DIGITAL INFORMATION REPRODUCING APPARATUS

Abstract: In a digital information reproducing apparatus a digital signal having passed through a band-limited transmission system is waveform-­ shaped at a threshold level controlled to its optimal value so that the information can be properly read out. For this purpose, the apparatus is equipped with a threshold level control circuit for controlling the threshold level to shape the waveform of the input signal. This threshold level control circuit comprises a first control circuit (8, 10,11) for controlling the threshold level with reference to a read clock extracted from the input signal, and a second control circuit (5, 9, 10, 11) for controlling the threshold level of a shaped signal which is obtained by waveform-shaping the input signal so that the DC component of the shaped signal becomes a predetermined value. The control of the threshold level is performed by cooperation of the first (8,10,11) and second control circuits (5, 9, 10, 11). The first control circuit is adapted to control the threshold level in accord­ ance with a difference between the shaped signal obtained by waveform-shaping the input signal and the read-out signal obtained by synchronizing the shaped signal with the read clock. Thus, the threshold level for waveform-shaping a modulated channel code signal can be controlled optimally regardless of whether the modulation system is DC-free or not. The second control circuit serves to prevent spurious lock of the first control circuit.

Description: The present invention relates to a digital information reproducing apparatus for reading out an information from a digital signal obtained through a band-limited transmission system, and more particularly to an apparatus capable of controlling the threshold level of a waveform shaper so as to correctly read the digital signal. 2. Description of the Prior Art: For the purpose of transmitting digital signals at a high transfer rate or recording them at a high density, there is often employed recently a technique of digital modulation which transmits or records digital signals within a base band. In such a signal transmission using a base band, it is generally necessary to transmit even the lowest frequency range including a DC component and is further necessary to shape the rounded waveform which is deformed in the transmission line to a proper shape at a predetermined threshold level. However, there exists a problem that an optimal threshold level is drifted because some low frequency noise is prone to mix into the input signal due to temperature drift in a circuit or nonuniformity of a recording medium used. For solving such problem, there is known a conventional method which controls the threshold level to an optimal value for elimination of any harmful influence that results from the drift of the threshold level caused by low frequency noise. As regards a channel code signal processed by a DC-free modulation system according to which the DC component of the signal to be transmitted is not modulated, it is possible to attain an optimal threshold level by controlling the threshold level in such a manner that the DC component of the waveform-shaped signal obtained from the transmitted input signal becomes a predetermined value. An exemplary waveform shaper for controlling the threshold level as mentioned above is disclosed in Japanese Patent Application of Laid-open No. 57 (1982)-78611. In such a conventional waveform shaper, however, controlled retention of the threshold level at an optimal value is not achievable in case the input is not a channel code signal processed by the DC-free modulation system. For example, in the so-called 2-7 code modulation employed frequently in the recent magnetic storage system and so forth, there occurs variation in the DC component of the modulated channel code signal, and consequently it is impossible to attain optimal control of the threshold level by the method mentioned. Also with respect to the channel code signal modulated by some other system than the DC-free modulation, it is still possible to accomplish optimal control of the threshold level by detecting the threshold level error with reference to a read clock extracted from the channel code signal. An exemplary waveform shaper for controlling the threshold level in this manner is disclosed in Japanese Patent Application of Laid-open No. 59 (1984)-113529, wherein the threshold level is controlled in accordance with a difference obtained from the phase difference between the rising edge of the shaped input signal and the read clock and the phase difference between the falling edge and the read clock. However, in the method which detects the threshold level error with reference to the read clock as mentioned above, if there occurs a variation in the threshold level in such a manner that the pulse duration changes by a length comparable to the read clock period, the signal with such pulse duration change may be erroneously recognized as the original signal. In such a case, there is induced a state of spurious lock where the threshold level is controlled to an improper value. A circuit for providing a comparison signal representing phase relationship between a clock pulse train and a variable input pulse train is disclosed in US patent 4,191,976. The circuit includes a phase difference measuring circuit providing a first signal proportional to the phase difference between the input pulse train and clock pulse train, the signal having related discontinuities at the points of minimum and maximum phase difference between input and clock pulse trains. A second signal is generated representing a selected value of phase difference, the selected value being greater than the minimum phase difference and less than the maximum phase difference. The first and second signals are compared to provide a signal representing the difference between measured and selected values of phase difference. A waveform-shaping circuit is disclosed in JP-A59-5740 which comprises a comparator, a low-pass filter, an amplifier and a constant voltage source. When a signal A is applied to an input of the comparator, the uninverted input side (threshold value) of the voltage comparator is set to zero and a regenerated signal is output therefrom. With the high-level and low-level voltages of the regenerated signal at V e and zero respectively, the dc component of the regenerated rectangular wave signal E appears on the output of the low-pass filter and its voltage V f is (t₁/t₁+t₂) V e . The reference voltage V g as the output voltage G from the constant current source and the dc component voltage V f are both inputted to the dc amplifier 10. The difference V h (=V f -V g ) appears at the output of the dc amplifier and is inputted to the uninverted side (threshold value) of the voltage comparator. It is an object of the present invention to provide a digital information reproducing apparatus which is capable of controlling the threshold level optimally for shaping the waveform of a modulated channel code signal regardless of whether the modulation system is DC-free or not, and is further capable of averting spurious lock in control of the threshold level. The present invention provides a digital information reproducing apparatus comprising; a threshold level control means for controlling a threshold level to shape a waveform of an input signal; a waveform shaping means for outputting a reproduced digital signal by shaping the waveform of the input signal in accordance with the threshold level outputted from said threshold level control means; and a clock extracting means for extracting read clock to read out data from the input signal, characterized in that said threshold level control means comprises: a first control means for detecting a threshold level error from the reproduced digital signal with reference to the read clock and controlling the threshold level so as to cancel said error; and a second control means for controlling the threshold level so that DC component of said reproduced digital signal becomes a predetermined value, wherein said second control means forces said first control means to settle the threshold level at a correct value in order to prevent said first control means from locking the threshold level at a spurious value. In this construction, the first control means is capable of optimally controlling the threshold level even if the input signal is the one processed by any modulation system that may cause variation in the DC component, and the second control means is adapted to prevent spurious lock of the first control means. Fig. 1 is a block diagram of a preferred embodiment of the present invention; Fig. 2 is a waveform chart of signals produced in the embodiment of Fig. 1; Fig. 3 is a block diagram of another embodiment of the invention; Fig. 4 is a conceptual diagram of an exemplary format on a recording medium; Fig. 5 is a block diagram of a further embodiment of the invention; and Fig. 6 graphically shows the gain characteristic of a threshold-level control loop in the embodiment of Fig. 5. This invention is useful both for digital data communication systems and for digital data storage systems, and will be described herein with reference to the latter. In Fig. 1, a waveform comparator 1 is supplied with an input digital signal deformed in waveform through a band-limited transmission system, which may be a communication channel or a recording-reproducing channel. Here a description will be given on a recording-reproducing channel as an example. The comparator 1 shapes the waveform of the input signal to produce a shaped signal. A clock extracting circuit 12 extracts a read clock from the shaped signal. The clock extracting circuit 12 is composed of a phase-locked loop (PLL) comprising a phase comparator 2, a filter 13 and a voltage controlled oscillator (V.C.O) 3. A D-flip-flop 4 latches the shaped signal synchronously with the read clock. The synchronized read-out signal thus outputted from the D-flip-flop 4 is the same as the original signal recorded on a recording medium. The read-out signal and read clock are sent to a well-known controller 50. The controller 50 produces a mode selection signal for controlling a transfer gate 5. When a mode selection signal is in its "H" state, the read-out signal is permitted to pass through a transfer gate 5. Low-frequency components are extracted from the shaped signal and the read-out signal respectively through low-pass filters 8 and 9. A differential amplifier 10 detects a difference between the two low-frequency components. A variable gain amplifier 11 amplifies this difference and feeds it back to the comparator 1 as a threshold level. Thus, the control loop for controlling the threshold level to an optimal value with reference to the read clock constitutes a first control means. Referring now to the signal waveform chart of Fig. 2, the operation of the first control means will be described in further detail. In this chart, A is a write clock and B is an original digital signal to be recorded synchronously with the clock A. Denoted by C, D, E, F and G are signals obtained at points c, d, e, f and g in Fig. 1, respectively. When the digital signal B is recorded and reproduced, generally the reproduced signal is deformed in waveform as represented by C. A shaped signal D obtained by shaping the waveform of the reproduced signal C at an optimal threshold level S1 thereof comes to have the same waveform as that of the digital signal B. In the clock extracting circuit 12, a read clock F is extracted from the shaped signal D. The read clock F is substantially opposite in phase to the recording clock A. The read-out signal G produced in the D-flip-flop 4 by synchronizing the shaped signal D with the read clock F is equivalent to the one obtained by delaying the shaped signal D. Consequently, the difference between the outputs of the low-pass filters 8 and 9 is reduced to nul. However, if the threshold level is S2 which is higher than the optimal threshold level S1, the shaped signal outputted from the comparator 1 becomes such as represented by E, in which the duty ratio is smaller than that of the digital signal B. When the shaped signal E is synchronized with the clock F in case the threshold level error is not so great as to bring about a read error, the read-out signal G is obtained similarly to the foregoing case where the threshold level is S1. Therefore, the output of the low-pass filter 8 becomes smaller than that of the low-pass filter 9, whereby the threshold level of the comparator 1 is caused to change in the negative direction. Thus, the threshold level of the comparator 1 is controlled with negative feedback. Fig. 3 shows another embodiment of the present invention which controls the threshold level by a first control means. A first phase comparator 40 and a second phase comparator 41 respectively compare the rising edge and the falling edge of a shaped signal which is outputted from a waveform comparator 1 with a read clock outputted from a voltage controlled oscillator 3. An adder 42 serves to add the outputs of the first and second phase comparators 40 and 41 to each other. The first phase comparator 40, the second phase comparator 41 and the adder 42 are equivalent to the aforesaid phase comparator 2 in Fig. 1 and constitute a phase-clocked loop (PLL) in combination with the filter 13 and the voltage controlled oscillator 3. Similarly to the case of Fig. 1, the output of the voltage controlled oscillator 3 is used as the read clock. If the threshold level becomes higher, as will be apparent from comparison of the shaped signal D with another shaped signal E shown in Fig. 2, the rising edge lags while the falling edge advances. In case the threshold level becomes lower to the contrary, such phenomenon is rendered reverse. The threshold level error can be detected by first taking out a difference between the outputs of the first and second phase comparators 40 and 41 by a differential amplifier 44, and then extracting the low-frequency component through a low-pass filter 45. The threshold level of the waveform comparator 1 can be controlled by feeding back the error thus obtained. By referring to the read clock as mentioned above, it becomes possible to attain optimal control of the threshold level if the original of the input signal is a channel code signal processed by some other system than the DC-free modulation system. However, if the threshold level has a great deviation from its optimal value to such an extent that the pulse duration of the shaped signal varies by a length of one clock pulse or so as compared with the original digital signal, a read error is generated as a result, and the threshold level control comes to be stabilized in a state where the pulse duration of the read-out signal G has a variation of one clock pulse in comparison with that of the original digital signal. Such state is defined here as "spurious lock". Although the spurious lock is scarcely induced once the threshold level is controlled to its proper value, it may often occur at the beginning of the control operation. For the purpose of preventing the spurious lock, the embodiment of Fig. 1 is so operated that the controller 50 changes the mode selection signal to "L" state to place the transfer gate 5 in a high-impedance condition. Since the input terminal of the low-pass filter 9 is connected to a power supply 6 of a voltage V through a resistor 7, the low-pass filter 9 provides an output DC voltage corresponding to the supply voltage V. The differential amplifier 10 compares the output of the low-pass filter 8 with the output DC voltage of the low-pass filter 9, and feeds the difference back to the threshold level. Accordingly, the threshold level is so locked that the DC level of the shaped signal becomes a predetermined value. In this case, a second control means is constituted by the closed loop which controls the threshold level by comparing the low-frequency component of the shaped signal with the DC voltage V. Although there exists no possibility that the aforementioned spurious lock is induced under the control of the second control means, the threshold level is not always exactly controlled to its optimal value because the low-frequency component of the information-carrying digital signal does not always retain a fixed voltage. However, in case the recorded digital signal is such that the DC value thereof is constant and none of low-frequency component is contained, the threshold level can be kept at its optimal value by the second control means. When recording an information by dividing a recording medium into a plurality of sectors, a repetition signal of a predetermined pattern is recorded often at the beginning of the data signal so as to maintain the clock extracting circuit in synchronism. For this reason, the repetition signal is termed here a "SYNC signal". In the SYNC signal portion, the threshold level can be kept at its optimal value by the second control means. Fig. 4 shows an exemplary format where each sector includes an ID area 22 for recording an address and so forth, and a data area 23 for recording a data signal which carries the information to be recorded. The individual areas are separated from each other by a gap 20. At the time of recording an ID signal or data signal in such areas, a SYNC signal 21 is added to the beginning of each area, so that the SYNC signal 21 and the following ID signal or data signal are recorded continuously. When reading out the information from such recording medium, the controller 50 initially sets the mode selection signal to "L" state to set the second control means in an active condition. In this stage, the gain of the variable gain amplifier 11 is increased by the mode selection signal so that the threshold-level control loop is ready for fast response. Upon arrival of the SYNC signal 21, the phase-locked loop of the clock extracting circuit 12 and the threshold-level control loop are promptly synchronized so that the predetermined pattern of the SYNC signal 21 is recognized by the controller 50. The controller 50, when recognized the SYNC signal 21, changes the mode selection signal to "H" state so that the threshold level is controlled by the first control means. Thus, a shaped signal of the optimal level can be obtained despite any variation of the low-frequency component in the data signal or ID signal. In this stage, since the variable gain amplifier 11 is switched to a small-gain condition by the mode selection signal, the threshold level control operation is hardly affected by any noise or the like to consequently ensure stable and accurate read of the information. Incidently, it is known the controller 50 set a SYNC detected signal (which has been initially reset) when detects the SYNC signal 21, and reset the SYNC detected signal after reading the following data. This known SYNC detected signal can be directly used as the mode selection signal. Alternatively, the controller 50 may produce the mode selection signal which is made "H" in response to a first coming SYNC signal after the start of the information reproducing operation. Furthermore, the mode selection signal may not be necessary to be controlled in response to the SYNC signal. The mode selection signal may be controlled by a timer which shows a time when the operation of the apparatus is stabilized after the start time. Also, the mode selection signal may be controlled in accordance with any other proper manner by the controller 50. Moreover, an information for controlling the mode selection signal may be contained in the input signal of the apparatus. Fig. 5 shows a further embodiment of the invention effective for a channel code signal processed by a DC-free modulation system which controls the original of an input signal in such a manner that its DC component becomes a predetermined value. In this diagram, a differential amplifier 10 compares the output of a low-pass filter 8, obtained by extracting the low-frequency component of a shaped signal with the output of another low-pass filter 9 obtained by extracting the low-frequency component of a read-out signal to detects a threshold level error in a first control means. Another differential amplifier 32 compares the output of the low-pass filter 8 with a fixed voltage V to detects a threshold level error in a second control means. The threshold level of a waveform comparator 1 is controlled in accordance with the sum of the output signal from an integrator 30 which integrates the threshold level error detected by the second control means and the threshold level error detected by the first control means. Fig. 6 graphically shows exemplary gain characteristics of the control loops in the respective modes. In this graph, A and B represents the gain characteristic of the first control means and that of the second control means, respectively. The circuit configuration is so designed that, as shown in Fig. 6, the second control means is dominant in a direct current while the first control means is dominant at a cross-over frequency fo where the open-loop gain of the first control means becomes 0 dB. The threshold level is controlled by the first control means in a band from a frequency fc where the gain characteristics A and B cross each other to the frequency fo, so that optimal control of the threshold level can be attained even when the original digital signal contains some frequency component of this band. Since the second control means is dominant in the frequency range below fc, if spurious lock is induced due to some reason in the threshold level control performed by the first control means, there exists no possibility that such state is retained for a long time, because the spurious lock is promptly eliminated by the second control means to eventually resume the proper threshold level. According to the present invention, as described hereinabove, the threshold level in shaping the waveform of the input signal can be controlled optimally with reference to a read clock extracted from the input signal even if the original channel code signal is not the one processed by DC-free modulation, and a combination with the threshold level control performed by the second control means realizes solution of the known problem that the threshold-level control loop is prone to cause spurious lock. It is to be understood that the aforementioned drawback of readily inducing spurious lock of the first control means is not inherent in the above-described constitution, and it results essentially from reference to the read clock. Therefore the first control means in the embodiment mentioned is merely illustrative, and it is obvious that the present invention is effective with respect to any other modified constitution as well.

Claims: A digital information reproducing apparatus comprising; a threshold level control means (5-11) for controlling a threshold level to shape a waveform of an input signal (C); a waveform shaping means (1) for outputting a reproduced digital signal by shaping the waveform of the input signal in accordance with the threshold level outputted from said threshold level control means; and a clock extracting means (12) for extracting read clock to read out data from the input signal, characterized in that said threshold level control means comprises: a first control means (4,8,9,10) for detecting a threshold level error from the reproduced digital signal with reference to the read clock and controlling the threshold level so as to cancel said error; and a second control means (6,7,8,10) for controlling the threshold level so that DC component of said reproduced digital signal becomes a predetermined value, wherein said second control means forces said first control means to settle the threshold level at a correct value in order to prevent said first control means from locking the threshold level at a spurious value. An apparatus as claimed in claim 1, wherein said first control means has means (4) for producing a synchronized signal from the reproduced digital signal by synchronizing the reproduced digital signal with the read clock, and means (10) for producing a difference signal between the reproduced digital signal and the synchronized signal, the difference signal being used to control the threshold level. An apparatus as claimed in claim 1, wherein said first control means has a first phase comparator (40) for comparing the phase of a rising edge of the reproduced digital signal with the phase of the read clock, a second phase comparator (41) for comparing the phase of a falling edge of the reproduced digital signal with the phase of the read clock, means (44) for producing a difference signal between outputs of said first and second phase comparators, the difference signal being used for controlling the threshold level, and an oscillator for extracting the read clock, wherein the oscillation frequency of the oscillator is controlled by an output signal of an adder which substantially adds the outputs of said first and second phase comparators. An apparatus as claimed in claim 1, wherein said threshold control means has a selector means (50,5) for activating said second control means (6,7,8,10) in a header area of a repetitive predetermined pattern inputted prior to an information-carrying area of the input signal, and activating said first control means (4,8,9,10) in said information-carrying area. An apparatus as claimed in claim 4, wherein said second control means (6,7,8,10) is faster in response than said first control means (4,8,9,10). An apparatus as claimed in claim 4, further having a three-state gate (5) an output terminal of which is connected to a voltage source (6) of a predetermined voltage through a resistor (7) to provide an output signal which is obtained by means synchronizing the reproduced digital signal with the read clock, wherein said first (4,8,9,10) and second (6,7,8,10) control means control the threshold level in accordance with a low-frequency component of a difference between said reproduced digital signal and the output signal of said gate, and said selector means (50) places said gate (5) in an active state at the time of selecting said first control means (4,8,9,10) and places said gate in a high-impedance state at the time of selecting said second control means (6,7,8,10). An apparatus as claimed in claim 1, wherein said first (4,8,9,10) and second (6,7,8,10) control means operate simultaneously and have such characteristics that an open-loop gain of said second control means (6,7,8,10) is higher than that of said first control means (4,8,9,10) in a direct current but becomes smaller than 1 at a cross-over frequency where the open-loop gain of said first control means is 1.

Inventors: KOUDO, TOSHIKAZU, KUSHISAKA, TOHRU, TANAKA, SHINICHI
Applicants: MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1987
Main classification: G11B20
IPC codes: G11B20