JPH0240101A - Envelope detection circuit - Google Patents

Abstract

PURPOSE:To perform a comparative operation stably for a disturbance noise by providing a variable DC offset voltage generation circuit to offset the potential of a comparison signal input terminal to the one to mask a noise level corresponding to the magnitude of the noise level of a comparison signal inputted to a comparator. CONSTITUTION:The title circuit is provided with a peak hold circuit 2 to hold the peak value of an input envelope detecting signal, a variable attenuator circuit 20, and the variable DC offset voltage generation circuit 30. The variable DC offset voltage generation circuit 30 offsets the comparison signal input terminal of the comparator 4 to DC potential to mask the noise level, not depending on the level of the input envelope detecting signal. Therefore, it is possible to reduce the output of an erroneous comparison signal due to the noise even when the envelope signal with large disturbance noise is inputted. In such a way, a stable comparative operation can be obtained.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention is applicable to two rotary heads having different azimuth angles, for example.
The present invention relates to an envelope detection circuit that detects the presence or absence of a signal reproduced by the rotary head or the presence or absence of a signal area that is normally reproduced in a magnetic recording and reproducing apparatus having more than 100 yen.

[Prior art] Figure 3 is a diagram showing a conventional envelope detection circuit.
) is reproduced by a rotating head of a magnetic recording and reproducing device.
00K) Iz ~ 2) l) The high frequency reproduction signal a of lz is A
AM detection circuit that performs M detection and outputs an envelope detection signal, (2) is the peak value of the envelope detection signal,
A peak hold circuit that outputs the voltage signal C held for a sufficiently long time, (3) is a capacitor connected to the peak hold circuit (2) and determines the time constant for holding the peak value, and (4) is an envelope detection signal The output voltage signal C of the peak hold circuit (2) is input to the reference signal input terminal, and a binary comparator of H level or L level is output depending on the level difference between the two input signals. a comparator that outputs a signal f;
(5), (6), and (7) have resistance values R1,
A reference voltage signal Vd, which is obtained by reducing the output signal C of the peak hold circuit (2) by a predetermined ratio, is input to a comparator (4) using resistors R2 and R3 to determine a threshold level. (8) is a switch that grounds the resistor (7) and switches the reference voltage signal Vd to a low value.The resistors (5), (8), (7) and switch (8) connect the variable attenuator circuit (20). It consists of

FIG. 4 is a signal waveform diagram of each part for explaining the operation of this conventional example, and FIGS. 4(a) to (fl) are a -
The input/output signal waveforms at point f are shown.

Next, the operation will be explained. When the signal is reproduced by a rotating head while feeding the magnetic tape at a speed faster than the standard reproduction speed (hereinafter referred to as "high-speed reproduction"), the reproduction signal a
As shown in FIG. 4(a), the reproduced high frequency component has a signal waveform that is AM modulated with the rotation period of the rotary head.

The AM detection circuit (1) detects 500
High frequency components are removed under conditions such that the ripple of the double-wave rectified signal of about KHz is a sufficiently small value with respect to the envelope (7) amplitude (AM modulation degree), and the ripple is adjusted according to the fluctuation of the envelope of the reproduced signal a A waveform envelope detection signal shown in FIG. 4(b) is output. This envelope detection signal is input to the peak hold circuit (2), and the peak value vb is held by the capacitor (3) for a period sufficiently longer than the amplitude fluctuation period of the envelope.The peak bold voltage of the waveform shown in FIG. Signal C is output.

As a result, the peak hold voltage signal C becomes approximately the DC level of the voltage vb as long as the envelope detection signal continues at a sufficiently shorter period than the discharge time constant of the capacitor (3), and can be used as a reference voltage signal for the comparator (4). .

Next, the operation of the comparator (4) will be explained.

The comparator (4) generally outputs either H1 or L binary level if there is even a slight difference between the comparison signal and the reference voltage signal.

Therefore, if you set one of the two input signals to the level you want to compare in advance, that value will be used as the threshold value to convert the comparison signal, which is an analog signal, into a binary signal of H9L. .

In conventional circuits, the threshold value is determined by a resistor (
5), (13), and (7) and the opening/closing of switch (8).
8) is closed, the comparator (4) operates under the following conditions.

If the peak value of the envelope detection signal is Vb, then Vd = Vb (R2/(RI+R2)) (as shown in FIG. 4(d)) is input to the reference voltage signal input terminal of the comparator (4). Therefore, the comparator (4) determines that the level of the envelope detection signal is Vb(
During a period greater than R2/(R1+R2)), a binary comparison signal f as shown in FIG. 4(f) which is at H level is output.

Furthermore, when the switch (8) is open, the reference voltage signal Vdl input to the comparator (4) is
Figure 4 (di) 4;: Indication, Vd1=V b(R
2+R3) /(R1+R2+R3) Tonari, since the threshold value has been increased, the comparator (4
) is a binary comparison signal f1 as shown in FIG. 4 (fl). This switch (8) is closed during standard speed playback where the AM modulation degree of the envelope detection signal is large, and is opened during high speed playback where the AM modulation degree is small. The reference voltage signal is switched so that low level detection is performed without error.

[Problems to be Solved by the Invention] Since the conventional envelope detection circuit is configured as described above, when a no-signal (no-recording) area continues for a long time, the envelope detection signal continues to be at 0 level, and this Along with this, the output voltage signal C of the peak hold circuit (2) also becomes the θ level. Therefore, the reference voltage signal d input to the comparator (4) also becomes O level.
A binary comparison signal f is generated depending on the state of the DC offset voltage generated inside the circuit of the comparator (4).

There was a problem in that vignetting noise was generated in fl as shown in FIGS. 4(f) and (fl), resulting in an unstable state.

Also, if a certain level of noise is reproduced in a no-signal area, [Reference voltage - Noise level X R2/(
R1+R2)]<[comparison signal=noise level], and there is also a problem in that the comparator (4) outputs a binary comparison signal f indicating the presence of a signal even though there is no signal.

This invention was made to solve the above-mentioned problems, and it is possible to ensure stable comparator operation even when a no-signal area continues for a long time, and even in the case of an envelope detection signal with a lot of disturbance noise. The purpose of this invention is to obtain an envelope detection circuit that can ensure stable comparator operation.

[Means for Solving the Problems] The envelope detection circuit according to the present invention offsets the potential of the comparison signal input terminal to a potential that masks the noise level in accordance with the magnitude of the noise level of the comparison signal input to the comparator. It is characterized by being equipped with a variable DC offset voltage generation circuit.

[Function] The variable DC offset voltage generation circuit in this invention has the following features:
Since the comparison signal input terminal of the comparator is offset to a DC potential that masks the noise level without depending on the level of the input envelope detection signal, even if an envelope detection signal with large disturbance noise is input, the noise will cause an incorrect comparison signal. will be output less.

[Embodiment of the Invention] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. 1st
In the figure, (9) is a resistor (10) with a resistance value R4 connected between the output terminal of the AM detection circuit (1) and the comparison signal input terminal of the comparator (4).

(11) is a constant current I1 connected to the comparison signal input terminal of the comparator (4). First and second constant current sources that flow I2, (12) is a switch that opens and closes the constant current source (11), resistor (9), constant current source (10),
(11) and the switch (12) constitute a variable DC offset voltage generation circuit (30).

FIG. 2 is a waveform diagram for explaining the operation of this embodiment, and FIGS. 2(a) to (fl) show the input and
The output signal waveform is shown.

Next, the operation will be explained.

The operations of the AM detection circuit (1), the peak hold circuit (2), and the variable attenuation circuit (20) are the same as in the conventional example shown in FIG. 3, and therefore their explanations will be omitted.

In the case of a system in which the magnetic tape is wound around a rotating drum by 90' C, the AM modulation degree of the envelope waveform of the playback signal (a) is normally synchronized with the rotation of the drum to which the rotating head is attached during playback. As shown in FIG. 5(a), since signal areas and non-signal areas are output every 90', the degree of modulation is large, resulting in almost 100% modulation. In addition, in a system to which this envelope detection circuit is applied, the width of the rotating head is usually wider than the recording track width, so when playing back a magnetic tape at high speed, within the signal area of 90°, as explained in the conventional example, A.M.
This results in a signal waveform with a small degree of modulation.

FIG. 5 is a diagram showing the waveform of the reproduction signal for one rotation of the rotary drum, and FIG. 5(a) is the signal waveform during standard speed reproduction,
FIG. 5(b) shows the signal waveform during high-speed reproduction.

Therefore, during standard speed playback, the switch (8) is closed and the reference voltage is set to a low level VdV 1 (R2/(R
1+R2)) and switch (8) for high-speed playback.
Open and set the reference voltage to high tz Jt, vat
= Vl (R2+R3)/(R1+R2+R3) 4: Set.

On the other hand, the envelope detection signal is inputted to the comparison signal input terminal of the comparator (4) via the resistor (9). This envelope detection signal is connected to the switch (12).
When is open, as shown in Figure 2(e), e=R4I
I is given by the negative DC offset voltage e,
When the switch (12) is closed, as shown in FIG. 2 (el), a negative DC offset voltage el is applied by e 1 = R4 (I 1 + I 2 ). In addition, II
and I2 are current values flowing into the constant current sources (10) and (11), which are constant regardless of the level of the envelope detection signal.

Therefore, the switch (1) is opened for no signal input, the comparison signal input terminal of the comparator (4) is offset to a small negative DC potential e=R4It, and the comparator (4)
Even if there is some DC offset in the internal circuit, the output of the comparator signal f when there is no signal can be fixed at L level.

Furthermore, in an actual magnetic recording/reproducing device, the reproduction signal from the rotating head contains impedance noise of the rotating head, noise of the head amplifier system, etc. even in a non-recording area.
Furthermore, a motor drive amplifier for magnetic tape running system (M
White noise and pulse noise are included due to noise from DA). These noise levels vary greatly depending on the running condition of the magnetic tape, and are small during standard speed playback, and are caused by increased current to the MDA and instability of the interface between the rotating head and magnetic tape during high speed playback. It gets bigger. Since this noise level is continuously reproduced at a -like level in terms of time, it becomes the output level of the AM detection circuit (1) minus the output level of the peak hole 1 circuit (2). Therefore, during normal playback, switches (8) and (12) are closed, and [(noise level during normal playback]) - (R4I 1.or
R4 (I 1 + I 2))], and during high-speed playback, open switches (8) and (12) to set [(noise level of noise during high-speed playback) - (R411orR
4(I I+I 2))] R4, II, I
If set to 2, malfunctions due to convergence will be eliminated even with respect to noise contained in the reproduced signal a.

This condition is, for example, when the noise level during standard speed playback is 0. IV, the reference voltage Vd is 50% of the peak hold circuit (2) output level e, the noise level during high-speed reproduction is 0.3 V, and the reference level Vdl is 75% of the peak hold circuit (2) output level C. In that case, if R4 is IKΩ, then during standard speed playback, (0, IVX O, 5)) (0, IV-10' X I
)-, I) 50 p-A During high-speed playback, (0,3VX O,75))(0,3V-103X I
), ", I:>75 p-A, and during standard speed playback, set 11 to 50 JLA and close switch (12), and during high speed playback, close switch (12) and set I2 to 25 PA.
It would be a good idea to do this.

In the above embodiment, the switch (12) was closed during high-speed playback, but depending on the state of the noise level and how the reference voltage is applied, the switch (12) may be closed during standard-speed playback.
) can also be configured to be closed.

[Effects of the Invention] As described above, according to the present invention, depending on the magnitude of the noise level contained in the envelope detection signal input to the comparison signal input terminal of the comparator, the comparison signal input terminal is connected to the noise level. Since the variable DC offset voltage generating circuit is provided to offset the DC potential that masks the level, an envelope detection circuit that stably performs a comparator operation against disturbance noise can be obtained.

[Brief explanation of the drawing]

FIG. 1 is a block circuit diagram of an embodiment of the present invention, FIG. 2 is a signal waveform diagram of each part for explaining the operation of the embodiment, and FIG. 3 is a block circuit diagram of a conventional envelope detection circuit.
FIG. 4 is a signal waveform diagram of each part of this conventional circuit, and FIG. 5 is a diagram showing waveforms of reproduction signals during standard speed reproduction and high speed reproduction. (1)...AM detection circuit, (2)...Peak hold circuit, (4)...Comparator, (20)...Variable attenuation circuit, (30)...Variable DC offset voltage generation circuit . In each figure, the same reference numerals indicate the same or corresponding parts.

Claims (1)

[Claims] (1) A peak hold circuit that holds the peak value of the input envelope detection signal, and if the envelope detection signal is a signal with a large AM modulation degree, a low reference that reduces the peak hold voltage by a first ratio. a variable attenuator circuit that outputs a voltage and outputs a high reference voltage reduced by a second ratio when the envelope detection signal is a signal with a low AM modulation degree;
a comparator whose comparison signal input terminal receives the AM detection signal and whose reference voltage signal input terminal receives the output voltage of the variable attenuator circuit; The comparison signal input terminal of the comparator is offset to a DC potential of a small value that masks the noise, and if the envelope detection signal is a signal with a high noise level, the input terminal of the comparison signal of the comparator is offset to a DC potential of a large value that masks the noise. An envelope detection circuit equipped with a variable DC offset voltage generation circuit.