GB1590302A

GB1590302A - Rotational speed changeover device

Info

Publication number: GB1590302A
Application number: GB3632877A
Authority: GB
Original assignee: Victor Company of Japan Ltd; Nippon Victor KK
Current assignee: Victor Company of Japan Ltd; Nippon Victor KK
Priority date: 1976-09-06
Filing date: 1977-08-31
Publication date: 1981-05-28
Also published as: DE2739996C3; AU2857377A; AU503117B2; JPS5332003A; DE2739996B2; DE2739996A1; JPS6160630B2

Description

(54) ROTATIONAL SPEED CHANGEOVER DEVICE (71) We, NIPPON VICTOR KABUSHIKI KAISHA, of No.12, 3 Chome, Moriya-Cho, Kanagawa-Ku, Yokohama-City, Kanagawa-Ken, Japan, a Japanese Corporation, do hereby declare the invention for which we pray that a patent may be granted to us and the method by which it is to be performed to be particularly described in and by the following statement: The present invention relates generally to devices for changing rotational speed of a rotary structure, such as a device for changing the rotational speed of rotary heads according to a selected video signal system in an apparatus for recording and/or reproducing a video signal on/or from a recording medium by means of the rotary heads.

Heretofore, there has been a demand for the realization of a video signal recording and/or reproducing apparatus capable of selectively recording and/or reproducing NTSC (National Television System Committee) system color video signals and PAL (Phase Alternation by Line) system color video signals. In the case where color video signals of two systems are to be selectively recorded and/or reproduced by rotary heads in a single of two systems are to be selectively recorded and/or reproduced by rotary heads in a signal apparatus, it is necessary to set the rotational speed of the rotary heads to conform to that of the selected system of the color video signal.

For example, in the case where two rotary heads are used to record a video signal of one field per oblique track alternately on a tape, it is necessary to rotate the rotary heads at 30 revolutions per second in the case of the NTSC system and at 25 revolutions per second in the case of the PAL system.

Accordingly, as an apparatus heretofore realized to have the capability of selectively recording and/or reproducing color video signals of the above mentioned two systems, there has been an apparatus of an arrangement wherein a motor exclusively for the NTSC system and a motor exclusively for the PAL system are provided, and the exclusive motor corresponding to the selected color video signal system is selectively rotated, and the rotary heads are rotated by this selected motor. In this apparatus, however, since two motors are used for driving the rotary heads, the price of the recording and/or reproducing apparatus becomes high, and, moreover, the weight of the apparatus becomes large, whereby the apparatus has undesirable features such as its unsuitableness as a small recording and/or reproducing apparatus for home use.

Furthermore, there has been another known apparatus wherein two pulleys of different diameters are provided for a single motor, and the operative position of an endless belt for these pulleys is changed in accordance with the selected viedo signal system thereby to selectively switch the rotational speed of the rotary heads. In this apparatus, however, the shifting of the belt operative position with respect to the pulleys can be carried out only in the play mode of operation wherein the pulleys are rotating and cannot be carried out in the stopped mode of operation. For this reason, it is not possible to rotate the rotary heads from the beginning at the changed speed of a desired system, and there arise instances wherein a tape recorded by a certain system cannot be properly reproduced from the beginning.

According to the invention, there is provided a rotational speed changeover device comprising: first and second rotary members driven simultaneously by a single motor rotating at a specific speed thereby to rotate in the same direction respectively at rotational speeds N1 and N2, where N1 < N2; a third rotary member provided between said first and second rotary members and operating to transmit torque to a rotary structure being rotated; a first clutch mechanism provided interrelatedly with said first and third rotary members and constituting a one-way clutch for transmitting torque from said first rotary member to said third rotary member and for preventing torque from being transmitted from said third rotary member to said first rotary member; a second clutch mechanism provided interrelatedly with said second and third rotary members for assuming selectively an engaging state and disengaging state, the engaging state being assumed as a result of external control action to cause said second rotary member to rotate unitarily said third rotary member at the rotational speed N2, said engaging state being held so long as torque continues to be transmitted from the second rotary member to the third rotary member, said second clutch mechanism returning to assume the disengaging state by itself when the second rotary member lowers the rotational speed thereof so that the torque transmitted from the second rotary member to the third rotary member become zero, said first clutch mechanism causing said first rotary member to rotate unitarily said third rotary member at the rotational speed N1 when said second clutch mechanism assumes said disengaging state; and control means for producing said control action to cause the second clutch mechanism to assume said engaging state.

Thus, the rotational speed of a rotary structure being rotated can be changed over between specific different speeds by using a single motor rotating at a specific speed, and the entire apparatus can be made relatively inexpensive and light in weight. Furthermore, since the rotational speed can be changed over irrespective of the operational mode of the apparatus, the rotary structure can be rotated at a desired rotational speed from the start of recording or reproducing operation of the recording and/or reproducing apparatus. Since there are provided clutches which may be placed into their operative state and inoperative state by action due to centrifugal force, there is no necessity of providing a mechanism for changing over a clutch between operative and inoperative states for each clutch, whereby the entire apparatus can be simplified.

The invention will be further described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a bottom view of one embodiment of the rotational speed changeover device according to the invention as applied to a video signal recording and/or reproducing apparatus; Figure 2 is an elevation view orthogonal to Figure 1 showing the same device; Figure 3 is an elevation view, partly in section taken along planes indicated by line III - III in Figure 1, showing a rotational speed changeover mechanism constituting an essential part of the device shown in Figures 1 and 2; Figures 4A and 4B are respectively exploded perspective views of the rotational speed changeover mechanism illustrated in Figure 3; Figures 5A and 5B are bottom views respectively in section taken along line V - V in Figure 3, showing the rotational speed changeover mechanism in the PAL system mode and NTSC system mode, respectively; and Figures 6A and 6B are bottom views respectively in section taken along line VI VI in Figure 3, showing the rotational speed changeover mechanism in the PAL system mode and NTSC system mode, respectively.

Referring first to Figures 1 and 2, a guide drum assembly 10 comprising a rotary drum 11 and a stationary drum 12 is provided on a chassis base 13. The rotary drum 11 is provided with two rotary magnetic heads 14a and 14b for video signal recording and/or reproducing fixed thereto at positions on diametrically opposite sides thereof. A magnetic tape (not shown), as it travels along its travel path, is in wrapped contact obliquely around the guide drum assembly 10 over an expanse thereof slightly greater than one half of its circumference, and a video signal is recorded or reproduced on or from this magnetic tape with one field per track by the rotary magnetic heads 14a and 14b. The rotary drum 11 is supported on the upper end of a rotary shaft 15 having at its lower end a pulley 16 and is thus rotated at 50 revolutions per second (rps.) in the case of a video signal of the PAL system and at 60 rps. in the case of a video signal of the NTSC system.

A single AC motor 17 is mounted on the chassis base 13 and, in operation, rotates at a constant speed of 25 rps. irrespective of the video signal system. This motor 17 has a rotor shaft 18 on which pulleys 19 and 20 for drum rotation and pulleys 21 and 22 for capstan rotation and reel rotation are fixedly supported. A rotational speed changeover mechanism 23 is mounted by a holder 24 on the lower surface of the chassis base 13 in the vicinity of the pulleys 19 and 20.

The rotational speed changeover mechanism 23 comprises, essentially, as indicated in Figure 3, a rotary shaft 25 rotatably journaled by bearings 26a and 26b supported by the holder 24, a pulley 27 for the PAL system provided rotatably relative to the shaft 25 by a sleeve bearing 33, a pulley 28 for drum rotation transmission provided rotatably relative to the shaft 25 by a sleeve bearing 34, and a pulley 29 for the NTSC system fixed to the shaft 25. Square endless belts 30 and 31 are respectively passed around the pulleys 19 and 27 and the pulleys 20 and 29. A flat belt 32 is passed around the pulleys 28 and 16.

As shown in Figures 3 and 4A, the pulley 27 for the PAL system is provided therein at a position offset from its axis with a hole 35 into which is rotatably fitted one bent end part 36a of a crank-shaped pin 36. On the lower face of the pulley 28 for drum rotation transmission, is formed a cam groove 37 comprising stepped parts 37a and 37b and an arcuate surface joining these stepped parts. At the time of rotation of the pulley 27, the pin 36 rotating with the pulley 27 is rotationally displaced about the bent end part 36a by centrifugal force, and its other bent end part 36b contacts the wall of the cam groove 37.

Furthermore, as shown in Figures 3 and 4B, the pulley 28 for drum rotation transmission is provided on its upper face at a position offset from its axis with a hole 38, in which one bent end part 39a of a crankshaped pin 39 is rotatably fitted. On the lower face of the pulley 29 for the NTSC system are formed a circular cam surface 40 and a cam 41 having, for example, six engagement projections 41a, whereby a cam groove 42 is formed. The other bent end part 39b of the pin 39 is fitted in this cam groove 42. The above mentioned pin 36 and the stepped parts 37a and 37b of the cam groove 37 form one one-way clutch, while the above mentioned pin 39 and the cam 41 form another one-way clutch.

A changeover lever 50 is pivotally supported by a pivot pin 52 on a holding bracket 51 fixed to the lower surface of the chassis base 13. One arm of this level 50 at an intermediate part thereof is connected through a pin 53 to the outer end of a plunger 55 of a solenoid 54 mounted on the lower surface of the chassis base 13. A tension spring 56 is connected at its one end to the extreme outer end of the above mentioned arm of the lever 50, which is thereby urged to rotate in the counterclockwise direction in Figure 1. A changeover arm 57 is provided at the other arm of the lever 50 so as to rotate unitarily therewith.

An arcuate feeler 57a which can enter and swing out of a gap 43 between the pulleys 28 and 29 is formed at the outer end of the arm 57. This feeler 57a is adapted to contact the aforementioned bent part 39b of the crankshaped pin 39 thereby to rotate the same as described hereinafter.

The operation of the device of the above described construction will now be described, first, with respect to recording and/or reproducing a color video signal of the PAL system. The solenoid 54 is in its inoperative state, and the lever 50 is in its state wherein it has been turned counterclockwise by the force of the spring 56. The feeler 57a of the arm 57 is in its state of extraction from the gap 43.

When the motor 17 is started, the pulleys 27 and 29 are rotated by power transmitted from the motor 17 respectively through the pulleys 19 and 20 and the endless belts 30 and 31 and thus rotate simultaneously in the same direction. Here, the relationship between the diameters D1 and D2 respectively of the pulleys 27 and 29 is so set that D1 > D2, while the rotational speeds N1 and N2 respectively of these pulleys 27 and 29 have the relationship N1 < N2.

As a consequence of the rotation of the pulley 27, the pin 36 rotating unitarily therewith is subjected to a centrifugal foice, and its bent part 36b rotationally displaces outward about the bent part 36a thereby to contact the cam surface of the cam groove 37 of the pulley 28, as shown in Figure 5A.

The bent part 36b of the pin 36 thereupon contacts the stepped part 37a (or 37b) of the cam groove 37 of the pulley 28 thereby to push the same, and the pulley 28 is rotated unitarily with the pulley 27 at the rotational speed N1 through the pin 36.

As a consequence of the rotation of the pulley 28, the pin 39 rotating unitarily therewith is subjected to a centrifugal force, and the bent part 39b of this pin 39 rotationally displaces outward about the other bent part 39a and contacts the circular cam surface 40 of the pulley 29, as shown in Figure 6A. Here, the pulley 29 is also rotating in the same direction at the rotational speed N2, but the pulley 29 is completely free of engagement by the pin 39, and these parts are not exerting any force with each other. As a result, the circular cam surface 40 is merely in sliding contact with the bent part 39b of the pin 39 with a speed difference (N2-N1).

Accordingly, no torque whatsoever is transmitted to the pulley 28 from the pulley 29, and only the torque of the pulley 27 is transmitted to the pulley 28, whereby the pulley 28 rotates unitarily with the pulley 27 at the rotational speed N1. This rotation of the pulley 27 at the rotational speed N1 is transmitted by way of the belt 32 and the pulley 16 to the rotary drum 11, which rotates at a speed of 50 rps. Thus, the recording of reproducing of the color video signal of the PAL system is carried out by the rotary heads 14a and 14b.

The operation in the case of recording or reproducing a color video signal of the NTSC system in which, while the original intention was that the rotary heads be rotated at the rotational speed conforming to the NTSC system, the operation has been erroneously started at the rotational speed conforming to the PAL system, and the rotational speed is to be changed over to that conforming to the NTSC system will now be described.

With, with the pulley 28 in a state of rotation unitarily with the pulley 27 at the rotational speed N1, a changeover button (not shown) provided on a control panel of the recording and/or reproducing apparatus is operated, the solenoid 54 is supplied with electrical power for 0.5 second, for example, and is temporarily energized. As a consequence of this energization of the solenoid 54, the plunger 55 is retracted, whereby the lever 50 is rotated clockwise as viewed in Figure 1 counter to the force of the spring 56.

Consequently, the arm 57 also rotates in the same direction, and its feeler 57a enters the gap 43 between the pulleys 28 and 29 to contact the bent part 39b of the pin 39 and causes it to rotate inward against the centrifugal force due to its rotation unitarily with the pulley 28. The bent part 39b of the pin 39 rotating together with the pulley 28 at the rotational speed N1 is thus pushed by the feeler 57a and rotationally displaced inward about the bent part 39a. During this action, the bent part 39b is arrested as indicated in Figure 6B by one of the engaging projections 41a of the cam 41 of the pulley 29 rotating at the rotational speed N2, which is higher than the speed N1.

Thereafter, the pin 39 is pushed by the cam 41, and the pulley 28, being intercoupled through the pin 39 to the pulley 29, is rotated unitarily with the pulley 29 at the rotational speed N2.

At this time, while the pulley 27 continues to rotate at the rotational speed N1, the pulley 28 rotates at the rotational speed N2.

For this reason, the bent part 36b of the pin 36 rotating unitarily with the pulley 27 merely slides at a difference speed of (N2 N1) in the direction opposite to that for engagement with the stepped part 37a in contact with and relative to the cam surface of the cam groove 37, whereby there is no obsturction whatsoever of the independent rotations respectively of the pulleys 28 and 27. Thus, the pulley 28 is rotated at the rotational speed N2 independently of the pulley 27 and unitarily with the pulley 29.As a consequence, the rotary drum 11 rotates at the speed of 30 rps., and recording or reproducing of the NTSC system color video signal of a field frequency of 60 Hz is carried out by the rotary heads 14a and 14b.

In the case where recording or a color video signal of the NTSC system is to be carried out from the beginning, a play button (not shown) is pushed with the aforementioned changeover switch (not shown) in the operated state. As a consequence, the motor 17 is started, and the pulleys 27 and 29 start to rotate. At the same time, the solenoid 54 is temporarily activated. As a consequence of this operation of the solenoid 54, the levers 50 and 57 are rotated clockwise as viewed in Figure 1, and the feeler 57a enters into the gap 43 between the pulleys 28 and 29 and presses the bent part 39b of the pin 39 against the cam 41 of the pulley 29. Consequently, the engaging projection 41a of the cam 41 arrests the bent part 39b of the pin 39 immediately after the start of the rotation of the pulley 29, and, thereafter, the pulleys 29 and 28 rotate unitarily. As a result, the rotary drum 11 is rotated at a speed of 30 rps., and recording or reproducing of the color video signal of the NTSC system is carried out.

The changeover operation in the case where, while the original intention was to carry out recording or reproducing of a color video signal of the PAL system, the rotary drum has been rotated at the rotational speed conforming to the NTSC system will now be described. In this case, the power supply to the motor 17 is temporarily interrupted for a period of the order of, for example, 0.5 second. The load on the motor 17 is large, being a combination of the load due to the rotary drum 11 and additional loads due to moving parts such as a capstan and a reel (both now shown). For this reason, when the power supply to this motor 17 is cut off, the motor speed instantly undergoes a large drop, and the rotational speeds of the pulleys 27 and 29 also decrease immediately. In contrast, the rotational speed of the pulley 28 does not immediately undergo a large drop because of the inertial rotation (i.e., rotational momentum) of the rotary drum 11.

Consequently, together with the cutting off of the power supply to the motor 17, the rotational speed of the pulley 29 becomes lower than that of the pulley 28, and the pin 39 disengages from its state of arrest by the cam 41, its bent part 39b then contacting the cam surface 40. As a consequence, the pulley 28 is released from its state of connection with the pulley 29, and these two pulleys rotate independently of each other.

Furthermore, the rotational speed of the pulley 27 also decreases below that of the pulley 28, but the bent part 36b of the pin 36 merely slidingly contacts the cam surface of the cam groove 37, and the pulleys 27 and 28 also rotate independently of each other.

When the motor 17, after the temporary stoppage of its power supply, is again supplied with power, it again starts to rotate at its rated speed, and the pulleys 27 and 29 again rotate respectively at the speeds N1 and N2. On the other hand, no torque is transmitted to the pulley 28, which merely rotates under its remaining momentum, whereby its rotational speed gradually decreases. When the rotational speed of the pulley 28 becomes lower than N1, the bent part 36b of the pin 36 rotating together with the pulley 27 at the speed N1 engages with the stepped part 37a or 37b of the cam groove 37, and thereafter the pulleys 27 and 28 rotate unitarily at the speed N1. As a consequence, the rotary drum 11 is rotated at a speed of 25 rps., and a state wherein recording or reproducing a video signal of the PAL system can be carried out is attained.

It is to be noted that, in the present embodiment of the invention, by changing over a changeover switch operated by the aforementioned changeover button from the PAL system side to the NTSC system side, the solenoid 54 is temporarily supplied with electric power, while, by changing over the changeover switch from the NTSC system side to the PAL system side, the power supply to the motor 17 is temporarily cut off. Thus the selection between the two system modes can be carried out simply by a single changeover switch.

While, in the present embodiment of the invention, changing over of the rotational speed of the rotary drum between speeds conforming respectively to the NTSC system and the PAL system is carried out, a similar changing over of speeds conforming respectively to the NTSC system and the SECAM system can also be carried out.

Furthermore, the changeover device of the present invention is not limited in application to the changeover of the rotational speed of a rotary drum such as that described above but can be applied also to the changeover, for example, of the rotational speed of a capstan in a tape recorder for audio signals.

Further, this invention is not limited to these embodiments but various variations and modifications m y be made without departing from the scope of the invention.

WHAT WE CLAIM IS: 1. A rotational speed changeover device comprising: first and second rotary members driven simultaneously by a single motor rotating at a specific speed thereby to rotate in the same direction respectively at rotational speeds N1 and N2, where N1 < N2; a third rotary member provided between said first and second rotary members and operating to transmit torque to a rotary structure being rotated; a first clutch mechanism provided interrelatedly with said first and third rotary members and constituting a one-way clutch for transmitting torque from said first rotary member to said third rotary member and for preventing torque from being transmitted from said third rotary member to said first rotary member; a second clutch mechanism provided interrelatedly with said second and third rotary members for assuming selectively an engaging state and a disengaging state, the engaging state being assumed as a result of external control action to cause said second rotary member to rotate unitarily said third rotary member at the rotational speed N2, said engaging state being held so long as torque continues to be transmitted from the second rotary member to the third rotary member, said second clutch mechanism returning to assume the disengaging state by itself when the second rotary member lowers the rotational speed thereof so that the torque transmitted from the second rotary member to the third rotary member become zero; said first clutch mechanism causing said first rotary member to rotate unitarily said third rotary member at the rotational speed N1 when said second clutch mechanism assumes said disengaging state; and control means for producing said control action to cause the second clutch mechanism to assume said engaging state.

2. A rotational speed changeover device as claimed in claim 1 in which: said first clutch mechanism comprises a first engagement member provided on the first rotary member and a first cam member which has a cam groove and is formed on the third rotary member and which has a shape such that when the third rotary member is stopped or is rotating at the rotational speed lower then the speed N1 said first cam member is engaged by said first engagement member rotating together with the first rotary member at the rotational speed N1, and, when the rotational speed of the third rotary member is higher than the speed N1, said first cam member is not engaged by the first engagement member; and said second clutch mechanism comprises a second engagement member provided on the third rotary member and a second cam member which has a cam groove and is formed on the second rotary member and which has a shape such that it engages with said second engagement member when the second engagement member is forcibly displaced by said control action and continues to engage with second engagement member so long as torque continues to be transmitted from the second rotary member to the third rotary member, and such that the second cam member disengages from said second engagement member when the second rotary member lowers the rotational speed thereof so that the torque transmitted from the second rotary member to the third rotary member becomes zero.

3. A rotational speed changeover device as claimed in claim 2 in which: said first engagement member comrpises a first pin bent into a crank shape with two bent parts, one bent part being rotatably provided in

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. whereby its rotational speed gradually decreases. When the rotational speed of the pulley 28 becomes lower than N1, the bent part 36b of the pin 36 rotating together with the pulley 27 at the speed N1 engages with the stepped part 37a or 37b of the cam groove 37, and thereafter the pulleys 27 and 28 rotate unitarily at the speed N1. As a consequence, the rotary drum 11 is rotated at a speed of 25 rps., and a state wherein recording or reproducing a video signal of the PAL system can be carried out is attained. It is to be noted that, in the present embodiment of the invention, by changing over a changeover switch operated by the aforementioned changeover button from the PAL system side to the NTSC system side, the solenoid 54 is temporarily supplied with electric power, while, by changing over the changeover switch from the NTSC system side to the PAL system side, the power supply to the motor 17 is temporarily cut off. Thus the selection between the two system modes can be carried out simply by a single changeover switch. While, in the present embodiment of the invention, changing over of the rotational speed of the rotary drum between speeds conforming respectively to the NTSC system and the PAL system is carried out, a similar changing over of speeds conforming respectively to the NTSC system and the SECAM system can also be carried out. Furthermore, the changeover device of the present invention is not limited in application to the changeover of the rotational speed of a rotary drum such as that described above but can be applied also to the changeover, for example, of the rotational speed of a capstan in a tape recorder for audio signals. Further, this invention is not limited to these embodiments but various variations and modifications måy be made without departing from the scope of the invention. WHAT WE CLAIM IS:

1. A rotational speed changeover device comprising: first and second rotary members driven simultaneously by a single motor rotating at a specific speed thereby to rotate in the same direction respectively at rotational speeds N1 and N2, where N1 < N2; a third rotary member provided between said first and second rotary members and operating to transmit torque to a rotary structure being rotated; a first clutch mechanism provided interrelatedly with said first and third rotary members and constituting a one-way clutch for transmitting torque from said first rotary member to said third rotary member and for preventing torque from being transmitted from said third rotary member to said first rotary member; a second clutch mechanism provided interrelatedly with said second and third rotary members for assuming selectively an engaging state and a disengaging state, the engaging state being assumed as a result of external control action to cause said second rotary member to rotate unitarily said third rotary member at the rotational speed N2, said engaging state being held so long as torque continues to be transmitted from the second rotary member to the third rotary member, said second clutch mechanism returning to assume the disengaging state by itself when the second rotary member lowers the rotational speed thereof so that the torque transmitted from the second rotary member to the third rotary member become zero; said first clutch mechanism causing said first rotary member to rotate unitarily said third rotary member at the rotational speed N1 when said second clutch mechanism assumes said disengaging state; and control means for producing said control action to cause the second clutch mechanism to assume said engaging state.

the first rotary member, the other bent part being caused to rotationally displace outward by a centrifugal force accompanying rotation thereof together with the first rotary member, said first cam member being formed to have a cam surface comprising stepped parts adapted to be engaged by said other bent parts of said first pin and an arcuate surface disposed between and joining said stepped parts; and said second engagement member comprises a second pin bent into a crank shape with two bent parts, one bent part being rotably provided in the third rotary member, the other bent part being caused to rotationally displace outwardly by a centrigfugal force accompanying rotation thereof together with the third rotary member, said second cam member being formed to have outwardly a circular cam surface and inwardly a cam engaged by said other bent part of said second pin displaced by said control means.

4. A rotational speed changeover device as claimed in claim 2 in which said control means has a member for temporarily entering into a gap between the second and third rotary members and pushing and displacing said second engagement member of the second clutch mechanism.

5. A rotational speed changeover device as claimed in claim 1 in which said rotary structure being rotated is provided with rotary heads for recording and/or reproducing a video signal on and/or from a recording medium, and the rotational speed of the third rotary member is changed over between N1 and N2 so that said rotary heads will rotate at a rotational speed in correspondence with the field frequency of the system of the video signal.

6. A rotational speed changeover device as claimed in claim 1 in which said rotary structure comprises two rotary heads for recording or reproducing a video signal of the NTSC system or the PAL system selec tively and alternately on or from each track, and the rotational speed of the third rotary member is changed over between N1 and N2 so that said rotary heads will rotate at 30 revolutions per second for a video signal of the NTSC system and at 25 revolutions per second for a video signal of the PAL system.

7. A rotational speed changeover device substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.