Tracking error detection

Abstract: Tracks scanned by a playback head (H) have a first signal (f₁) recorded in every nth track (T), a second signal (f₂) recorded in a track (T) which precedes and is adjacent to each nth track (T) and a third signal (f₃) recorded in a track which follows and is adjacent to each nth track (T). When a preceding track (T) is scanned, the second signal (f₂) recorded therein is detected; and at a time delayed from the detection of that second signal (f₂), the first signal (f₁) picked up as a cross-talk component is sensed. When a following track (T) is scanned, the third signal (f₃) recorded therein is detected; and at a time delayed from the detection of the third signal (f₃), the first signal (f₁) picked up as a cross-talk component is sensed. The difference between the first signal (f₁) sensed when a preceding track (T) is scanned and the first signal (f₁) sensed when a following track (T) is scanned is indicative of a tracking error.

Description: This invention relates to tracking error detection. Tracking error control arrangements are known for controlling the position of one or more heads as those heads scan tracks previously recorded on a record medium. For example, in video recording, video signals are recorded on a magnetic tape in successive oblique tracks by a helical scan rotary head assembly. When the video signals are reproduced, either by the same head assembly or by different apparatus, the very same oblique tracks that had been recorded must be followed accurately for proper signal reproduction, and some form of tracking control generally is used for this purpose. Similar tracking control is normally provided when audio signals are recorded in oblique tracks, such as in digital audio tape (DAT) recording. In one type of tracking control arrangement, proper positioning of the heads with respect to the oblique tracks is carried out with the assistance of a control signal recorded along a longitudinal edge of the magnetic tape. This longitudinally recorded control signal is reproduced by a stationary head, and tape movement is adjusted if there is a phase difference between the reproduced control signal and a position signal representing the rotary position of the heads. For example, proper positioning of a head relative to a track is indicated if the head position signal, which is produced when the head rotates to a predetermined position (for example, to the beginning) of a track, coincides with a reproduced control signal. In the absence of coincidence, the tape is driven either faster or slower until the head is brought into proper alignment with a track, as represented by such coincidence. A disadvantage of this technique is the requirement for a stationary head which is dedicated to a single function. This head adds to the overall cost of the signal recording/playback apparatus and, more importantly, it impedes miniaturization of the apparatus. If the stationary head can be eliminated, the overall size of the apparatus can be reduced. It has been proposed to mix tracking control signals with information signals in the same oblique tracks. This has the advantage of eliminating the stationary head and relies upon the transducer which reproduces information to reproduce the tracking control signals as well. Digital video and digital audio recording are particularly receptive to this approach because digital information signals are easily multiplexed with control signals without loss of useful information, they may be time base compressed easily and they may be readily processed. Hence, in a particular track, a tracking control signal may be recorded in a restricted region that is normally preceded and followed by digital information signals. In one implementation of this proposal, a pilot signal of predetermined frequency is recorded in alternate tracks, and each pilot signal region is preceded and followed by digital signals. Advantageously, digital video and digital audio signals may be recorded in tracks having no guard bands between them. While this increases the amount of magnetic tape made available to record useful information, it also emphasizes the need for precise tracking control as playback heads scan the respective tracks. Typically, the heads used for recording and/or reproduction exhibit a gap length larger than the width of a track, which means that an edge of a preceding track is overwritten when the next adjacent track is recorded. This also means that when a playback head is in proper alignment with a prerecorded track, the signals recorded in both adjacent tracks are picked up as cross-talk components. This phenomenon of cross-talk pick-up is used to detect tracking errors and to provide proper tracking control over the scanning heads. In the arrangement mentioned above, wherein pilot signals of a predetermined frequency are recorded in restricted regions of alternate tracks, a pair of heads is used to scan two adjacent tracks substantially simultaneously. Hence, when one head scans the track in which the pilot signal is recorded, the other head scans the adjacent track which has no pilot signal therein. These two heads are mounted on the same rotary head assembly, such as a rotary head drum, and are relatively closely spaced thereon. The head which scans the track having no pilot signal therein will pick up, as a cross-talk component, the pilot signal recorded in an adjacent preceding track as well as the pilot signal recorded in an adjacent following track. If the magnitude of the cross-talk pilot signal picked up by this head from the preceding track is equal to the magnitude of the pilot signal picked up from the following track, the heads are in proper alignment with respect to the tracks scanned thereby. A tracking error is detected if the cross-talk pilot signal magnitudes from the preceding and following tracks differ from each other. In that event, the tape transport is accelerated or decelerated to bring the heads back into proper tracking alignment. In the foregoing implementation, wherein pilot signals are recorded in restricted regions in alternate tracks, it will be appreciated that when a head scans a track having no pilot signal therein, the pilot signal recorded in the following track will be picked up as a cross-talk component before the pilot signal recorded in the preceding track is picked up. Assuming that the pilot signals are recorded in substantially equal regions in each track, it will be further appreciated that the pilot signals picked up as cross-talk components from the following and preceding tracks will be present concurrently for a significant period of time, and that the pilot signal picked up from the following track will terminate first. Thus, during the scan of a track in which no pilot signals are recorded, there will be distinct intervals of time during which the pilot signal is picked up as a cross-talk component from one adjacent track and then from the other. However, these time intervals are relatively short. Consequently, there is the possibility that the cross-talk pilot signal picked up from one of the adjacent tracks during such short interval may not be detected satisfactorily. Furthermore, this brief time interval is reduced if the track pitch is decreased; and a reduction in track pitch desirably leads to greater recording density. Consequently, if the recording medium is subjected to jitter, the time interval during which one cross-talk pilot signal is picked up may be so small as to inhibit adequate detection. Thus, a tracking error may not be properly sensed and tracking control is degraded. EP-A-0 219 303 discloses a tracking control technique using pilot signals recorded at different track positions in accordance with a repeating sequence of tracks. According to the present invention there is provided a method of detecting a tracking error in a playback head which helically scans tracks across a record medium having a first signal recorded in a restricted region in every nth track where n is at least four, a second signal recorded in a restricted region in a track which precedes, in the head scanning direction of the tape, and is adjacent to each track in which said first signal is recorded, and a third signal recorded in a restricted region in a track which follows, in the head scanning direction of the tape, and is adjacent to each track in which said first signal is recorded, said method comprising the steps of: scanning a preceding track by said playback head to reproduce the signals recorded therein and cross-talk components picked up from adjacent tracks; detecting said second signal reproduced from the scanned preceding track; detecting at a time delayed from the detection of said second signal the amplitude of the first signal reproduced by said playback head as a cross-talk component when said playback head scans said preceding track; scanning a following track by said playback head to reproduce the signals recorded therein and cross-talk components picked up from adjacent tracks; detecting said third signal reproduced from the scanned following track; detecting at a time delayed from the detection of said third signal the amplitude of the first signal reproduced by said playback head as a cross-talk component when said playback head scans said following track; and sensing a difference between the detected amplitude of the first signal detected when said playback head scans said preceding track and the detected amplitude of the first signal detected when said playback head scans said following track, said sensed difference being indicative of said tracking error. According to the present invention there is also provided apparatus for detecting a tracking error in transducer means which helically scans tracks across a record medium having a first signal recorded in every nth track where n is at least four, a second signal recorded in a track which precedes, in the head scanning direction of the tape, and is adjacent to each nth track, and a third signal recorded in a track which follows, in the head scanning direction of the tape, and is adjacent to each nth track, said apparatus comprising: second signal detecting means for detecting said second signal when said transducer means scans a preceding track; third signal detecting means for detecting said third signal when said transducer means scans a following track; first signal detecting means operable when said transducer means scans said preceding track to detect, at a time delayed from the detection of said second signal, the amplitude of the first signal picked up as a cross-talk component by said transducer means, and operable when said transducer means scans said following track to detect, at a time delayed from the detection of said third signal, the amplitude of the first signal picked up as a cross-talk component by said transducer means; and sensing means for sensing a difference between the detected amplitude of the first signal detected by said first signal detecting means when said transducer means scans said preceding track and the detected amplitude of the first signal detected by said first signal detecting means when said transducer means scans said following track, said sensed difference being indicative of said tracking error. Thus the present invention provides a technique for detecting a tracking error using a transducer which scans tracks across a record medium having a first signal recorded in every nth track, a second signal recorded in a track which precedes and is adjacent to each nth track, and a third signal recorded in a track which follows and is adjacent to each nth track. When a preceding track is scanned, the second signal recorded therein is detected; and the first signal, picked up as a cross-talk component when this preceding track is scanned, is detected at a time delayed from the detection of the second signal. When a following track is scanned, the third signal recorded therein is detected; and the first signal, picked up as a cross-talk component when this following track is scanned, is detected at a time delayed from the detection of the third signal. A difference between the first signal detected when the preceding track is scanned and the first signal detected when the following track is scanned is sensed and provides a tracking error indication. In a preferred embodiment, n= 4. Also, the first signal is of a predetermined frequency and functions as a pilot signal, whereas the second and third signals exhibit frequencies different from the first signal and function as synchronizing signals. In one embodiment, the frequencies of the second and third signals are equal to each other, and in another embodiment these frequencies are different. The first, second and third signals may be detected by sensing their respective frequencies. A sampling pulse may be generated when the second signal is detected, and after being delayed this sampling pulse is used to sample the first signal picked up as a cross-talk component. Similarly, a sampling pulse may be generated when the third signal is detected, and after being delayed this sampling pulse is used to sample the first signal picked up as a cross-talk component. A tracking error may be determined by sensing the difference between these respective samples of the first signal. The delay imparted to the respective sampling pulses assures that the first signal, picked up as a cross-talk component, will be sampled at some point of the restricted region in which it is recorded. This delay may be increased or decreased, as desired, and permits accurate detection notwithstanding jitter, reduced track pitch or a particular pilot signal recording format. The invention will now be described by way of example with reference to the accompanying drawings, throughout which like parts are referred to by like references, and in which; Figure 1 is a schematic representation of a rotary head assembly that may be used both with the above prior proposal as well as with the present invention; Figure 2 is a schematic representation of recording tracks scanned by a pair of playback heads in the above prior proposal; Figure 3 is a schematic representation of recording tracks scanned by a pair of playback heads in an embodiment of the present invention; Figure 4 is a block diagram of a preferred embodiment of apparatus in accordance with the present invention; and Figures 5A to 5E are timing diagrams. Before describing the present invention, a more detailed explanation of the above prior proposal for detecting tracking errors will be explained in conjunction with Figures 1 and 2. Figure 1 is a schematic representation of a rotary head assembly wherein a pair of playback heads H L and H T are mounted on a rotary head drum 1 about which is partially wrapped a magnetic tape 2. The heads H L and H T are designated leading and trailing heads, respectively, and are closely spaced so as to be angularly separated by a relatively short distance GL. In addition, and as will be best appreciated by reference to Figure 2, the heads H L and H T are offset from each other in the axial direction of the drum 1. This offset is substantially equal to the pitch of the tracks scanned thereby. Although Figure 1 illustrates the tape 2 as being wrapped about a peripheral surface of the drum 1 by an angular extent of more than 90°, it will be appreciated that this wrap angle may be much greater, for example, greater than 300° and preferably greater than 330°. As a result of a large wrap angle and closely spaced heads, the leading and trailing heads H L and H T function to scan substantially simultaneously a pair of tracks at each rotation of the drum 1. If the wrap angle is large enough, only two heads need be used to scan successive pairs of tracks. The arrangement shown in Figure 1, wherein a pair of heads is mounted on a rotatable drum in closely spaced but axially offset relationship offers significant advantages over previously proposed mounting arrangements wherein two heads are mounted 180° apart. Tracks which are recorded by diametrically opposed heads may not be in alignment with each other, because the rotary movement of the drum to scan one head across the tape may not be precisely the same as the rotary movement thereof to scan the other head. This difficulty is, of course, avoided when the heads are closely spaced because both heads will be subjected to the very same irregularities that may be present in the drum rotation. Hence, the scanning trace of the head H L is practically identical to the scanning trace of the head H T , and the resultant tracks scanned thereby are in precise alignment. Good effects are achieved even if the track pitch is made very small. The track pattern scanned by the heads H L and H T in Figure 1 is illustrated schematically in Figure 2. In this track pattern, the pilot signal of frequency f₀ is recorded in alternate tracks, such as in odd tracks T₁, T₃, T₅, etc. In accordance with the proposal made heretofore, the duration of the pilot signal is controlled such that the pilot signal is recorded in a restricted region in a track. The frequency of the pilot signal is several hundreds of kHz, but its frequency is such as to exhibit relatively small azimuth loss. As is typical in video recording, the heads H L and H T have different azimuths such that the phenomenon of azimuth loss minimizes the cross-talk component picked up by one head from a track that was recorded by a different head. For example, if it is assumed that the leading head H L scans the odd tracks T₁, T₃, T₅, etc. during both recording and reproducing operations, and if it also is assumed that the trailing head H T scans the even tracks T₂, T₄, etc., during both recording and reproducing, then azimuth loss will minimize the signals picked up by the head H L from adjacent even tracks and, similarly, azimuth loss will minimize the signals picked up by the head H T from adjacent odd tracks. That is, when the head H L scans the track T₃ during a playback operation, azimuth loss will minimize the signals picked up as cross-talk components from the tracks T₂ and T₄. Similarly, when the head H T scans the track T₂ during a playback operation, azimuth loss will minimize the cross-talk components picked up by the head H T from the tracks T₁ and T₃. However, the frequency of the pilot signal f₀ is not subjected to significant azimuth loss and will be picked up as a cross-talk component by the head H T when it scans an even track. When the heads H L and H T scan the odd and even tracks, as illustrated in Figure 2, the pilot signal f₀ is reproduced when the head H L reaches a point to in the track T₁. Because of the spacing GL between the heads H L and H T , the head H T reaches a point t₀' in the track T₂ when the head H L first detects the pilot signal. It is seen that the points t₀ and t₀' are spaced apart by the distance GL, the same spacing as between the heads H L and H T . It is recognized that, when the head H L first detects the pilot signal, the head H T is sufficiently distant therefrom as not to pick up a pilot signal cross-talk component, as yet. Rather, it is until the head H T advances to the position t₁ in the track T₂ that a cross-talk pilot signal component will be picked up by the head H T from the adjacent track T₃. It is seen that the head H T reaches the point t₁ at a delayed time t a from the point t₀'. That is, at this delayed time t a , the head H T picks up a cross-talk pilot signal component from only one adjacent track, namely the right track T₃. From Figure 2, it is apparent that the head H T reaches a portion of the track T₂ whereat the pilot signals f₀ are picked up as cross-talk components simultaneously from the left adjacent track T₁ and from the right adjacent track T₃. During the time that the pilot signal is picked up from both adjacent tracks, a determination cannot be made with respect to a tracking error. Hence, the tracking error detecting circuitry normally used with the arrangement shown in Figure 2 is disabled or otherwise inhibited during the period of time that the head H T picks up pilot signal cross-talk components from both tracks adjacent to the track being scanned thereby. When the head H T reaches the point t₂ as it scans the track T₂, the pilot signal cross-talk component picked up from the right adjacent track T₃ no longer is detected, and the pilot signal cross-talk component is now picked up only from the left adjacent track T₁. Figure 2 illustrates that the head H T arrives at the point t₂ at a delayed time t b from the point t₀′. Preferably, the pilot signal cross-talk component picked up from, for example, the track T₃ is detected separately from the pilot signal cross-talk component picked up from the track T₁. A tracking error is determined by sensing the difference between these separately detected pilot signal cross-talk components. For example, if the magnitude of one pilot signal cross-talk component exceeds the magnitude of the other, it is concluded that the heads are not properly centred on the tracks and the tape 2 is accelerated or decelerated to effect a shifting the heads relative to the tracks scanned thereby. Hence, tracking errors are corrected. In the proposal represented by Figure 2, a pilot signal of only one frequency is recorded in alternate tracks. However, as discussed above, the time interval during which the pilot signal may be detected by the head H T as a cross-talk component picked up from only one adjacent track is relatively brief. For example, the pilot signal picked up from the right adjacent track is detected only during the relatively brief interval t c , and the pilot signal picked up from the left adjacent track is detected only during the relatively brief interval t d . Between these intervals, the pilot signal is picked up from both adjacent tracks. This brief interval t c or t d is a function of the difference d between the start of successive tracks. This difference d becomes smaller as the track pitch is reduced; and this results in decreasing the interval t c or t d during which the pilot signal may be detected from only one adjacent track. Indeed, this interval t c or t d may be expressed as follows: t c = t d = 2(v T /f D ) cos  v R [seconds] where v T is the speed at which the tape 2 is transported, f D is twice the drum rotation rate,  is the recording angle (that is, the angle of a track relative to the longitudinal direction of the tape) and v R is the speed of the head relative to the tape 2 as both the head and tape 2 move. As the time interval t c or t d is reduced, jitter in the tape movement may prevent accurate detection of the pilot signal picked up from an adjacent track, thus degrading the tracking error operation. Embodiments of the present invention can overcome this difficulty associated with the proposal shown in Figure 2 by recording pilot and synchronizing signals in accordance with the track pattern shown in Figure 3. It will be appreciated that the heads H L and H T , which are used to scan two adjacent tracks substantially simultaneously, may be mounted on a rotary drum 1 consistent with the configuration shown in Figure 1. Thus, the heads are closely spaced and offset from each other in the axial direction of the drum 1 by an amount substantially equal to the pitch of the tracks. Furthermore, the heads have different azimuth angles so as to attenuate cross-talk picked up from adjacent tracks, except that a pilot signal picked up as a cross-talk component will not be significantly attenuated. In the embodiment illustrated in Figure 3, it is assumed that the head H L scans odd tracks T₁₁, T₁₃, T₁₅, etc. and the head H T scans even tracks T₁₂, T₁₄, T₁₆, etc. A pilot signal f₁ of a frequency that is not significantly attenuated by azimuth loss is recorded in a restricted region in every nth track, where n is an even number equal to or greater than 4. In the preferred embodiment, as illustrated in Figure 3, n = 4. Thus, the pilot signal f₁ is recorded in the tracks T₁₃, T₁₇, etc. The pilot signal f₁ is recorded for a predetermined duration and commences at a fixed interval from the beginning portion of the track in which it is recorded. A synchronising signal f₂ is recorded in a restricted region in a track which precedes the track containing the pilot signal f₁. Thus, the synchronizing signal f₂ is recorded in the tracks T₁₂, T₁₆, etc. The synchronizing signal has a predetermined frequency different from the pilot signal frequency, and preferably is greater than the pilot signal frequency so as to be subjected to azimuth loss. The duration of the synchronizing signal is not critical, and in the illustrated embodiment this duration is less than the duration during which pilot signal f₁ is recorded. As will be appreciated below, the length and position of the synchronizing signal f₂ is such that, after the signal has been detected by, for example, the head H T , the pilot signal f₁ is picked up as a cross-talk component by the head H T . Similarly, a synchronizing signal f₃ is recorded in a restricted region in a track which follows and is adjacent to each track in which the pilot signal f₁ is recorded. As shown, the synchronizing signal f₃ is recorded in the tracks T₁₄, T₁₈, etc. The synchronizing signal preferably is of a frequency higher than the frequency of the pilot signal f₁ and, thus, is subject to azimuth loss. In one embodiment, the synchronizing signals f₂ and f₃ are of the the frequency. In an alternative embodiment, the frequencies of the synchronizing signals f₂ and f₃ differ from each other. The duration of the synchronizing signal f₃ and its particular location relative to the pilot signal f₁ recorded in the adjacent track are not critical; however, it will become apparent that the synchronizing signal f₃ is located such that after it has been detected by, for example, the head H T , the head H T picks up the pilot signal f₁ as a cross-talk component. In the illustrated embodiment, the duration of the synchronizing signal f₃ is shorter than that of the pilot signal f₁. It will be recognized that in those regions of each track which precede and follow a respective pilot or synchronizing frequency, useful information is recorded, such as digital video or digital audio information. Of course, useful information is recorded in substantially the entire length of those tracks having neither a pilot signal nor a synchronizing signal recorded therein. As illustrated, the tracks T₁₁, T₁₅, etc., have such useful information recorded therein. One example of apparatus for detecting a tracking error from the pilot and synchronizing signal pattern illustrated in Figure 3 is shown as a block diagram in Figure 4. For the purpose of this description, it assumed that the head H T scans the even tracks to reproduce the synchronizing signals f₂ and f₃ and to pick up the pilot signal f₁ as a cross-talk component. It will be appreciated, however, that the head H L may be used to record and reproduce these even tracks. The tracking error detector shown in Figure 4 comprises individual signal detectors connected in common to the head H T by a playback preamplifier 11. These detectors include a synchronizing signal detector to detect the synchronizing signal f₂, a synchronizing signal detector to detect the synchronizing signal f₃, and a pilot signal detector to detect the pilot signal f₁. To discriminate these signals reproduced or picked up by the head H T , filters are used, each tuned to a respective frequency and thus operable to pass either the synchronizing signal f₂, the synchronizing signal f₃, or the pilot signal f₁. These filters are illustrated as band-pass filters having centre frequencies tuned to the frequency of the particular signal to be passed thereby. The pilot signal detector comprises a band-pass filter 12 coupled to the output of the preamplifier 11 and passing the pilot signal frequency. The output of the band-pass filter 12 is coupled to an envelope detector 13 which produces a signal whose amplitude is proportional to the magnitude of the pilot signal picked up by the head H T as a cross-talk component. The detected envelope of the pilot signal is supplied in common to sample-and-hold circuits 14 and 15 which function to sample the pilot signal envelope. The sampling pulses used by these sample-and-hold circuits are generated by the synchronizing signal detectors, as will be described. A difference circuit 16, such as a comparator, is coupled to the sample-and-hold circuits 14 and 15 to determine the difference between the pilot signal samples produced thereby. This difference is indicative of a tracking error and is supplied to an output terminal 18 by way of a switch 17. The switch 17 is closed by a sampling pulse generated by one of the synchronizing signal detectors. The synchronizing signal detector to detect the synchronizing signal f₂ includes a band-pass filter 21 which is coupled to the output of the preamplifier 11 and is tuned to the frequency of this synchronizing signal. The envelope of the synchronizing signal passed by the band-pass filter 21 is detected by an envelope detector 22, and the level of the detected envelope is compared with a reference level by a comparator 23. It will be appreciated that the comparator 23 functions as a threshold circuit and generates an output pulse when the envelope of the synchronizing signal f₂ exceeds the threshold level represented by the reference signal supplied to this comparator. The pulse produced by the comparator 23 is coupled to the sample-and-hold circuit 14 as a sampling pulse after being delayed by a delay circuit 24. The delay circuit 24 imparts a predetermined time delay to the sampling pulse generated by the comparator 23 for a purpose soon to be described. The synchronizing signal detector to detect the synchronizing signal f₃ is of substantially the same construction as the synchronizing signal detector used to detect the synchronizing signal f₂. As shown, a band-pass filter 31, tuned to the frequency of the synchronizing signal f₃, is coupled to the output of the preamplifier 11 and passes the synchronizing signal f₃ to an envelope detector 32. The envelope of the synchronizing signal f₃, which represents the magnitude thereof, is compared by a comparator 33 with a threshold level supplied thereto by a reference signal source. The comparator functions as a threshold detector to produce a sampling pulse when the envelope of the synchronizing signal f₃ exceeds this threshold. The sampling pulse generated by the comparator 33 is supplied to the sample-and-hold circuit 15 via a delay circuit 34. This sampling pulse also is used to close the output switch 17. It will be appreciated that, for the embodiment wherein the frequencies of both the synchronizing signals are the same, the bandpass filters 21 and 31 may be substantially identical. Conversely, if the frequencies of the synchronizing signals differ from each other, the centre frequencies of the respective band-pass filters 21 and 31 likewise differ from each other. Still further, delays imparted by the delay circuits 24 and 34 to the sampling pulses generated by the comparators 23 and 33, respectively, may be equal. As mentioned above, the purpose of these delays is discussed below, and it will become apparent that such delays need not be equal to each other. The manner in which the tracking error detector shown in Figure 4 operates now will be described in conjunction with Figure 3 and the timing diagrams of Figures 5A to 5E. Let it be assumed that the head H L scans the odd tracks shown in Figure 3, and that the head H T scans the even tracks. Figure 5A is a timing waveform representing a switching pulse, and it is assumed that the heads H L and H T begin the scanning of their respective tracks at the positive transition of the switching pulse. It will be appreciated that the period of the switching pulse corresponds to the rotational period of the drum 1 and that the wrap angle of the tape 2 deployed about the drum 1 is such that the pair of heads H L and H T scan successive pairs of tracks across the tape 2. Thus, the heads H L and H T scan track pairs T₁₁, T₁₂; T₁₃, T₁₄; T₁₅, T₁₆; etc. In the illustrated embodiment, the synchronising and pilot signals are recorded in their respective tracks at the vicinity of the beginning portion of such tracks. Useful information may precede as well as follow the restricted regions in which these respective synchronizing and pilot signals are recorded. When the head H T scans the track T₁₂, the synchronizing signal f₂ is detected when the head H T reaches the point t₁₀. The synchronizing signal f₂ reproduced by the head H T is passed to the envelope detector 22 by the band-pass filter 21 and compared with a threshold level by the comparator 23. Assuming that the magnitude of the synchronizing signal f₂ exceeds this threshold level, the comparator 23 generates an output pulse as shown in Figure 5B. As the head H T scans the track T₁₂, the pilot signal f₁ recorded in the right adjacent track T₁₃ is picked up and is passed by the band-pass filter 12. The magnitude of the pilot signal cross-talk component is detected by the envelope detector 13 and is supplied to the sample-and-hold circuit 14. From the track pattern shown in Figure 3, it is expected that the pilot signal f₁ is picked up before the synchronizing signal f₂ is detected, and the pilot signal f₁ is continuously picked up for a substantial period of time, even after the head H T passes beyond the synchronizing signal f₂. The delay circuit 24 delays the sampling pulse shown in Figure 5B by a time delay t A such that when the head H T reaches the point t₁₁ (shown in Figure 3), the delayed poling pulse is generated, as represented by the timing waveform of Figure 5D. If the time delay t A is greater than the interval during which the synchronizing signal f₂ is reproduced, interference between the synchronizing signal f₂ and the pilot signal cross-talk component is avoided. At the delayed time t A , that is, when the head H T reaches the point t₁₁, the pilot signal magnitude supplied by the envelope detector 13 to the sample-and-hold circuit 14 is sampled. This pilot signal sample is held at least until the sample-and-hold circuit 15 is operated. In any event, the sample held by the sample-and-hold circuit 14 may be reset prior to the next sampling operation thereof. Now, when the heads H L and H T scan the next track pair, shown as the tracks T₁₃ and T₁₄, a similar operation is carried out with the synchronizing signal detector used to detect the synchronizing signal f₃. That is, when the head H T now reaches the synchronizing signal f₃ in the track T₁₄, as represented by the point t₂₀, the synchronizing signal f₃ is reproduced, passed to the envelope detector 32 by the band-pass filter 31 and compared with a threshold level by the comparator 33. Assuming that the magnitude of the synchronizing signal f₃ exceeds this threshold level, the comparator 33 generates a sampling pulse as illustrated in Figure 5C. At this time, the head H T has not yet reached the vicinity of the region in which the pilot signal f₁ is recorded. Hence, a pilot signal cross-talk component is not yet picked up from the adjacent track T₁₃. Of course, as the head continues its scan of the track T₁₄, this pilot signal cross-talk component will be picked up. The delay circuit 34 delays the sampling pulse produced by the comparator 33 by a time delay t B , as illustrated in Figure 5E. Accordingly, the delayed sampling pulse is produced by the delay circuit 34 when the head H T reaches the point t₂₁ in the track T₁₄, as shown in Figure 3. At this time, the head H T has passed beyond the synchronizing signal f₃ and picks up the pilot signal f₁ as a cross-talk component from the adjacent track T₁₃. It will be seen that, at the point t₂₁, there is no interference between the pilot signal f₁ and the synchronizing signal f₃. This delayed sampling pulse now samples the magnitude of the pilot signal cross-talk component picked up from the adjacent track T₁₃. This sample of the pilot signal, produced when the head H T scans the right adjacent track T₁₄ is now compared with the sample of the pilot signal cross-talk component picked up when the head H T scanned the left adjacent track T₁₂, which sample is stored in the sample-and-hold circuit 14. Any difference between these pilot signal samples is determined by the difference circuit 16 and supplied to the output terminal 18 as a tracking error indication. The sample-and-hold circuit 15 may be reset before the next sampling operation thereof. Thus, a pilot signal sample produced when the head H T scans the track which precedes and is adjacent to the track T₁₃ in which the pilot signal f₁ is recorded is compared with the pilot signal sample produced when the head H T scans the track which follows and is adjacent to the track T₁₃. If the heads exhibit proper track alignment, the magnitudes of these samples will be equal. However, if the alignment of the heads is left-of-centre, the pilot signal sample produced when the following track T₁₄ is scanned will be greater than the pilot signal sample produced when the preceding track T₁₂ is scanned, resulting in a tracking error indication. The tracking error signal produced at the output terminal 18 is supplied to a servo circuit (not shown) which, for example, controls the operation of a capstan motor to transport the tape 2 in a manner to correct this tracking error. Conversely, if the pilot signal sample produced when the head H T scans the preceding track T₁₂ is greater than the pilot signal sample produced when the head H T scans the following track T₁₄, it is concluded that the alignment of the heads is right-of-centre; and the tracking error signal produced by the difference circuit 16 is indicative thereof. The aforementioned servo circuit responds to this tracking error signal to control the tape transport and thereby correct the tracking error. From the foregoing, it will be seen that the detection of the cross-talk pilot signal is not dependent upon the track format, as is the case in the arrangement shown in Figure 2. Hence, the duration of the time period during which the cross-talk pilot signal is picked up and detected is relatively longer than that discussed above in conjunction with Figure 2. Consequently, the possibility of jitter will not prevent proper detection of the pilot signal cross-talk component. In the embodiment discussed with reference to Figures 3 and 4, it has been assumed that the frequencies of the synchronizing signals f₂ and f₃ differ from each other. As an alternative, these frequencies may be equal. Of course, when a synchronizing signal is detected, it must be known whether the track in which this synchronizing signal is recorded precedes or follows the track in which the pilot signal is recorded. But, since the head H L scans the track in which the pilot signal is recorded during the same pass that the head H T scans the following, adjacent track, the determination of whether the head H T is scanning the preceding or following track is easily determined. That is, it is known that the head H T scans a preceding track if the head H L does not reproduce the pilot signal; and it is known that the head H T scans the following track when the head H L reproduces the pilot signal. The configuration of the heads mounted on the drum 1 need not be limited solely to the representation shown in Figure 1. If the drum and tape movements can be controlled sufficiently so as to exhibit accurate scanning, the heads may be mounted 180° apart, provided that the heads are capable of reproducing the synchronizing and pilot signals recorded in the pattern shown in Figure 3. That is, such diametrically opposed heads may be used, whereby one head scans the odd tracks T₁₁, T₁₃, T₁₅, etc. and the other head scans the even tracks T₁₂, T₁₄, T₁₆, etc.

Claims: A method of detecting a tracking error in a playback head (H) which helically scans tracks (T₁₂, T₁₃, T₁₄) across a record medium (2) having a first signal (f₁) recorded in a restricted region in every nth track (T₁₃) where n is at least four, a second signal (f₂) recorded in a restricted region in a track (T₁₂) which precedes, in the head scanning direction of the tape, and is adjacent to each track (T₁₃) in which said first signal (f₁) is recorded, and a third signal (f₃) recorded in a restricted region in a track (T₁₄) which follows, in the head scanning direction of the tape, and is adjacent to each track (T₁₃) in which said first signal (f₁) is recorded, said method comprising the steps of: scanning a preceding track (T₁₂) by said playback head (H) to reproduce the signals recorded therein and cross-talk components picked up from adjacent tracks (T₁₃); detecting said second signal (f₂) reproduced from the scanned preceding track (T₁₂); detecting at a time (t₁₁) delayed from the detection of said second signal (f₂) the amplitude of the first signal (f₁) reproduced by said playback head (H) as a cross-talk component when said playback head (H) scans said preceding track (T₁₂); scanning a following track (T₁₄) by said playback head (H) to reproduce the signals recorded therein and cross-talk components picked up from adjacent tracks (T₁₃); detecting said third signal (f₃) reproduced from the scanned following track (T₁₄); detecting at a time (t₂₁) delayed from the detection of said third signal (f₃) the amplitude of the first signal reproduced by said playback head (H) as a cross-talk component when said playback head (H) scans said following track (T₁₄); and sensing a difference between the detected amplitude of the first signal (f₁) detected when said playback head (H) scans said preceding track (T₁₂) and the detected amplitude of the first signal (f₁) detected when said playback head (H) scans said following track (T₁₄), said sensed difference being indicative of said tracking error. A method according to claim 1 wherein n=4. A method according to claim 1 or claim 2 wherein said first signal is a pilot signal of predetermined frequency and said second and third signals have frequencies different from said first frequency. A method according to claim 3 wherein the frequency of said second signal differs from the frequency of said third signal. A method according to claim 3 wherein the frequency of said second signal is equal to the frequency of said third signal. A method according to any one of the preceding claims wherein each step of detecting comprises sensing the respective frequency of said first, second and third signals. A method according to claim 6 wherein said step of detecting the first signal when said playback head (H) scans said preceding track (T) comprises filtering the frequency of said first signal, generating a first sampling pulse at a predetermined time delay from the sensing of the frequency of said second signal, and using said first sampling pulse to sample the filtered first signal. A method according to claim 7 wherein said step of detecting the first signal when said playback head (H) scans said following track (T) comprises filtering the frequency of said first signal, generating a second sampling pulse at a predetermined time delay from the sensing of the frequency of said third signal, and using said second sampling pulse to sample the filtered first signal. A method according to claim 8 wherein said step of sensing a difference comprises holding samples of the filtered first signal sampled by the respective first and second sampling pulses, and obtaining the difference between the held samples. A method according to claim 9 wherein said step of sensing a difference further comprises providing as an output signal representative of a tracking error the difference between the held samples in response to said second sampling pulse. Apparatus for detecting a tracking error in transducer means (1) which helically scans tracks (T₁₂, T₁₃, T₁₄) across a record medium (2) having a first signal (f₁) recorded in every nth track (T₁₃) where n is at least four, a second signal (f₂) recorded in a track (T₁₂) which precedes, in the head scanning direction of the tape, and is adjacent to each nth track (T₁₃), and a third signal (f₃) recorded in a track (T₁₄) which follows, in the head scanning direction of the tape, and is adjacent to each nth track (T₁₃), said apparatus comprising: second signal detecting means (21) for detecting said second signal (f₂) when said transducer means (1) scans a preceding track (T₁₂); third signal detecting means (31) for detecting said third signal (f₃) when said transducer means (1) scans a following track (T₁₄); first signal detecting means (12) operable when said transducer means (1) scans said preceding track (T) to detect, at a time (t₁₁) delayed from the detection of said second signal (f₂), the amplitude of the first signal (f₁) picked up as a cross-talk component by said transducer means (1), and operable when said transducer means (1) scans said following track (T) to detect, at a time (t₂₁) delayed from the detection of said third signal (f₃), the amplitude of the first signal (f₁) picked up as a cross-talk component by said transducer means (1); and sensing means (16) for sensing a difference between the detected amplitude of the first signal (f₁) detected by said first signal detecting means (12) when said transducer means (1) scans said preceding track (T₁₂) and the detected amplitude of the first signal (f₁) detected by said first signal detecting means (12) when said transducer means (1) scans said following track (T₁₄), said sensed difference being indicative of said tracking error. Apparatus according to claim 11 wherein n=4. Apparatus according to claim 11 or claim 12 wherein said first signal is a pilot signal of predetermined frequency and said second and third signals have frequencies different from said first frequency. Apparatus according to claim 13 wherein the frequency of said second signal differs from the frequency of said third signal. Apparatus according to claim 13 wherein the frequency of said second signal is equal to the frequency of said third signal. Apparatus according to any one of claims 11 to 15 wherein each of said detecting means (12, 21, 31) includes frequency sensing means (12, 21, 31) for sensing the respective frequency of said first, second and third signals. Apparatus according to claim 13 wherein said first signal detecting means (12) comprises filtering means (12) tuned to the frequency of said first signal for filtering said first signal, first sampling pulse generating means (22 to 24) for generating a first sampling pulse at a predetermined time delay from the detection of said second signal, second sampling pulse generating means (32 to 34) for generating a second sampling pulse at a predetermined time delay from the detection of said third signal, and sampling means (14, 15) for sampling the filtered first signal with said first and second sampling pulses. Apparatus according to claim 17 wherein said sampling means (14, 15) comprises first sample-and-hold means (14) responsive to said first sampling pulse for sampling and holding the filtered first signal, and second sample-and-hold means (15) responsive to said second sampling pulse for sampling and holding the filtered first signal. Apparatus according to claim 18 wherein said sensing means (16) comprises difference means (16) coupled to said first and second sample-and-hold means (14, 15) for obtaining a difference between the samples held thereby. Apparatus according to claim 19 wherein each of said second and third signal detecting means (21, 31) includes filter means (21, 31) tuned to a respective frequency of the second and third signals for filtering said respective second or third signal. Apparatus according to claim 20 wherein each of said first and second sampling pulse generating means (22 to 24, 32 to 34) includes envelope detecting means (22, 32) for detecting an envelope of the respective second or third signal, comparator means (23, 33) for generating a sampling pulse if the detected envelope exceeds a predetermined level, and delay means (24, 34) for delaying the generated sampling pulse. Apparatus according to claim 19 wherein said sensing means (16) further includes output switch means (17) responsive to said second sampling pulse for providing said difference between the samples as a tracking error signal. Apparatus according to any one of claims 11 to 22 wherein said transducer means (1) comprises one of a pair of closely spaced playback heads (H) operable to scan two adjacent tracks (T) substantially concurrently. Apparatus according to claim 23 wherein the pair of playback heads (H) have different azimuth angles. Apparatus according to claim 23 or claim 24 wherein said transducer means (1) comprises a rotary head drum (1) rotatable about an axis thereof, and said pair of playback heads (H) are mounted on said drum (1) offset from each other in the axial direction by an amount substantially equal to the pitch of said tracks (T).

Inventors: YANAGIHARA NAOFUMI, YANAGIHARA, NAOFUMI, YANAGIHARA, NAOFUMI, C/O PATENTS DIVISION
Applicants: SONY CORP, SONY CORPORATION
Application year: 1991
Main classification: G11B15
IPC codes: G11B15