JP3641792B2 - Audio tape playback circuit - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a technology effective when applied to an audio tape playback circuit, and further to an audio tape playback device for playing back an audio signal magnetically recorded on an audio cassette tape, and is used for, for example, a cassette deck for a car stereo. And effective technology.
[0002]
[Prior art]
The audio tape reproducing circuit performs an equalizing operation for amplifying the reproduction output of the magnetic head while performing frequency compensation in order to perform audio reproduction with a balanced frequency characteristic.
[0003]
The magnetic head traces the recording track of the magnetic tape while contacting the magnetic tape driven to run at a constant speed, thereby converting the audio signal magnetically recorded on the magnetic tape into an electric signal and outputting it. .
[0004]
At this time, the frequency characteristics of the magnetic tape and the reproducing head are not flat, and in particular, the converted output of the reproducing head exhibits a frequency characteristic in which the level decreases in the lower sound range. Therefore, a natural reproduced sound cannot be obtained even if this is amplified and reproduced as it is.
[0005]
In order to correct the frequency characteristics of the reproducing head, the audio tape reproducing circuit is configured to perform frequency compensation so-called equalization (equalization processing) that selectively reinforces the frequency range in which the level decreases (for example, Segawa). Fuyuki “The Fun of Audio” (published by Shinchosha), see pages 368-371).
[0006]
FIG. 5 shows a configuration of an audio tape reproducing circuit developed by the inventor prior to the present invention.
[0007]
The main part of the reproducing circuit shown in the figure is formed by an equalizer amplifier 1. The equalizer amplifier 1 is formed by an operational amplifier circuit 11 having a differential input (+/−) and a frequency selective feedback circuit 12, and operates as a low-frequency emphasis type equalizer amplifier in which the amplification gain increases in the low sound range. .
[0008]
The operational amplifier circuit 11 is a kind of bipolar differential circuit configured using pnp bipolar transistors Q11 to Q17, npn bipolar transistors Q21 to Q25, resistors R1 to R3, a phase compensation capacitor C1, a constant current source I1, and the like. The reproduction signal generated in the winding (coil) of the magnetic head 2 is amplified and transmitted, and a low-frequency emphasis type equalization operation is performed by frequency selective negative feedback by the feedback circuit 12 and the external resistors R5 and R6. Reference numeral 21 denotes a magnetic tape which is driven to travel while being in contact with the magnetic head 2. Vb is a bias voltage source.
[0009]
If necessary, the output of the equalizer amplifier 1 passes through a post circuit 31 such as a tape noise removal system (for example, “Dolby System”), and is then amplified by the main amplifier 32 and supplied to the speaker 33. As a result, the frequency characteristics of the reproduction head are compensated, and a natural reproduction sound with a balanced frequency can be obtained.
[0010]
[Problems to be solved by the invention]
However, the present inventors have revealed that the above-described technique has the following problems.
[0011]
That is, since this type of conventional audio tape reproducing circuit has a low-frequency emphasis type equalization characteristic as described above, it also has a function as a kind of low-pass filter. The audio tape reproducing circuit having the low-pass filter characteristic is believed to be hardly affected by noise because it acts as a kind of noise filter for high-frequency noise. At least, an audio tape reproducing circuit having a low frequency emphasis type frequency characteristic hardly causes a problem due to a high frequency noise.
[0012]
However, according to what the present inventor has learned, the audio tape reproducing circuit, which should have been originally excellent in noise resistance due to the low-pass filter characteristics, can generate UHF near 1 GHz or higher quasi-microwaves. It has been found that the radio wave (N) emitted from the mobile phone used suffers significant noise interference. Furthermore, this noise disturbance has not been a problem in the conventional two-way mobile phone using a continuous carrier, but it appears prominently in a time division multiplexing mobile phone sharing the same frequency channel for transmission and reception. Has been clarified by the inventor.
[0013]
In the field of mobile communication such as cellular phones, in order to make effective use of frequency resources, the conventional frequency division method is shifting to a multiplexing method using time division or code division. In this multiplex system mobile phone, as shown in FIG. 6, in order to use the same frequency for transmission and reception and to use the same frequency among a plurality of terminals, burst transmission of radio waves is performed at regular intervals (1 / fn). However, this burst transmission output (mobile phone output) appears as noise output (reproduction noise output) of the equalizer amplifier 1. Since this noise output overlaps with the low sound range where the gain of the equalizer amplifier 1 increases, the S / N ratio of the audio reproduction output is significantly impaired.
[0014]
In view of this, the present inventor inserts high-frequency bypass capacitors Cp1 and Cp2 in parallel on the input side of the equalizer amplifier 11 as shown in FIG. 7 in order to remove the high-frequency noise (N) due to the burst transmission. investigated.
[0015]
However, the bypass capacitors Cp1 and Cp2 are not only effective against the high-frequency noise, but the charge / discharge current Ip flowing through the capacitors Cp1 and Cp2 flows through the coil of the magnetic head 2 when the power is turned on / off. As a result, it has been found that the magnetic tape 21 in contact with the magnetic head 2 is erroneously recorded locally.
[0016]
In addition, as shown in FIG. 8, the installation of a noise filter using inductances L1 and L2 was also examined. This type of noise filter is said to have a higher noise removal effect than a capacitor alone, but has been found to be very expensive.
[0017]
As described above, in the audio tape reproducing circuit described above, noise disturbance due to burst transmission in a high frequency range above UHF is remarkable, while the conventional noise countermeasure technology using a capacitor and an inductance is less effective, and the magnetic tape. A new problem of erroneous recording occurs. In addition, since it is difficult to integrate a capacitor and an inductance used for the above-described noise removing means, it has been necessary to use a shield for the housing.
[0018]
SUMMARY OF THE INVENTION An object of the present invention is an audio tape reproducing circuit having a low-frequency emphasis type equalization characteristic, and does not depend on external components, and does not involve a risk of erroneous recording on a magnetic tape. It is an object of the present invention to provide a technique that greatly improves the noise resistance against UHF or higher burst transmission, which has been difficult to achieve with conventional noise countermeasure techniques.
[0019]
The above and other objects and features of the present invention will be apparent from the description of this specification and the accompanying drawings.
[0020]
[Means for Solving the Problems]
Of the inventions disclosed in this application, the outline of typical ones will be briefly described as follows.
[0021]
That is, the first and second bipolar transistors (Q11, Q12) are connected in common between the emitters, the common emitter is connected to the collector of the third bipolar transistor (Q14) forming a constant current source, and the first By providing a load circuit (Q21, Q22) on the collector side of the second bipolar transistor (Q11, Q12), a signal inputted between both bases of the first and second bipolar transistors (Q11, Q12) can be obtained. An operational amplification circuit (11) that is differentially amplified and taken out from the load circuit (Q21, Q22), and a negative feedback with frequency selectivity are applied to the operational amplification circuit (11), thereby providing a magnetic head (2). And a negative feedback circuit (12) that provides a low-frequency emphasis type equalization characteristic that compensates for the frequency characteristic. In the tape reproducing circuit, a resistor (R4) that suppresses the formation of a high-frequency path is connected in series between the common emitter of the first and second bipolar transistors (Q11, Q12) and the collector of the third bipolar transistor (Q14). The offset stability against high-frequency noise of the operational amplifier circuit (11) is enhanced by inserting it into the operational amplifier circuit (11).
[0022]
According to the above-described means, even if high-frequency noise jumps that are difficult to prevent with the conventional noise removing means, the DC offset generated by detecting the high-frequency noise between the electrodes of the bipolar transistor is suppressed. Therefore, it is possible to reliably reduce the noise generated and amplified in the low frequency range, so that the audio tape playback circuit having the low frequency emphasis type equalization characteristic depends on the external parts. In addition, there is no risk of erroneous recording on the magnetic tape, and the noise resistance against burst transmission of UHF or higher, which is difficult to achieve with conventional noise countermeasure technology, is greatly improved with only the configuration in the semiconductor integrated circuit. The objective is achieved.
[0023]
Preferably, a resistor (R4) inserted in series between the common emitter of the first and second bipolar transistors (Q11, Q12) and the collector of the third bipolar transistor (Q14) includes an operational amplifier circuit (11). In the range that does not hinder the normal operation of (), a resistance value effective to suppress the formation of a high-frequency path in a frequency range sufficiently higher than the operating frequency range of the operational amplifier circuit (11) is provided. As a result, it is possible to selectively enhance the noise removal performance for high frequency burst noise in the vicinity of 1 GHz or higher.
[0024]
Further, a resistance (R4) inserted in series between the common emitter of the first and second bipolar transistors (Q11, Q12) and the collector of the third bipolar transistor (Q14) is determined by the parasitic resistance of the bipolar transistor. Also, make sure that the resistance value is sufficiently high. As a result, it is possible to reliably obtain noise removal performance with respect to high-frequency burst noise in the vicinity of 1 GHz or higher.
[0025]
Furthermore, pnp bipolar transistors may be used as the first and second bipolar transistors (Q11, Q12) and the third bipolar transistor (Q14), respectively.
[0026]
In addition, output buffer circuits (Q15 to Q17, Q23 to Q25) for buffer-amplifying outputs taken from the collector load circuits (Q21, Q22) of the first and second bipolar transistors (Q11, Q12), The output of the output buffer circuit may be negatively fed back.
[0027]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, preferred embodiments of the present invention will be described with reference to the drawings.
[0028]
FIG. 1 shows an embodiment of an audio tape reproducing circuit to which the technique of the present invention is applied. In the figure, 1 is an equalizer amplifier, 2 is a magnetic head, 31 is a post circuit, 32 is a main amplifier, and 33 is a speaker.
[0029]
The magnetic head 2 traces the recording track of the magnetic tape 21 driven to run at a constant speed, thereby converting the audio signal magnetically recorded on the magnetic tape 21 into an electric signal and outputting it. The post circuit 31 is a tape noise removal system (for example, “Dolby system”), and the main amplifier 32 amplifies the tape reproduction signal output via the post circuit 31 and supplies the amplified signal to the speaker 33.
[0030]
The equalizer amplifier 1 performs an equalizing operation for amplifying the reproduction output of the magnetic head 2 while performing frequency compensation. The equalizer amplifier 1 is composed of an operational amplifier circuit 11 having a differential input (+/−) and a frequency selective feedback circuit 12, and operates as a low frequency emphasis type equalizer amplifier in which the amplification gain increases in the low frequency range. To do.
[0031]
The operational amplifier circuit 11 is basically a kind of bipolar differential circuit configured using pnp bipolar transistors Q11 to Q17, npn bipolar transistors Q21 to Q25, resistors R1 to R4, a phase compensation capacitor C1, a constant current source I1, and the like. The reproduction signal generated in the winding (coil) of the magnetic head 2 is amplified and transmitted, and the low-frequency emphasis type equalization operation is performed by the frequency selective negative feedback by the feedback circuit 12 and the resistor R6. Reference numeral 21 denotes a magnetic tape which is driven to travel while being in contact with the magnetic head 2. Vb is a bias voltage source.
[0032]
The pnp bipolar transistors Q11 and Q12 are connected in common at the emitters to form a differential pair. The pnp bipolar transistor Q13 transmits the reference current supplied from the constant current source I1 to the pnp bipolar transistors Q14 and Q15 by a current mirror. Thereby, Q14 and Q15 each form a constant current source for supplying a constant collector current. Resistors R1, R2, and R3 are interposed between the emitters of Q13, Q14, and Q15 and the power supply potential Vcc, respectively. Depending on the values of the resistors R1, R2, and R3, current is transmitted from Q13 to Q14 and Q15. Each coefficient can be set.
[0033]
Q11 and Q12 forming a differential pair are connected to the collector of Q14 whose common emitter is a constant current source, and a load circuit composed of npn bipolar transistors Q21 and Q22 is connected to the collector side of Q11 and Q12. The reproduction signal of the magnetic head 2 input between both bases is differentially amplified. Q21 and Q22 are such that the collector current of one of the differential transistors Q11 and Q12 (Q11) is current mirror controlled by the collector current of the other (Q12), and the difference between the collector currents of the differential transistors Q11 and Q12 is Creates a collector load circuit that converts the voltage difference.
[0034]
The pnp bipolar transistors Q15 to Q17 and the npn bipolar transistors Q23 to Q25 form a buffer output circuit for buffering and amplifying the differential amplification output taken out from the load circuit.
[0035]
The feedback circuit 12 has frequency selectivity based on the time constant of the resistor and the capacitor, and negatively feeds back the output of the operational amplifier circuit 11 taken out from the nodes (common emitter) of Q25 and Q17 to the input side, thereby the magnetic head. A low-frequency emphasis type equalization characteristic that compensates for the frequency characteristic of 2 is added.
[0036]
In addition to the above configuration, a resistor R4 is inserted in series between the common emitter of the pnp bipolar transistors Q11 and Q12 forming a differential pair and the collector of the pnp bipolar transistor Q14 forming a constant current source.
[0037]
Next, the operation will be described.
[0038]
In FIG. 1, high frequency noise (N) due to burst transmission of a mobile phone rides on the input line of the equalizer amplifier 1 and reaches the base of the transistor Q11.
[0039]
Here, if there is no particular obstacle to the transmission of high-frequency noise between the common emitter of Q11 and Q12 and the collector of Q14, the parasitic capacitance Cs of Q13 to Q15 between the emitter of Q11 and the power supply lines (Vcc and GND). A high-frequency path (high-frequency transmission path) is formed. When this high frequency path is formed, high frequency noise reaching the base of Q11 is detected between the base and emitter of Q11, and an offset current flows between the base and emitter on the Q11 side by this detection. Then, as shown in FIG. 2, when the offset current is turned on / off at the burst period (1 / fn) of the high frequency noise, a large audible noise is superimposed on the low frequency range where the gain of the equalizer amplifier 1 is increased. It comes to occur.
[0040]
However, in the circuit shown in FIG. 1, the resistor R4 interposed in series between the common emitters of Q11 and Q12 and the collector of Q14 forms a high-frequency path between the emitter of Q11 and the power supply line (Vcc). Is suppressed. Thereby, as shown in FIG. 3, the offset current due to the detection current of the high frequency noise is reduced, and the audible noise generated in the burst period (1 / fn) of the high frequency noise is greatly reduced.
[0041]
In this case, the resistor R4 has a frequency range (UHF band) sufficiently higher than the operating frequency range of the operational amplifier circuit 11 within a range that does not significantly hinder the normal operation of the operational amplifier circuit 11 in order to obtain the above-described effect. It is necessary to have a resistance value that is effective in suppressing the formation of a high-frequency path in the above), but for this purpose, the parasitic resistance that bipolar transistors normally have is not sufficient and is sufficiently higher than that. It is necessary to have a high resistance value in KΩ units.
[0042]
FIG. 4 shows a change curve of the low frequency audible noise output level with respect to the high frequency input level.
[0043]
In the figure, a curve A shows a case where there is no resistor R4, that is, a case where there is only a parasitic resistance of a transistor, and a curve B shows a case where there is a resistor R4 (3 KΩ).
[0044]
As described above, with a simple configuration in which the resistor R4 is simply added to the operational amplifier circuit 11, there is no risk of erroneous recording on the magnetic tape, and a burst of UHF or more that has been difficult to achieve with conventional noise countermeasure technology. Noise resistance against transmission can be greatly improved.
[0045]
As mentioned above, the invention made by the present inventor has been specifically described based on the embodiments. However, the present invention is not limited to the above embodiments, and various modifications can be made without departing from the scope of the invention. Not too long.
[0046]
In the above description, the case where the invention made by the present inventor is applied to the audio tape reproducing circuit, which is the field of use behind it, has been described. However, the present invention is not limited to this, for example, it has an audio recording track. The present invention can also be applied to a video tape reproducing circuit or a data tape reproducing circuit.
[0047]
【The invention's effect】
The effects obtained by the representative ones of the inventions disclosed in the present application will be briefly described as follows.
[0048]
In other words, in an audio tape reproducing circuit having a low-frequency emphasis type equalization characteristic, the configuration within the semiconductor integrated circuit does not depend on external parts and does not involve the risk of erroneous recording on the magnetic tape. The noise resistance against UHF or higher burst transmission, which has been difficult to achieve with conventional noise countermeasure technology, can be greatly enhanced.
[Brief description of the drawings]
FIG. 1 is a circuit diagram showing an embodiment of an audio tape reproducing circuit to which the technology of the present invention is applied.
FIG. 2 is an operation waveform diagram when no resistor is inserted in the audio tape reproducing circuit according to the present invention.
FIG. 3 is an operation waveform diagram in the case where a resistor is inserted in the audio tape reproducing circuit according to the present invention.
FIG. 4 is a characteristic diagram showing a difference in noise resistance performance depending on the presence or absence of resistance in the audio tape reproducing circuit according to the present invention.
FIG. 5 is a circuit diagram showing a configuration of an audio tape reproducing circuit studied prior to the present invention.
6 is a waveform diagram showing the operation timing and noise waveform of the audio tape reproducing circuit of FIG. 5. FIG.
7 is a circuit diagram showing an example in which conventional external noise countermeasure means is provided in the audio tape reproducing circuit of FIG. 5. FIG.
8 is a circuit diagram showing an example in which another conventional external noise countermeasure means is provided in the audio tape reproducing circuit of FIG.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Equalizer amplifier 11 Operational amplifier circuit 12 Feedback circuit 2 Magnetic head 21 Magnetic tape 31 Post circuit (tape noise removal system)
32 Main amplifier 33 Speakers Q11 to Q17 pnp bipolar transistors Q21 to Q25 npn bipolar transistors R1 to R6 Resistor C1 Phase compensation capacitor I1 Constant current source Vb Bias voltage source Vcc Power supply potential Cs Parasitic capacitance Cp Bypass capacitor L1, L2 Inductance

Claims (5)

The emitters of the first and second bipolar transistors are commonly connected to each other, the common emitter is connected to the collector of the third bipolar transistor forming a constant current source, and on the collector side of the first and second bipolar transistors. An operational amplifier circuit that differentially amplifies a signal input between both bases of the first and second bipolar transistors by providing the load circuit and extracts the signal from the load circuit, and frequency selection for the operational amplifier circuit An audio tape reproducing circuit including a negative feedback circuit for providing a low-frequency emphasis type equalizing characteristic that compensates for a frequency characteristic of a magnetic head by applying a negative feedback of the first and second bipolar circuits. Formation of a high-frequency path between the common emitter of the transistor and the collector of the third bipolar transistor By inserting suppressing resistor in series, the audio tape reproducing circuit is characterized in that so as to increase the offset stability against high-frequency noise of the operational amplifier. The resistor inserted in series between the common emitter of the first and second bipolar transistors and the collector of the third bipolar transistor has an operating frequency range of the operational amplifier circuit as long as it does not interfere with the normal operation of the operational amplifier circuit. 2. The audio tape reproducing circuit according to claim 1, wherein the audio tape reproducing circuit has a resistance value effective to suppress the formation of a high-frequency path in a sufficiently higher frequency range. The resistance inserted in series between the common emitter of the first and second bipolar transistors and the collector of the third bipolar transistor has a resistance value sufficiently higher than the parasitic resistance of the bipolar transistor. The audio tape reproducing circuit according to claim 1 or 2, characterized in that: 4. The audio tape reproducing circuit according to claim 1, wherein pnp bipolar transistors are used as the first and second bipolar transistors and the third bipolar transistor, respectively. 2. An output buffer circuit for buffer-amplifying an output extracted from the collector load circuit of the first and second bipolar transistors, and negatively feeding back the output of the output buffer circuit. 5. The audio tape reproducing circuit according to any one of 4 above.

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