Method of Recording Video Signals Within a Predetermined Bit/Unit Time Recording Rate

The present invention relates to the recording of digitally encoded video data on a recording medium such as magnetic tape, in which a head wheel rotates in a gap of a recording cylinder about which the tape is guided in a spiral path, so that recorded tracks which are inclined with respect to the major axis of the tape will be formed thereon.

Various types of inclined track recording systems and methods are known, in which the tape is looped about a recording cylinder or drum with a looping angle of about 180.degree.. The digitized video signal is distributed to a plurality of channels and is recorded with several magnetic heads on adjacent magnetic tracks. The number of magnetic recording heads rises with the number of channels. Thus, to keep the number of recording heads and thus the construction of the recording transducer unit reasonably simple, the number of channels should be as low as possible. A suitable number is two channels, which is most desirable.

Data rates of digital video signals may be in the order of 200 megabits per second (200 Mbit/s); this means that the data rate of 100 Mbit/s per channel will be needed, which means a band width of about 50 MHz. Currently used recording speeds, however, cause an unacceptable increase in wear of the head, wear on tapes, and errors if a recording rate of about 80 Mbit/s is exceeded.

It is an object to provide a method of recording at a rate of 200 Mbit/s in a two-channel multiplex system with acceptable error rates, without recording, within one channel, at a rate over 80 Mbit/s.

Briefly, the signals are time-transformed in the recording/reproduction process; specifically, prior to recording the signals are time-expanded; upon reproduction, the signals are time-compressed and the tape is passed about the scanning drum at an angle of over 180.degree..

The method and system has the advantage that the entire stream of data can be subdivided on two channels, while yet permitting recording in any one channel at a rate of data transfer, per unit time, which is acceptable, and then correcting for the expanded rate by time compression upon reproduction.

FIG. 1a is a schematic representation of recorded track on magnetic tape in accordance with the prior art;

FIG. 1b is a view similar to FIG. 1a, showing the recording in accordance with the present invention;

FIG. 2 is a highly schematic view of a head wheel, and the tape looped thereabout;

FIG. 3, in graphs a to d, is a series of time diagrams illustrating the signals being recorded; and

FIG. 4 is a schematic circuit diagram of a time transformation circuit.

In accordance with well known technology, as illustrated in FIGS. 1a and 2 a magnetic tape 1 is looped about a head wheel carrier or recording cylinder or recording drum, so that a recording track 2 will be formed thereon, having a length L, and extending at an angle .beta.1 with respect to the longitudinal edge of the tape. In accordance with the invention, the same quantity of data should have more time for recording available to it, that is, in other words, the track should be longer. As will be explained in detail in connection with FIG. 2, the magnetic tape is carried about the recording head by a distance in excess of that obtainable by a looping angle of 180.degree.. Since the width of the tape is predetermined and standardized, for example 1 inch, the additional length of track can be obtained by changing the angle of the track. As seen in FIG. 1b, the track 3 will have a length L' which is longer by the distance .DELTA.L than the length L, and extends at an angle .beta.2 with respect to the longitudinal edge of the tape 1.

The recording head wheel to carry out this extended recording is shown in FIG. 2; a magnetic tape 1 is looped about a head wheel by an angle 180.degree.+.DELTA..alpha.. Magnetic heads 11, 12, 21, 22 are located at the circumference of the head wheel 4. The arrangement of FIG. 2 permits distribution of the digital data on two parallel channels. Two oppositely positioned magnetic heads, each, for example heads 11, 21, are associated with one channel.

The present invention is not limited to use of two channels, but may equally be used with single-channel recording, in which case the head wheel needs only two oppositely positioned heads, rather than four. This is well known, for example, in magnetic recording apparatus for recording analog video signals in accordance with the B-standard.

All elements of the magnetic recording apparatus not necessary for an understanding of the present invention have been omitted. Such elements are well known in the art, and include, for example, tape guide arrangements, further components of the scanning apparatus, an upper and a lower scanning drum, head wheel motors, rotary transducers, and the like; they all may be constructed in accordance with well known technology and as described in the literature.

In accordance with a feature of the invention, the digital information applied to the magnetic heads in two-channel mode is time-expanded. Application of the time-expanded digital information is per track. The temporal relationships are best seen in FIG. 3, in which the time axis--abscissa corresponds to the angular position of the head wheel, and the respective time is entered. N defines that number of video lines which are recorded on any one track. The time indication of 64 microseconds is referenced to the CCIR standard. A corresponding time duration can be used for other television standards.

Heads 11 and 21 form channel 1, which is recorded, alternatingly, with head 11 and head 21 for the time N.multidot.64 .mu.s. Due to the above referred-to time expansion, recording is effected not from one head wheel position of 0.degree. to 180.degree., but rather 180.degree.+.alpha..degree.. Head 11 records lines 1 to N, that is, N lines. At the next rotation of the head wheel, head 11 records line 2N to 3N.

The head 21 starts one-half head wheel rotation later to record the lines N+1 to 2N; at the next head wheel rotation it starts at line 3N+1.

Information in the second channel, that is, channel 2, is recorded by the head wheels 12 and 22. As with the signals for channel 1, the signals are time-expanded and distributed to the heads 12 and 22, in which the designation 1/2N and 3/3N shows the offset of the second head wheel by 90.degree. with respect to the heads of the first group, heads 11 and 21, respectively. The graphs a to d of FIG. 3 illustrate the time relationship.

A suitable time transformation system to carry out the method of the present invention is shown in FIG. 4, in which the figure illustrates such a system for either one of the channels 1 or 2.

The received signals to be recorded are connected to an input terminal 5. Four memories, 6, 7, 8, 9, are provided which receive clock signals of different repetition rate or frequency T1, T2 from inputs 10, 13. The frequency f1 of the clock signal T1 usually corresponds to the clock rate of the received digital TV signal applied to the signal input 5; the clock signal T2 has a different frequency, namely

The digital input signals and the clock signals are transferred by switches 14,15,16,17,18, and 19 to the memories 6,7,8, and 9. These switches as well as the switches 20 and 24 which are connected to the outputs of the memories are controlled by a control unit 26 which is supplied with suitable pulses derived from a head wheel position transducer. The timing of the control signals generated by the control unit 26 depends on the timing of the signals to be recorded schematically shown in FIG. 3.

The switches are shown in positions which they have during the position of the head wheel between 180.degree. and 180.degree.+.alpha.. During the corresponding time interval the memory 6 is connected with the head 11 through the switch 20 and the parallel serial converter 23. Further, memory 6 is controlled by clock pulses T.sub.2 via the switch 16. Because switch 19 is in the lower position memory 6 does not get new information. The information of the television lines 1 to N is transferred to head 11. During the same time the transfer of the information of TV lines N+1 to 2N takes place by providing memory 8 with clock pulses T.sub.2 via switch 14 and connecting head 21 with the output of the memory 8 by means of switch 24 and a second parallel serial converter 25. During the time interval between the head wheel positions 180.degree. and 180.degree.+.alpha. the input 5 is connected to the input of memory 7 by the switch 19. At the same time memory 7 is supplied with clock pulses T.sub.1 with the result that the input signals are written into memory 7. Since during this time interval memory 8 works in a read-out mode and memory 9 shall be kept clear to receive data occuring during the next half-revolution switch 15 is in the middle position so that memory 9 does not get any clock pulses. During the third half revolution of the head wheel information of lines 2N+1 to 3N is read out from memory 7. Therefore, switch 20 is brought into the lower position. Memory 6 is disconnected from any clock pulses by switch 16; memory 8 is operated in read-out mode upto the end of the recording of channel 1 (360.degree. plus .alpha.), and memory 9 is supplied with clock pulses T.sub.1 for storing new information.

Various changes and modifications may be made, and the particular instrumentation illustrated in FIG. 4 is only one example, which is particularly suitable, and preferred, to obtain time transformation. Memories of the first-in, first-out (FIFO) type are particularly suitable for time expansion or time compression, respectively, in which the signals entered can be read in at a certain clock rate, and can be read out or reproduced at a clock rate independent from the input clock rate. Such memories are available as integrated circuits, for example the Texas Instrument 74 SN 225 N.

Time compression of the signals, upon reproduction, can be effected by a structure almost identical to that of FIG. 4, in which, only, the inputs and outputs as well as the clock frequencies have to be interchanged.