Zoned Equalization for a Magnetic Disc

1. Field of the Invention

This invention relates in general to magnetic recording apparatus and, in particular, to apparatus for use in the playback of information recorded on a magnetic disc.

2. Description Relative to the Prior Art

It is well known in the field of magnetic recording to employ what are called equalization circuits. Equalization, as employed in the playback of magnetically recorded information, serves to correct playback inequities which stem from the fact that playback signal processing is, in general, frequency dependent: With a typical inductive playback head that is dependent on recorded signal flux rate of change, the playback voltage-versus-log frequency response curve rises with frequency, only to fall off eventually as the playback wavelength approaches the gap length of the playback magnetic head. Further, because the inductive reactance of the aforenoted head is frequency dependent (i.e. X.sub.L =2.pi.fL), differing signal delays inherently become associated with the various signal components which comprise, say, a recorded pulse that is played back by means of such head; these differing signal delays manifest themselves as phase disparities among the various signal components which comprise the played back pulse. To effect a flattened, or otherwise shaped, voltage-versus-log frequency curve, amplitude equalization is employed, whereby (typically) lower and higher signal frequencies receive more gain than mid-range signal frequencies; to bring the various signal frequency components which comprise a played back signal into proper phase relationship with respect to each other, frequency dependent phase equalization is employed.

In variable speed magnetic tape recorders, it is known to vary the degree of equalization as a function of tape speed: At one tape speed, the equalization would be set one way; at any other tape speed, the equalization would be set another way. Lowman, in his text "Magnetic Recording", McGraw-Hill Book Company, pg. 178, describes such a variable equalization circuit in which "speed gates" select one or another pair of gain and time delay circuits, depending on which of six recorder tape speeds is employed.

Currently, there appears in the literature various disclosures for an electronic still camera in which a magnetic disc is spun at relatively high speed in relation to a record head within the camera. As each picture is "snapped", an encoded video signal is recorded in a respective (or grouping of) circular track(s) about the rotary axis of the disc. The disc, therefore, ultimately comprises a plurality of recorded tracks . . . and these, incidentally, reside in a relatively narrow band around the disc. Typically, there would be upwards of eighty video tracks per disc. During playback of video information recorded on such a disc, a playback head is positioned respective of the track whose encoded picture is to be displayed. (While the invention, as will appear below, is cast in the environment of an 80-track "video" disc, it is not so limited and may, if appropriate, be accommodated to digital and other forms of analog recording.)

Other art which may be germane to the present invention may be found in:

Because the lineal speed of a pre-recorded disc relative to a playback head depends on the radial position of the head with respect to the disc, the present invention calls for equalization that correspondingly varies as a function of the radial position of the playback head. Since the recorded tracks of the aforedescribed disc reside in a relatively narrow band about the disc--and because of the relatively large number of tracks within such band--an additional concept of the invention is to divide the band into head-position zones--each such zone corresponding to a different grouping of recorded tracks--and to employ equalization that is respective of the specified zones. Such a concept is viable because of the close proximity of the adjacent tracks to each other (i.e. there is little difference between the requisite equalizations associated with closely spaced tracks), despite the fact that tracks in different zones may be sufficiently far apart to require different amounts of equalization. Attendant upon such an equalization strategy is the economy of avoiding (in connection with the disc in question), eighty separate equalization arrangements for each of the recorded disc tracks.

The invention will now be described with reference to the figures of which

FIG. 1 illustra1tes head curves which reflect concepts embodied by the invention,

FIG. 2 is a system block diagram of playback apparatus according to the invention, and

FIG. 3 indicates a table illustrating weighting constants for the apparatus of FIG. 2.

Referring to FIG. 1, the voltage-versus-log frequency response curves for a playback head cooperating with an 80-track rotary disc span a range of zones which have been labeled a through h. The innermost track of the disc (lowest lineal speed) exhibits the lowest output, and lowest bandedge frequency; contrarily, the outermost track (greatest lineal speed) exhibits greatest output, and greatest bandedge frequency. Rather than provide respective amplitude and phase equalization for each of the eighty tracks, the invention (as noted above) provides equalization respective of zones . . . not tracks . . . thereby to provide playback uniformity that is substantially independent of the selected-track head position. With the showing of FIG. 1 in mind, then, reference should be had to FIG. 2:

A playback head 10--positionable to one of eighty video tracks of a magnetically recorded disc, depending on the command of a controller 11--applies its output signal to an equalizing circuit 12, and thence to a preamplifier 14. The equalizing circuit 12, which effects both amplitude and phase equalization, constitutes a transversal filter. Although transversal filters can take a variety of forms, a filter such as that described in "Modern Filter Theory and Design", Temes and Mitra, John Wiley and Sons, pg. 508, is indicated for the practice of the invention: The filter comprises a number of discrete signal delay element 16, and an output summing device 18 adapted to receive the "qualified" input and output of the delay elements 16 . . . thereby to form an equalized "sum-signal" version of the signal output of the playback head 10. To modify the filter 12 characteristics, the input signal to, and the output signal from, the delay elements 16 are (before being summed) "qualified" by weighting constants A.sub.o to A.sub.N . . . such constants being respectively applied to multipliers 20 along with the delay element input and output signals. Attendantly, the product signals appearing at the outputs of the multipliers 20 serve (as they are summed) to form an equalized "sum-signal" that is customized according to the weighting constants A.sub.o to A.sub.N.

In accordance with the invention, respective sets of weighting constants A.sup.o to A.sup.7, corresponding to the "zoned" track-related response curves a through h (See FIG. 3), are stored in a memory matrix 22, which may take the form of a read-only-memory (ROM). Depending on the binary count of a 3-bit input to a 1-of-8 decoding circuit 24 (which may take any of a variety of well known forms), one or another of the eight sets (A.sup.o to A.sup.7) of weighting constants is applied to the respective multipliers 20 of the equalizing circuit 12, thereby to customize the equalization provided by the circuit 12 in accordance with the selected set of weighting constants.

The controller 11, which may take the form of a microprocessor such as the TMS 1000, available from Texas Instruments, is functionally indicated in FIG. 2 by the state-of-the-art manner in which such microprocessor is programmed for operation: In general, a person operating a disc player incorporating the invention selects a given track. After an in-processor comparison between the instant track location occupied by the positionable head 10 and the given track, the controller 11 positions the head 10--one way or the other--pursuant to such comparison. Should the selected and instant track locations be the same, the controller produces a 3-bit binary count depending on which of eight 10-track zones contains the selected track . . . and it is this 3-bit count which serves to select (24) the particular set of equalizing circuit weighting constants that corresponds to the selected track. Attendantly, the operation of the equalizing is customized for the selected track.

The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention. While the aforedescribed apparatus and the concept which it embodies are directed to the playback side of a recorder, it is also within the purview of the invention to qualify the record side of a recorder so as to practice the inventon. For example, assuming the aforesaid eighty tracks are bias-recorded, amplitude and phase equalization (respective of zoned tracks) may be impressed upon signals as they are recorded in such tracks.