This invention relates to a 4-head sequential switch scanning type magnetic recording/reproducing apparatus for recording and reproducing video signals.
Remarkable progress has been made in recent years in reduction of the size of magnetic recording/reproducing apparatuses for recording and reproducing video signals (hereinafter referred to as "VTRs") and compact, light-weight and portable VTRs with a built-in video camera have been developed in the standards using a 1/2 in.-wide magnetic tape.
VTRs of this type are disclosed, for example, in Japanese Patent Publication No. 20403/1970. In accordance with this prior art reference, a magnetic tape is wound helically around about 3/4 of the periphery of a cylinder to which four heads are mounted with spacings of 90.degree. between them and the number of revolution of the cylinder is set to be 3/2 times that of an ordinary 2-head type VTR so that the tape is sequentially scanned by each head to record or reproduce video signals. This recording/reproducing system is referred to as a "4-head sequential switch scanning system", and provides the advantage that the cylinder diameter can be reduced to 2/3 of the ordinary 2-head type VTRs and the system can be made compact.
Therefore, the system is extremely suitable for VTRs with a built-in video camera which are carried around outdoors for recording and reproduction.
Incidentally, when recording or reproduction of the tape is made by the heads, the tape travels while generally being oscillated. This vibration results from vibration of the outer peripheral surface of the rotary cylinder, the projecting quantity of the heads from the rotary cylinder and the shapes of the projecting portions, the fitting positions of the heads to the rotary cylinder, imbalance of the inertial mass of the rotary cylinder at the rotating position, distortion of the rotary axis of the rotary cylinder, and so forth. In addition, non uniform rotation of the rotary cylinder occurs due to the ripple of a motor. If video signals are recorded on the tape which travels while being oscillated as described above by the heads which are mounted to the rotary cylinder having by itself non-uniform rotation, the signals thus recorded have come to possess a time base strain having the number of revolution of the rotary cylinder as the fundamental frequency.
FIG. 10 is a waveform diagram showing the time base strain in the recording and reproducing modes when the recording head and the reproducing head are the same for each track as will be described elsewhere. In this diagram solid line a represents the relationship between a scanning time t and the time base strain .delta. of the signal recorded on the tape. Signals having such a time base strain are scanned and recorded on the tape by each head.
The time base strain resulting from the revolution of the cylinder is in synchronism with the phase of revolution of the cylinder. Therefore, it has substantially the same amplitude and phase characteristics in the recording and reproducing modes. When each track is scanned and reproduced by the same head as in the recording mode, the time base strain occurs in substantially the same way as in the recording mode as represented by dash line b in the diagram so that the time base strain in the recording mode is offset and the time base strain of the reproduced video signal falls to a very low level.
An azimuth recording system has been employed for ordinary 2-head type VTRs and each head is subject to a servo-mechanism in such a manner as to reproduce and travel the track which is in conformity with its azimuth angle. For this reason, the head used in the recording mode for each track is the same as the head in the reproducing mode and the level of the time base strain of the reproduced video signal is very low. In other words, the time base strain can be eliminated sufficiently by signal processing.
In the 4-head sequential switch scanning type VTRs, however, the azimuth angles of the two heads which are adjacent to each other with the 90.degree. spacing between them are different, but the azimuth angles of the two heads facing each other with the 180.degree. spacing between them are equal to each other. Therefore, every other track formed on the tape is formed sequentially by the head having the same azimuth angle.
Accordingly, since conventional VTRs employ a servo-mechanism so that each head reproduces and scans the track which is in conformity with the azimuth angle of the head, the track on the tape is sometimes reproduced and scanned by a different head which faces the head used for recording at 180.degree. but has the same azimuth angle. In such a case, the phase of rotation of the rotary cylinder in the reproducing mode comes to be inversed by 180.degree. with respect to the phase in the recording mode As shown in FIG. 11 which is a characteristic diagram showing the time base strain when the recording head and the reproducing head are different for each track as will be described elsewhere, the time base strain in the recording mode due to the rotation of the rotary head has a phase represented by dash line b which is different by 180.degree. from the phase of the time base strain in the recording mode represented by solid line a. The time base strain of the reproduced video signal corresponds to the difference between these time base strains a and b and hence becomes an extremely high level.
It is very difficult to sufficiently eliminate the time base strain of the reproduced video signal occurring in the manner described above by signal processing, so that jitter and color non-uniformity develop on the reproduced picture and quality of the reproduced picture is extremely deteriorated.
FIG. 12 shows an example of measured jitter frequency distribution of reproduced video signals. Solid line in the drawing represents the case where the head for scanning each track is the same in both recording and reproducing modes and dash line represents the case where the head is different. It can be understood from t he diagram that deterioration of the number of revolution of the rotary cylinder and its higher harmonic components is remarkable when the head is different in the recording and reproducing modes.