JPS6125056Y2 - - Google Patents

Description

[Detailed Description of the Invention] The present invention relates to a magnetic reproducing circuit used in magnetic card readers and the like.

Generally, when reproducing magnetic recording, the characteristics of the reproduced output waveform differ depending on the recording density, so depending on whether the magnetic reproducing circuit is made for low or high recording density, malfunctions may occur in the opposite case. It becomes easier to wake up. That is, as shown in A in Figure 5, when information is read from a recording medium with a high recording density, the output waveform is a logic "1", which is twice the frequency of a logic "0", due to the frequency characteristics of the magnetic head that reads the magnetic recording. ” playback output level may drop as shown in (b). Therefore, it is preferable for the magnetic reproducing circuit to use a peak detection method in which the output of the reproducing circuit is inverted for each peak value of the reproduced waveform. Also, as shown in D in Figure 6, when the recording density is low, the bit interval T
becomes longer, creating a flat part in the reproduced waveform, and if noise like A is added to that part, the peak detection method described above will invert the output at the peak value of this part, causing malfunction. Therefore, it is preferable for the magnetic reproducing circuit to use a level detection method in which the output of the reproducing circuit is inverted every time the reproduced waveform reaches a certain reference value.

Conventionally, the magnetic reproducing circuit of a magnetic card reader uses either the peak detection method or the level detection method described above, so when reading information from recording media with different magnetic recording densities, it is necessary to read information from recording media with different magnetic recording densities. This has the disadvantage of causing malfunctions in either area.

The present invention eliminates these drawbacks and enables stable reproduction regardless of whether the recording density of the recording medium is high or low.The present invention uses a peak detection method when reproducing high recording density media, and a peak detection method when reproducing low recording density media. In this case, we provide a magnetic reproducing circuit that can be switched using a simple method such as a level detection method.

Next, one embodiment of the present invention will be described based on the drawings. In FIG. 1, both ends of a magnetic head HD that reads information from a magnetic recording medium are connected to the input ends of an amplifier AMP1 via resistors R ₁ and R ₂ , respectively, and the output end and the inverting input end of the amplifier AMP1 are connected through a resistor R. The non-inverting input terminal of the amplifier AMP1 is grounded through the resistor _R3 , and the resistors _R1 , _R2 , _R3 , _R4 and the amplifier AMP1 form a differential amplifier ₁ .
Connect the output end of amplifier AMP1 to capacitor C ₁ and resistor R ₁
The non-inverting input terminal of amplifier AMP2 is grounded via a resistor _R1 , and the two diodes _D1 , _D2 are connected to the inverting input terminal of the amplifier AMP2 through a series circuit with
are connected in series in the same direction, and the other two diodes D ₃ and D ₄ are connected in series in the same direction, and are connected in parallel in opposite directions to form an amplifier.
Connect between the output terminal of AMP2 and the inverting input terminal, connect one end of the peak detection/level detection changeover switch SW to the inverting input terminal of amplifier AMP2, and connect the other end of the switch SW to the amplifier AMP via capacitor _C3 .
It is connected to the output terminal of the amplifier AMP1 through a series circuit of a capacitor C ₂ and a resistor R ₆ . Connect the output end of amplifier AMP2 to the amplifier
The output terminal of amplifier AMP3 is connected to the inverting input terminal of AMP3, and the non-inverting input terminal is connected via a resistor R ₁₀ , and the non-inverting input terminal is grounded via a _resistor R _{9 .} A Schmitt trigger circuit 2 is formed by the amplifier AMP3.

Also, the connection point of diodes D ₁ and D ₂ and the diode
The connection point of D ₂ and D ₄ is made common and connected to the non-inverting input terminal of the amplifier AMP3 via the resistor R ₃ .
This is to stabilize the operation of the amplifier AMP2. That is, when the inverting input terminal of the amplifier AMP2 is in a no-signal state, the amplifier is connected via the resistor _R8 .
A bias current is applied to the inverting input terminal of AMP2, and a Schmitt trigger circuit 2 is applied to the output of amplifier AMP2.
This is to prevent noise that would reach the hysteresis level. This is because when the noise is viewed as a current source, the current is extremely small compared to when the reproduced signal is viewed as a current source.
The bias current to the inverting input terminal of AMP2 becomes effective.

Next, to explain the circuit configuration when the changeover switch SW is opened and closed, when the changeover switch SW is opened, the series circuit of capacitors C ₂ and C ₃ and resistor R ₆ is connected between the outputs of amplifiers AMP1 and AMP2. Therefore, it can be virtually ignored, and the circuit shown in FIG. 1 can be rewritten as shown in FIG. 2. Here, capacitor C ₁ , diode D ₁ to _{D 4} , amplifier AMP2
A well-known peak detection circuit 3 (see, for example, Japanese Patent Application Laid-Open No. 130917/1983 filed by the present applicant) is formed by this. Note that the circuit consisting of diodes _D1 to _D4 may be a circuit that is cut off for voltages smaller than each of the positive and negative constant voltages, and conductive for voltages greater than the respective constant voltages, and is a constant voltage circuit. A circuit in which diodes are connected in series with opposite directions may also be used. Note that the resistor R ₅ is a protection resistor, and the capacitor C ₁ is set to a constant for peak detection in the peak detection circuit 3. Also, the amplifier
A reference potential is applied to the non-inverting input terminal of AMP2 via a resistor _R1 . Next, when the changeover switch SW is closed, the circuit shown in FIG. 1 can be rewritten into a circuit as shown in FIG. 3. capacitor here
_C2 is a capacitor for cutting off low frequencies, but if the capacitance of this capacitor is increased, the amplifier AMP1
Most of the output current is supplied to the input of the amplifier AMP2 through the capacitor C ₂ and the resistor R ₅ . Therefore, if the values of the resistor R ₆ and the capacitor C ₃ are set as constants to form the desired integrating circuit, the well-known circuit with the time constants R ₆ and C ₂ can be formed by the resistor R ₆ , the capacitor C ₂ , and the amplifier AMP2. A speed compensated integral amplifier circuit 4 is formed. At this time the diode
D ₁ to _{D 4} act as amplitude compression elements to stabilize the above-mentioned integral operation.

In FIGS. 2 and 3, the same symbols as in FIG. 1 represent the same elements, and the symbols A to F are check points of waveforms to be described later.

To explain the operation, when reading a medium with high recording density, the switch SW is opened. Then, the circuit shown in FIG. 1 becomes the circuit shown in FIG. 2. When the output from the magnetic head HD is amplified by the differential amplifier 1, it becomes a waveform as shown in FIG.
When the output is inverted for each peak value of the A waveform, a B waveform is obtained, which is further input to the Schmitt trigger circuit 2 for waveform shaping, and a rectangular wave like the C waveform is obtained. In other words, even if the output level is low as shown in waveform A, since peak detection is performed, a reproduced output with accurate timing can be obtained without malfunction. Next time when reading a medium with low recording density, close the switch SW. Then, the circuit shown in FIG. 1 becomes the circuit shown in FIG. 3. When the output from the magnetic head HD is amplified by the differential amplifier 1, it becomes a waveform as shown in Fig. 6D. When this is input to the integral amplifier circuit 4, since the capacitance of the capacitor _C2 is large, most of the input signal is transmitted to the capacitor C. ₂ and the amplifier AMP2 via resistor R ₆
is input to the inverting input terminal of . Therefore, the series circuit of the capacitor C ₁ and the resistor R ₅ can be virtually ignored, and since the capacitor C ₃ is connected between the output terminal and the inverting input terminal of the amplifier AMP2, This integral amplifier circuit 4 does not operate as the peak detection circuit shown in _FIG .
It operates as an integrating circuit whose amplitude is compressed by D ₄ and whose time constant is determined by resistor R ₆ and capacitor C ₃ . Therefore, from the output terminal of the amplifier AMP2, that is, the integral amplifier circuit 4, the time constant R ₆ ,
According to C ₃ , an integral waveform like E is obtained, which is waveform-shaped by the Schmitt trigger circuit 2 and converted to F.
A square wave reproduction output like this can be obtained. In other words, since the integral output of the integral amplifier circuit 4 is input to the Schmitt trigger circuit 2 and the output is inverted, when the D waveform in Fig. 6 reaches a level that is a certain percentage of its peak value, , integral amplifier circuit 4
The integrated output becomes a constant value, and the output of the Schmitt trigger circuit 2 can be inverted. That is,
Since the level detection method is used in which the output of the Schmitt trigger circuit 2 is inverted every time the D waveform reaches a certain ratio Vref with respect to the peak value, there will be no malfunction even if there is a noise peak as shown in A of the waveform.

FIG. 4 shows another embodiment of the present invention, in which the same reference numerals as in FIG. 1 represent the same elements. This feature is based on the integral amplifier circuit 4 in FIG.
Increase the capacitor C ₂ in the amplifier AMP
As mentioned earlier, most of the output current from C1 is input to amplifier AMP2 through resistor _R6 , but in this case, current also flows through capacitor _C1 and resistor _R5 , so the integral operation is stable. We have improved this point because there are times when it does not work. In other words,
Connect the resistor R ₁₁ between the resistor R ₅ and the input terminal of the amplifier AMP2, and connect the resistor R ₅ between the connection point with the resistor R ₁₁ and the amplifier.
The output of AMP2 is connected to the output of amplifier AMP2 through diodes _D1 to _D4 , and the current flowing through capacitor _C1 and resistor _R5 is passed through diodes _D1 to _D4 to prevent it from flowing into the input terminal of amplifier AMP2, thereby performing an integral operation. It has been stabilized.

As mentioned above, the present invention allows one magnetic reproducing circuit to be switched to the peak detection method or the level detection method by a simple external operation, so the detection method can be changed depending on the recording density of the recording medium such as a magnetic card. If selected, stable playback output without malfunction can be obtained at any time, which is an excellent practical effect.

[Brief explanation of the drawing]

Part 1 is a circuit diagram of an embodiment of the present invention, Figure 2 is an equivalent circuit diagram when the switch SW is open in Figure 1, and Figure 3 is an equivalent circuit diagram when the switch SW is closed in Figure 1. , FIG. 4 is a circuit diagram of another embodiment of the present invention, and FIGS. 5 and 6 are operational waveform diagrams. C ₁ ... Capacitor, C ₃ ... Capacitor, D ₁ .
D ₂ , D ₃ , D ₄ ... Diode, R ₆ ... Resistor, AMP
2...Amplifier, SW...changeover switch.

Claims (1)

[Scope of utility model registration request] A magnetic head HD for reading information from a magnetic recording medium, a first amplifier 1 for amplifying the output from the magnetic head, and one end connected to the output terminal of the first amplifier. a first peak detection capacitor _C1 , and a second amplifier AMP2, one input terminal of which is supplied with a reference potential, and the other input terminal of which is connected to the other end of the first capacitor.
, each of the positive and negative constant voltages connected between the other input terminal of the second amplifier and the output terminal of the second amplifier causes a cut-off state for a small voltage. circuits D ₁ to _{D 4} that become conductive to a voltage greater than a constant voltage; a changeover switch SW connected at one end to the other input terminal of the second amplifier; a second integrating capacitor _C3 connected between the output terminal of the second amplifier and the other end of the switch and the first amplifier;
A magnetic regeneration circuit having an integrating resistor _R6 connected between one end of the capacitor.