compensation becomes increasingly necessary.

FIG. 7 shows an embodiment in which the present invention is applied to a two-carrier audio FM multiplex recording system. Since this embodiment uses a two-carrier system, there is a possibility that the amount of DC offset becomes large. Therefore, a fixed adjustment circuit with a wide adjustment range is used for coarse adjustment, and an automatic adjustment circuit is used for fine adjustment. The main parts of this embodiment are the same as the block diagram of the VTR shown in FIG. 3, and the common parts are given the same numbers. 120 and 121 are FM
Each modulator has a different center frequency. 122, 12
3 is a low-pass filter, 124, 125 is a mixer, 126, 127 is a recording amplifier, 128, 12
9 is BPF, 130, 131 are each 120,
FM demodulators compatible with 121, 132 and 133 are
LPFs 134 and 135 are level adjustment circuits for adjusting the reproduction levels of both tracks, 136 and 137 are DC off-set fixed compensators, and 138 is a DC off-set automatic compensation circuit. The reason why the center frequencies of the two FM modulators 120 and 121 are changed from each other is that the signals of adjacent tracks are converted to BPF 128,
This is because it can be removed with 129 lbs., so it is strong against adjacent damage prevention. Since the operation of this circuit is the same as that shown in FIG. 3, a detailed explanation of the operation will be omitted.

After the reproduced audio signals output from the FM demodulators 130 and 131 have unnecessary bands removed by LPFs 132 and 133, level adjustment circuits 134 and 1 fixedly adjust variations in the reproduction level of each channel.
After passing through 35, it passes through DC offset fixed regulators 136 and 137 that fixedly compensate for DC offset, and completely corrects DC offset due to temperature characteristics etc. in an automatic DC offset compensation circuit 138.

FIG. 7 shows an example in which the reproduction level adjustment circuits 134, 135 and the DC off-set fixed compensation circuits 136, 137 are attached to both channels CH-1 and CH-2, but depending on the specific implementation circuit, In some cases, it may be sufficient to attach it to only one channel.

FIG. 8 shows a concrete example of the reproduction level adjustment circuit and the DC off-set fixing adjustment circuit. 14
0 and 141 are the input terminals of the playback level adjustment circuit for the playback signals of CH-1 and CH-2, _Q5 to _Q7 are transistors, _R5 to _R9 are resistors, VR1 and VR2 are variable resistors, and Vcc is the power supply. It is voltage. The signal input from terminal 141 passes through an amplifier consisting of transistor Q ₅ and resistors R ₅ and R ₆ , and then is connected to terminal 101 for input to the automatic DC off-set compensation circuit shown in FIG. The reproduction level of the signal input from the terminal 140 is adjusted by a variable gain amplifier consisting of transistors Q ₆ , R ₇ and VR1 to match the reproduction level of the transistor Q ₅ . Transistors Q ₇ and R ₈ constitute a current source, and by changing VR2, the current value of the current source is changed, and by changing the voltage drop of R ₇ , the collector potential of Q ₇ is controlled, and the DC Offset can be adjusted.
The above is a case in which the DC offset is roughly adjusted, but if this output signal is input to the terminal 100 in FIG. 6, the DC offset can be automatically finely adjusted based on temperature characteristics and the like.

The above explanation has been given using the recording and reproduction of monaural audio signals as an example, but as shown in FIG. 9, the present invention can also be applied to stereo or bilingual recording and reproduction. In this embodiment, two carriers are provided for one channel, and a total of four carriers are used. 160,
161 are recording audio input terminals for stereo L and R or bilingual main audio and sub audio, respectively;
2 and 163 are paired with 194 and 195, respectively, 162 and 163 are amplitude compression circuits for noise removal, 194 and 195 are amplitude expansion circuits, and 164
-167 are FM modulators, 168-171 are LPFs that remove unnecessary bands, 172 and 173 are mixers for FM-modulated audio and video signals, 180-18
3 is a BPF for audio signal extraction, 184 to 187 are audio signal FM demodulators, 188 to 191 are LPFs that pass only audio signals, and 192 and 193 are for audio signals reproduced by the first head 8 and the second head. 9
Switching circuit equipped with a noise removal function that switches the audio signal reproduced by the 196, 1 without noise
Reference numerals 97 are playback audio signal output terminals corresponding to 160 and 161, respectively.

The operation of the apparatus shown in FIG. 9 is essentially the same as the operation of the apparatus shown in FIG. 3, and will therefore be briefly described. The audio signals input from 160 and 161 pass through amplitude expansion circuits 162 and 163, respectively, and then are FM modulated by FM modulators 164 to 167.
Signals whose center frequencies are recorded by the first head 8 and the second head 9, and L, R (or main audio, sub audio)
This results in ₃ to ₆ different modulated signals. Audio signals ₃ and ₄ are recorded in the first head 8 after removing unnecessary bands of this modulated signal by LPFs 168 to 171.
and the video signal are mixed in the mixer 172, and then
The data is recorded on the recording medium 10 by the magnetic recording head 8 via the recording amplifier 126. The same applies to the signals recorded by the second head 9.

During reproduction, the signals reproduced by the magnetic reproduction heads 8 and 9 are amplified by preamplifiers 11 and 12, and then
BPF180-183 corresponding to center _{frequency 3-6}
After passing through, the signal is demodulated by FM demodulators 184 to 187 corresponding to FM modulators 164 to 167, respectively, and then passes through LPFs 188 to 191 to remove the carrier component and extract only the audio signal. DC off/
Set automatic compensation circuits 192 and 193 respectively.
The audio signals of CH-1 and CH-2 are connected except for DC off-set, and passed through amplitude expansion circuits 194 and 195 corresponding to 162 and 163, so that the audio can be reproduced without discontinuity due to DC off-set.

As described above, by using the present invention, it is possible to prevent large amplitude noise caused by discontinuity of the FM signal at the time of switching the reproduction track, and waveform distortion caused by holding the previous value. Furthermore, by using the present invention, it is possible to almost completely eliminate discontinuities in the audio signal that occur when switching reproduction tracks with a simple circuit configuration, which is highly effective.

[Brief explanation of drawings]

Figure 1 is a block diagram of a video tape recorder to explain the conventional audio FM multiplexing system, Figure 2 is a recorded signal frequency spectrum diagram of an example of the audio FM multiplexing system, and Figure 3 is a diagram of a recorded signal frequency spectrum using the present invention. A block diagram showing an embodiment of the audio FM multiplexing system, Figure 4 A to F are waveform diagrams for explaining its operation, Figure 5 is a block diagram of a switching circuit with a DC off-set removal function, and Figure 6. is a circuit diagram showing a specific example thereof, Fig. 7 is a block diagram showing an example using the present invention in an audio FM multiplexing system with two FM modulators, and Fig. 8 is a circuit diagram showing a reproduction level adjustment circuit and a DC off circuit.・Circuit diagram showing a specific example of a set fixed compensation circuit,
FIG. 9 is a block diagram showing an embodiment for recording stereo audio or bilingual audio. Explanation of symbols, 3...FM modulator, 8, 9...Magnetic head, 10...Magnetic recording medium, 15...Band pass filter, 16...FM demodulator, 18...Previous value holding circuit, 22, 23...Band pass filter,
24, 25...FM demodulator, 28...Switching circuit, 102...DC off-set compensation circuit,
103... Subtraction circuit, 120, 121... FM modulator, 134, 135... Level adjustment circuit, 13
6,137...DC off-set fixed compensator, 1
38...DC off-set automatic compensation circuit, 16
4,165,166,167...FM modulator, 1
80, 181, 182, 183... Band pass filter, 184, 185, 186, 187... FM
Demodulator, 192, 193...DC off-set automatic compensation circuit.

Claims (1)

[Scope of Claims] 1. A plurality of playback heads that frequency-modulate a dynamic range compressed audio signal and read out the frequency-modulated audio signal from a magnetic recording medium sequentially recorded by the plurality of heads; a plurality of frequency demodulators that demodulate the frequencies of signals reproduced from the reproduction heads; a switching circuit that switches the output signals of the plurality of frequency demodulators within a period in which the signals are simultaneously output from the plurality of reproduction heads; A noise removal device comprising: an amplitude expansion circuit for expanding the dynamic range of the output signal of the switching circuit; 2. A plurality of playback heads that read out frequency-modulated audio signals from a magnetic recording medium that frequency-modulates a dynamic range compressed audio signal and sequentially records the audio signal on a plurality of heads, and reproduces the audio signal from these multiple playback heads. a plurality of frequency demodulators for frequency demodulating each signal; a switching circuit for switching the output signals of the plurality of frequency demodulators during a period in which the signals are simultaneously output from the plurality of playback heads; an amplitude expansion circuit that extends the dynamic range; a DC offset potential detection circuit that detects the difference potential between the output signals of the plurality of frequency demodulators during a period in which signals are simultaneously output from the plurality of heads; A noise removal device comprising: a DC offset compensation circuit that subtracts a DC offset amount from the output signal of one frequency demodulator based on the output signal of one frequency demodulator according to the output signal of the circuit.