GB2117548A

GB2117548A - Magnetic recording systems

Info

Publication number: GB2117548A
Application number: GB08205118A
Authority: GB
Inventors: Gordan Albert Clark
Original assignee: COMPACT BUSINESS MACHINES LIMI
Current assignee: COMPACT BUSINESS MACHINES LIMI
Priority date: 1982-02-22
Filing date: 1982-02-22
Publication date: 1983-10-12
Also published as: GB2117548B

Abstract

A magnetic recording system, such as an audio frequency magnetic tape recorder, comprises a pulse width modulator 8-15 which receives the input signal and a magnetic recording transducer 51 which receives the pulse width modulated signal. Thus, the full dynamic range of the recording medium can be utilized to minimise noise and the distortion induced by the transfer characteristic is substantially eliminated, particularly at high signal levels. The modulator is a Schmitt oscillator with amplitude limited by zener diode 15. The oscillator frequency reduces to accommodate lengthening pulse widths of negative polarity without pulse widths being inconveniently short for positive polarity signals. <IMAGE>

Description

SPECIFICATION Improvements in or relating to magnetic recording systems The present invention relates to magnetic recording systems, such as tape recorders.

In known tape recorders, for instance in the field of high fidelity equipment, and audio signal to be recorded is superimposed on a supersonic bias signal so as to overcome the non-linearity of the input-output characteristics of the magnetic recording medium, such as conventional magnetic tape. However, this system does not entirely cancel out the distortion caused by the transfer characteristic, which distortion limits the maximum recorded level if acceptable quality of reproduction is to be obtained. The system also intrinsically limits the reproduced output level to about a third of the theoretical maximum level. Further, careful setting of the bias signal level is necessary in order to obtain good results.

According to a first aspect of the invention, there is provided a magnetic recording system, comprising a pulse width modulator whose input is arranged to receive a signal to be recorded, and a magnetic recording transducer arranged to receive the output of the modulator.

It is thus possible to provide a magnetic recording system, for instance in an audio frequency tape recorder, capable of providing low distortion performance without the need for a supersonic bias system. The recording medium may be driven into or near to saturation by the pulse width modulated signal so that use can be made of the full magnetic capacity of the medium. As the noise level of such a system is at least as low as that in conventional recording systems, the dynamic range of the system is greater because a higher maximum reproduced level can be obtained. Further, the distortion performance at high levels is substantially reduced with respect to conventional systems.

During reproduction from the recording medium, it is generally possible for the reproducing transducer to extract the recorded signal directly, as the frequency of the "carrier wave" of the pulse width modulated signal is substantially higher than the frequency band which the transducer is capable of reproducing. For instance, in the case of an audio tape recorder required to record and reproduce signals in a frequency band from 20 H3 to 20 KH3, the carrier wave frequency may be in the region of 125 KH3, although this frequency need not be constant. The reproducing head reproduces the audio signal directly, but is incapable of reproducing the carrier frequency to any significant extent. If necessary, additional filtering may be provided to reduce any reproduced carrier wave to an insignificant level.

According to a second aspect~of the invention, there is provided a pulse width modulator comprising an operational amplifier whose non-inverting input is connected via a first resistor to a modulating signal input terminal and via a second resistor to the output of the amplifier, and whose inverting input is connected via a capacitor to a common line and via a resistor to the output of the amplifier.

Preferably the amplifier output is provided with level limiting means. The level limiting means may comprise a diode bridge whose positive and negative terminals are connected to a zener diode and whose other terminals are connected in series with a third resistor between the amplifier output and the common line, the first and second resistors being connected to the connection between the third resistor and the diode bridge.

Preferably, the recording system according to the first aspect of the invention utilises the pulse width modulator according to the second aspect of the invention.

The invention will be further described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a circuit diagram of a tape recorder recording stage constituting a preferred embodiment of the invention; and Figure 2 is a circuit diagram of a preferred pulse width modulator.

Fig. 1 shows the circuit diagram of a record amplifier/driver for an audio frequency magnetic tape recording system. An input buffer amplifier comprises an operational amplifier 1 whose non-inverting input is connected via a capacitor 2 to an audio input terminal and via a resistor 3 to a 15 volt supply line. A common input terminal is connected via a capacitor 4 to the 15 volt supply line. The inverting input of the amplifier 1 is connected via a feedback resistor 5 to its output and via a series connection of a resistor 6 and a capacitor 7 to the 15 volt supply line.

The output of the amplifier 1 is connected to the input of a pulse width modulator including an operational amplifier 8. The noninverting input of the amplifier 8 is connected via a resistor 9 to the output of the amplifier 1 1 and via a series connection of two resistors 10 and 1 1 to the output of the amplifier 8.

The inverting input of the amplifier 8 is connected via a capacitor 12 to the 15 volt supply line and via a resistor 13 to the connection between the resistors 10 and 11.

A diode bridge 14 has a zener diode 15 connected between its positive and negative terminals with its other terminals connected between the 15 volts supply line and the connection between the resistors 10, 11 and 13.

The output of the amplifier 8 is connected to an input resistor 16 of a buffer amplifier comprising an operational amplifier 17. The other end of the resistor 16 is connected to the non-inverting input of the amplifier 17 and via a resistor 18 to the 15 volt supply line. The inverting input of the amplifier 17 is connected to the output thereof via a resistor 19.

The positive and negative supply terminals of the operational amplifiers 1, 8 and 17 are connected to a 30 volt supply line and to a zero volt supply line, respectively, the 15 volt supply line being decoupled by a capacitor 20 and the 30 volt supply line being decoupled by a capacitor 21.

The output of the amplifier 17 is connected via a capacitor 22 and via series connected resistors 23 and 24 to the zero volt supply line. The 30 volts supply line is connected via a resistor 25 to a zener diode 26 and to the base of transistor 27, whose emitter is connected via a resistor 28 to the zero volt supply line. The transistor 27 thus functions as a constant current source providing a constant current of approximately 30 mA.

The collector of the transistor 27 is connected to the emitters of transistors 29 and 30, which are arranged as a long-tail pair. The connection between the resistors 23 and 24 is connected to the base of the transistor 29 and, via a diode 31, to the emitter of the transistors 29 and 30. The base of the transistor 30 is connected via a zener diode 32 to the zero volt line and via a resistor 33 to the 30 volt supply line. The collector of the transistor 29 is connected via diodes 34 and 35 to the collector of a transistor 36. Similarly, the collector of the transistor 30 is connected via diodes 37 and 38 to the collector of a transistor 39. The collector of the transistor 29 is further connected via a diode 40 and a parallel circuit of a resistor 41 and a capacitor 42 to the base of the transistor 39, which is also connected via a resistor 43 to a 48 volt supply line.Similarly, the collector of the transistor 30 is connected via a diode 44 and a parallel circuit comprising a resistor 45 and a capacitor 46 to the base of the transistor 36, which is also connected via a resistor 47 to the 48 volt supply line. The emitters of the transistors 36 and 39 are connected together and via a resistor 48 to the 48 volt supply line. The 48 volt supply is connected via a resistor 49 to a 50 volt supply line and is decoupled by a capacitor 50. One side of a record head 51 is connected to the connection between the diodes 34 and 35 whereas the other side of the record head is connected to the connection between the diodes 37 and 38.

The record amplifier/driver shown in Fig. 1 operates as follows. An audio signal to be recorded is supplied to the input terminals and the amplifier 1 which provides impedance matching and amplification. For instance the gain of the amplifier 1 may be set to be sufficient for an input signal level nominally of 1 50 mV. The amplified audio signal is supplied to the pulse width modulator, which functions as hereinafter described with reference to Fig. 2 of the drawings. The pulse width modulation signal is then supplied via the buffer amplifier 17, connected as a unity gain buffer, to the transistor 29. The pulse width modulated signal thus causes the transistors 29 and 30 to conduct alternately.The connections of the bases of the transistors 36 and 39 to the collectors of the transistors 30 and 29 cause the transistors 36 and 39 to conduct alternately also, so that the transistor 26 conducts when the transistor 30 conducts and the transistor 39 conducts when the transistor 29 conducts. These for transistors thus form a conventional bridge driver arrangement with the record head 51 connected to the output thereof. The value of the current set by the constant current source comprising the transistor 27 is selected so as to drive the record head 51 to such an extent that the magnetic tape is saturated or nearly saturated in opposite directions corresponding to the two levels of the pulse width modulated signal.

The pulse width modulator 8 is arranged to oscillate at a supersonic frequency, for instance in the region of 125 kHz, so that it is capable of being modulated by the full audio band width of from approximately 20 Hz to approximately 20 kHz. Although the frequency of the pulse width modulator is essentially constant for relatively low input signal levels, the frequency decreases somewhat for higher signal levels. Indeed, the pulse width modulator is of a type where the smaller pulse never reduces below 50% of its value with no modulation, provided the input remains within the range of the output switching excursion.

Instead, the frequency of oscillation reduces so as to accommodate lengthening pulse widths of one polarity without the pulse width of the opposite polarity becoming inconveniently short. In order to reproduce a magnetic tape which has been recorded by means of the amplifier/driver of Fig. 1, it is merely necessary to use a conventional replay tapehead connected to an audio amplifier, because the tape head re'produces the audio signal but substantially filters out the pulse width modulation "carrier frequency" of around 125 kHz. If necessary, additional filtering may be provided in the replay signal path to reduce the break through of the carrier signal to a sufficiently low level.

Because the pulse width modulated signal supplied to the tape head 51 is essentially a square wave of variable mark-space ratio with fast rising and falling edges, it is desirable for the record head 51 to have a relatively low self inductance in order to avoid the generation of large back-EMF when the polarity of the current through the record head changes abruptly. The diodes 34, 35, 37 and 38 provide a degree of isolation from this back EMF for the transistors 29, 30, 36 and 39.

The record amplifier/driver obviates the need for a bias oscillator, as is required in conventional tape recorders. Thus, there is no need for careful adjustment of bias levels, or for different bias levels for different types of magnetic tape or different tape speeds. Further, it is unnecessary to provide signal level adjustment for different types of tape as the amplifier/driver shown in Fig. 1 is merely required to supply a signal to the record head 51 which is sufficient to saturate or nearly saturate the magnetic recording tape. Because of the pulse width modulated nature of the signal being recorded on the magnetic tape, the non-linearity of the input-output characteristic of the magnetic tape does not substantially cause distortion of the type associated with conventional tape recorders.Further, because the whole magnetic range of the tape is utilized, the recorded signal level on the magnetic tape is substantially greater than that of a conventional tap recorder, so that the dynamic range of the system is relatively good.

The noise level of a signal reproduced from tape is essentially dictated by noise generated in the electronic circuits and by the intrinsic tape noise and so is not inferior to that achieved by conventional tape recorders.

Fig. 2 shows the circuit arrangement of the pulse width modulator of Fig. 1 with the same reference numerals referring to the same components. However, the power supply lines are shown as plus and minus 15 volts with respect to the common line, as is conventional with operational amplifiers. Also, component values have been shown in this Figure purely by way of example to illustrate the operation of the pulse width modulator. The usual assumption that the operational amplifier 8 has infinite gain and band width will be made to simplify the explanation of the operation.

When the input terminal A is connected to the common line, the circuit functions as a standard arrangement known as a Schmitt Oscillator. However, the diode bridge 14 and the 9 volt zener diode 15 limit the positive and negative excursions at the output D so as to avoid relying on saturation of the amplifier 8, which saturation tends not to be well defined. When the point D is at plus 10 volts, the point B is at 1 volt. The capacitor 12 is changed towards plus 10 volts by the current through the resistor 13. When the point C reaches plus 1 volt, the output signal at D changes to minus 10 volts. Thus, the level at the point D changes to minus 1 volt and the capacitor 12 is charged in the opposite direction until the point C reaches minus 1 volt, so that the amplifier output switches polarity and the point D goes to plus 10 volts, whereafter the operation is repeated.

The period between successive changes is determined by the current through the resistor 13, by the value of the capacitor, and by the ratio of the resistors 9 and 10. When the point C is at minus 1 volt and the point D is at plus 10 volts, there is initially a potential of 11 volts across the resistor 13 so that the initial charging current of the capacitor 12 is 1.1 mA. As the point C approaches the switching point, the potential across the resistor 13 is near to 9 volts so that the charging current is approximately 0.9 mA. Thus, initially the rate of rise in potential across the capacitor 12 is 0.55 volts per microsecond but this reduces to 0.45 volts per microsecond at the switching point, the voltage at C following a portion of an exponential curve.

Thus, the time taken for the point C to change from the negative switching point to the positive switching point is approximately 4 microseconds. As the circuit is symmetrical with regard to polarity, a similar period is taken for the point C to change from the positive to the negative switching point. Thus, a symmetrical square wave output signal is produced at D with a frequency of approximately 125 kHz.

When the signal level at A is at plus 5 volts, the circuit continues to oscillate but, when D is at plus 10 volts, B is at plus 5.5 volts and when D is at minus 10 volts, B is at plus 3.5 volts. When D switches to plus 10 volts, C is at plus 3.5 volts so that the potential across the resistor 13 is 6.5 volts.

Thus, the rate of rise in the potential across the capacitor 12 is initially 0.325 volts per microsecond falling to 0.225 volts per microsecond as the threshold at plus 5.5 volts is reached. Thus D remains at plus 10 volts for approximately 7.3 microseconds. When D has switched to minus 10 volts, the potential across the resistor 13 is initially 15.5 volts and falls to 13.5 volts at the switching threshold. Thus, the rate of fall of the potential across the capacitor 12 is now much greater than the rate of rise, and the output D remains at minus 10 volts for approximately 2.8 microseconds.

Similarly, if the input A is connected to a negative potential, the negative output periods at D are longer than the positive output periods. When the potential at the input A is plus or minus 5 volts, the frequency of oscillation becomes approximately 100 kHz.

The ratio of positive pulse lemgth to negative pulse length is obviously a function of the input at A.

The function T+~R T+ +Tis the average rate of integrated output, where T+ is the duration of positive pulses and T- is the duration of negative pulses. This function is equivalent to passing the pulses through a low pass filter, or recording them on tape and replaying them through a head/amplifier sys tem with limited bandwidth.

It can be shown mathematically that this function is remarkably linear up to about 80% of the max. permissable input ( 1 or), and is only 1.5% out of 90%.

Hence the system described gives a high output combined with very low distortion.

Claims

1. A magnetic recording system, comprising a pulse width modulator whose input is arranged to receive a signal to be recorded, and a magnetic recording transducer arranged to receive the output of the modulator.

2. A magnetic recording system as claimed in claim 1, in which the pulse width modulator comprises an operational amplifier whose non-inverting input is connected via a first resistor to a modulating signal input terminal and via a second resistor to the output of the amplifier, and whose inverting input is connected via a capacitor to a common line and via a resistor to the output of the amplifier.

3. A magnetic recording system as claimed in claim 2, in which the amplifier output is provided with level limiting means.

4. A magnetic recording system as claimed in claim 3, in which the level limiting means comprises a diode bridge whose positive and negative terminals are connected to a zener diode and whose other terminals are connected in series with a third resistor between the amplifier output and the common line, the first and second resistors being connected to the connection between the third resistor and the diode bridge.

5. A magnetic recording system as claimed in any one of the preceding claims, in which the output of the pulse width modulator is connected to the input of a transistor bridge amplifier circuit whose output is connected to the magnetic recording transducer.

6. A magnetic recording system as claimed in claim 5, in which the transistor bridge comprises first and second PNP transistors and first and second NPN transistors, the collectors of the first NPN and PNP transistors being connected to a first terminal of the transducer, the collectors of the second NPN and PNP transistors being connected to a second terminal of the transducer, the emitters of the NPN transistors and the emitters of the PNP transistors forming to first and second power supply inputs, respectively.

7. A magnetic recording system as claimed in claim 6, in which a constant current source is connected between a power supply terminal and one of the power supply inputs.

8. A magnetic recording system as claimed in claim 7, in which the base of one of the first transistors is connected to the output of the modulator, the collector of the one first transistor is connected via a potential divider to the base of the second transistor of opposite type to the one first transistor, the vase of the other first transistor is connected via a further potential divider to the collector of the other second transistor connected to a constant voltage source.

9. A magnetic recording system substantially as hereinbefore described with reference to and as illustrated in Fig. 1 of the accompanying drawings.

10. An audio frequency magnetic tape recorder including a system as claimed in any one of the preceding claims.

11. A pulse width modulator comprising an operational amplifier whose non-inverting input is connected via a first resistor to a modulating signal input terminal and via a second resistor to the output of the amplifier, and whose inverting input is connected via a capacitor to a common line and via a resistor to the out the output of the amplifier.

12. A pulse width modulator as claimed in claim 11, in which the amplifier output is provided with level limiting means.

1 3. A pulse width modulator as claimed in claim 12, in which the level limiting means comprises a diode bridge whose positive and negative terminals are connected to a zener diode and whose other terminals are connected in series with a third resistor between the amplifier output and the common line, the first and second resistors being connected to the connection between the third resistor and the diode bridge.

1 4. A pulse width modulator substantially as hereinbefore described with reference to and as illustrated in Fig. 2 of the accompanying drawings.