GB2292648A - Recording video data and replaying digitally recorded video data - Google Patents

Abstract

Video data is recorded in real time by compressing each frame in accordance with the J-PEG video compression recommendation. Compressed frames are alternately written to one of three compressed frame buffers 26, 27, 28 at frame rate. The compressed frame buffers are alternately read at substantially half of said frame rate, out of phase with the writing of said buffers. Frames are read from the frame buffers alternately to two SCSI disk drives 21, 22. Thus, for each cycle, a first half frame is written to a disk drive, a second half frame is written to the other of said disk drives and a complete frame is written to the remaining compressed frame buffer. <IMAGE>

Description

RECORDING VIDEO DATA AND REPLAYING DIGITALLY RECORDED VIDEO DATA The present invention relates to digitally recording video data and to replaying digitally recorded data.

Equipment is known for recording full bandwidth digital video data in real time onto video tape. In addition, it is also known to record full bandwidth professional standard video data onto magnetic disks, thereby providing a level of random accessibility, as is well known in the computer art. However, a known problem with recording full bandwidth video data onto randomly accessible video disks is that said data must be recorded in parallel and a highly sophisticated and therefore expensive system is required in order to facilitate parallel recording.

More recently, standard computer disks have improved and compression techniques have also improved, allowing compressed video data to be written serially standard computer hardware. However, using presently available general purpose computer hardware, the level of compression required is such that a significant degree of degradation is introduced into the video signal. Thus, using known systems of this type, editing facilities are provided which, ultimately, result in a list of editing operations being produced which are then performed on the full bandwidth video tape. The process is generally known as off-line editing and the resulting list of instructions is referred to as an edit decision list.Thus, off-line systems of this type are significantly less expensive than full bandwidth editing facilities with random accessibility but they require full bandwidth equipment to be used for the on-line process, once the edit decision list has been produced. Thus, although these editing systems are quite sophisticated, they are not complete in themselves and require a further editing process to be executed.

According to a first aspect of the present invention, there is provided a method of digitally recording video data, wherein a real-time video signal consists of video portions generated at portion rate, comprising compressing portions of an input video signal; alternately writing said compressed portions to one of three compressed portion buffers at said portion rate; alternately reading said compressed portions at substantially half of said portion rate, out of phase with the writing of said buffers; writing portions read from said portion buffers alternately to one of two data storage devices, wherein, for each cycle, a first half portion is written to a storage device, a second half portion is written to the other of said storage devices and a complete portion is written to said remaining compressed portion buffer.

Thus, the present invention allows standard data storage devices to be used, such as standard SCSI NINE Giga byte disk drives. Each portion may typically consist of a video frame or a video field. When working with frames, compressed frames are produced at frame rate and written to compressed frame buffers at said rate. Three compressed frame buffers are provided such that, while one of said buffers is being written to, the remaining two stores may be read from. Frames are then read from the compressed frame stores are written to the magnetic disks at half frame rate, resulting in a data rate compatible with available disk transfer rates.

The invention will now be described by way of example only, with reference to the accompanying drawing, in which: Figure 1 shows a system for digitally recording video data, including compressing devices and storage devices; Figure 2 details compressing devices and storage devices identified in Figure 1, including compressed frame buffers and SCSI disk storage devices; and Figure 3 illustrates the order in which data is transferred between the compressed frame stores and the SCSI drives shown in Figure 2.

A system for digitally recording video data is illustrated in Figure 1. Processing electronics and storage devices are housed within a PC type tower 15, therefore the hardware resembles that of a conventional personal computer. However, it should be emphasised that this similarity is the full extent of the resemblance and that the circuitry within the tower is of a purpose-built hardware design and is not compatible with the operations of a personal computer.

The tower 15 provides connections to a video monitor 16, video tape recorder 17, a keyboard 18 and a mouse 19.

In addition, audio devices are provided, allowing audio signals to be supplied to the system and received from the system, although these are not detailed in Figure 1.

The monitor 16 is a conventional television-type monitor arranged to display video signals. In addition to displaying video clips, the monitor 16 also displays a menu and items may be selected from said menu by manual operation of the mouse 19. The keyboard 18 plays a rather limited role in the operation of the system and is provided essentially to allow titles and file names etc.

to be edited by an operator.

Two video outputs are provided such that a video output supplied to the video tape recorder 17 consists of video data, i.e. derived from video clips etc., without the titles menu displayed on the monitor 16. Thus, during operation, it is not necessary for an operator to effectively switch-off titles displayed on the monitor 16, given that this titling information is not supplied to the video tape recorder 17.

In the first preferred embodiment, basic editing facilities are provided, under which video clips recorded in PAL or NTSC format are recorded by the recording system held within Tower 15. Recorded clips are then modified under operator control, allowing an editing sequence to be generated in digital form. In the first preferred embodiment, the system is capable of recording up to one hour of good quality digital video, which may then be replayed and recorded. Thus, it is intended that actual video recordings are supplied into the system, which in turn provides an editing environment from which a finished product is derived.Thus, the first preferred embodiment is an on-line system which, although invoking a level of lossy compression, requires so little compression that the level of degradation is effectively unnoticeable, such that the quality of the edited video output is substantially similar to the un-edited video source of the material.

In other words, the digital editing function only introduces losses which are small compared to the bandwidth or the source material.

Major hardware components of the recording system contained within tower 15 of Figure 1 are shown in Figure 2. A first four Giga byte hard disk drive 21 is provided, along with a second similar disk drive 22. These drives are written to and read from over a small computer serial interface and may therefore be referred to as SCSI A and SCSI B respectively.

Full bandwidth video is supplied to a video input/output circuit 23. The input/output device 23 converts between analog signals and 4:2:2 CCIR 601 digital video signals. Double-buffering of said digital signals is effected via a first full bandwidth digital frame store, in parallel with a second similar store.

Alternatively, a single shared store 24, with a time multiplexed bus may be provided, to allow simultaneous access for two full rate component signals.

Transfers between full bandwidth stores 24 and 25 and the SCSI drives 21 and 22 is effected via a J-PEG compressing circuit 25 and three compressed frame buffers 26, 27 and 28.

During a writing operation, analogue video signals are supplied to the video input/output device 23 over an input line 29. The input circuitry of device 23 converts said analogue signal into a 4:2:2 digital video signal, which, in the first preferred embodiment, is processed as discrete video frames. Thus, reading and writing is effected by alternating portions of the video data and, in the first preferred embodiment, said portions consist of whole video frames. However, in an alternative embodiment, it may be preferable to effect portion transfer as video fields, or groups or sub-group of fields or frames.

A first digitised video signal may be written to a first portion of store 24, whereafter the next frame is written to a second portion of store 24. The next subsequent frame would again be written to the first portion, the data previously stored in said portion having been read and supplied to the J-PEG compressor 26, while the second portion was being written to.

The J-PEG compressor 25 is arranged to provide fourfold data compression, thus data written to the J-PEG compressor at 20 Megabyte per second results in output frames compressed to 5 Megabyte per second. This level of compression introduces minimal degradation to the video signals, such that the compression/decompression procedure is effectively lossless. However, it is not possible to write a 5 Megabyte per second stream to conventional SCSI disk drives, with each frame/field portion being randomly accessible, as would be required within a conventional sequential programmable computer. Thus, in most computing environments, a much greater level of compression is required which, as previously stated, will introduce losses and degrade the video signal to such an extent that it could not be used as an on-line source.

In order to reduce the rate at which data is written to the SCSI drives 21 and 22, two of said drives are provided, as shown in Figure 2, which are both written to simultaneously at half normal transmission rate. Thus, the 5 Megabyte per second stream is recorded by recording two 2.5 Megabyte streams simultaneously, 2.5 Megabyte being compatible with SCSI disk transfer speeds.

In order to effect the transfer of a single stream of 5 Megabyte per second to a dual stream at 2.5 Megabyte per second, the three compressed frame buffers 27, 28 and 29 are provided. On a first cycle, for example, the J-PEG compressor 25 may write data to compressed frame buffer 26 at 5 Megabyte per second. On the next cycle, the J-PEG compressor 25 writes a second frame to compressed frame buffer 27 at 5 Megabyte per second. As this second transfer starts, compressed frame buffer 26 starts to write compressed video data to drive 21 at 2.5 Megabyte per second such that, when a complete transfer has been made from the J-PEG compressor 25 to the compressed frame buffer 29, the transfer of data from compressed frame buffer 26 to the disk drive 21 is only half complete.

On the next cycle, a third compressed frame is transferred from the J-PEG compressor 25 to the third compressed frame buffer 28. While this is taking place, the second half of compressed frame buffer 26 is read, thereby resulting in the second half of said compressed frame being written to the SCSI drive 21. Simultaneously with this operation, compressed frame buffer 27 is read and data is written to SCCI drive 22 at 2.5 megabytes per second. At the end of the third cycle, when data has been written to compressed frame buffer 28 at 5 Megabyte per second, thereby filling said frame buffer, frame buffer 26 will have effected a complete transfer of one frame to drive 21 and half of the frame stored in frame buffer 27 will have been transferred to drive 22. Thus, buffer 26 is now available to receive the fourth compressed frame from compressor 25, compressed frame buffer 27 completes the transfer of the second half of its stored frame to drive 22 and compressed frame buffer 28 initiates a transfer to drive 21.

Thus, it can be appreciated from the above that frames alternate in their writing to drives 21 and 22 and each write operation is effected at half frame rate.

Claims (12)

1. A method of digitally recording video data, wherein a real time video signal consists of video portions generated at portion rate, comprising compressing portions of an input video signal; alternately writing said compressed portions to one of three compressed portion buffers at said portion rate; alternately reading said compressed portions at substantially half of said portion rate out of phase with the writing of said buffers; writing portions read from said portion buffers alternately to two data storage devices, wherein, for each cycle, a first half portion is written to a storage device, a second half portion is written to the other of said storage devices, and a complete portion is written to said remaining compressed portion buffer.

2. A method according to claim 1, wherein said video data is supplied in composite or S-video format.

3. A method according to claim 1, wherein said video data is supplied in full bandwidth digital D1 format.

4. A method according to any of claims 1 to 3, wherein said video signal is compressed in accordance with the J PEG video compression recommendation.

5. A method according to claim 4, wherein said J-PEG compression is effected at a compression ratio of between 3:1 and 12:1.

6. A method according to claim 5, wherein said J-PEG compression is effected at a compression ratio of substantially 4:1.

7. A method according to any of claims 1 to 6, wherein said compressed portion buffers are read at substantially 2.5 Megabyte per second.

8. A method according to any of claims 1 to 7, wherein said portions are video frames.

9. A method according to any of claims 1 to 7, wherein said portions are video fields.

10. A method according to any of claims 1 to 9, wherein said compressed video data read from said portion buffers is written to SCSI disk drives.

11. A method of replaying digitally recorded video data, wherein said recorded video data has been recorded in accordance with any of claims 1 to 10.

12. A method of digitally recording video data and replaying said digitally recorded data substantially as herein described with reference to the accompanying drawings.