1. Field of the Invention
The present invention relates to a helical-scan video tape recorder (hereinafter referred to as "VTR"), and more particularly to such a VTR capable of playing back recorded video signals at various desired speeds ranging from a normal speed in reverse reproduction to a threefold normal speed in forward reproduction.
2. Description of the Prior Art
When a magnetic tape is to be played back at a speed different from a normal reproduction speed (a mode hereinafter referred to as a "special reproduction"), a video head scans the magnetic tape along tracks different from recorded tracks thereon. Therefore, in such a special reproduction mode, it is impossible to scan a single recorded track accurately. As a result, there is created a period characterized by a reduced reproduced signal level.
Where such a period of lower reproduced signal level is positioned in the vicinity of a vertical synchronous signal, a television receiver reproducing the video signals is substantially free of noise bars and hence capable of generating good reproduced images. Speeds at which the magnetic tape can be fed while producing noise-free images in the special reproduction mode are limited to fixed speeds such as a normal speed, a stationary image, a double normal speed, and a threefold normal speed. When the magnetic tape is fed along at other speeds such as those higher than the threefold normal speed or intermediate speeds between a normal speed in reverse reproduction (-normal speed) and the threefold normal speed, such for example as a 0.4 normal speed in reverse reproduciton (-0.4 normal speed), a 0.6 normal speed, a 1.5 normal speed, and a 2.5 normal speed, the television receiver produces fields in which the reproduced signal level is much lowered, resulting in noise bars on reproduced images with a failure in reproducing good images.
Such a condition will be described with reference to FIG. 1 of the accompanying drawings. FIG. 1 (a) shows repeated recorded-track patterns of a 2-head helical-scan VTR. A graduation on the horizontal axis indicates a period of one field and a timing of a head changeover switch. The vertical axis indicates the distance of movement of a magnetic tape, with a single graduation showing the distance of travel of the tape during a single field period in a recording mode or a normal reproducing mode. The single graduation on the vertical axis also represents the width of a recorded track. The two video heads have slightly different head gap angles for azimuth recording.
In the patterns of recorded tracks shown in FIG. 1, those tracks recorded with a head A are odd-numbered, while those tracks recorded with a head B are even-numbered. The odd-numbered tracks can be reproduced only by the head A, while the even-numbered tracks can be reproduced only by the head B. The thicker lines G are indicative of paths of the heads at the time of signal reproduction at a speed that is 0.6 times the normal reproducing speed. FIG. 1 (b) shows an envelope waveform in the 0.6 normal speed reproduction. FIG. 1 (c) illustrates an envelope waveform in a stationay-image reproduction and a double normal speed reproduction, and (d) an envelope waveform at a fourfold normal speed reproduction and a reverse twofold normal speed reproduction. With the envelope (c), it is possible to position a period of a lowered reproduced signal level in the vicinity of times at which the fields are changed. With the envelope (b), however, there will be fields in which the reproduced signal level is lowered. The envelope (d) results in fields in which the reproduced signal level is reduced in the vicinity of the center of the field. Accordingly, only the envelop (c) produced in the still and twofold normal speed reproduction modes is capable of reproducing good video images.
It is therefore quite difficult to perform variable-speed reproduction in special reproduction modes. There are known methods of displacing a playback magnetic head in order to follow recorded tracks on a magnetic tape for variable-speed reproduction. However, such known arrangements are complex in construction and high in cost.