HELICAL TAPE SCANNER

Abstract: A helical tape scanner (10) in which the stationary lower drum (16) includes a narrow upper ridge (18) which is offset from the axis of the rotatable upper drum (14) in a direction toward the end of scan. The video tape moves in a helical path from the stationary lower drum (16) over the offset upper ridge to the rotatable upper drum (16) which includes the video read/write heads (22) to transfer signals to and from the tape (12).

Description: HELICAL TAPE SCANNER REFERENCE TO PRIOR APPLICATION This is a continuation-in-part application of co-pending United States application serial number 07/717,138, filed on June 18, 1991. BACKGROUND OF THE INVENTION This invention relates generally to magnetic tape recorders and more particularly to a structure providing an improved head-to- tape interface in helical scan rotary magnetic head and cylindrical tape guide assemblies used, for example, in wideband video and data tape record and /or reproduce apparatus. The rotary head segmented scan wideband tape record and/or reproduce apparatus is an exceedingly complex assembly. In helical scan tape record and/or reproduce apparatus of this kind, information is recorded on and reproduced from a magnetic tape by one or more magnetic heads rotated at a high velocity relative to the tape, which is transported at a much lower velocity along a helical path about a cylindrical drum tape guide. The heads and cylindrical tape guide usually are an assembly commonly referred to as a scanning assembly, or simply a scanner. The information is recorded on the tape in discrete parallel tracks that diagonally extend along the tape at an angle, typically of a few degrees relative to the length of the tape, so that the length of a recorded track greatly exceeds the tape width. Rotary head helical scan tape record and /or reproduce apparatus employ a variety of different tape handling apparatus, rotary head handling apparatus and tape guide apparatus. Some use a single head to record and/or reproduce information relative to the tracks on the tape, while others use a plurality of two or more heads to do so. Regardless of the number of heads, usually they are mounted to rotate in a circular path as information is transferred in respect of the tape. The cylindrical drum tape guide is commonly formed of two axially displaced cylindrical drum sections. In a widely used helical scan rotary magnetic head and cylindrical tape guide assembly, the upper drum is mounted for rotation and the lower drum is mounted stationary, with the record and/or reproduce heads mounted to the upper rotatable drum to be rotated thereby. Other helical scan rotary magnetic head and cylindrical tape guide assemblies have stationery upper and lower drum sections, with the rotary heads mounted to a separate rotatable structure, such as a disk or third drum section, for rotation between the stationary upper and lower drum sections. In helical scan tape-record and/or reproduce apparatus, two recognized alternatives of guiding the tape helically about the cylindrical drum tape guide are commonly used, which are generally referred to as the "alpha" and the "omega" wrap apparatus. In the alpha wrap apparatus, the tape is introduced to the helical path about a cylindrical drum tape guide for scanning by a rotary head from one direction relative to the drum guide and is wrapped completely around the drum so that it exists in the opposite direction relative to the drum guide. It is referred to as alpha wrap apparatus for the reason that the helical tape path about the cylindrical drum tape guide generally conforms to the Greek symbol alpha when one views the tape path about the cylindrical drum in a direction along the axis of the cylindrical drum. In omega wrap apparatus, the tape is introduced to the helical path about the cylindrical drum tape guide along a path extending towards the drum in a direction generally radially to the cylindrical drum, passes around an entrance guide that directs the tape onto the surface of the drum, helically extends about the drum, and passes around an exit guide to be directed thereby away from the drum in a direction generally radial thereto. The tape path about the cylindrical drum tape guide generally conforms to the shape of the Greek symbol omega when it is viewed in a direction extending along the axis of the cylindrical drum. In all of these helical scan magnetic tape record and/or reproduce apparatus, the tape is directed about the cylindrical drum tape guide along a helical path, with the tape exiting the path about the drum tape guide at a different position axially displaced along the drum surface relative to its position of entry to the path. In some of these apparatus, the tape follows a full-wrap helical path about the cylindrical tape guide of near 360°. In other of these apparatus, the tape follows a path of much less than 360°, a common path being near 180° called "half-wrap". These helical scan magnetic tape record and/or reproduce apparatus have provided a practical means of achieving the high head-to-tape speeds necessary for recording and reproducing wideband data, such as television video information. - 2- TiTϋTE SHEET The parameter of the rotary head helical scan tape record and/or reproduce apparatus critical to high quality wideband signal transfers is the head- to-tape interface. This is particularly so in such apparatus for tape record formats having narrow tracks of recorded information, either with narrow or no record- free space or guard bands between them, recorded and reproduced at high relative head-to -tape speeds, such as in digital video tape record and /or reproduce apparatus constructed according to either of the well known Dl or D-2 formats. The head-to-tape interface is determined to a considerable extent by the point of contact between the head and the tape, which must be of an optimum configuration for optimum signal transfers. Optimum head-to-tape interface is achieved by establishing a perfectly uniform and symmetric head tip penetration relative to the tape surface over the entire scan of the tape by the head, while the tape is guided accurately and stably about the cylindrical tape guide. More particularly, the tip of the head where signal transfers occur must engage the tape in a manner which maintains a consistent degree of tip penetration relative to the tape along the entire scan of tape to optimize the quality of the signal transfer. Unfortunately, the dynamics of guiding flexible tape about a cylindrical tape guide and rotating a head in contact with the guided flexible tape have prevented achieving perfectly uniform and symmetric head tip penetration along the entire scan of the tape. As is known, in rotary head helical scan tape record and /or reproduce apparatus the head tip engages the tape such that a slight lift or "tent" is created in the tape at the point of contact with the head tip. It is desirable to support the tape at the head tip so that the profile of the tent remains uniform and symmetrical at the head tip throughout the scan of the tape by the head and so that the head tip engages the tent of the tape throughout the scan of the tape by the head . Too much lift or "tenting," such that the apex of the tape tent remains away from the portion of the tip of the head where signal transfers occur, results in a loss of signal output to or from the head, and a concomitant drop in the output signal-to-noise ratio (S/N). Changes in the configuration of the tent at the head tip also leads to a loss in signal output and drop in output signal-to- noise ratio. Typically, rotary head helical scan tape record and/ or reproduce apparatus are arranged to generate an air film or bearing between the surface of the cylindrical drum tape guide and the tape, which supports the tape off the - 3 ' surface of the tape guide to facilitate the control of the transport of the tape about the tape guide. In the more common rotary head helical scan tape record and/or reproduce apparatus, the air film is created by an arrangement of upper rotating and lower stationary cylindrical drum tape guide sections. The upper rotating drum section generates an air flow which is drawn between the surfaces of the two drum sections and the tape transported thereabout to form the air film that supports the tape over the surface of the tape guide. If the thickness of the air film is uniform across the width of the tape and over the length of tape supported over the surface of the tape guide, a uniform space is maintained between the tape and the surface of the tape guide. With such an air film, the tape tent profile will be uniform and symmetrical at the head tip throughout the head scan with the head tip engaging the tent in the desired manner. In practice, however, an air film of perfectly uniform thickness is not established throughout the head scan. Commonly, the air film between the (_y-indrical drum tape guide and the tape in the region near the the location at which the head enters the scan of the tape is thicker than it is in the region near the location at which the head exits the scan of the tape . This non-uniformity of air film thickness is caused principally by the loss of air from the air film established between the tape and surface of the cylindrical drum tape guide. Several other factors can lead to a non-uniform air film thickness, such as variations in tape tension, variations in pressure at the gap between the axially displaced sections of the cylindrical drum tape guide and at the windows provided in the tape guide for the magnetic heads, and variation in the distance from the head to the tape edge over the scan of the tape by the head. Such variations singly or in combination contribute to changes in the degree of tip penetration relative to the tape as the head scans the tape, which results in an undesirable non-uniform tape tent profile. The composite tape tent profile obtained over the scan of the tape has a relatively thick air film and consequent low head tip penetration in the region near the location at which the head enters ihe scan of the tape and a relatively thin air film and consequently greater head tip penetration in the region near the location at which the head exits the scan of the tape. This variation in the thickness of the air film and concomitant variation in head tip penetration results in changes in the contour of the tape tent profile at the head-to-tape interface, which contribute significantly to losses in signal output and degradation of signal-to-noise ratio. -4- SUBSTiTUTE SHEET While head tip penetration was less important in helical scan tape record and /or reproduce apparatus designed for analog signals, the newer helical scan tape record and/or reproduce apparatus designed for digital signals require higher data rates and relative head-to-tape speeds to record more data per unit area of tape, and use thinner tapes for storage of more data on a reel of tape. With relatively thick tapes, moderate tape speed and slow head rotation speeds found in properly designed record and/or reproduce apparatus for analog signals, there is no significant deterioration in signal output and degradation of signal-to- noise ratio due to the presence of a variation in the thickness of the air film and resultant variation in head tip penetration that alters the contour of the tape tent profile at the head-to-tape interface. Moreover, after a "wear-in" period for the head and tape, it is found that the signal output for such record and/or reproduce apparatus actually improve for the "used" tape. There is, however, no corresponding resultant improvement experienced with the thinner tapes and the newer, faster record and/or reproduce apparatus designed for digital signals, such as digital video tape recorders constructed according to either of the well known Dl or D-2 formats. While "wear-in" still occurs in these newer, faster record and /or reproduce apparatus, the resultant change in tip penetration relative to the position of the head in the scan of the tape can produce over time a decreasing taper in the signal output level along a scan of the tape from a maximum signal level at the head scan position near the location at which the head enters the scan of the tape to a minimum at the head scan position near the location at which the head exits the scan of the tape. To achieve a more uniform and symmetric head tip penetration relative to the tape surface over the entire scan of the tape by the head, while the tape is guided accurately and with stability about the cylindrical tape guide, some rotary head helical scan tape record and/or reproduce apparatus have employed a magnetic head and cylindrical tape guide assembly having non-coaxial stationary and rotable cylindrical drum tape guide sections, with the axis of the stationary section displaced a small amount relative to the axis of the rotatable section in a direction towards the location at which the head exits the scan of the tape. For * e precise tape guidance required in aforedescribed record and /or reproduce pparatus for digital signals, however, it has not been possible to displace the stationary guide section a sufficient amount to remove all undesirable decreasing taper in the signal output level without losing stability of tape guidance about the cylindrical drum tape guide. When the displacement of the stationary guide - 5 - section becomes too large, the radial distance to the tape extending about the stationary guide section becomes larger than the radial distance to the tape extending about the rotatable section over a substantial length of the scan of the tape by the head. This creates forces that act on the tape as it is transported about the cylindrical drum tape guide, which drive the tape axially along the tape guide away from the tape edge guiding elements commonly provided on the stationary cylindrical drum tape guide section. There exists, therefore, a significant need for a helical scan rotary magnetic head and cylindrical tape guide assembly which establishes a more uniform and symmetric head tip penetration relative to the tape surface over the entire scan of the tape by the head without risking the loss of stability and accuracy of tape guidance about the cylindrical tape guide. The present invention fulfills these needs and provides further related advantages. SUMMARY OF THE INVENTION In accordance with the present invention, a helical scan rotary magnetic head and cylindrical tape guide assembly of a kind having a magnetic head mounted for rotation juxtaposed a stationary cylindrical drum tape guide is provided having a structure for improving the head-to-tape interface. The stationary cylindrical drum tape guide is constructed to have at least two non- coaxial cylindrical segments. The segment closest to the juxtaposed rotary magnetic head has its axis displaced relative to the axis of the other segment (or segments) a small amount generally towards the location at which the rotary head exits the scan of the tape, and extends along its axis for a length which is smaller than the length of the other non-coaxial cylindrical segment (or segments) of the stationary tape guide section. Hence, the offset segment projects outward from the axis of rotation of the head a greater distance at the location where the rotary head exits the scan of the tape than elsewhere about the locus of rotation of the head. As previously described, the air film established between the cylindrical drum tape guide and the tape helically transported about the guide is normally thicker in the region where the head enters the scan of the tape and gradually decreases to its thinnest dimension in the region where the head exits the scan of the tape. The drum segment acts locally near the tip of the rotary head as the head scans the tape to aid in supporting the tape where of the air film thickness decreases at a greater elevation above the cylindrical drum tape guide than the air film would do alone. In accordance with the present - 6 - SUBSTITUTE SHEET invention, however, the amount and direction of the offset of the axes of the drum segments and the relative axial lengths of the drum segments are selected to maintain, at least locally near the tip of the rotary head, a constant average elevation of the tape over the surface cylindrical tape guide, and thereby maintain the depth of head penetration at a constant level relative to the tape over the entire scan. Moreover, these parameters are selected to maintain the desired downward force on the tape as it is transported about the cylindrical drum tape guide to bias the tape against the tape edge guide elements typically provided on the cylindrical drum tape guide. In one preferred embodiment, the stationary cylindrical tape guide section is also offset relative to the axis of head rotation in a direction to counteract any unwanted tape guiding instability introduced by the offset of the cylinrical drum segment juxtaposed the rotary magnetic head. This offset further enhances the guiding of the tape about the cylindrical drum tape guide by contributing to the maintenance of the downward force on the tape that biases it against the tape edge guide elements . Other features and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the invention. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a front perspective view of a preferred embodiment of the helical scan rotary magnetic head and cylindrical tape guide assembly of the present invention; FIG. 2 is a schematic view of the portion of the tape helically wrapped about assembly of FIG. 1; FIG. 3 is a top plan view of a prior art helical scan rotary magnetic head and cylindrical tape guide assembly, illustrating the typical separation between the tape 20' and the surface 25' of the assembly 10 * in an exaggerated depiction for sake clarity; FIG. 4A is a vertical section of the prior art assembly of FIG. 3 taken at a location near the point 26' at which the head enters the scan of the tape 20' helically transported about the assembly 10'; FIG.4B is an expanded view of the head and tape of FIG.4A; FIG. 5A is a vertical section of the prior art assembly of FIG. 3 taken at a location near the midpoint 21' of the head's scan of the tape 20' helically transported about the assembly 10' ; FIG. 5B is an expanded view of the head and tape of FIG. 5A; FIG. 6A is a vertical section of the prior art assembly of FIG. 3A taken at a location near the point 28' at which the head exits the scan of the tape 20' helically transported about the assembly 10'; FIG. 6B is an expanded view of the head and tape of FIG. 6A; FIG. 7 is a top plan view of an embodiment of the helical scan rotary magnetic head and cylindrical tape guide assembly of the present invention; FIG. 8 is a vertical section taken generally on the line 8-8 of FIG. 7; FIG. 9 is a vertical section taken generally on the line 9-9 of FIG. 7; and FIG. 10 is, a top plan view of another embodiment of the helical scan rotary magnetic head and cylindrical tape guide assembly of the present invention. DETAILED DESCRIPTION In the following description of embodiments of the present invention, like elements will be designated by like reference numerals, and the description of similar elements may not be repeated with reference to subsequent drawing figures related to the various embodiments of the invention. Broadly stated, the present invention is directed to a rotary magnetic head and cylindrical tape guide assembly for helical scan magnetic tape record and/or reproduce apparatus for transferring information signals with respect to a magnetic tape record medium, which includes a stationary cylindrical tape guide section of a design that establishes a head-to-tape interface that provides - 8 - SUBSTITUTE SHEET throughout the scan of the tape by the head a more uniform relative tip penetration of the rotary magnetic head with respect to the average elevation of the magnetic tape over the surface of the cylindrical tape guide in the proximity of the rotary magnetic head. The rotary magnetic head and cylindrical tape guide assembly can take various forms, which determine the particular design of the stationary cylindrical tape guide section needed to establish the head-to-tape interface required to obtain the desired uniform relative head tip penetration. As will be appreciated from the foregoing and the following detailed description of preferred embodiments of the present invention, the apparatus of the present invention can be implemented in record and/or reproduce arrangements employing either alpha or omega wrap tape guiding configurations and utilizing either full-wrap or half-wrap arrangements. Furthermore, the apparatus of the present invention is adaptable to any helical scan rotary magnetic head and cylindrical tape guide assembly having a stationary cylindrical drum tape guide located proximate the rotary magnetic head, such as those previously described herein, whether the rotary magnetic head is supported by rotatable cylindrical drum tape guide or independent of the cylindrical drum tape guide. Moreover, it should also be understood that the apparatus of the present invention is adaptable to helical scan magnetic tape record and/or reproduce arrangements having one or more ihe rotary heads, and in which the rotary heads are rotatable in either rotational direction, the tape is introduced into the helical path about the rotating heads either above or below the exit point and the tape is transported about the helical path in either direction. The relationships of head rotation, tape transport direction and manner of tape guiding relative to the helical path represents a variety of different configurational relationships of which only one will be specifically described herein. In general, and with reference to FIG. 1, a rotary magnetic head and cylindrical tape guide assembly 10 for transferring information signals with respect to a magnetic tape record medium 12 in accordance with the present invention includes a rotary magnetic head 22 disposed for magnetic coupling with the tape record medium 12 transported in a helical path about a cylindrical drum tape guide 15, as information signals are transferred between the head and the tape. The preferred embodiments described herein are in the form of omega half-wrap tape guiding configurations, such as employed in rotary head helical scan tape record and/or reproduce apparatus constructed according to the well known D-2 format. In such configurations, the tape 12 is introduced to the helical path about the cylindrical drum tape guide 15 along a path extending towards the drum guide 15 in a direction indicated by arrow 11 mat is generally radial to the cylindrical drum tape guide, is directed therefrom onto the surface of the drum guide 15 by an entrance guide 34, helically extends about ihe drum guide 15, and is directed away from the drum guide 15 by an exit guide 36 in a direction generally radial thereto. In the described embodiments, the tape enters the helical path about the cylindrical drum tape guide surface at a location axially displaced along the drum surface below the location at which the tape exits the helical path, and the rotary heads 22 rotate in a direction indicated by arrow 13 opposite the normal direction 11 of transport of the tape 12 about the cylindrical drum tape guide 15. The heads 22 record and/or reproduce information signals relative to discrete parallel tracks 24 that diagonally extend along the tape at an angle, a, (FIG. 2) relative to the longitudinal direction of the tape. For a given helical scan magnetic tape record and reproduce apparatus construction, the angular orientation of the recorded tracks is a function of the velocity of transport of the tape about the cylindrical drum tape guide, ihe speed of rotation of the rotary head and the direction of tape transport about the drum guide relative to the direction of rotation of the rotary head, as well as the angle relative to the plane of rotation of the rotary head at which the tape enters the helical path about the cylindrical drum tape guide. The angle at which the tape 12 enters the helical path, the direction 11 of tape transport about the cylindrical drum tape guide 15, the direction 13 of head rotation and the relative head-to- tape speed determine the length of the tape 12 occupied by each recorded track and the spacing or pitch, between adjacent tracks. In most helical scan magnetic tape record and reproduce apparatus, including the preferred embodiments described herein, the rotary head 22 is carried by the cylindrical drum tape guide 15, which in turn is formed by two axially displaced cylindrical sections 14 and 16, one of which (usually the upper most section 14) rotates while the other section 16 remains stationary. Typically, the heads are secured to the upper rotatable cylindrical drum tape guide section 14 to project through scanner windows 20 provided in the rotatable drum section to permit the rotary heads to engage the tape as it is helically transported about the helical scan rotary magnetic head and cylindrical tape guide assembly 10. In rotary head helical scan tape record and/or reproduce apparatus employing such tape guide assemblies, the heads scan from the bottom of tape 12 toward the top, i.e., the scan begins where the tape 12 exits the helical path about the cylindrical - 10 - SUBSTITUTE SHEET tape guide assembly 10 over the upper drum 14 and ends where the tape 12 enters the helical path about the cylindrical drum tape guide assembly over the lower stationary drum 16. The resulting track 24 that a head 22 scans along the tape 12 is shown in a dashed line in FIGS. 1 and 2, starting at the head entry point 26 (the intersection of the scanned track 24 with the tape exit tangent line 30) and extending to the head exit point 28 (the intersection of scanned track 24 with the tape entry tangent line 32). The tape 12 is guided onto and off of the cylindrical drum tape guide assembly 10 by means of an entry guidepin 34 and an exit guidepin 36, both mounted to a stationary support plate (not shown) that also supports the scanner 10. The plane from which the tape enters the helical path about the drum assembly 10 is identified by numeral 38, while the plane onto which the tape leaves or exits the helical path is identified by numeral 40. As the tape 12 moves along the helical path about the cylindrical drum tape guide assembly 10, the upper drum tape guide section 14 rotates counter to the movement of tape 12 so that the heads 22 carried by the rotating drum 14 scan the tape to record and/or reproduce information in respect of tracks along the tape. As described hereinbefore, the relative movement of tape 12 and heads 22 carried by the upper drum section 14 serves to generate a hydrodynamic air film between the tape 12 and the surfaces 25a, 25b and 25c of the cylindrical drum tape guide sections 14 and 16 forming ihe cylindrical drum tape guide assembly 10. The hydrodynamic air film supports the tape 12 spaced from and over the surface of the cylindrical drum tape guide assembly 10. An exemplary prior art cylindrical drum tape guide assembly 10' of this type is illustrated in FIGS. 3 - 6, but without the offset drum segments 18 and 19 forming the drum section 16. As previously described, prior art cylindrical drum tape guide assemblies of this type create air films of non-uniform and unsymmetrical thickness between the tape 12' and the surface 25' of the drum guide assembly 10', with the thickest air film at the head entry point 26'. The thinnest air film occurs at the head exit point 28'. A better appreciation of the improvement resulting from the apparatus of this invention is obtained through a better understanding of the head-to-tape interface problem caused by the varying thickness of the air film established between the tape 12' and cylindrical drum tape guide sections 14' and 16'. FIGS. 4A/4B, 5A/5B and 6A/6B depict the poor head tip penetration characteristic of the head-to-tape interface provided by the prior art cylindrical - 11 - drum tape guide assembly of FIG. 3 from ihe start of the scan of the tape 12' by a head 22' (FIGS. 4A/4B) to the end of the tape scan by the head (FIGS. 6A/6B). In the embodiment illustrated in FIG. 4, the magnetic heads 22' has a non-magnetic gap at its tip 23' that effects the recording and/or reproducing of information relative to the tape 12'. The non-magnetic gap is defined by opposing, spaced apart faces of a pair of pole members, which in FIGS. 4 - 6 are aligned with the pole faces lying in planes parallel to the plane of FIGS. 4 - 6 and spaced apart in the direction of view of the figures. Hence, the width of the non-magnetic gap at the tip 23' of the head 22', which defines the width of the active transducing region 23a' of the head 22' and the width of the tracks of information along the tape 12' lies in the plane of the tape and extends horizontally in FIGS. 4 - 6. Referring to FIG.4A, at the start of ihe scan of the tape 12' by a head 22', tip 23' of head 22' creates a slight lift or tent in tape 12' at the point where tape 12' passes over head tip 23'. FIG. 4B is an enlarged view of the region of head tip 23' and tape 12' shown in FIG. 4A showing tape 12' lying against the active transducing region 23a' of head 22'. At the start of ihe scan, shown by FIG. 4B, tape 12' has little contour, because ihe thickness of the air film established between the tape 12' and the surface of the scanner 10' provides the desired support of the tape, and the head creates only a slight lift to the tape. Consequently, most of the active transducing region 23a' of the head 22' is contacted by the tape 12'. Referring to FIG. 5A, which illustrates ihe head-to-tape interface at the approximate midpoint 21' of the scan of the tape 12' by the head 22', more lifting of the tape by the head results because the thickness of the air film between tape 12' and drum 14' is reduced by the various previously described effects. The result of the increase in tape lifting by the head is seen in FIG. 5B, which is an enlarged view of head tip 23' and tape 12' shown in FIG. 5A. As the lifting of ihe tape by the head increases, lateral curvature of tape 12 relative to the head tip 23' occurs. And, at or near the end of the scan, shown by FIG. 6A, even more lifting results from the further reduction in the thickness of the air film between tape 12' and the stationary drum 14'. The result of the increase in lifting is seen in FIG. 6B which is an enlarged view of head tip 23' and tape 12' shown in FIG. 6A. At this point in the tape path about the rotary magnetic head and cylindrical tape guide assembly 10', the lateral curvature of tape 12' increases to the point where a physical gap is created between tip 23' and tape 12' at the very center of the width of the head 22'. The loss of contact that 12 - SUBST-TUTE SHEET results from this gap causes a corresponding drop of output signal level and signal-to-noise ratio. In the prior art helical scan rotary magnetic head and cylindrical tape guide assembly 10' illustrated in FIGS. 3 - 6, it is seen that the two cylindrical drum tape guide sections 14' and 16' are coaxially aligned, with the upper rotatable drum section 14' having a larger radius than the lower stationary cylindrical drum section 16'. This construction of the two cylindrical drum sections creates the desired downward forces that act on the tape 12' as it is transported about assembly 10' to bias the tape 12' against the tape edge guide band 17' extending helically about the perimeter of the lower stationary cylindrical drum section 16', whereby the tape 12' is transported about the assembly 10' with required precise guidance. As previously described, some rotary head helical scan tape record and/or reproduce apparatus have employed a magnetic head and cylindrical tape guide assembly having non-coaxial stationary and rotable cylindrical drum tape guide sections, with ihe axis of the stationary section displaced a small amount relative to the axis of the rotatable section in a direction towards ihe location at which the head exits the scan of the tape, in an attempt to reduce the drop in the signal output level to an acceptable level, without losing stability of tape guidance about ihe cylindrical drum tape guide. However, as previously discussed, this has not proved successful, particularly, in rotary head helical scan tape record and/or reproduce apparatus for digital signals. The improvement in the scanner structure to ameliorate this degradation of head-to-tape interface is depicted in FIGS. 7 - 10. FIGS. 8 and 9 depict the configuration assumed by the tape 12 over an upper rotatable drum section 14 and stationary lower drum section 16, which includes two non-coaxial segments 18 an 19, as the tape 12 is transported about the rotary magnetic head and cylindrical tape guide assembly 10. FIG. 8 illustrates the tape configuration near the start of scan of the tape 12 by head 22, i.e., close to the point otherwise referred to as the head entry point 26 located at the tape exit tangent line 30 (FIG. 1). FIG. 9 illustrates the tape configuration near the end of scan of the tape 12 by head 22, i.e., close to head exit point 28 located at the exit tangent line 32 (FIG. 1). At the location where the head 22 starts the scan of the tape 12, most of tape overlaps the upper rotating drum section 14 while a small portion of the tape overlaps the upper segment 18 of the lower stationary drum section 16 at the - 13 - edge of stationary lower drum section proximate the rotary head 22. In this embodiment, the rotating head is fastened to the upper rotatable drum section 14 for rotation by the rotatable drum. Although both portions of the tape that are over the drum sections 14 and 16 are separated by an air film, the portion of tape 12 over upper rotatable drum sectiion 14 is displaced a greater distance from the surface 25a of the drum section 14 as a result of the thicker air film developed in this region. As the head 22 commences its scan of the tape 12, tip 23 of head 22 that projects through the drum window 20 beyond the surface 25a of the drum section 14 creates a slight lift or tent in tape 12 at the interface between the tape 12 and the head tip 23. This lift or tent results in an average radial penetration "Rs(ave)" of head tip 23 into tape 12 of ϊ^ .-aiϊfi wte, Rsi = the relative penetration of the head tip 23 into the tape 12 on the upper drum section 14 side of head 22 at the start of the scan of the tape 12, and Rs2 = the relative penetration of the head tip 23 into the tape 12 on ihe lower drum section 16 side of head 22 at the start of the scan of thetape 12. The slight depression or notch on the upper drum section 14 side of ihe head 22 is caused by the loss of air from the air film by bleeding through ihe window 20 provided in the upper rotatable drum 14 for the head 22. As the air film between the tape 12 and the cylindrical drum tape guide assembly 10 dissipates towards the end of the head's scan of the tape 12, as shown in FIG. 9, this notch in the tape also dissipates. As the head 22 proceeds through the scan of the tape 12, the thick air film between upper rotatable drum section 14 and tape 12 diminishes significantly along the scan as the result of leakage of air from the air film, such that at the end of the scan of the tape 12 by the head 22, as shown in FIG. 9, tape 12 is displaced from the surface 25a of the upper rotatable drum 14 a much lesser distance than at the start of the head's scan of the tape. Also, the axial location of the tape 12 relative to the surface 25a, 25b and 25c defined by the two cylindrical drum tape guide sections 14 and 16 shifts due to the helical path followed by the tape about the cylindrical drum tape guide assembly 10 to reside mostly -over the - 14- SUBSTITUTE SHEET lower stationary drum section 16 and only slightly -on over the upper rotatable drum section 14. Thus, at the end of the scan of the tapel2 by the head 22, head tip 23 lifts tape 12 a greater distance on the upper drum section 14 side of head 22 than at the start of scan (i.e., REI > Rsi)- At the mid-point 21 of the scan of the tape by the head 22, the head tip 23 lifts tape 12 about the same distance on both sides of the head 22. With, however, upper segment 18 of the stationary lower drum section 16 displaced or offset from the centerline (axis "A") of the coaxial upper drum section 14 and lower drum segment 19 of the drum section 16 in the region at which the head 22 ends its scan of the tape 12 , tape 12 is maintained radially farther out from the axis "A". As a result, tape 12 is held radially outward by upper drum segment 18 so that the lift on the lower drum-section 16 is not as great as at the start of ihe head's scan of the tape 12, (i.e., RE2 < Rs2)- In this manner, the radial penetration of head tip 23 "RE(ave)" mto t P e 2 at the end of the tape scan will be substantially equal to the "Rs(ave)" radial penetration of head tip 23 at the start of the tape scan [i.e., RE(ave) ® RS(ave)]- Furthermore, the offset drum segment 18 has the beneficial effect of maintaining the lateral curvature of tent in the tape 12 at the head tip 23 approximately the same at the start of the head's scan of the tape 12 as it is at the end of the scan of the tape 12, as shown in FIGS. 8 and 9. The relationship of the axes and relative dimensions of upper rotatable drum section 14, the lower segment 19 of the lower stationary drum section 16, and the upper segment 18 of the lower stationary drum section 16 is best illustrated in FIG. 7. Point "A" identifies the centerline or axis for ihe coaxial rotatable upper drum section 14 and the lower segment 19 of the lower stationary drum 19, while point "B" is the centerline or axis for the offset upper segment 18 of the lower drum section 16. For stability and accuracy of guidance of the tabe about the cylindrical tape guide assembly 10, the diameter of the upper rotatable drum section 14 is greater than the diameter of lower segment 19 of the lower stationary drum section 16. For purposes of establishing the angular relationships, the head scan entry point 26 at tape exit tangent line 30 is chosen as zero degrees. Relative to this entry point 26, the head scan exit point 28 at the tape entry tangent line 32 is displaced in a clockwise direction an angle "B" of approximately 188°. The centerline or axis of the offset upper segment 18 of the lower drum section 16 is displaced a distance "X" at a clockwise angle of "c" from the head scan entry point 26. - 15 - The upper drum segment 18 of the stationary drum section 16 closest to the juxtaposed rotary magnetic head 22 extends along its axis for a length which is smaller than the length of the other non-coaxial lower drum segment 19 of the stationary drum tape guide section 16, and forms a small ridge member about the stationary drum tape guide section. The two drum segments 18 and 19 can be integral members forming a single piece stationary drum tape guide section 16. Alternatively, the two drum segments 18 and 19 can be separate pieces secured together by a suitable fastening device. In any event, ihe drum segment 18 acts locally near the tip 23 of the rotary head 22 as ihe head scans the tape 12 where the air film thickness decreases to aid in supporting ihe tape at a greater elevation above the cylindrical drum tape guide 10 than the air film would do alone. In accordance with the present invention, the amount and direction of the offset of the axes of the drum segments 18 and 19 and the relative axial lengths of the drum segments are selected to maintain, at least locally near the tip 23 of the rotary head 22, a constant average elevation of the tape over the surface cylindrical drum tape guide 10, and thereby maintain the depth of head penetration at an constant average level relative to the tape over the entire scan. Moreover, these parameters are selected to maintain the desired downward force on the tape 12 as it is transported about the cylindrical drum tape guide 10 to bias the tape against ihe tape edge guide band 17 provided on the stationary cylindrical drum tape guide section 16. For clarity of illustration in ihe drawings, the differences in diameters between upper rotatable drum 14 and the lower drum segment 19 of the lower stationary drum section 16 and the offset or displacement of the axis "B" for ridge member 18 of the drum section 16 are greatly exaggerated. In actual practice of one preferred embodiment constructed in accordance with the present invention, the offset or displacement dimension "X" was on the order of about 1.7 millimeters (mm) and the lower drum ridge member 18 had a diameter on the order of about 2.4 mm greater than the diameter of the lower segment 19 of the lower stationary drum section 16. The angle "g" is on the order of between 140° to 170° (in one embodiment, about 159°) such that the offset of the ridge member 18 is generally in front of the head exit point 28 at the end of head's scan of the tape 12. The width of the offset upper ridge 18 is preferably on the order of 1% to 15% of the width of tape 12. - 16 - SUBSTITUTE SHEET In the alternative embodiment of the apparatus of the present invention illustrated in FIG. 10, upper rotatable drum section 14" and the lower drum segment 19" of the lower stationary drum 16" are not coaxial, but are instead axially displaced. Point "A" represents the centerline or axis of rotation of the upper rotatable drum 14", with lower ridge member 18" displaced generally in the same manner as in the embodiment of FIG. 7, namely a distance "X" along a radial line at an angle "g" in a clockwise direction from head entry point 26". The centerline or axis "C" of the lower segment 19 of the lower stationary drum section 16' is, however, displaced a distance "Y" along a radial line an angle "v" clockwise from the head entry tangent line 30". The angle "v" is normally approximately 230° with the distance "Y" on the same order as the distance "X". As has been described, ridge member 18" acts locally near head tip 23" at the end of scan of the tape 12 to help hold the tape 12 up where the air film has decreased and thereby provides a more constant average effective tip penetration throughout the tape scan and eliminates the output signal taper between the start and end of the tape scan. Since lower drum segment 19" of the lower drum section 16" is slightly smaller in diameter than either rotatable upper drum section 14 or the offset upper ridge member 18", and also is much wider than upper ridge 18", the area of tape 12" in contact with lower drum 16" is larger than the area of tape 12" in contact with ridge 18". Thus, the net guiding forces are downward throughout the scan. Simply offsetting the axis of stationary lower drum 16" from the axis of rotatable upper drum 14" without stationary lower drum 16" having the offset ridge member 18" will result in a substantial region of scan where the effective diameter (size plus air film) of lower drum 16" will be larger than the effective diameter of upper rotatable drum 14". As described previously, this condition can produce forces to undesirably drive tape 12" upward away from the edge guiding elements. Thus, while the offset ridge member 18" on stationary lower drum section 16" can be utilized with either coaxial or non-coaxial upper and lower drums 14" and 16", non-coaxial or offset upper and lower drums 14" and 16" are preform more satisfactorily with the offset ridge member 18" on stationary lower drum section 16". A wide variety of modifications and improvements to the helical tape scanner described herein are believed to be apparent to those skilled in the - 17 art. For example although the invention has been described with respect to a video recorder, the invention has equal utility and applicability to a data recorder. Accordingly, no limitation on the present invention is intended by way of the description herein, and while the invention has been described with particular reference to preferred embodiments, the protection sought is to be limited only by the terms of the claims which follow. - 18 - SUBSTITUTE SHEET

Claims: What is claimed is: 1. A scanner assembly for a magnetic tape recorder having a helical path for a magnetic tape, comprising: a stationary lower cylindrical drum disposed on a central axis of the scanner assembly and having an upper cylindrical ridge; a rotatable upper cylindrical drum mounted above the stationary lower cylindrical drum, said upper cylindrical drum having at least one magnetic head mounted at the periphery of the rotatable upper cylindrical drum, said head having a tip projecting from the rotatable upper cylindrical drum for reading/ writing to magnetic tape wrapped onto said upper cylindrical drum; and means to define a helical path for magnetic tape from said stationary lower cylindrical drum to said rotatable upper cylindrical drum, said helical path for said tape having a magnetic head entrance region at the beginning of scan and a magnetic head exit region at the end of the scan; said cylindrical ridge being disposed in proximity to the rotatable upper cylindrical drum on an axis offset from ihe central axis toward the end of the scan such that the upper cylindrical ridge projects outward from the periphery of said stationary lower cylindrical drum for at least the end of the scan. 2. The scanner assembly of claim 1 wherein said rotatable upper cylindrical drum rotates about said central axis of said stationary lower cylindrical drum. 3. The scanner assembly of claim 1 wherein said rotatable upper cylindrical drum rotates about an axis displaced from the central axis of said stationary lower cylindrical drum. 4. The scanner assembly of claim 3 wherein the displacement of the axis of said rotatable upper cylindrical drum is away from the area of scan. 5. The scanner assembly of claim 2 wherein the axis of said upper cylindrical ridge is offset at a clockwise angle of between 150° and 160° from the beginning of scan. 19 - 6. The scanner assembly of claim 5 wherein ihe axis of said upper cylindrical ridge is offset at a clockwise angle of about 159° from the beginning of scan. 7. The scanner assembly of claim 6 wherein the offset of the axis of said upper cylindrical ridge is generally 0.00066 inches (0.001676 cm) and ihe diameter of the upper cylindrical ridge is generally 0.00095 inches (0.00241 cm) greater than the diameter of said stationary lower cylindrical drum. 8. The scanner assembly of claim 2 wherein the width of said upper cylindrical ridge is between 1% and 15% of the width of said tape. 9. The scanner assembly of claim 8 wherein the width of said upper cylindrical ridge is generally 0.050 inches (0.127 cm). 10. The scanner assembly of claim 4 wherein the axis of said upper cylindrical ridge is offset at a clockwise angle of between 140° and 170° from the beginning of scan. 11. The scanner assembly of claim 10 wherein the axis of said upper cylindrical ridge is offset at a clockwise angle of about 159° from ihe beginning of scan. 12. The scanner assembly of claim 11 wherein ihe offset of the axis of said upper cylindrical ridge is offset generally 0.00066 inches (0.001676 cm) and the diameter of the upper cylindrical ridge is generally 0.00095 inches (0.00241 cm) greater than the diameter of said stationary lower cylindrical drum. 13. The scanner assembly of claim 4 wherein ihe width of said upper cylindrical ridge is between 1% and 15% of the width of said tape. 14. The scanner assembly of claim 13 wherein the width of said upper cylindrical ridge is generally 0.050 inches. 15. A scanner assembly for a magnetic tape recorder having a helical path for a magnetic tape, comprising: -20 - SUBSTITUTE S a stationary lower cylindrical drum disposed on a central axis of the scanner assembly and having an upper cylindrical ridge; a rotatable upper cylindrical drum mounted above the stationary lower cylindrical drum and having a greater diameter than said stationary lower cylindrical drum; means to define a helical path for said magnetic tape from said stationary lower cylindrical drum to said rotatable upper cylindrical drum; and a plurality of magnetic heads mounted at the periphery of the rotatable upper cylindrical drum for reading and writing to the magnetic tape with each head having a tip radially projecting from the rotatable upper cylindrical drum into the magnetic tape, said helical path for said tape having a head entrance region at the beginning of scan and a video head exit region at the 15 end of scan; said upper cylindrical ridge having a width of between 1% and 15% of the tape width disposed in proximity to the rotatable upper cylindrical drum on an axis offset from the central axis at a clockwise angle of about 159° from the beginning of scan whereby the upper cylindrical ridge projects outward from ihe periphery of said stationary lower cylindrical drum. 16. The scanner assembly of claim 15 wherein said rotatable upper cylindrical drum rotates about said central axis of said stationary lower cylindrical drum. 17. The scanner assembly of claim 15 wherein said rotatable upper cylindrical drum rotates about an axis displaced from said central axis of said stationary lower cylindrical drum in a direction away from the area of scan. 21

Inventors: ESHEL ABRAHAM, MAGNUSSON STEVEN L, ESHEL, ABRAHAM, MAGNUSSON, STEVEN, L.
Applicants: AMPEX SYSTEMS CORP, AMPEX SYSTEMS CORPORATION
Application year: 1992
Main classification: G11B21
IPC codes: G11B5, G11B15