VIDEO SIGNAL RECORDING/REPRODUCING APPARATUS

Abstract: A helical scan type magnetic recording/reproducing apparatus in which a magnetic tape (1) is guided helically around a rotary magnetic head assembly (5) over an angular range approximately to 360°. Two magnetic heads (6, 7) are disposed relatively closely to each other at an angle smaller than 180°. The rotary magnetic head assembly (5) performs a single complete rotation for a unit period, for example, one-field period. In recording, video signal of the unit period undergoes time axis compression to be compressed to a period corresponding to a scanning period of the heads during which the magnetic heads (6, 7) are in contact with the magnetic tape (1) in each rotation of the head assembly (5) and supplied to the magnetic heads which are so designed that the azimuth angles of the respective gaps differ from each other. The time-axis-compressed video signal is sup­ plied to the two magnetic heads alternately in each of successive rotations of the head assembly (5). Alternatively, the video signal is separated into a luminance signal and a chrominance signal, wherein the former is supplied to one (6′) of the magnetic heads while the latter is simultaneously supplied to the other (7′). The record tracks (65, 66, 67, 68) are formed on the magnetic tape in a pattern identical with the track pattern formed by a conventional two-head helical scan type video recorder in which the magnetic heads are disposed in opposition to each other with an angular distance of 180° therebetween and in which no time axis conversion is performed. Thus, the magnetic tape recorded according to the proposal can be reproduced by using the conventional magnetic recording/reproducing apparatus of two-head helical scan type.

Description: The present invention in general relates to a helical scan type video signal recording/reproducing apparatus according to the preamble of claim 1. As a video signal recording/reproducing apparatus, the helical scan type magnetic recording/reproducing apparatus for domestic use enjoys increasingly popularization. In the magnetic recording/reproducing apparatus, two magnetic heads scan alternately a magnetic tape slant-wise for recording video signal on field-by-field basis through each of the magnetic heads in an alternating manner. As the result of development of the magnetic head having high performance, a reproduced image of high quality can be obtained. Further, because of the azimuth recording system as adopted, the recording of video signal can be accomplished with a significantly increased density. In the US 3 925 816 a magnetic recording and reproducing apparatus is disclosed in which first and second signals, such as, the chrominance and luminance signals of a color television signal, are recorded by first and second magnetic heads in respective first and second tracks which are made to overlap partly, so as to increase the utilization of the tape and the signals to be recorded. Further helical scan-tape record/replay system comprising a special head unit with two individual gaps disposed in tandem substantially at the same location as disclosed in GB-A 2 076 579. GB 2 059 135 discloses an apparatus for recording and reproducing a tape format having both video and audio information on the same tracks. The apparatus uses one conventional head. The earlier application EP-A-0 102 811 discloses a video signal recording/reproducing apparatus of helical scan type, in which record tracks are formed on a magnetic tape with inclination. This apparatus comprises rotary magnetic head means including a pair of magnetic heads having respective gaps of azimuth angles differing from each other, said magnetic heads being disposed at an angular distance smaller than 180 o , said head means being rotated with a period corresponding to a unit period of a video signal and being wound helically with the magnetic tape over a range greater than 180 o . Video signal generating means are provided for generating a video signal of said unit period within a period during which said magnetic tape is scanned by said magnetic heads in each complete rotation of said rotary magnetic head means. Second means are provided for recording the output signal of said video signal generation means by said magnetic heads, and third means are provided for supplying the output signal of said video signal generating means to said pair of magnetic heads alternately for every unit period of said video signal. The magnetic heads have a smaller angular distance than the angular distance between the points in which the magnetic tape is out of contact with the rotary magnetic head means. In recent years the technological approach for combining integrally the helical scan type magnetic recording/reproducing apparatus with a video camera has attracted great concern. As fundamental requirements for accomplishing such an integral combination, the helical scan type magnetic recording/reproducing apparatus should desirably be realized in small size and light weight while assuring a high quality of reproduced image. In order to satisfy the requirements mentioned above, it will be most preferable to increase the number of rotations of the head cylinder or drum to thereby correspondingly increase the relative speed between the magnetic head and the magnetic tape for a given recording density, and implement the magnetic head cylinder or assembly in a reduced size. However, in the case of the two-head helical scan type magnetic recording/reproducing apparatus, the increasing of the relative speed means a corresponding increase in the diameter of the head cylinder, involving a large size of the apparatus to disadvantage, although the image quality can be improved. In contrast, in the case of one-head helical scan type magnetic recording/reproducing apparatus, the relative speed between the magnetic head and the magnetic tape can be increased about twice as high as the relative speed in the two-head helical scan type apparatus, when the diameter of the head cylinder of the former is selected equal to that of the head cylinder of the latter, provided that the head cylinder is rotated at a period corresponding to one field of the video signal. Further, when the relative speed between the magnetic head and the magnetic tape is made equal to that of the two-head helical scan type magnetic recording/reproducing apparatus, it is possible to decrease the diameter of the head cylinder to a half, to allow the apparatus to be realized in a reduced size, whereby the requirements concerning the high quality of the reproduced image and implementation in small size and light weight can thus be satisfied. The same holds true in application to a so-called 1.5-head helical sacan type magnetic recording/reproducing apparatus for broadcasting purpose in which an auxiliary head is provided for recording and reproducing the vertical blanking period of the video signal. Although the one-head or 1.5-head helical scan type magnetic recording/reproducing apparatus satisfies the aforementioned requirements to a reasonable degree, it is impossible to adapt the azimuth recording system because the magnetic tape is sequentially scanned by one and the same magnetic head. As the consequence, it is indispensable to dispose guard bands between the record tracks, which results in a corresponding decrease in the recording density, being accompanied with unduly increasing of the amount of magnetic tape as used. This is undesirable particularly in the magnetic recording/reproducing apparatus for home use. An object of the present invention is to provide a video signal recording/reproducing apparatus of helical scan type which is capable of reproducing pictures with an improved image quality and which can be realized in reduced size and weight while assuring a high recording density. This object is solved by the recording/reproducing apparatus according to claim 1. The present invention will be apparent from the following detailed description taken in conjuction with the accompanying drawings, in which: Fig. 1 is a view for schematically illustrating a magnetic tape feeding mechanism used in a video signal recording/reproducing apparatus according to the present invention; Figs. 2a and 2b show in detail a structure of a rotary head assembly of the recording/reproducing apparatus shown in Fig. 1 in a plan view and an elevational view, respectively; Fig. 3 is a view for illustrating a pattern of record tracks formed on a magnetic tape; Fig. 4 shows in a block diagram a general arrangement of a recording/reproducing circuit according to an embodiment of the invention; Figs. 5a to 5d show timing charts for explaining an operation (without overlap recording) of the recording/reproducing circuit shown in Fig. 4; Fig. 6 shows in a block diagram an exemplary embodiment of a time axis conversion circuit used in the magnetic recording/reproducing apparatus according to the invention; Figs. 7a to 7c show timing diagrams for illustrating an embodiment of the magnetic recording/reproducing apparatus according to the present invention on the basis of figures 4 and 6; Fig. 8 is a view showing rotational phases or timing of magnetic heads for illustrating the operation together with Figs. 7a to 7c; Figs. 9a to 9c show timing diagrams for illustrating still another aspect of operation of the magnetic recording/reproducing apparatus shown in Fig. 4; Figs. 10a and 10b are views showing in detail a structure of rotary magnetic head assembly for use in the magnetic recording/reproducing apparatus according to another embodiment of the invention; Fig. 11 is a block diagram showing a general arrangement of a magnetic recording/reproducing apparatus according to still another embodiment of the invention; Figs. 12a to 12c show timing charts for illustrating operation of the apparatus shown in Fig. 11; Fig. 13 is a view showing a pattern of record tracks formed on a magnetic tape by the apparatus shown in Fig. 11; Fig. 14 is a view showing in detail an exemplary disposition of two magnetic heads; Now, the invention will be described in detail in conjunction with the preferred embodiments thereof by referring to the drawings. Fig. 1 is a top plan view showing schematically an arrangement of a magnetic tape feeding mechanism of a video signal recording and reproducing apparatus according to an embodiment of the invention. In the figure, the apparatus is shown in the state in which a tape cassette 2 is placed therein with the apparatus being ready for recording or reproducing (replay or playback) operation. A reference numeral 5 denotes a rotary head assembly which is composed of a tape guide of a cylindrical or drum-like configuration and magnetic heads mounted rotatably along the cylindrical surface of the tape guide drum. More specifically, the rotary head assembly may be constituted, for example, by a stationary or fixed cylinder, a rotatable cylinder disposed coaxially with the fixed cylinder and supported rotatably, and magnetic heads fixedly mounted on the rotatable cylinder at the side opposing to the fixed cylinder. Alternatively, the rotary head assembly may be constituted by a pair of stationary fixed cylinders disposed coaxially with each other with a small gap being interposed therebetween, and rotatable magnetic heads which project outwardly slightly from the gap. In the tape cassette 2 which is not in the state placed in the magnetic recording/reproducing apparatus, a magnetic tape 1 extends from a supply reel 4 by way of a tape guide 8 disposed at the exit side and a tape guide 9 disposed at the entry side. When the tape cassette 2 is placed in the magnetic recording/reproducing apparatus which is subsequently set to a recording or playback mode, movable tape guides 12, 13, 14 and 15 cooperate to lead outwardly the magnetic tape 1 from the tape cassette 2 to wind the magnetic tape 1 helically around the rotary head assembly 5 at a wrap angle of more or less about 360°(e.g. ca. 300°). At the same time, the magnetic tape 1 is caused to bear on an erasing head 11 and a voice control head 16 and sandwiched between a pinch roller 17 and a capstan 18 under a predetermined tension applied by a tensioning post 10. The magnetic tape 1 travels in the direction indicated by an arrow A as driven by the capstan 18 and the pinch roller 17, while the magnetic heads 6 and 7 of the rotary head assembly 5 are rotated in the direction indicated by an arrow B, whereby recording or reproduction is effected by the magnetic heads 6 and 7 alternately changed over for every rotation. Figs. 2a and 2b show a typical structure of the rotary head assembly 5 in a plan view and an elevational view, respectively. In the figures, a reference symbol 5a denotes a rotatable upper cylinder, while 5b denotes a stationary lower cylinder. Parts corresponding to those shown in Fig. 1 are denoted by same reference numerals. Referring to Figs. 2a and 2b, the magnetic tape 1 is helically wound around the outer periphery of the rotary head assembly 5 at a wrap angle 360° minus °. The range of this wrap angle is determined by the tape guides 13 and 14. The upper cylinder or drum 5a is rotated in the direction indicated by the arrow B, while the magnetic tape 1 travels in the direction indicated by the arrow A. The magnetic heads 6 and 7 having respective gaps of azimuth angles differing from each other are mounted on the rotatable upper cylinder 5a with an offset of a value approximately equal to the pitch P of the record tracks (not shown) multiplied by ϑ°/360° in the direction of the axis of rotation of the upper cylinder 5a and at an angular distance of ϑ° between the heads 6 and 7. In this connection, it is noted that the angle ϑ° is selected slightly greater than the angle ° mentioned above. The upper cylinder 5a and hence the magnetic heads 6 and 7 are adapted to perform a single complete rotation in a period corresponding to one field of the video signal in a standard system. In the case of NTSC system, by way of example, the upper cylinder 5a is rotated at an angular speed of 3600 rpm. In the case of the embodiment being described, the recording or reproduction is effected by the magnetic heads 6 and 7 alternately changed over to each other for each of successive rotations of the rotary head assembly 5. The recording or reproducing operation takes place in an angular range of (360° ϑ°) extending from a point P to a point Q in which range the magnetic heads 6 and 7 are brought into contact with the magnetic tape 1 for scanning the latter, where ϑ° represents an angular range extending from the point Q to the point P and including the angular range defined by the angle ° in which the magnetic tape 1 is out of contact with the rotary head assembly 5. In the recording operation, the magnetic heads 6 and 7 are, respectively, supplied with one field of video signal during each rotation of the head assembly 5 in the alternating manner. On the other hand, in the replay operation, one field of the video signal is picked up by the magnetic heads 6 and 7 alternately for reproduction during each of successive rotations of the head assembly 5, respectively. Describing more specifically in this connection, the magnetic head 6 is supplied with one field of video signal during the period in which the magnetic head 6 travels from the point P to the point Q as the rotary magnetic head assembly 5 performs a single rotation, whereby a record track 19 illustrated in Fig. 3 is produced. At the moment the magnetic head 6 has reached the point Q, the recording operation is stopped. Subsequently, when the magnetic head 7 has attained the point P, the latter is supplied with the video signal of the succeeding field, the recording of which is stopped at the point Q, whereby a record track 20 shown in Fig. 3 is formed. In this way, the magnetic heads 6 and 7 are alternately brought into the recording state in the angular range defined between the points P and Q in each of successive rotations of the magnetic head assembly, whereby the record tracks containing video information are alternately formed on the field-by-field basis through the alternate recording operations of the magnetic heads 6 and 7, respectively. As described hereinbefore, the magnetic heads 6 and 7 have the respective head gaps of azimuth angles which differ from each other. Accordingly, the record tracks formed by the magnetic heads 6 and 7, respectively, can be so realized that both tracks are partially superposed onto each other. As the result of this, the record track can be produced with a width smaller than that of the gap of the magnetic heads 6 and 7, rendering it unnecessary to provide the guard band to thereby allow the recording density to be significantly increased. In the reproducing or playback operation, the magnetic heads 6 and 7 are caused to scan, respectively, the record tracks having directions of magnetization matching with the respective azimuth angles in the course of travel from the point P to the point Q through the tracking control technic employed in the hitherto known recording/reproducing apparatus of two-head helical scan type. Since the width of the record track is narrower than the gap width of the magnetic head (6,7), the latter will also scan a part of the adjacent record track simultaneously. However, reproduction of the video signal from the adjacent track is effectively supressed due to the azimuth loss. Further, even when the magnetic head (6, 7) is displaced more or less in the direction widthwise of the relevant record track, the reproduced output signal is prevented from being degraded. By the way, when the period of one field of the video signal in standard system is represented by T f , the magnetic head (6, 7) is periodically rotated at the frequency corresponding to T f , whereby the recording or reproduction of the video signal of one field is carried out during a time span from the point P to the point Q, i.e. for the time given by T f (360° ϑ°/360°). Accordingly, for the recording operation, the standard video signal must be compressed in respect of the time base or axis thereof by a factor of (360° ϑ°/360°). On the other hand, upon reproduction, the reproduced video signal has to be expanded in respect of the time axis by a factor of 360°/(360°-ϑ°). Additionally, it should be mentioned that when the magnetic head 7 has attained the point Q starting from the point P, the magnetic head 6 has reached the point P. However, when the magnetic head 6 has subsequently reached the point Q starting from the point P, the magnetic head 7 is still on the way to the point Q. In order that the magnetic head 7 reaches the point P, the head 7 has to be further rotated by 2ϑ°. In this way, the magnetic heads 6 and 7 differ from each other as to the chronographical position in the recording or reproducing period extending from the point P to the point Q in each rotation of the magnetic head assembly 5. For these reasons, there are required means for establishing the timing at which the video signal of one field is supplied to the magnetic heads 6 and 7 in the recording operation and/or means for establishing continuity between the video signals produced by the magnetic heads 6 and 7, respectively, in the reproducing operation. Referring to Fig. 4, there is shown in a block diagram a general arrangement of a recording/reproducing circuit of the magnetic video signal recording/reproducing apparatus according to an embodiment of the invention, which circuit is designed to control the recording and/or reproducing operation of the magnetic heads 6 and 7 in consideration of the facts mentioned above. In this connection, it should be noted that only a main portion of the recording/reproducing circuit is illustrated for clarifying the characteristic features of the invention. Referring to Fig. 4, a reference numeral 21 denotes an input terminal to which video signal to be recorded is applied. A numeral 22 denotes a time axis conversion circuit for compressing the input video signal in respect to the time axis or base in the recording operation on one hand and expanding the video signal in respect to the time axis in the reproducing operation on the other hand. The time axis conversion circuit 22 has an output terminal at which the time-axis compressed signal is produced and to which is connected a frequency modulator circuit 23 for modulating the time-axis compressed signal so as to be suited for the recording. The modulated signal output from the frequency modulator 23 is supplied to recording amplifier circuits 26 and 27 by way of gate circuits 24 and 25, respectively, which are controlled in synchronism with the rotational phases of the magnetic heads 6 and 7. Reference numerals 36, 37, 38 and 39 denote switches for changing over the recording mode and the reproducing (playback) mode to each other. In the recording operation, these switches 36 to 39 are thrown to respective contacts labelled R, whereby the output signals of the recording amplifiers 26 and 27 are supplied to the magnetic heads 6 and 7, respectively. On the other hand, when the switches 36 to 39 are thrown to respective contacts P, the magnetic heads 6 and 7 are connected to pre-amplified to an adder circuit 32 by way of gate circuits 30 and 31, respectively. These gate circuits are opened or enabled in coincidence with periods during which the magnetic heads 6 and 7 trace the record tracks, respectively. The output signal of the adder circuit 32 is applied to the input of a demodulator circuit 33 for demodulating the frequency-modulated signal. The modulated signal undergoes the time-axis expansion processing in the time axis conversion circuit 22 to be restored to the original signal which is then outputted from an output terminal 35. For controlling the timing of operations performed by the gate circuits 24, 25, 30 and 31 and the time axis conversion circuit 22, a tach. signal or tacho-signal is made use of which is produced by a tacho-signal (or tach. signal) generator circuit 34 on the basis of the rotation of the rotary head assembly 5. Next, operation of the recording/reproducing circuit shown in Fig. 4 will be elucidated. Referring to Fig. 4, the video signal in a standard system is supplied to the time axis conversion circuit 22 from the input terminal 21. As described hereinbefore in conjuction with Figs. 2a and 2b, the video signal of a standard system in which the period of one field is represented by T f must be recorded with in a period of (360° ϑ°)·T f /360° during which the magnetic heads 6 and 7 are, respectively, brought into contact with the magnetic tape 1 over the range extending from the point P to the point Q in each of successive rotation of the magnetic head assembly 5. To this end, the video signal supplied to the time axis conversion circuit 22 is compressed in respect to the time axis or base at the ratio of (360° ϑ°)/360°. The time axis conversion circuit 22 may be constituted, for example, by a storage such as a random access memory to which writing and reading operations can be conducted at arbitrary timing and speed, an analog-to-digital (A/D) converter circuit and a digital-to-analog (D/A) converter circuit. The video signal supplied to the time axis conversion circuit 22 is converted into a corresponding digital signal which is subsequently written in the storage or memory device, which is followed by the reading of the stored signal from the memory effected at a speed corresponding to the writing speed multiplied by 360°/(360° ϑ°), whereby the digital signal whose time axis or base is compressed by a factor of (360° ϑ°)/360° can be obtained. Through the digital-to-analog conversion of the time-axis-compressed digital signal, the analog video signal whose time axis is correspondingly compressed can be obtained. Further, the time axis conversion circuit 22 is supplied with the tacho-signal which is generated by the tacho-signal generator 34 in synchronism with the rotational phases of the magnetic heads 6 and 7, whereby the time axis conversion circuit 22 is so controlled that the recording of the video signal may be carried out at the timing described hereinbefore by referring to Figs. 2a and 2b, to thereby establish the timing at which the time-axis-compressed video signal is read out. A typical example of the circuit configuration of the time axis conversion circuit 22 is shown in Fig. 6. The video signal supplied from the input terminal 21 and the video signal output from the domodulator circuit 33 in the reproducing operation can be selectively changed over by means of a switch circuit 221. This switching operation is controlled by a mode signal supplied to a mode signal input terminal 50 from an operation mode selector unit (not shown) and indicating the recording mode or alternatively the reproducing mode. The mode signal may consist of such a signal which assumes a "H" level when the recording mode is selected while taking a "L" level in response to the selection of the reproducing or playback mode. The video signal output from the switch circuit 221 is applied to a horizontal synchronizing signal separator circuit 222 on one hand and to an analog-to-digital (A/D) converter circuit 224 constituting a part of the storage or memory unit 220 on the other hand. In the horizontal synchronizing signal separator circuit 222, the horizontal synchronizing signal is separated. The output signal of the separator circuit 222 is supplied to a memory driver circuit 223 of the memory unit 220. The memory driver circuit 223 serves to prepare a reference clock signal which is in synchronism with the applied horizontal synchronizing signal. On the other hand, the video signal supplied to the analog-to-digital converter circuit 224 of the memory unit 220 is sampled by a sampling pulse signal which is synchronized with the reference clock signal produced by the memory driver circuit 223 to be thereby converted into a digital signal which is then written in a random access memory 225 constituting a main part of the memory unit 220. The signal written in the random access memory 225 is read out at the timing of a clock signal produced by the memory driver circuit 223 and supplied to a digital-to-analog (D/A) converter circuit 226 to be converted into an analog signal. As will be seen, the writing and reading operation of the random access memory 225 are controlled by the clock signal supplied from the memory driver circuit 223. Accordingly, by differentiating the writing speed and the reading speed from each other, i.e. by making the frequency of the clock signal for the writing operation differ from that of the clock signal for the reading operation, the time axis compression or alternatively the time axis expansion can be correspondingly realized. The timing for the writing operation and the reading operation to and from the memory unit 220 is controlled by the tacho-signal supplied from the tacho-signal generator circuit 34. The video signal output from the digital-to-analog converter circuit 226 is applied to a switch circuit 227 to be supplied to a frequency modulator circuit 23 for recording or alternatively to a reproduced signal output terminal 35 in dependence on whether the mode signal applied to the switch circuit 227 commands the recording operation or alternatively the reproducing operation. In the case of the embodiment shown in Figs. 4 and 6, the time axis conversion circuit is so designed as to serve for both the recording and reproducing operations with the same circuit configuration. It is however obvious that a time axis compression circuit only for the recording operation and a time axis expansion circuit only for the reproducing operation may be separately provided. The video signal thus compressed in respect to the time axis and produced by the time axis conversion circuit 22 are supplied to the gate circuits 24 and 25 after having been frequency-modulated by the frequency modulator 23. The gate circuits 24 and 25 are operated in synchronism with the tacho-signal in such a manner in which the gate circuit 24 is enabled or opened during the travel of the magnetic head 6 from the point P to the point Q (refer to Fig. 2a), while the gate circuit 25 is opened so long as the magnetic head 7 is moving from the point 7 to the point Q in the succeeding rotation of the head assembly. In other words, the gate circuits 24 and 25 are opened alternately with each other for every successive field of the time-axis-compressed video signal. As the result, the recording amplifiers 26 and 27 are supplied with the video signal alternately on the field-by-field basis, the amplified video signal outputs of these amplifiers being supplied to the relevant magnetic heads 6 and 7, respectively. Next, description will be made of the reproducing operation. In the reproducing or playback mode of operation, the change-over switches 36, 37, 38 and 39 are thrown to the respective contacts labelled P. The travel of the magnetic tape is controlled in a similar manner as is in the case of the conventional magnetic video recording/reproducing apparatus so that the magnetic heads 6 and 7 may scan for reproduction the record tracks having the directions of magnetization coinciding with the azimuth angles thereof, respectively. As the result, the preamplifier circuits 28 and 29 are supplied with the time-axis-compressed video signal from the associated magnetic heads alternately for each of the successive fields. The amplified video signal outputs from the preamplifiers 28 and 29 are supplied to the gate circuits 30 and 31, respectively. The gate circuits 30 and 31 are operated in synchronism with the tacho-signal produced by the tacho-signal generator in such a manner in which the gate circuit 30 is opened or enabled during the period in which the magnetic head 6 travels from the point P to the point Q (refer to Fig. 2a), scanning for reproduction the record track appropriate to the azimuth angle of the magnetic head 6, while the gate circuit 31 is opened in the succeeding rotation of the magnetic head assembly so long as the magnetic head 7 is travelling from the point P to the point Q for scanning the record track appropriate to the azimuth angle thereof for reproducing the recorded video information. In this way, only the video signal as desired are allowed to pass through the gate circuits 30 and 31, whose output video signals are added together by the adder circuit 32 to be subsequently demodulated by the frequency demodulator circuit 33. The demodulated video signal is then supplied to the time axis conversion circuit 22. The time axis conversion circuit 22 is controlled by the tacho-signal produced by the tacho-signal generator 34 in the manner described hereinbefore. More specifically, the supplied video signal is written in the memory unit 220 of the time axis conversion circuit 22 at a writing speed equal to the reading speed in the recording operation and subsequently read out from the memory unit 220 at a reading speed equal to the writing speed multiplied by (360° ϑ°)/360°. Accordingly, there makes appearance at the output terminal the video signal expanded in respect to the time axis or base by a factor of 360°/(360°-ϑ°), i.e. the video signal having the same time axis as that of the standard system. The memory unit 220 of the time axis conversion circuit 22 destined for performing compression and expansion of the time axis of video signal as described above should have a storage capacity corresponding to a product of the storage capacity required for storing at least one field of the video signal multiplied by 2ϑ°/360°. Next, description will be made on the time axis compression of the video signal and the timing at which the video signal is supplied to the magnetic heads 6 and 7 on the assumption that the memory unit 220 as employed has the memory capacity mentioned above. Referring to Fig. 2a, it is assumed that a point Q' lies at an angular distance ϑ° from the point Q on the side opposing to the point P and that the rotational phase of the head assembly 5 is synchronized with the video signal of the standard system in such a manner in which one field of the video signal of the standard system is started from a time point at which the magnetic head 6 is positioned at the point Q and thus the magnetic head 7 is positioned at the point Q'. From this time point, the writing operation of one field of video signal in the memory unit 220 is initiated, while the read-out operation of that field of video signal from the memory unit 220 is started at the moment when one of the magnetic heads 6 and 7 which is to effect the recording operation has attained the position P. On these conditions, the time-axis compressing operation performed by the memory unit as well as the timing of read-out operation will be described in more detail by also referring to Figs. 5a and 5b wherein Fig. 5a shows a timing diagram for illustrating the video signal of a standard system supplied to the memory unit 220 from the input terminal 21 and Fig. 5b shows a timing diagram for illustrating the time-axis-compressed video signal obtained through the memory unit 220. In Fig. 5a, each of numerals 40 to 45 denotes one field of video signal of the standard system in concern, respectively, while 40a to 45a in Fig. 5b denote the corresponding fields of the time-axis-compressed video signal. Assuming now that the field 40 of video signal (hereinafter simply referred to as the field) is written in the memory unit 220 after the magnetic heads 6 and 7 have reached the points Q and Q', respectively, the field 40 is written at addresses in a sequential order. In this connection, it is assumed that the memory unit is so arranged that when the video information is written at the last address thereof, the write-in operation is performed continuously and sequentially, starting again from the first address. On the assumption, the read-out operation is initiated at the reading speed mentioned hereinbefore, i.e. at the speed corresponding to the writing speed multiplied by 360°/(360°-ϑ°) from the first address where the writing operation was made at first, at the moment the magnetic head 7 has reached the point P. In this way, the writing and reading operations are repeated in the memory unit in a manner in which the writing operation is followed up by the reading operation at higher speed. Accordingly, the field 40 inputted to the memory unit 220 is outputted therefrom as the field 40a which is compressed in the time axis by a factor of (360°-ϑ°)/360° and which is supplied to the magnetic head 7 in the manner described hereinbefore. When the magnetic head 7 has attained the point Q', the whole field 40 has been completely written in the memory unit 220 whereupon writing of the succeeding field 41 is initiated from the next address. At that time point, the magnetic head 6 is at the point Q. Further, at that time point, the last portion of the field 40 corresponding to the period of T f ·ϑ°/(360-ϑ°) remains to be read, This portion of the field 40 is completely read out from the memory unit 220 during a period in which the magnetic head 7 is rotated from the point Q' to the point Q. Upon reaching of the magnetic head 7 at the point Q, the reading of the whole field 40 has been completed. At that time point, the magnetic head 6 is located at the point P and a first portion of the field 41 corresponding to the period of T f ·ϑ°/360° is stored in the memory unit 220 while the reading of the field 41 is initiated, starting from the address where it was written at first. The writing operation of the field 41 is performed continuously starting again from the first address upon reaching the last address. In this way, the writing and reading operation being followed up by the reading operation at a higher speed. As the result, the field 41 inputted to the memory unit 41 is outputted therefrom as the field 41a which is compressed in the time axis by a factor of (360°-ϑ°)/360° as compared with the input field 41. The time-axis-compressed field 41a (of video signal) is then supplied to the magnetic head 6 in the manner described hereinbefore. When the magnetic head 6 has reached the point Q, the whole field 41 has been completely read out from the memory unit, and at the same time the writing of the succeeding field 42 is initiated, starting from the address succeeding to the one where the field 41 has been written. Upon reaching of the magnetic head 7 at the point p, the reading of this field 42 is started, whereby the field 42a which corresponds to the field 42 compressed in the time axis is obtained to be supplied to the magnetic head 7. As will now be understood, the timing for reading the video information from the memory unit 220 is determined in association with the positions of the magnetic heads 6 and 7. This timing for the reading operation is established on the basis of the tacho-signal generated by the tacho-signal generator 34, as described hereinbefore. In this connection, it should be noted that although the video signal obtained from the memory unit 220 is continuous between the field 40a supplied to the magnetic head 7 and the field 41a supplied succeedingly to the magnetic head 6, there exists an interval ΔT between the field 41a and the field 42a which is succeedingly supplied to the magnetic head 7, as can be seen from Fig. 5b. This interval ΔT is required for the writing operation effected during the travel of the magnetic head 7 from the point Q' to the point P after the time-axis-compressed video signal to be supplied to the magnetic head 6 has been completely read out, as described hereinbefore, and defined as follows: ΔT = 2·T f ·ϑ°/360° To say in another way, this means that the memory unit performs completely the time axis compression of video signal of the standard system at every second field, i.e. at every frame. Next, the time-axis expanding operation of the memory unit 220 will be elucidated by also referring to Figs. 5b and 5c. Since the relative timing at which the fields of video signal are reproduced by the magnetic heads 6 and 7, respectively, can be considered to be identical with the relative timing at which the fields of video signal are supplied to the magnetic heads 6 and 7, respectively, a series of the fields illustrated in Fig. 5c may be duly regarded as representing the time-axis-compressed video signal reproduced by the magnetic heads 6 and 7 and supplied to the memory unit 220 of the time axis conversion circuit 22. On the other hand, Fig. 5c illustrates the time-axis-expanded video signals obtained through the memory unit 220. It will be noted that 40b to 45b represent the fields resulting from the time axis expansion of the fields 40a to 45a, respectively. Assuming now that the magnetic head 7 is travelling from the point P to the point Q for reproducing the record track having the direction of magnetization matched with the azimuth angle thereof, the memory unit 220 performs the writing operation of the field 40a in the similar manner as the case of the recording operation, while executing simultaneously the reading operation at a reading speed corresponding to the writing speed multiplied by (360° ϑ°)/360°, starting from the first address where the video information was first written. Thus, the writing and reading operations are repeated in such a manner in which the writing operation is followed up by the reading operation which is performed at a lower speed than the former. When the magnetic head 7 has attained the point Q, the reproduction of the field 40a through the magnetic head 7 comes to an end. At that time point, however, the last portion of the field 40a which corresponds to the period of T f ·ϑ/(360°-ϑ°) remains to be read out. This remaining portion of the field 40a is readout by the magnetic head 7 during the rotation thereof from the point Q to the point P. In this way, the field 40b resulting from the time axis expansion of the field 40a can be obtained from the memory unit 220. On the other hand, at the aforementioned time point at which the magnetic head 7 has reached the point Q, the magnetic head 6 is at the point P. During the rotation of the magnetic head 6 from this point P to the point Q, the head 6 scans the record track having the direction of magnetization matched to the azimuth angle thereof, whereby the field 41a is reproduced. This field 41a is sequentially written in the memory unit 220 starting from the address located next to the last address where the video signal of the field 40a has been written. When the magnetic head 6 has attained the point P' distanced from the point P by the angle ϑ° in the rotating direction of the head, the read-out operation of the field 41a is started. The writing and reading operations of the field 41a are repeatedly performed in the manner described hereinbefore, resulting in that upon reaching of the magnetic head 6 at the point Q, the writing operation of the field 41a is completed while the reading of the field 41a is continued until the magnetic head 6 has attained the point P'. In this way, the field 41b is produced from the memory unit 220 as the result of the time axis expansion of the field 41b. When the magnetic head 6 has reached the point P', the magnetic head 7 is located at the point P from which the magnetic head 7 starts the reproduction of the field 42a, whereby the field 42b is produced through the time-axis-expanding operation performed by the memory unit 220. In the similar manner, the fields 42b, 43b, 44b, 45b and so forth are obtained through the time axis expansion. It goes without saying that the video signal illustrated in Fig. 5c which has undergone the time axis expansion is the signal of the standard system in concern. There is illustrated in Fig. 5d an example of the tacho-signal produced by the tacho-signal generating circuit 34. As described hereinbefore, the tacho-signal is generated in synchronism with the rotation of the magnetic heads 6 and 7. For example, such tacho-signal may be produced in principle in the manner similar to the generation of the switching signal utilized for changing-over of the magnetic heads in the conventional two-head helical scan type of magnetic recording/reproducing apparatus. More specifically, means for detecting rotation of the magnetic heads 6 and 7 such as, for example, electro-magnetic sensor element may be mounted on a member rotatable together with the magnetic heads at a predetermined position relative thereto, wherein the rotation detecting signal output of the sensor element is shaped into the tacho-signal. In case the angular distance ϑ between the magnetic heads 6 and 7 shown in Fig. 2a is, for example, equal to 60°, no recording is made during one sixth of a rotation of the head assembly. Accordingly, the radio of compression required for the recording of video signal of standard system in concern is 5/6, which in turn means that the frequency of video signal must be inceased by a factor of 1.2. This value can be readily realized by improving the performance or properties of the magnetic heads, the magnetic tape and the like to some degree. In this case, the memory unit 220 of the time axis conversion circuit 22 may be of the storage capacity which permits one third (or 2ϑ°/360°) of one field of the standard system to be stored. By way of example, assuming that the video signal is sampled at a sampling frequency to be converted into the 8-bit digital signal for being written in the memory unit 220, the latter may have the storage capacity of 56 K bytes, because 1 60 x 1 3 x 10 x 10⁶ = 56 x 10³ (bytes) The memory unit of such storage capacity can be inexpensively realized in the present-day state of memory technology. In this connection, it should be mentioned that the reading speed is 1.2 times as high as the writing speed in the recording operation and vice versa in the reproducing operation. It will be now understood that in the case of the embodiment described in the foregoing, the number of rotations of the head assembly is increased as compared with the conventional magnetic recording/reproducing apparatus of the two-head helical scan type, which means that the cylindrical magnetic head assembly can be realized in a reduced diameter in view of the relative speed between the magnetic heads and the magnetic tape. By virtue of this feature, the magnetic recording/reproducing apparatus according to the illustrated embodiment can be manufactured in a small size and a light weight while assuring a satisfactory image quaIity of the reproduced picture. Further, since the azimuth recording system can be adopted, the recording density can be increased and the reproduction of the recorded video signal can be accomplished without any loss or drop-out of video information. In case the audio or sound signal is recorded on the magnetic tape in the longitudinal direction thereof, there arises deviation ΔT in time between the reproduced audio and video signals, as can be seen from Figs. 5a to 5c. Such deviation is however negligibly small, giving rise to no practical problem. By the way, in case the aforementioned ratio a of compression is 5/6, the deviation ΔT is on the order of 3 m sec and involves no practical problem in consideration of the fact that deviation of one field is usually permissible. In the embodiment described above, the video signal of the standard system may be either for the broadcasting or from a video camera. In this connection, it should be pointed out that in the case of a magnetic recording/reproducing apparatus which incorporates a video camera, the time axis compression of the video signal can be omitted in the recording operation provided that the video camera is so designed as to perform a special scanning upon recording the video signal supplied from the video camera, as will be mentioned below. A magnetic recording/reproducing apparatus embodied according to the invention with the above consideration taken into account will now be described. The video camera may be regarded as a sort of the storage or memory unit. By taking advantage of this fact, the scanning operation for reading out the video signal from the imaging device is synchronized with the rotation of the magnetic heads 6 and 7 (Fig. 2a). In that case, the scanning speed is so selected that the period required for scanning one frame is equal to the period required for two rotations of the magnetic head (6, 7) and multiplied by a factor of (360° ϑ°)/360°, while the scanning is interrupted for a period ΔT = 2·T f ·ϑ°/(360°-ϑ°) in every frame. Through the scanning operation effected in this manner, the video signal whose time axis is compressed as shown in Fig. 5b can be obtained from the video camera. The video signal may be directly supplied to the frequency modulator 23 shown in Fig. 4. In the reproducing operation, the video signal of the standard system can be obtained at the output terminal after having been processed by the reproducing circuitry shown in Fig. 4. The aforementioned scanning operation of the video camera can be readily realized through simple modification of the deflecting system in the case of a conventional type vidicon. When a solid-state imaging device is employed, the process of scanning the pixels has to be slightly modified. So long as the scan interrupt period ΔT is short, unnaturalness will not be observed in the reproduced picture to any appreciable degree. By the way, the record tracks (Fig. 3) formed on the magnetic tape in the magnetic recording/reproducing apparatus according to the embodiments of the invention described in the foregoing can be produced in the pattern utterly identical with that of the record tracks formed by the hitherto known two-head helical scan type magnetic recording/reproducing apparatus for domestic use in which the azimuth recording is adopted. Accordingly, the record tracks produced by the inventive apparatus can be reproduced by using the conventional magnetic recording/reproducing apparatus for home use. To this end, the diameter ' and the lead angle ϑ'₀ of the head cylinder of the conventional recording/reproducing apparatus for home use should be so selected as to satisfy the following conditions: and πα sinϑ₀ = π 2 ' sin ϑ'₀ (2) where  and ϑ₀ represent, respectively, the diameter and the lead angle of the head assembly drum of the magnetic recording/reproducing apparatus according to the invention. In the expression (1), the righthand member assumes the minus sign when the scanning direction of the magnetic head is same as the travelling direction of the magnetic tape, while taking the plus sign when both the directions are opposite to each other. In both of the above expressions (1) and (2), V t represents travelling speed of the magnetic tape, f V represents the field frequency, and α represents the ratio of compression. By way of example, in the case of a so-called VHS-system video tape recorder of two-hour mode, ' = 62 mm, ϑ'₀ = 5°56′ 7.4″, V t = 33.35 mm/sec, and f V = 59.94 (NTSC system) Accordingly, when α = 5/6,  = 37.167 mm, and ϑ₀ = 5°58′22″. Obviously, the diameter of the cylindrical head assembly of the magnetic recording/reproducing apparatus can be reduced to about 60 % of that of the magnetic head cylinder of the video tape recorder of VHS system and thus implemented in a miniature size, and the magnetic tape carrying the record tracks on which the video signal produced by the video camera incorporated integrally in the magnetic recording/reproducing apparatus can be replayed by the conventional video tape recorder of VHS system. It has been an important problem to realize the magnetic recording/reproducing apparatus incorporating integrally a video camera in a small size and light weight unit to enhance the portability for the convenience of use. Figs. 7a, 7b and 7c show timing diagrams for illustrating an embodiment of the magnetic recording/reproducing apparatus according to the present invention. In the figures, 41c, 42c and 43c denote overlap record signals (i.e. the signals recorded in overlap), Tr represents a period of one frame. T o represents an overlap recording period, and ΔT' represents the drop-out period. Parts corresponding to those shown in Figs. 5a to 5c are denoted by like reference symbols. Usually, in the magnetic recording/reproducing apparatus of two-head helical scan type, a part of the same video signal is recorded in overlap between the adjacent track with a view to preventing influence of variation in the switching position of the magnetic head from appearing on the reproduced picture. However, in the magnetic recording/reproducing apparatus described hereinbefore in conjunction with Figs. 5a to 5d, the overlap recording is not performed because the drop-out period ΔT is inserted in succession to the completed recording by the magnetic head 6 (Fig. 2a). According to the embodiment of the invention, it is intended to perform the overlap recording within a period corresponding to ΔT shown in Fig. 5b. To this end, an overlap period T o is set in every frame as is shown in Fig. 7a to thereby allow the same video signal to be recorded on the two adjacent tracks, respectively, within the period T o . The magnetic recording/reproducing apparatus according to the embodiment of the invention can be realized in principle in the same circuit configuration as those shown in Figs. 4 and 6. A difference from the latter is seen in the storage capacity of the memory unit 220 constituting a main part of the time axis conversion circuit 22. More particularly, according to the instant embodiment illustrated in Figs. 7a to 7c, the random access memory 225 shown in Fig. 6 must have a storage capacity greater than the value described in conjunction with Fig. 5 by a factor of (Tr + T o )/Tr. Further, the timing at which the individual fields are written in and read out from the memory unit 220 is slightly modified as mentioned below. Fig. 8 is a schematic plan view of the cylindrical magnetic head assembly for illustrating the timing for operation of the memory unit 220. In the figure, parts corresponding to those shown in Fig. 2a are denoted by same reference sumbols. Referring to Fig. 8, it is assumed that a point P₂ is located on the head cylinder 5 at an angular distance δ from the point P in the direction opposite to the rotating direction B of the head cylinder 5, and that point Q₂ and Q₃ are located at angular distances δ' and δ from the point Q in the directions opposite to and same as the rotating direction B of the head cylinder 5, respectively. One field of the video signal is read out from the memory unit to be recorded by the magnetic head 6 or 7 in the course of rotation of the head cylinder or assembly 5 from the point P to the point Q, as is in the case of the embodiments described hereinbefore. However, it is assumed that the writing of each field to the memory unit is started at the time point at which the magnetic head 6 is present at the point Q₂. The wrap angle of (360°- ϑ°) at which the magnetic tape is wound around the head cylidner 5 is so set that the magnetic tape (not shown) is scanned by the magnetic heads 6 and 7 in the course of rotation thereof at least from the point P₂ to the point Q₃. (refer to Figs. 2a and 8). Also referring to Figs. 7a and 7b, the field 40 (of video signal) is written in the memory unit by the magnetic head 6 starting from the point Q₂, and the reading of the field 40 from the memory unit is started at a higher speed when the magnetic head 7 has reached the point P, whereby the field 40a having the time axis or base compressed is recorded by the magnetic head 7. Upon reaching of the magnetic head 6 at the point Q₂, the writing operation of the succeeding field is started. However, the reading of the field 40 continues until the magnetic head 7 has attained the point Q. Upon arrival of the magnetic head 7 at the point Q, the magnetic head 6 reaches the point P, whereupon the reading of the field 41 from the memory unit is started, resulting in that the time-axis-compressed field 41a is supplied to the magnetic head 6. At the time point when the magnetic head reaches the point Q₂, the writing of the succeeding field 42 to the memory unit is initiated. When the magnetic head 6 reaches the point Q, the reading of the whole field 41 is completed. However, in the course of further rotation of the magnetic head 6 from the point Q to the point Q₃, the field 42 written in the memory unit is read out. Thus, the magnetic head 6 is supplied with the time-axis-compressed field 41a together with a signal part 42c (overlap record signal) of the time-axis-compressed field 41a to be recorded on the same track in overlapping relation. Next, upon arrival of the magnetic head 7 at the point P₂, the reading operation from the memory unit is started from the address at which the preceding field 41 was written rather than the first of the addresses at which the field 42 was written by skipping a number of the addresses from which the reading of video signal is performed in the course of rotation of the magnetic head 7 from the point P₂ to the point P. When the magnetic head has reached the point P, the reading of the field 42 is then started. In this way, the reading operation from the memory unit is started from the preceding predetermined address, wherein the reading of the field 42 is effected during a period required for the magnetic head 7 to attain the point Q. In the course of rotation of the magnetic head 7 from the point P₂ to the point Q, the overlap record signal 41 which constitutes a part of the preceding time-axis-compressed field 41 and the field 42a are read out and supplied to the magnetic head 7 to be recorded on the same recording track. The writing and reading of further fields to and from the memory unit are effected in the similar manner, whereby the record tracks containing the overlap portions, respectively, are formed on the magnetic tape by means of the magnetic heads 6 and 7. In reproducing operation, the field 41a and the overlap record signal, for example, are written in the memory unit during the scanning period of the magnetic head 6 corresponding to the rotation thereof from the point P to the point Q₃, which is followed by the writing of the overlap record signal 41c and the field 42a during the scanning period of the magnetic head 7 from the point P₂ to the point Q. The reading operation is performed by jumping from the address at which the field 41a was written to the one succeeding to the address where the overlap record signal carrying information corresponding to that of the field 41a was written or alternatively by jumping from the address where the overlap record signal 42c was written to the one succeeding to the address where the field 42a of the video information corresponding to that of the overlap record signal 42 was written, whereby the continuous original video signal of the standard system in concern can be obtained, as is illustrated in Fig. 7c. In this conjunction, it should be mentioned that the reproduction can also be accomplished by using a hitherto known magnetic recording/reproducing apparatus of the two-head helical scan type. In any case, influence of variations in the change-over or switching position of the magnetic head can be positively prevented from making appearance on the reproduced picture by virtue of the presence of the portion recorded in overlap as described above. Since the period T o for the overlapped recording is sufficiently short as compared with the one-frame period T r , it is sufficient to increase only a little the storage capacity of the memory unit for carrying out the overlapped recording. Figs. 9a, 9b and 9c are timing diagrams for illustrating a further embodiment of the magnetic recording/reproducing apparatus, wherein parts corresponding to those shown is Figs. 5a to 5c are denoted by same reference symbols. In the case of the embodiments described in conjunction with Figs. 5 and 7, the magnetic heads 6 and 7 are disposed with an angular distance therebetween which corresponds to the angular distance ϑ° between the points Q and P. In the instant embodiment mentioned below, it is intended to dispose the magnetic heads 6 and 7 with a smaller angular distance β° than the angle ϑ°. Except for this difference, other arrangement is same as in the case of the embodiment described before in conjunction with Figs. 5a to 5d. More specifically, referring to Figs. 9a to 9c in combination with Fig. 2a in which the magnetic heads 6 and 7 should however be assumed to be disposed at a smaller angular distance β° rather than ϑ°, the field 40 begins to be written in the memory unit 220 of the time axis conversion circuit 22 from the time point when the magnetic head 6 is located at the point Q, while the reading of the field 40 is started when the magnetic head 7 has reached the point P. At this time point, a part of the field 40 which corresponds to the period of T f . (ϑ° + β°)/360° has been stored in the memory unit. When the magnetic head 6 reaches the point Q, the writing of the succeeding field 41 to the memory unit is started, while the reading of the field 40 comes to an end upon arrival of the magnetic head 7 at the point Q. During rotation of the magnetic head 7 from the point P to the point Q, the time-axis-compressed field 40a is obtained from the memory unit to be recorded through the magnetic head 7. Next, at the moment the magnetic head 6 has attained the point P, the reading of the field 41 from the memory unit is started. At that time point, the field 41 of a period corresponding to T f ·ϑ°/360° has been stored. The time interval ΔT'' interposed between end of reading the preceding field 40 and the start of reading the succeeding field 41 is given by T f ·(ϑ° - β°)/360°. When the magnetic head 6 has reached the point Q, the writing and reading operations to and from the memory unit are completed substantially simultaneously, whereupon the writing of the next succeeding field 41 is started. In the course of rotation of the magnetic head 6 from the point P to the point Q, the time-axis-compressed field 41a is outputted from the memory unit to be recorded on the magnetic tape through the magnetic head 6. Upon reaching of the magnetic head 7 at the point P, the reading of the field 42 from the memory unit is started. The further succeeding fields are sequentially written in and read out from the memory unit in the similar manner to be alternately recorded by the magnetic beads 6 and 7. Also in this case, the time interval ΔT' intervaning between the completed reading of the preceding field 41 from the memory unit and the start of reading the succeeding field 42 is given by T f ·(ϑ° + β°)/360°. In this way, there can be derived through the memory unit the time-axis-compressed vided signal in which the drop-out intervals ΔT' and ΔT'' are alternately present at every field. It should be noted that the sum of ΔT' and ΔT'' is equal ΔT shown in Fig. 5b. In this case, ΔT' is shorter than ΔT''. Difference between the intervals ΔT' and ΔT'' becomes smaller, as the magnetic heads 6 and 7 are disposed closer to each other. It is sufficient for the memory unit to have the storage capacity capable of storing the video signal of the standard system in concern corresponding to T f ·(ϑ° + β°)/360°(=ΔT') . In the playback operation, the time-axis-compressed fields are reproduced by the magnetic heads 6 and 7 alternatively during the respective rotations thereof from the point P to the point Q, wherein the video signal (Fig. 9c) of the standard system in concern can be obtained through the time-axis-expansion effected by reading one field of the once stored and reproduced video signal from the memory unit in each rotation of the head cylinder or assembly in the similar manner as is in the case of the embodiment described hereinbefore in conjunction with Fig. 5 except that the angular distance between the points P and P' (Fig. 2a) is selected equal to β°. The magnetic recording/reproducing apparatus illustrated in Figs. 7 and 8 may be combined integrally with the video camera designed for performing the special scanning as described in conjunction with Fig. 6 to thereby accomplish the similar operation. Further, the record tracks as formed can also be reproduced by the hitherto known two-head helical scan type magnetic recording/reproducing apparatus. Next, description will be made of the magnetic recording/reproducing apparatus according to another embodiment of the invention in which the magnetic heads 6 and 7 are disposed closer to each other. In the preceding embodiments of the invention, the video signal of a unit period, e.g. one field is recorded and reproduced by the two magnetic head alternately. The embodiment described below is so designed that the video signal is recorded and reproduced by a pair of magnetic heads simultaneously. In this case, one of the magnetic head is used for recording and reproducing the luminance signal, while the other is used for recording and reproducing the chrominance signal. The two magnetic heads ae so disposed that the azimuth angles of the respective gaps are different from each other as in the case of the preceding embodiments. Now, description will be made on a concrete arrangement of the embodiment. Figs. 10a and 10b show a rotary head assembly 5 suited for use in the magnetic recording/reproducing apparatus according to the instant embodiment in a plan view and an elevational view, respectively. The magnetic tape 1 is wound around the rotary head assembly 5 at the wrap angle of (360° - °) as in the case of the preceding embodiments. The magnetic heads denoted by 6' and 7' and having respective gaps of azimuth angles differing from each other are disposed closer to each other with an offset or difference in height in the direction of the axis of rotation which substantially corresponds to the pitch of the record tracks (although not show in Fig. 10b). A rotatable cylinder 5a having the magnetic heads 6' and 7' mounted thereon is rotated at a period corresponding to one field of the color video signal in synchronism with the verticalsynchronizing signal of the color television signal of a standard system in concern. For example, in the case of the NTSC system, the head cylinder 5a is rotated at a speed of 3600 rpm. In each rotation of the head cylinder 5a, the magnetic head 6' records or reproduces the luminance signal during rotation from the point P to the point Q over an angular range of (360° - ϑ°) (where ϑ° > °) within the region in which the magnetic tape is wound around the head cylinder 5, while the magnetic head 7' simultaneously records or reproduces the chrominance signal during rotation thereof over the angular range of (360° - ϑ°). More specifically, when the period of one field of the color television signal according to the standard system such as NTSC, PAL, SECAM or the like system is represented by T f , each of the magnetic head 6' and 7' performs a signale complete rotation during the period T f , whereby the luminance signal and the chrominance signal of one field of the period T f are recorded or reproduced in each rotation of the head cylinder during a period defined below: T f ·(360° - ϑ°) (3) while neither recording nor reproduction takes place in the remaining period given by T f ·ϑ°/360°. Fig. 11 shows in a block diagram an exemplary embodiment of the recording/reproducing circuit for recording and/or reproducing the luminance signal in the manner described above. In Fig. 11, the components having functions equivalent to those shown in Fig. 4 are denoted by like reference symbols. In the instant embodiment, the time axis conversion circuit 22 can be implemented in the configuration similar to that of the preceding embodiment shown in Fig. 6. A reference numeral 51 denotes a separator circuit for separating the time-axis-compressed video signal into the luminance signal Y and the chrominance signal C. Numerals 52 and 53 denote frequency modulating circuits for modulating in frequency the luminance signal and the chraminance signal, respectively, so that they are suited for the recording. Numerals 56 and 57 denote demodulator circuits for demodulating the frequency-modulated luminance signal and the frequency-modulated chrominance signals, respectively, which are reproduced from the magnetic tape 1 by means of the magnetic heads 6' and 7', respectively, in the playback or replay operation. The demodulated signals outputted by the demodulators 56 and 57 are supplied to a synthesizing or addition circuit 58 to be added together for re-constituting the composite color video signal whose time axis is in the compressed state. Next, operation of the recording/reproducing circuit will be described. Referring to Fig. 11, the color video signal of a standard system is supplied to the time-axis-compression circuit 22 through an input terminal 21. As described hereinbefore, the luminance signal and the chrominance signal having a period T f corresponding to one field of the standard system in concern have to be recorded within a period in which the magnetic heads 6' and 7' are in contact with the magnetic tape in each rotation of the head cylinder, i.e. within a region extending from the point P to the point Q and expressed by (360° - ϑ°)·T f /360°. To this end, the color video signal supplied to the time axis conversion circuit 22 is compressed in the time axis or base at the ratio of (360° - ϑ°)/360°. The time axis conversion circuit 22 may be realized in the same configuration as the one shown in Fig. 6, as described hereinbefore. Accordingly, by reading the video signal written in the memory unit 220 at a reading speed corresponding to a product of the writing speed multiplied by 360°/(360° - ϑ°), the video signal whose time axis is compressed by a factor of (360° - ϑ°)/360° can be obtained. The tacho-signal produced by the tacho-signal generator 34 is supplied to the time axis conversion circuit 22 in synchronism with the rotational phases of the magnetic heads 6' and 7', whereby the timing at which the time-axis-compressed color video signal is read from the memory unit 220 of the time axis conversion circuit is so controlled that luminance signal and the chrominance signal may be recorded in a manner described above. The memory unit 220 of the time axis conversion circuit 22 may be of a storage capacity equal to at least the storage capacity capable of storing one field of the color video signal and multiplied by a value of ϑ°/360°. With such memory unit, the color video signal is sequentially stored in the random access memory 225 of the memory unit 220 starting from the first address thereof when the magnetic head 6' is in the course of rotation from the point Q to the point P shown in Fig. 10. Upon arrival of the magnetic head 6' at the point P, the video color signal has been stored to the last address. Simultaneously, the sequential reading of the color video signal from the random access memory 225 is initiated starting from the first address thereof at the reading speed corresponding to the product of the writing speed multiplied by 360°/(360° - ϑ°), while the color video signal succeeding to the one stored precedingly is sequentially stored in the random access memory 22 starting from the first address from which the preceding color video signal information has been read out. In this manner, the writing and reading operations are repeated over the first to last addresses of the memory unit 220 in the manner in which the reading operation is followed up by the writing operation at a lower speed than the former. When the magnetic head 6' has reached the point Q, the reading operation is once stopped, and the similar operations are repeated. In this connection, it should be noted that when the magnetic head 6' has reached the point Q, the address at which the color video signal information is written will substantially coincide with the address from which the video signal information is to be read. Accordingly, the succeeding writing operation initiated when the magnetic head 7' is at the point Q is made starting from the first address of the memory unit, whereby the similar operation is repeated. Figs. 12a, 12b and 12c show timing diagram for illustrating operation of the time axis conversion circuit 22 shown in Fig. 11, wherein Fig. 12a shows the color video signal supplied to the time axis conversion circuit 22 and Fig. 12b shows the color video signal resulted from the time axis compression performed by the time axis conversion circuit 22. A symbol T f represents the period of one field, and ΔT represents a time required for the magnetic head 6' shown in Fig. 10a to move from the point Q to the point P. As will be seen in Figs. 12a and 12b, the time-axis-compressed color video signal output from the time axis conversion circuit 22 has a period of (T f - ΔT) in one field and is delayed by ΔT relative to the color video signal inputted to the time axis conversion circuit 22. In the case of the embodiment shown in Fig. 10a, ΔT is equal to T f ·ϑ°/360°. Fig. 12c will be referred to later on. Turning back to Fig. 11, the time-axis-compressed color video signal output from the time axis conversion circuit 22 is supplied to the Y/C separator circuit 51 to be separated into the luminance signal Y and the chrominance signal C. The luminance signal Y is supplied to the frequency modulating circuit 52 to be outputted therefrom as the frequency-modulated luminance signal (hereinafter referred to as the FM luminance signal) which is subsequently supplied to the magnetic head 6' after having been amplified by a recording amplifier 26. On the other hand, the chrominance signal C separated through the separator circuit 51 is supplied to the frequency modulating circuit 53 to be outputted therefrom as the frequency-modulated chrominance signal (hereinafter referred to as the FM chrominance signal) which is then supplied to the magnetic head 7' after having been amplified by a recording amplifier 27. All of the change-over switches 36, 37, 38 and 39 are thrown to the respective contacts R so that the magnetic heads 6' and 7' can record the FM luminance signal and the FM chrominance signal during rotation thereof from the point P to the point Q. Fig. 13 shows a pattern of record tracks formed by the magnetic heads 6' and 7' through the procedure described above. The record tracks 65 and 67 are formed by the magnetic head 6', recording thereon the FM luminance signal, while the record tracks 66 and 68 are formed by the magnetic head 7' and record thereon the FM chrominance signal. It will be seen that the record tracks containing the FM luminance signal and those recording the FM chrominance signal are formed alternately in the longitudinal direction of the magnetic tape 1. The record tracks located adjacent to each other have directions of magnetization which differ from one another in accordance with the azimuth angles of the head gaps of the magnetic heads 6' and 7', respectively. Referring again to Fig. 11, description will be made of the reproducing operation of the recording/reproducing apparatus according to the instant embodiment. In this case, the change-over switches 36, 37, 38 and 39 are thrown to the respective contacts P, whereby the magnetic heads 6' and 7' are allowed to scan for reproduction the record tracks on the magnetic tape during rotation from the point P to the point Q (Fig. 10a). Referring to Fig. 13, the magnetic head 6' scans the record track 65 for reproduction of the color video signal while the magnetic head 7' scans the record track 66 simultaneously with the magnetic head 6', which is followed by the simultaneous scanning for reproduction of the record tracks 67 and 68 performed by the magnetic heads 6' and 7', respectively. In this way, the magnetic head 6' scans sequentially the record tracks on which the FM luminance signal is recorded, while the magnetic head 7' scans sequentially the record tracks on which the FM chrominance signal is recorded. The FM luminance signal reproduced through the magnetic head 6' is demodulated by the frequency demodulating circuit 56 after having been amplified by a preamplifier 28. On the other hand, the FM chrominance signal picked up by the magnetic head 7' is demodulated by a frequency demodulating circuit 57 after having been amplified by a pre-amplifier 29. The demodulated luminance signal Y and the demodulated chrominance signal C are mixed together through the synthesizing circuit 58, the output signal of which undergoes the time axis expansion through the time axis conversion circuit 22 to be reproduced as the color video signal of the standard system in concern and supplied to the output terminal 35. In the time axis conversion circuit, the same memory as employed for performing the time axis compression in the recording operation can be used for the reproduction, as is in the case of the preceding embodiments. The writing speed in the reproducing operation is set equal to the reading speed in the recording operation, while the reading speed in the reproducing operation is set equal to the writing speed in the recording operation. Accordingly, in the reproducing operation, the reading speed corresponds to the writing speed multiplied by (360° - ϑ°)/360°, resulting in that the color video signal outputted from the synthesizing circuit 58 is expanded in the time axis by a factor of 360°/(360° - ϑ°), whereby the standard color video signal is obtained at the output terminal 35. In the memory unit 220 of the time axis conversion circuit 22, the writing operation is effected at the first address whenever the magnetic head 6' reaches the point P (Fig. 10a) which corresponds to the beginning of the recording track. Substantially at the same time, the first address is read out (i.e. immediately succeeding to the writing operation at the first address). The reading and writing oeprations are repeated in the manner in which the writing operation is followd up by the reading operation at a lower speed than the former. At the time point when the magnetic head 6' has attained the point Q, the magnetic heads 6' and 7' are located at the end positions of the respective record tracks, whereupon the writing operation to the memory unit 220 is stopped. At that time, the address at which the color video information is to be written will substantially coincide with the reading address from which the video information is to be read out. The reading operation of the whole memory unit 220 is subsequently effected only once more. This reading operation takes place during the period in which the magnetic head 6' moves from the point Q to the point P i.e. in the region of T f ·ϑ°/360°, resulting in that a quantity of the color video signal information corresponding to the storage capacity of the memory unit 220 is read out. At the moment the magnetic head 6' has attained the point P, reproduction of the luminance signal and the chrominance signal by the magnetic heads 6' and 7' is started. Thus, the writing and reading operations are initiated again starting from the first address of the memory unit 220 of the time axis conversion circuit 22 in the similar manner. In this way, the time axis expansion of the reproduced color video signal is accomplished. Figs. 12b and 12c illustrate timing relation between the input color video signal and the output color video signal of the time axis conversion circuit 22. The beginning of the color video output signal (Fig. 12c) undergone the time axis expansion approximately coincides with the beginning of the time-axis-compressed color video signal (Fig. 12b) inputted to the time axis conversion circuit 22. However, the output color video signal is expanded in the time axis by ΔT (=T f ·ϑ°/360°) relative to the input color video signal. Accordingly, one field of the output color video signal has the period T f . When the timings of the time-axis-compressed color video signal and the time-axis-expanded color video signal produced through the time-axis compressing operation and the time-axis expanding operation of the time axis conversion circuit 22 are considered with reference to the timing of the color video signal of the standard system in concern, the timing relation is such as illustrated in Figs. 12a, 12b and 12c. More specifically, there arises a difference in time represented by ΔT between the color video signal of the standard system to be recorded (Fig. 12a) and the reproduced color video signal of the standard system. As the consequence, there is produced the time difference or deviation between the reproduced color video signal of the standard system and the reproduced audio or sound signal. However, since the difference ΔT is of a sufficiently small value, no problem of deviation does occur in practice between the reproduced picture and sound. Referring to Fig. 14 which is a view for illustrating in detail an example of positional relationship between the magnetic heads 6' and 7' shown in Fig. 10b, reference symbols b1 and b2 designate the gaps of the magnetic heads, respectively. In Fig. 14, parts corresponding to those shown in Fig. 10b are denoted by like reference symbols. The magnetic heads 6' and 7' having the respective gaps 61 and 62 of azimuth angles differing from each other have a gap width (track width) T X and are disposed closely to each other with an offset T Y (where T Y > T X ) in the direction perpendicular to the rotating direction B of the upper cylinder 5a. Information of the record tracks on the magnetic tape through the contact scanning effected by the magnetic heads 6' and 7', the record track associated with the magnetic head 6' is first formed, which is followed by formation of the record track by the magnetic head 7' in partial overlap or superposition on the first formed track with an overlap width of (T X - T Y ). Accordingly, the record track formed by the magnetic head 6' has a width T Y and the record track formed by the magnetic head 7' has a width T X . In the succeeding formation of the second record tracks by the magnetic heads 6' and 7' during a next rotation thereof, the record track associated with the magnetic head 6' is formed in partial overlap of width (T X - T Y ) on the record track formed by the magnetic head 7' in the preceding rotation thereof. This type of recording can be realized by appropriately selecting the speed of the magnetic tape. Consequently, the record track formed by the magnetic head 7' also has the width T Y . In this way, the record tracks are formed in partial overlap on the precedingly formed tracks, whereby the record track having a narrower width than the gap width of the magnetic heads 6' and 7' can be formed. By virture of the record track array described above, there arises no lowering of the level of the reproduced signal due to tracking error even when the magnetic heads 6' and 7' are deviated in position in the direction widthwise of the recording track in the reproducing operation. Since the magnetic heads 6' and 7' have the respective gaps 61 and 62 which differ from each other in respect to the azimuth angle, the record tracks positioned adjacent to each other have mutually different directions of magnetization. When the record track is scanned for reproduction by the magnetic head having the gap of azimuth angle matches with the direction of magnetization of that record track, a part of the adjacent record track is also scanned by the magnetic head. However, because of the feature mentioned above, the video signal is prevented from being reproduced from the adjacent record track due to the azimuth loss. Also in the case of the instant embodiment, the writing and reading timing in the recording and reproducing operations is established on the basis of the tacho-signal produced by the tacho-signal generator 34, as is the case of the preceding embodiments. In the case of the instant embodiment, however, the recording or reproducing operation is performed at the same timing on the field-by-field basis since the two magnetic heads 6' and 7' are operated simultaneously. Further, it is not always necessary to operate the memory unit 220 of the time axis conversion circuit 22 in such a manner in which the writing is initiated starting from the first address of the memory unit 220 when the magnetic head 6' has reached the point Q in the recording operation, while the writing is initiated starting from the first address of the memory unit at the moment when the magnetic head 6 has attained the point P in the reproducing operation. It is also possible to arrange such that the address from which the reading is to be started in the memory unit is designated on the basis of the address at which the writing has been made at the time when the magnetic head 6' has reached the point P in the recording operation, while the address from which the writing is to be started is designated on the basis of the address from which the reading was made when the magnetic head 6' had attained the position or point P in the reproducing operation. In the latter case, the writing can be made in continuation starting from the address which is found at the moment the magnetic head 6' has attained the point Q in the recording operation without returning to the first address of the memory unit, while in the reprocuding operation, the writing can be made continuously starting from the address found at the time when the magnetic head 6' has attained the point Q without returning to the first address of the memory unit.

Claims: Video signal recording/reproducing apparatus of helical scan type, in which record tracks are formed on a magnetic tape with inclination by rotary magnetic head means (5) including a pair of magnetic heads (6, 7; Se', 7') having respective gaps of azimuth angles differing from each other, said magnetic heads being disposed at an angular distance smaller than 180 o , said head means being rotated with a period corresponding to a field period of a video signal and being wound helically with the magnetic tape (1) over a range greater than 180 o ; characterized by first means (22; 77, 78, 79; 88; 84, 85) comprising time axis compressing means (22; 220; 77, 78, 79; 88) being controlled by a signal of a tacho signal generator (34) for generating sequential output signals, which depend on the position of the magnetic heads a period of each output signal corresponds to a period during which the magnetic tape is scanned by one of the magnetic heads (6, 7) during each complete rotation of the rotary magnetic head means (5) a first output signal of said output signals consists of a first field of the video signal and a portion of a succeeding second field of the video signal, whereas a succeeding second output signal consists of a portion of the first field and of the succeeding second field all fields of the video signal are time axis compressed by the time axis compressing means, second means (23, 26, 27; 51, 52, 53; 53a, 53b) for recording the output signals of said first means by said magnetic heads; and third means (24, 25; 34) are provided for supplying the output signals of said first means to said pair of magnetic heads alternately for every field period of said video signal. Video signal recording/reproducing apparatus according to claim 1, characterized by a memory unit (220) for storing said output signals, which constitutes a main part of said time axis compressing means. Video signal recording/reproducing apparatus according to claim 1 or 2, characterized in that said video signal generating means further includes separating means (51;85) for separating luminance signal and chrominance signal from each other from the time-axis compressed video signal and supplying substantially simultaneously said luminance signal and said chrominance signal to said pair of magnetic heads, respectively, said pair of magnetic heads forming substantially simultaneously two different record tracks for said luminance signal and said chrominance signal. Video signal recording/reproducing apparatus according to any of claims 1 to 3, characterized in that the time-axis compressing means are constituted by a video camera apparatus (84) which outputs the time-axis compressed video signal or the time-axis compressed luminance signal and the time-axis compressed chrominance signal within the period during which said magnetic heads scan magnetic tape.

Inventors: KOJIMA, NOBORU, SHIBATA, AKIRA, TERADA, TOSHIMICHI
Applicants: HITACHI, LTD.
Application year: 1983
Main classification: G11B5
IPC codes: H04N9, H04N5