This invention relates to a helical scan type video tape recorder capable of providing reliable start-ups of its revolving head drum.
Recently, for video tape recorders (hereinafter called VTRs), helical scan type VTRs have come to be widely employed, and are thus used in large quantities, not only as consumer oriented models, the so-called home VTRs, but also as open reel type helical scan VTRs using 1 inch wide magnetic tape for broad-casting and other professional applications.
This type of helical scan VTR is not only comparable in performance capabilities to conventional 4 head type VTRs, but is characterized by relative simplicity of structure and ease in attaining small size and light weight. On the other hand, it tends to present a problem in that the magnetic tape sticks to the revolving head drum (hereinafter called the drum), making start-up of the drum difficult.
FIG. 1 shows an example of the magnetic tape transport path configuration in such helical scan type VTRs as described above.
In the figure, 1 denotes a magnetic tape; 2, a drum; 3, guide posts; 4, a supply-side tension post; 5, a take-up-side tension post; 6, a supply reel; and 7, a take-up reel.
The magnetic tape 1 from the supply reel 6 is caused to run around the periphery of the drum 2 by a multitude of guide posts 3, and at this time is caused to wrap around the drum 2 in a nearly complete circle, attaining a wrap-around angle to the drum 2 of as much as 344.degree.. When the magnetic tape 1 and the drum 2 are at a stand-still, these tend to stick together, as a result of which start-up of the drum 2 becomes impossible.
VTRs of this kind are frequently used out-doors and in other variable environmental conditions as portable VTRs, through utilization of their advantageous small size and light weight, so that variations in humidity and temperature are liable to cause dew condensations on the surface of the drum 2, leading to the adherence phenomenon mentioned above.
Then, when adherence occurs and the drum 2 cannot be started up, the VTR naturally becomes unusable, a highly undesirable situation for professional VTRs required to be reliably operable at all times. Even in such circumstances it is desirable to be able to start up the drum 2.
For helical scan type VTR motor control systems heretofore, those structured, for example, as shown in FIG. 2, have been employed.
In the figure, 10 denotes a phase comparator; 11, an adder; 12, a switching circuit to select between start-up and stopping of the drum; 13, an error voltage amplifier; 14, a motor driving amplifier; 15, a drum motor; 16, a revolution detector; 17, a frequency discriminator; 18, a reel motor; 19, a motor driving amplifier; and 20, a designated-voltage controller.
Then, a denotes a drum start command signal that is supplied when controlled in a stand-by mode; h, a terminal voltage of the drum motor 15; i, a terminal voltage of the reel motor 18; m, a speed error signal; n, a detection signal by the detector 16; y, a reference signal; and c, a phase error signal.
In such helical scan VTRs, stand-by mode controls are employed prior to recording, reproducing, and other operations of the VTR, and in the stand-by mode only the drum is revolving while the magnetic tape is at stand still with a weak tension applied thereto to prevent slackening.
Now, if the VTR is operated in the stand-by mode, and a drum start command signal a is generated, the switch circuit 12 is switched to the illustrated state, that is, the mobile contact point (1) is switched to the fixed contact point (3), and signals from the error voltage amplifier 13 are supplied to the motor driving amplifier 14. At this time, as the number of revolutions of the drum motor 15 is zero and the detection signal n of the revolution detector 16 is consequently also zero, a relatively great voltage h is generated out of the motor driving amplifier 14 and is supplied to the drum motor 15. By this, the drum motor 15 is started up, whereby a detection signal n is generated by the revolution detector 16 and is supplied to the phase comparator 10 and the frequency discriminator 17, and the phase error signal c, obtained through phase comparision with the reference signal y, and the speed error signal m obtained through frequency discrimination, are supplied via the adder 11 to the error voltage amplifier 13, so that phase servo control and speed servo control are exercised, and the phase and the number of revolutions of the drum motor 15, and consequently of the drum 2, are controlled to meet a predetermined condition.
Meanwhile, a terminal voltage i that corresponds to the voltage designated by the voltage controller 20 is supplied simultaneously with the above to the reel motor 18 by the motor driving amplifier 19, whereby a relatively weak designated tension is applied to the magnetic tape to prevent slackening thereof.
Now, suppose that when the VTR is thus operated in the stand-by mode, and the above mentioned adherence of the magnetic tape to the drum occurs, the drum will then not start up, and, in the meantime, only the terminal voltage h that corresponds to a zero number of revolutions is continuously applied to the drum motor 15, and no specific operation of any kind to urge start-up of the drum motor 15 is performed.
Consequently, in the conventional VTRs, there exists the disadvantage that unless, by some method, adherence of the magnetic tape to the drum is removed, operation of the VTR cannot commence.
A method has been proposed to eliminate this disadvantage, whereby when, under the stand-by mode operation, despite provision of a voltage to the drum motor, the drum motor does not revolve, the voltage impressed on the drum motor is switched off and on a plurality of times, thereby urging start-up thereof.
However, even by this method, depending on conditions of the adherence, the drum motor is not always able to be made to start up, and the method is thus disadvantageous in not being capable of providing reliable start-ups at all times.