The present invention relates to a system and a method for recording video signals on a helical head recording apparatus.
In conventional magnetic recording systems, a spinning rotary disc includes a plurality of magnetic recording heads at spaced intervals for recording information on lines traversing a magnetic recording tape passing across the rotary disc. Typically, the rotary head spins at approximately 14,400 rpm and has positioned on its periphery four magnetic heads spaced at 90.degree. intervals. Each transverse line is recorded by one head and switching between the heads occurs near an end of each transverse line.
The information recorded on each transverse line of the tape is sufficient for about 16 to 17 horizontal lines of video display. As is known, the interlaced video display includes two fields each with 262.5 horizontal lines such that 16 recorded lines are required to produce each field of the video display. Although this requires switching between magnetic recording heads 16 times, the high quality of the electronics associated with the relatively expensive studio equipment permits switching with relatively imperceptible distortion on the reproduced display. Further, there is sufficient recorded video information on each transverse line so that switching between heads can occur near the end of each line which switching will occur off of the video display of the television receiver. As a result, switching between heads does not produce perceptible visual distortion.
With the introduction of portable equipment typically used for on-site news gathering, small portable cameras are employed with less expensive tape recording machines and the problem of head switching line distortion is prevelant. Typically, the recorder employed with the mini-cams employs a cylindrical recording drum with a pair of spaced magnetic recording heads. Instead of the magnetic recording tape passing parallel to the axis of rotation of the drum, the tape is passed around the periphery of the drum at a small angle to the perpendicular of the axis of rotation. As a result, the path of the tape is generally helical, producing a diagonal line of recorded signal or video information.
Typically, there is sufficient recorded information on each of the diagonal lines to produce one entire field or 2621/2 horizontal lines of video information. The switching between the pair of magnetic heads typically employed in such recording systems occurs near the end of each diagonal line of recorded material just before the occurrence of the vertical synchronization signals. The switching point appears as a line distortion at the bottom of the displayed video picture on a television receiver. Such distortion may include horizontal misalignment of displayed information below the switching point in addition to a horizontal line of poor quality video information across the displayed picture at the switching point.
Each recorded diagonal line also includes a vertical blanking interval during which there is no video information for display but instead camera generated synchronization pulses are present and recorded. The vertical synchronization signals occurring near the beginning of the blanking interval are particularly critical because incorrect synchronization signals during this period adversely affects reproduction and locking of the displayed video signal. Thus, head switching occurring during the vertical synchronization signals can distort the entire video presentation by the television receiver. To avoid synchronization problems, the portable cameras are designed such that the head switching point is not positioned in the vertical blanking interval but instead is retained in the displayed video portion of the signal leading to the still objectional line of distortion.
It is desirable, therefore, to delete the line distortion which occurs in helical scan recorders of the type employed for recording video information from portable cameras to retain the advantages of a small and portable helical recording system but without interfering with vertical synchronization. One method proposed to achieve this goal is represented in my co-pending patent application entitled TELEVISION CAMERA AND BLANKING SYSTEM, filed on Nov. 14, 1977, Ser. No. 851,105 (now abandoned). In this system, each of the cameras are modified to include an electrical circuit for compressing the vertical blanking interval such that additional video or signal information is recorded and the portion of the video signal including the switching point is not subsequently displayed by the television receiver. Although this system effectively eliminates the objectional line distortion without affecting vertical synchronization, it requires modification of each of the cameras and many cameras can be used with each tape recorder.