UNRELIABILITY DETECTOR APPARATUS AND REPRODUCTION APPARATUS PROVIDED WITH THE UNRELIABILITY DETECTOR APPARATUS

Abstract: The invention relates to an unreliability detector apparatus for generating an unreliable decision signal in response to an input signal. The apparatus comprises an input terminal (1) for receiving the input signal, a first comparator unit (2) for comparing the input signal with a first threshold value (+Vd) of positive polarity and for supplying a first comparator output signal in response thereto, the said first comparator output signal changing over from a first signal value (-1) towards a second signal value (+1) upon the input signal exceeding the first threshold value, and vice versa, a second comparator unit (4) for comparing the input signal with a second threshold value (-Vd) of negative polarity and for supplying a second comparator output signal in response thereto, the said second comparator output signal changing over from a third signal value (-1) towards a fourth signal value (+1) upon the input signal exceeding the second threshold value, and vice versa. The invention further relates to an apparatus for reading information from a record carrier, said apparatus being provided with the unreliability detection apparatus.

Description: Unreliability detector apparatus and reproduction apparatus provided with the unreliability detector apparatus. The invention relates to an unreliability detector apparatus and to a reproduction apparatus provided with the unreliability detector apparatus. Unreliability detector apparatuses are generally provided with two comparators in which the input signal is compared with a positive and a negative threshold level respectively that lie between the maximum positive signal level and the zero signal level and between the maximum negative signal level and zero respectively. In a bit detector, it is commonly decided whether the signal is positive or negative. A positive signal is said to correspond with a + 1 and a negative signal with a -1. Ideally, signal values at the decision moments are + 100 % or -100 %: that is, without noise and other disturbances. Decisions, which are taken from the signal around the + 100% or - 100% values are almost certainly correct. Decisions, which are taken from the signal near the zero level are uncertain. With two threshold levels, equal to +V d and -Vd respectively, a decision > +V d is said to correspond with + 1 and a decision < -V d is said to correspond with -1. Decisions between -V d and +V d are less certain. They are called erasures. The number of erasures depends strongly on the levels of the decision thresholds. For increasing threshold values, the erasure rate will increase very strongly. If we consider erasures as uncertain bit decisions, we can try to make these uncertain bit decisions more certain. This is what is done in a dual decision feedback equalizer (DDFE). DDFEs are well known in the art. Reference is made in this respect to international patent application no. IB97/00792 (PHN 15.894), in the name of the same applicant. The invention aims at providing an unreliability detector apparatus for detecting erasures. Such erasures occur as unreliable samples taken from the input signal, from which samples the bits are normally to be detected. The unreliability apparatus in accordance with the invention comprises an unreliability detector apparatus for generating an unreliable decision signal in response to an input signal, the apparatus comprising - input means for receiving the input signal, - first comparator means that carries out a function equivalent to comparing the input signal with a first threshold value (+V d ) of positive polarity and for supplying a first comparator output signal in response thereto, the said first comparator output signal changing over from a first signal value (-1) towards a second signal value (+ 1) upon the input signal exceeding the first threshold value, and vice versa, - second comparator means that carries out a function equivalent to comparing the input signal with a second threshold value (-V d ) of negative polarity and for supplying a second comparator output signal in response thereto, the said second comparator output signal changing over from a third signal value (-1) towards a fourth signal value (+ 1) upon the input signal exceeding the second threshold value, and vice versa, - distance value calculation means that carries out a function equivalent to calculating a distance value from the first and second comparator output signals, the said distance value having a relationship with the distance in an X-Y plane between a fixed point and a curve in said plane, said curve in said plane being obtained by plotting time equivalent signal values of the first and the second comparator output signals along the Y- and X-axis respectively of said X-Y plane, forming points in said X-Y plane that form the said curve, said fixed point (P 3 ) in said X-Y plane being obtained by plotting said first signal value along said Y-axis and said fourth signal value along said X-axis, - third comparator means that carries out a function equivalent to comparing said distance value with a distance threshold value and for generating the unreliable decision signal upon said distance value not exceeding said distance threshold value. Another embodiment, the apparatus in accordance with the invention comprises an unreliability detector apparatus for generating an unreliable decision signal in response to an input signal, the apparatus comprising - input means for receiving the input signal, - first comparator means that carries out a function equivalent to comparing the input signal with a first threshold value (+V d ) of positive polarity and for supplying a first comparator output signal in response thereto, the said first comparator output signal changing over from a first signal value (-1) towards a second signal value (+ 1) upon the input signal exceeding the first threshold value, and vice versa, - second comparator means that carries out a function equivalent to comparing the input signal with a second threshold value (-V d ) of negative polarity and for supplying a second comparator output signal in response thereto, the said second comparator output signal changing over from a third signal value (-1) towards a fourth signal value (+ 1) upon the input signal exceeding the second threshold value, and vice versa, - processing means for carrying out a function equivalent to determining whether a curve in an X-Y plane, said curve in said plane being obtained by plotting time equivalent signal values of the first and the second comparator output signals along the Y- and X-axis respectively of said X-Y plane, has at least one point in common with a predetermined line in said X-Y plane, said predetermined line lying in a half plane in said X-Y plane formed by a first line interconnecting a first fixed point and a second fixed point and in which half plane a third fixed point is located, said first fixed point (P j ) being obtained by plotting said first signal value along said Y-axis and said third signal value along said X-axis, said second fixed point (P ) being obtained by plotting said second signal value along said Y-axis and said fourth signal value along said X-axis, said third fixed point (P 3 ) in said X-Y plane being obtained by plotting said first signal value along said Y-axis and said fourth signal value along said X-axis, said predetermined line lying in said half plane between said first line and said third point, the processing means further being adapted to generate the unreliable decision signal upon detecting that said curve has at least one point in common with said predetermined line. The invention is based on the following recognition. Assuming an ideal behaviour of the first and second comparator means, which may be in the form of latches, the two comparator means arrive at the same decision. That is to say: assuming short response times of the comparator means compared to the bit frequency period, the decisions travel back and forth between the first and second points in the X-Y plane defined above. However, in a situation when the response times of the comparator means is of the same order as the bit period, the decision curve deviate from the straight line between the above defined first and second points in the X-Y plane. The third point P 3 , defined above corresponds with an erasure. Generally, the decision curves formed during each polarity transition of the input signal will run through the first and second points and lie inside the triangle formed by the first, second and third points. The smaller the distance from the curve to the point P 3 , the larger the chance of occurrence of an erasure. In accordance with the invention, during each polarity transition of the input signal, the distance from the point P 3 to the decision curve is determined and the said distance is compared with a distance threshold value. A sample taken from the input signal is considered unreliable for the detection of a bit, when it is taken from the input signal at a time instant when it is determined that the said distance is smaller than said distance threshold value. In the second embodiment, it is determined whether the decision curve has at least one point in common with the predetermined line in said X-Y plane. The sample taken from the input signal is considered unreliable for the detection of a bit, when it is taken from the input signal at a time instant when said curve has at least one point in common with said predetermined line. These and other aspects of the invention will be apparent from and elucidated with respect to embodiments described hereafter in the figure description, in which figure la shows a first embodiment and figure lb shows a second embodiment of the unreliability detection apparatus, figure 2 in figure 2a shows an example of the input signal of the detection apparatus as a function of time, in figure 2b the output signal of the second comparator unit of the detection apparatus in response to that input signal and in figure 2c the output signal of the first comparator unit in response to that input signal, figure 3 shows a fictitious X-Y plane with the output signals of the comparator units as given in the figures 2b and 2c plotted along the X and Y axis in this plane, figure 4 in figure 4a shows another example of the input signal of the detection apparatus as a function of time, in figure 4b the output signal of the second comparator unit of the detection apparatus in response to that other input signal and in figure 4c the output signal of the first comparator unit in response to that other input signal, figure 5 shows the fictitious X-Y plane with the output signals of the comparator units as given in the figures 4b and 4c plotted along the X and Y axis in this plane, figure 6 an elaboration of the processing unit 11 in the apparatus of figure lb, figure 7 shows the incorporation of the detection apparatus of figure 1 in an apparatus for reading information from a record carrier, and figure 8 again the X-Y plane with two curves characterizing two polarity transitions in the input signal. Figure 1 shows an embodiment of the unreliability detection apparatus. The apparatus comprises an input terminal 1 for receiving an input signal, which input terminal is coupled to a first input of a first comparator unit 2 and a second comparator unit 4. A second input of the first comparator unit 2 is coupled to a terminal 6 at which terminal a first threshold value +V d is available. A second input of the second comparator unit 4 is coupled to a terminal 8 at which terminal a second threshold value -V d is available. Outputs of the first and second comparator units 2 and 4 respectively are coupled to corresponding inputs of a distance calculation unit 10, which has an output coupled to a first input of a comparator unit 12. A second input of the comparator unit 12 is coupled to a terminal 14 at which terminal a threshold value T is available. An output of the comparator unit 12 is coupled to an output terminal 16 on which an unreliable decision signal is available. Figure lb shows another embodiment of the apparatus, which shows much resemblance with the apparatus of figure la. The apparatus of figure lb differs from the apparatus of figure la in that, instead of the distance calculator unit 10 and the comparator unit 12, a processing unit 11 is provided, which has an output coupled to the output terminal 16 for providing the unreliable decision signal. The functioning of both embodiments will be further described hereafter. The first comparator unit 2 compares the input signal with the first threshold value +V d and supplies a first comparator output signal in response thereto at its output. The second comparator unit 4 compares the input signal with the second threshold value -V d and supplies a second comparator output signal in response thereto at its output. The functioning of the first and second comparator units 2 and 4 will be further described with reference to figure 2 and 3, and with reference to figure 4 and 5. Figure 2a shows a portion of the input signal as a function of time, during a polarity reversal of the input signal. Figure 2a further discloses the first and second threshold values +V d and - V d respectively. Figure 2b shows schematically the output signal of the comparator unit 4 as a function of time and figure 2c shows schematically the output signal of the comparator unit 2 as a function of time. Upon crossing the -V d level at the time instant t 0 , the output signal of the comparator unit 4 changes from a negative level (the third signal level claimed) such as the level -1 , to a positive level (the fourth signal level claimed), such as + 1. Upon crossing the +V d level at the time instant ti , the output signal of the comparator unit 2 changes from a negative level (the first signal level claimed) such as the level -1, to a positive level (the second signal level claimed), such as + 1. Figure 3 shows a fictitious X-Y plane in which the output signal of the first comparator unit 2 is plotted along the vertical Y-axis and the output signal of the second comparator unit 4 is plotted along the horizontal X-axis. Time equivalent signal values of the output signals of the comparator units 2 and 4 are combined so as to form points in the X-Y plane, said points forming a curve between the points P 1 and P . The point Pt characterizes the situation prior to the time instant to, as well as at the time instant t g , where both output signals of the comparator units 2 and 4 have the output value -1. The point P 2 characterizes the situation after the time instant t 2 , as well as at the time instant t 2 , where both the output signals of the comparator units 2 and 4 have the output value + 1. Other points on the curve between the points > 1 and P 2 are the point P 4 (t 1 ), characterizing the situation at the time instant t, , when the input signal crosses the threshold value +V d , and the point Ps(t 3 ), characterizing the situation when the output signal of the comparator unit 4 reaches the level + 1. During the transition, the curve is thus followed in the direction from the point Pt to the point P 2 . Suppose that the portion of the input signal shown in figure 2a, which portion includes a polarity reversal from a negative polarity to a positive polarity, was mirror imaged around the time axis. Such mirror imaging would lead to a polarity reversal in the input signal from a positive polarity to a negative polarity. It will be evident that such polarity reversal would lead to the same curve in the X-Y plane, as shown in figure 3, which curve is now passed in a direction from the point P 2 to the point Pi . Figure 4a shows again a portion of the input signal as a function of time, during a polarity reversal. Compared to figure 2a, the polarity reversal in figure 4a is slower. Figure 4b shows again schematically the output signal of the comparator unit 4 as a function of time and figure 4c shows again schematically the output signal of the comparator unit 2 as a function of time. Figure 5 shows again the fictitious X-Y plane in which the output signal of the first comparator unit 2 is plotted along the vertical Y-axis and the output signal of the second comparator unit 4 is plotted along the horizontal X-axis. The point P, characterizes the situation prior to the time instant t 10 , as well as at the time instant t 10 , where both output signals of the comparator units 2 and 4 have the output value -1. The point P 2 characterizes the situation after the time instant t 13 , as well as at the time instant t 13 , where both the output signals of the comparator units 2 and 4 have the output value + 1. Other points on the curve between the points P j and P 2 are the point P 4 (t π ), characterizing the situation at the time instant t π , when the input signal crosses the threshold value +V d , and the point P 5 (t 12 ), characterizing the situation when the output signal of the comparator unit 4 reaches the level + 1. The curves in the figures 3 and 5 have a certain distance D from the point P 3 . Comparing figure 3 with figure 5 makes clear that the distance D of the curve from the point P 3 is larger in figure 3 than in figure 5. The smaller the distance D of the curve from the point P 3 , the larger is the unreliability of the sample, when it is taken from the input signal during the time interval between the points P 4 and P 5 . The distance calculator unit 10 in figure la is adapted to calculate the said distance value D, which distance value is supplied to the first input of the comparator unit 12. The comparator unit 12 compares the distance D calculated with a threshold distance value T. When the distance D calculated is smaller than said threshold value T, the comparator unit 12 generates an unreliable detection signal at the output terminal 16. The curves shown in the figures 3 and 5 are ideal curves. More realistic curves are shown in figure 8. Figure 8 shows two curves Ci and C 2 , between the points P j and P 2 , characterizing two polarity transitions in the input signal of different shape. For detecting whether a sample taken from the input signal is unreliable or not, in a way different from what has been explained above in relation to the embodiment of figure la, the embodiment of figure lb can be used. The functioning of the processing unit 11 of the embodiment of figure lb will now be explained hereafter. The processing unit 1 1 is adapted carry out a function equivalent to determining whether a curve, such as the curves C j and C 2 in figure has at least one point in common with a predetermined line in said X-Y plane. This line is given in figure 8 by the line 1. The line 1 is defined as lying in a half plane in said X-Y plane formed by a line interconnecting the points P j and P 2 , in which half plane the point P 3 is located, see figure 8. More specifically, the line 1 lies in said half plane between said line through the points P j and P 2 and said point P 3 . From figure 8, it is clear that the curve C 2 has at least one point in common with the line 1. The processing unit 11 is now adapted to generate the unreliable decision signal upon establishing that the curve C 2 has at least one point in common with the line 1. A sample taken from the input signal during a time interval corresponding to the time interval in which the curve C 2 is followed between the points P 3 and P 4 , is now considered unreliable. Figure 6 shows an elaboration of the processing unit 11 of figure lb. The unit 11 comprises a first transistor pair Tr, ,Tr 2 , having interconnected emitters, and a second transistor pair Tr 3 ,Tr 4 , also having interconnected emitters. The transistors are in the form of NPN transistors. The bases of the transistor pair Tr, ,Tr 2 are coupled to the terminals denoted 22, being the first input of the processing unit 11. The bases of the transistor pair Tr 3 ,Tr 4 are coupled to the terminals denoted 24, being the second input of the calculator unit 10. The interconnected emitters of both transistor pairs are each coupled via a corresponding current source I to a point of constant potential, such as ground The collectors of the transistors TR j and Tr 4 are interconnected and coupled to a second point of constant potential (+) via an impedance R. The collectors of the transistors Tr 2 and Tr 3 are also coupled to the second point of constant potential (+) via an impedance R. The interconnected collectors are further coupled to terminals, denoted 26, that form the output of the processing unit 11. Further, a current source, denoted T, is available which is coupled between the first point of constant potential and the interconnected collectors of the transistors TR 2 and Tr 3 . The current source T could be identified as the current that sets the position of the line 1 with respect to the point P 3 , as shown in figure 8. In the present embodiment, the line 1 is perpendicular to the line O-P 3 . Varying the current amplitude of the current source T results in shifting the line 1 in the direction O-P 3 . During the polarity transition characterized by the curve Ci , the polarity of the signal at the output terminal 26 does not change. This is an indication that the bit(s) detected during this transition is (are) reliable. During the polarity transition characterized by the curve C 2 , curve C 2 intersects the line 1. This results in the polarity of the signal at the output terminal 26 to change. This is an indication that the bit(s) detected in response to samples taken from the input signal during this transition may not be reliable. The unreliability detector apparatuses described above can be used in an apparatus for reading data from a record carrier. This apparatus is shown in figure 7. The reading apparatus comprises a reading unit 30 for reading a signal from a record carrier 32, such as a magnetic record carrier. The reading unit 30 comprises a read head 34 for reading the information from the record carrier 32. The information thus obtained is supplied via an output 36 to the input terminal 1 of the unreliability detector apparatus 20, as well as to a bit detector unit 40, well known in the art. The output terminal 16 of the unreliability detector apparatus 20 is coupled to a control signal input 42 of the bit detector unit 40. An output 44 of the bit detector unit 40 is coupled to the output terminal 46 of the reading apparatus. The output signal of the unreliability detector apparatus 20 is supplied to the control signal input 42 of the bit detector unit 40. In response to an unreliability detection signal supplied by the apparatus 20, the bit detector unit 40 will either issue a bit unmodified, when the unreliability detector apparatus generates no detection signal, or will carry out a correction step on the bit, in order to correct the bit in the case an unreliable decision signal has been generated. Whilst the invention has been described with reference to preferred embodiments thereof, it is to be understood that these are not limitative examples. Thus, various modifications may be come apparent to those skilled in the art, without departing from the scope of the invention, as defined by the claims. As an example, in the claims, it is defined that the first comparator means carries out a function equivalent to comparing the input signal with the first threshold value of positive polarity and for supplying a first comparator output signal in response thereto, and that the second comparator means carries out a function equivalent to comparing the input signal with the second threshold value of negative polarity and for supplying a second comparator output signal in response thereto. The reason for this is that various ways of realizing this behaviour are possible. One of those, is the way described above with reference to figure 1. But other ways are also possible, such as adding a voltage +Vd to the input signal and comparing the signal thus obtained with a zero threshold value in the comparator unit 4 and subtracting voltage +Vd from the input signal and comparing the signal thus obtained with a zero threshold value in the comparator unit 2. Further, the invention lies in each and every novel feature or combination of features.

Claims: CLAIMS: 1. Unreliability detector apparatus for generating an unreliable decision signal in response to an input signal, the apparatus comprising - input means for receiving the input signal, - first comparator means (2) that carries out a function equivalent to comparing the input signal with a first threshold value (+V d ) of positive polarity and for supplying a first comparator output signal in response thereto, the said first comparator output signal changing over from a first signal value (-1) towards a second signal value (+ 1) upon the input signal exceeding the first threshold value, and vice versa, - second comparator means (4) that carries out a function equivalent to comparing the input signal with a second threshold value (N d ) of negative polarity and for supplying a second comparator output signal in response thereto, the said second comparator output signal changing over from a third signal value (-1) towards a fourth signal value (+ 1) upon the input signal exceeding the second threshold value, and vice versa, - distance value calculation means that carries out a function equivalent to calculating a distance value from the first and second comparator output signals, the said distance value having a relationship with the distance in an X-Y plane between a fixed point and a curve in said plane, said curve in said plane being obtained by plotting time equivalent signal values of the first and the second comparator output signals along the Y- and X-axis respectively of said X-Y plane, forming points in said X-Y plane that form the said curve, said fixed point (P 3 ) in said X-Y plane being obtained by plotting said first signal value along said Y-axis and said fourth signal value along said X-axis, - third comparator means that carries out a function equivalent to comparing said distance value with a distance threshold value and for generating the unreliable decision signal upon said distance value not exceeding said distance threshold value. 2. Unreliability detector apparatus for generating an unreliable decision signal in response to an input signal, the apparatus comprising - input means for receiving the input signal, - first comparator means (2) that carries out a function equivalent to comparing the input signal with a first threshold value (+V d ) of positive polarity and for supplying a first comparator output signal in response thereto, the said first comparator output signal changing over from a first signal value (-1) towards a second signal value (+ 1) upon the input signal exceeding the first threshold value, and vice versa, - second comparator means (4) that carries out a function equivalent to comparing the input signal with a second threshold value (-V d ) of negative polarity and for supplying a second comparator output signal in response thereto, the said second comparator output signal changing over from a third signal value (-1) towards a fourth signal value (+ 1) upon the input signal exceeding the second threshold value, and vice versa, - processing means for carrying out a function equivalent to determining whether a curve in an X-Y plane, said curve in said plane being obtained by plotting time equivalent signal values of the first and the second comparator output signals along the Y- and X-axis respectively of said X-Y plane, has at least one point in common with a predetermined line in said X-Y plane, said predetermined line lying in a half plane in said X-Y plane formed by a first line interconnecting a first fixed point and a second fixed point and in which half plane a third fixed point is located, said first fixed point (P j ) being obtained by plotting said first signal value along said Y-axis and said third signal value along said X-axis, said second fixed point (P 2 ) being obtained by plotting said second signal value along said Y-axis and said fourth signal value along said X-axis, said third fixed point (P 3 ) in said X-Y plane being obtained by plotting said first signal value along said Y-axis and said fourth signal value along said X-axis, said predetermined line lying in said half plane between said first line and said third point, the processing means further being adapted to generate the unreliable decision signal upon detecting that said curve has at least one point in common with said predetermined line. 3. Apparatus as claimed in claim 2, wherein said processing means comprises - first input terminals for receiving said first comparator output signal and second input terminals for receiving said second comparator output signal, - a first transistor pair having interconnected first main electrodes and having their control electrodes coupled to said first input terminals, - a second transistor pair having interconnected first main electrodes and having their control electrodes coupled to said second input terminals, the second main electrode of one transistor of the first and second transistor pairs being interconnected, the second main electrode of the other transistor of the first and second transistor pairs being interconnected. 4. Unreliability detector apparatus for generating an unreliable decision signal in response to input signal, the apparatus comprising - input means for receiving the input signal, - first comparator means (2) that carries out a function equivalent to comparing the input signal with a first threshold value (+V d ) of positive polarity and for supplying a first comparator output signal in response thereto, the said first comparator output signal changing over from a first signal value (-1) towards a second signal value (+ 1) upon the input signal exceeding the first threshold value, and vice versa, - second comparator means (4) that carries out a function equivalent to comparing the input signal with a second threshold value (N d ) of negative polarity and for supplying a second comparator output signal in response thereto, the said second comparator output signal changing over from a third signal value (-1) towards a fourth signal value (+ 1) upon the input signal exceeding the second threshold value, and vice versa, - processing means, comprising (a) first input terminals for receiving said first comparator output signal and second input terminals for receiving said second comparator output signal, (b) a first transistor pair having interconnected first main electrodes and having their control electrodes coupled to said first input terminals, (c) a second transistor pair having interconnected first main electrodes and having their control electrodes coupled to said second input terminals, the second main electrode of one transistor of the first and second transistor pairs being interconnected, the second main electrode of the other transistor of the first and second transistor pairs being interconnected, (d) an output for supplying the unreliable decision signal. 5. Apparatus as claimed in claim 3 or 4, wherein the processing means further comprise a current generator which is coupled to one of the interconnected second main electrodes. 6. Apparatus for reading data from a record carrier, the apparatus comprising - read means for reading a signal from said record carrier, - detector means for detecting bits in said signal, - output means for supplying said bits as the data, wherein the apparatus is provided with the unreliability detector apparatus as claimed in claim 1 , 2 or 4.

Inventors: BERGMANS JOHANNES WILHELMUS MA, VOORMAN JOHANNES OTTO, BERGMANS, JOHANNES, WILHELMUS, MARIA, VOORMAN, JOHANNES, OTTO
Applicants: KONINKL PHILIPS ELECTRONICS NV, PHILIPS SVENSKA AB, KONINKLIJKE PHILIPS ELECTRONICS N.V., PHILIPS AB
Application year: 1998
Main classification: H04L27
IPC codes: G11B20, H04L25, H04L27