JPS6367251B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for measuring electromagnetic conversion characteristics of a magnetic head.

In recent years, the recording density of home VTRs has increased rapidly and there is a trend toward smaller size. In the VHS type VTR, the tape speed is 3.33 [cm/S], the head-tape relative speed is 5.5 [m/S], and the guard band between video tracks on the magnetic tape is omitted by azimuth recording. Improved density. To perform this azimuth recording, a pair of video magnetic heads (hereinafter abbreviated as magnetic heads) with different gap inclinations are used.
In this case, when trying to obtain the best VTR playback image,
It is necessary that the electromagnetic conversion characteristics of these two magnetic heads be the same, and it becomes necessary to select the optimum pair of magnetic heads. These electromagnetic conversion characteristics include optimal recording current characteristics and frequency characteristics, and the electromagnetic conversion characteristics in the several MHz band have been visually measured using an oscilloscope, but the visual error is large and the number of measurements increases. I couldn't do it. For this reason, it has become necessary to efficiently measure the electromagnetic conversion characteristics of magnetic heads. Conventionally, the electromagnetic conversion characteristics of a magnetic head have been measured by using an oscilloscope to measure the peak-to-peak value (PP value) of a magnetic head reproduction signal. Therefore, in order to maintain compatibility with conventional measurement methods, the magnetic head electromagnetic conversion characteristic testing apparatus of the present invention also employs a method of measuring peak-to-peak values of reproduced signals. There are two ways to measure the P-P value: one is to calculate and display the average value or effective value of this signal, and the other is to directly perform envelope detection of the signal and display it. The former method of performing this calculation and displaying the P-P value is effective for signals with a single frequency input waveform as shown in Figure 1B, but as shown in Figure 1A, If the input signal contains harmonic components and the harmonic level differs depending on the input signal, the harmonic components contained in the signal will vary depending on the quality of the electromagnetic conversion characteristics of the magnetic head, and the calculation constant will not be determined. Not valid. For this reason, it is effective to measure the electromagnetic conversion characteristics of a magnetic head by using the latter method of detecting the envelope of the magnetic head reproduction signal and displaying the P-P value. However, the problem here is the characteristics of the envelope detection circuit. Since test magnetic heads are tested by recording and reproducing broadband signals up to several hundred KHz, the characteristics of envelope detection must be constant in the range of several hundred KHz to several MHz. A method of detection that combines an operation amplifier and a diode is considered, but there is no operation amplifier that can obtain high gain over several MHz, and if an envelope detection circuit is configured with such an operation amplifier, the detection characteristics will be input. It is not valid because it depends on the signal frequency.

Therefore, it is converted into a DC voltage using a diode envelope detection circuit as shown in Figure 2. Here, D ₁ and D ₂ are shot key barrier diodes. This diode-type envelope detection
As shown in FIG. 3, in the AC-DC conversion characteristic, a low level signal is detected in a forward non-linear period V _f , so the true value is not indicated. It is necessary to amplify the input level so that it is at least higher than V _f .
Furthermore, even if a shot key barrier diode with a small V _f is used, the voltage will not be less than 100 + [mv]. usually
Since the VHS-VTR magnetic head output is about 100 [μV _pp ], the gain of the regenerative amplifier needs to be 70 [dB] or more. Creating such a wide-band, high-gain amplifier involves problems such as oscillation, which makes the measuring instrument unstable. It is also necessary to keep environmental changes such as temperature constant.

The present invention eliminates this unstable element and provides a method that can linearly perform AC-DC conversion at a low level.

An embodiment of the present invention will be described based on the drawings.
In FIG. 4, the test head 1 is attached to a rotating cylinder like a VHS type VTR and rotated at high speed, or the test head 1 is fixed and the magnetic tape 2 is run at high speed.
Set it so that it matches the relative speed of the VTR you are using.
In the recording state, the oscillation frequency of the reference oscillator 3 is f _S1
The output is input to the attenuator 4, and the oscillator output is adjusted and sent to the switching circuit 5.
is input. The output is power amplified by a recording amplifier 6, and a recording current I ₁ [mA _pp ] is supplied to the test head 1 through a switch SW-1 to be recorded on the magnetic tape 2. Here, the attenuator 4 automatically adjusts the level of the output of the reference oscillator 3 so that the recording current supplied to the test head 1 reaches the set value.

The switching circuit 5 switches the level-adjusted signal f _S1 through the attenuator 4 according to the signal of the oscillation frequency f _Q1 of the sampling oscillator 7, and generates a recording current given to the test head 1 as shown in FIG. 5A. I have to. Here, the oscillation frequency f _Q1 of the sampling oscillator 7 is oscillated while maintaining the relationship shown in the following equation with the oscillation frequency f _S1 of the reference oscillator 3.

f _S1 −nf _Q1 = f _C (constant) n is an integer value of the result of dividing f _S1 by f _Q1 .
When f _S1 > f _Q1 , the remaining frequency signal after dividing f _S1 by f _Q1 appears as the output frequency f _C obtained by sampling f _S1 with f _Q1 . Output of sampling oscillator 7 oscillated under these conditions
The phase of f _Q1 shifts at a constant speed with respect to the reference oscillator 3 output f _S1 , and the frequency of this periodic phase shift is f _C. Therefore, we can expand the time axis of the current waveform in Figure 5 A. When I looked at the signal, the signal appeared as shown in Figure 5 (b).
The switching timing of f _S1 changes sequentially at a cycle of 1/f _C. Such a signal is supplied to the test head 1 and recorded. Note that the sampling oscillator 7
The oscillation frequency f _Q1 may be any value as long as it satisfies the relational expression with f _S1 described above, but considering that it can be easily processed later and can be used with an operation amplifier, it is set to 100 KHz. You will get better results if you make the audio band signal below. When the recording of the signal with the frequency f _S1 and the recording current I ₁ is completed in this way, the current setting value of the attenuator 4 is changed to I ₂ and the current setting value of the attenuator 4 is changed to I 2 and the The signals f _S1 and _I2 as shown in Figure 5 A are sent to the test head 1.
is supplied to the magnetic tape 2 and recorded on the magnetic tape 2. Thereafter, the current setting value was changed sequentially to I ₃ , I ₃ ...I _o , and test head 1 was tested.
We will supply it to Next, change the oscillation frequency of the reference oscillator 3 to f _S2 and change the current setting of the attenuator 4 to I ₁
The magnetic tape 2 is returned and supplied to the test head 1.
The current setting value is then changed sequentially to I ₂ , I ₃ , . . . I _o and recorded. At this time, the oscillation frequency of the sampling oscillator 7 is also changed to f _Q2 . In this way, recording is performed one after another until the oscillation frequency f _So and the recording current I _o are recorded.

Once everything has been recorded, it will automatically go into playback mode.
The switch SW-1 is switched to the P/B side, the magnetic tape 2 is rewound at high speed, and stops at the recording start position of the first recorded frequency f _S1 and recording current I ₁ , and then starts playback operation with normal running. , frequency on magnetic tape 2
f _S1 , the frequency f _So from the signal recorded at the recording current I ₁ ,
The recorded signal is sequentially reproduced using the recording current _Io . Here, the signal regenerated by the test head 1 is input to the regenerative amplifier 8 via the switch SW-1. Here, the playback output of test head 1 is several +
Since it is a very small signal ranging from [μV _pp ) to several hundred [μV _pp ], it is amplified to a level greater than the number [mV _pp ] that can be processed in subsequent stages. When the output of the regenerative amplifier 8 is observed here, it becomes an intermittent burst-like signal as shown in FIG. 6A. An enlarged view of the time axis of this burst-like reproduction signal is shown in FIG. 6B. This reproduced signal f _S1 is transmitted to the test head 1.
By changing the relative speed of magnetic tape 2 and magnetic tape 2, △f _S1
The entire playback signal oscillates in the time direction because it includes a time axis fluctuation component (jitter) of [Hz], but as shown in Figure 6 (b), the waveform switching position of the playback reference signal sequentially moved as in the case of recording. It becomes a waveform. This signal is input to the sample hold circuit 12 and also to the envelope detection circuit 9. This envelope detection circuit 9 is for detecting the envelope of the signal level shown in Figure 6B, shaping the waveform, and creating a rectangular wave signal as shown in Figure 6C. Because I don't
There is no need to manage the linearity of the detection characteristics, temperature and humidity characteristics, etc. of this envelope detection circuit 9, and a simple circuit is sufficient.

The signal obtained by the envelope detection circuit 9 becomes a frequency signal (f _Q1 ±f _Q1 ) [Hz] which includes the time axis _variation Δf _Q1 in the output signal f Q1 [Hz] of the sampling oscillator 7 . Since this rectangular wave signal is extracted from Figure 6B, the ratio of time axis fluctuation △f _Q1 /f _Q1 is the ratio of time axis fluctuation of the reproduction reference signal △
The value is similar to f _S1 /f _S1 .

Next, the rectangular wave output of the detection circuit 9 (FIG. 6C) is input to the delay circuit 10. The delay circuit 10 generates a pulse slightly delayed from the rising edge of the output of the detection circuit 9 as shown in FIG. 6D, and determines the sampling position of the reproduced signal shown in FIG. 6B. Further, the output of the delay circuit 10 is input to a pulse generation circuit 11. As shown in FIG. 6E, this pulse generating circuit 11 generates a pulse from the falling portion of the output of the delay circuit 10 and uses it as a sampling pulse. This sixth
The sampling pulse frequency of the sampling pulse in Figure E is (f _Q1 + Δf _Q1 ), and this sampling pulse causes the frequency (f _S1 + Δf Q1) shown in Figure 6 C.
The sample and hold circuit 12 samples the reproduced signal Δf _S1 ). In this sampling, since the phase of the reproduction reference signal deviates from the sampling pulse at a constant speed, the reproduction signal is frequency-converted by the sampling pulse, and the frequency-converted signal frequency is shown below.

(f _S1 +△f _S1 )−n(f _Q1 ±f _Q1 )=(f _S1 −nf _Q1 )
±(△f _S1 −n△f _Q1 )=f _C ±△f _C As a result, the sample and hold circuit 12 output is the 6th
It is converted into a signal f _C ±△f _C in the audio band as shown in the figure. The time axis fluctuation of this frequency-converted signal is ±(△f _S1 −n△f _Q1 )/(f _S1
−nf _Q1 ) = ±△f _C /f _C becomes ±△f _S1 /f _S1 = ±△
Since f _Q1 /f _Q1 , ±△f _S1 /f _S1 = ±△f _C /f _C , which is almost the same value as the time axis fluctuation ratio before frequency conversion. Normally, the amount of jitter in a VTR is several [% _pp ]
Since the following, △ for the frequency-converted f _C
f _C has a value of several [% _pp ], but if this is the value, the subsequent processing will operate stably. The signal frequency-converted in this way is input to the absolute value circuit 13, and P-P
The value signal is expressed as 0-P value (zero-P value) as shown in Figure 6.
The peak value) is input to the peak hold circuit 14, where it is converted to a DC voltage as shown in FIG. Since the converted signal is a low frequency signal in the audio band, this peak hold circuit 14
No special circuit is required, and a common circuit consisting of an operational amplifier and a diode will provide sufficient response. This peak hold circuit output is A/D converter 1
5, which is converted into a digital quantity and calculated, and the head output is digitally displayed in the display circuit 16. In this way, the reference signal frequency f _S1 ,
From the recorded current value I ₁ f _S1 , I ₂ ; ...f _S1 , I _o ; f _S2 , I ₁ ;
...The head output is displayed up to f _{So and} I _o , and test head 1 is inspected.

Here, the circuit systems 9 to 14 correspond to the envelope detection circuit system of the conventional example, and the AD-
The DC conversion characteristics are shown in Figure 7, and even minute signals of 100 [mV _pp ] or less can be linearly converted. Moreover, the conversion characteristics do not change over a wide band of 500KHz to 5MHz. Therefore, the regenerative amplifier 5
There is no need to increase the gain either; about 30 to 40 [dB] is sufficient. Therefore, it is possible to realize a stable magnetic head electromagnetic conversion characteristic measuring circuit that is resistant to unstable factors such as oscillation and resistant to environmental changes such as temperature and humidity.

According to the present invention, the following advantages can be obtained.

(1) In AC-DC conversion, a constant conversion gain can be obtained over a wide band of several MHz, and moreover, it can be performed linearly from o [mV _pp ].

(2) Since AC-DC conversion can be performed linearly from o [mV _pp ], there is no need to increase the gain of the regenerative amplifier as in the conventional case, and the stability of the measurement circuit system such as oscillation can be increased. .

(3) It is possible to realize a measurement circuit system that is stable against environmental changes such as temperature.

(4) Since high-frequency minute signals can be converted from AC to DC with a simple circuit, manufacturing of measuring instruments can be facilitated.

[Brief explanation of drawings]

Figure 1 is a waveform diagram of recording/reproduction signals by a magnetic head, Figure 2 is a circuit diagram of a conventional AC-DC converter,
FIG. 3 is an AC-DC conversion characteristic diagram of a conventional example, FIG. 4 is a block diagram showing an embodiment of the present invention, FIG. 5 is a recording signal waveform diagram of the present invention, and FIG. 6 is a diagram of each block of the present invention. Figure 7 is a time chart of the AC of the present invention.
It is a DC conversion characteristic diagram. 1...Test head, 2...Magnetic tape, 3...
Reference oscillator, 4... Attenuator, 5... Switching circuit, 7... Sampling oscillator, 8...
Reproduction signal, 9...Envelope detection circuit, 11...
...Pulse generation circuit, 12...Sample hold circuit, 14...Peak hold circuit, 15...A/
D converter.

Claims (1)

[Claims] 1. Either the magnetic head for inspection or the magnetic recording medium is slid at a certain speed, and the frequency and level of the reference signal applied to the inspection head are sequentially changed stepwise during recording to record on the magnetic recording medium. , a method of measuring the electromagnetic conversion characteristics of the inspection head by the level of the reproduced signal during reproduction, in which the sampling signal (oscillation frequency f _Q ) is converted to a reference signal (oscillation frequency f _S ) so that their frequencies are f _S
-nf _Q = f _C (f _C : constant frequency),
A reference signal is switched using the sampling signal and recorded on a magnetic recording medium via a magnetic inspection head, and upon reproduction, a switching-like reproduction reference signal obtained from the inspection head is envelope-detected and created from the detected signal. A method for measuring electromagnetic conversion characteristics of a magnetic head, characterized in that the reference signal portion of the switching-like reproduction reference signal is sampled and held using a sampling pulse, the peak of the sample-and-hold signal is held, and the signal level is converted into a digital quantity. .