DIGITAL STORAGE

ORDINARY DIGITAL STORAGE- Fact From Hype

There is no question as to the benefits of digital recording for event recording, ATMs, etc. This article is looking to the future of continuous recording as we currently seew the VCR fading into oblivion.

A lot of progress has been behind the scene with developments and availability of various compression techniques to, create more efficient storage of data with smaller file sizes. It may be worth revising some of the techniques involved in digital recording. The following is a brief extract from The Principles and Practice of CCTV 2^nd edition.

In digital recording each field is divided in to an array of individual points or pixels. At each one of these points, analogue to digital converters convert voltages representing the colour and brightness at that point to a binary digital number. This array of binary digital numbers can then be stored digitally in a file with a name cross referenced against time and date. A single frame of monochrome video needs about 450Kb (Kilobytes) of space for storage and single frame of colour needs about 650Kb. This is the uncompressed size that would be needed for storage on hard disc or other storage medium.

Consequently to store the same number of images as a videotape, a total storage capacity of about 280Gb (Gigabytes) would be needed for one camera. This is considerably larger than hard discs and other media generally available and would also be tremendously expensive. Consequently some means is required of reducing the amount of space required without adversely affecting picture quality. The technique of reducing the amount of space required is generally referred to as compression.

The video frame contains a large amount of redundant information that can be eliminated without a great loss in perceived picture quality. Consequently, common types of compression used are known as "lossy compression" because the redundant information is discarded. Most compression methods are effective up to a certain point, or "Knee", beyond which the image quality quickly degrades.

To assist in reducing the amount of size required for storage the video signal can be represented in a form known as YUV. The YUV format consists of the Y (luminance) and UV (colour difference) signals (for further descriptions of luminance and video signal components see section 2). The advantage of using YUV format is that fewer bytes are needed to digitise the video. Normally, recording all of the colour components; red, green, blue (RGB recording) would need three bytes, one byte for each colour. By using YUV format the luminance can be digitised as one byte and the colour difference signal as one byte. Consequently only two bytes are needed rather than three, a saving of one third of the storage space required. This technique can be used together with compression to minimise the amount of space required for storage

Types of Compression.

The technology for compressing video pictures originated in the storage of still photographs on computers. The most commonly used standard, JPEG, takes its name from the Joint Photographic Expert Group by whom it was developed. Using JPEG compression, the knee occurs at about 8:1 compression. The most commonly used standard is Motion JPEG for which the knee occurs at about 15:1 compression. Consequently, M-JPEG reduces a 450Kb file to only 30Kb. While this is still too large to fit the same number of images as a video tape on to a hard disk it is small enough to permit, say, 2 frames per second to be recorded for 24 hours on to a 6Gb hard disk, which is a size generally available, costing a few hundred pounds.

Another more recent compression standard was devised by the Motion Picture Expert Group specifically for the digitisation of moving images. This standard is given the name MPEG. This standard makes use of the redundancy between adjacent frames.

MPEG-1 contains three types of encoded frames. Intracoded frames (I-frames) contain all of the video information required to make a complete picture. Predicted frames (P-frames) are generated by previous I-frames or P-frames and are used to generate future P-frames. Bi-directional Predicted frames (B-frames) are generated using both previous and future frames. A complete sequence of frames is made up of a series of these different frame types with more than one I-frame for every 10 P- or B-frames. This process is known as inter-frame correlation and allows compression ratios of 100:1 to be achieved.

MPEG-2 is the format used in the latest Digital Video Disk (DVD) technology, which can store about 90 minutes of VHS quality video and audio on to only 650Mb of storage space, such as a CD-ROM. However there are a number of disadvantages to MPEG compression. Firstly, in order for MPEG to achieve high compression it needs the video signal not to change abruptly from frame to frame. Since many video recording applications require multiplexing because more than one camera must be recorded, the rapid change from frame to frame as cameras are switched defeats the inter-frame correlation technique used in MPEG.

There is now a far greater range of recording devices available at easily affordable prices than before. This is a brief review of the main characteristics for each.

This is not intended to praise or condemn the humble VCR, simply to include it in the list of available devices. To record 16 cameras over 24 hours will provide a picture update time of 5.12 seconds. The tape can then be removed and stored for as long as the Code of Practice requires. This is frequently 31 days but sometimes 90 days. With S-VHS resolution can be up to 500 lines, depending on multiplexers, cameras, lenses, transmission, etc. Rewind time for a E180 tape is in the order of three minutes, this would be the time to locate a scene at the opposite end of the tape. The stored signal is an analogue video signal and can be replayed on any other make of VCR. The information stored on tape is permanent and can normally only be deliberately wiped off. VCRs require regular, relatively expensive maintenance and frequent replacement of tapes.

There are other types of higher quality analogue video recorders such as U-Matic, but these are rarely used in CCTV systems, they are mainly the province of broadcast television.

As discussed earlier, there are many different formats of compression and analogue to digital conversion and different compressed file sizes. There are also many varying claims for the resolution produced for these combinations. A future article will attempt to compare these to a common base. For simplicity of comparisons, this article will be based on a final file size of 20Kb which is a compression ratio of about 30:1 for a colour picture. There will be 16 cameras with a picture update time of 5.12 seconds to compare with conventional recording.

HDDs are found in every computer and have evolved to be extremely reliable devices requiring virtually no maintenance. There are no touching components in a HDD although there are mechanical parts to rotate the disc and move the read/write head. Seek time is virtually instantaneous to retrieve a scene from any part of the disc by many search parameters. It is possible, although unlikely, to accidentally delete all the data from a hard disc. Current disc capacity is up to 36Gb, with 18Gb being readily available, this is likely to increase dramatically over the next few years.

The example would require 5.4Gb per 24 hours, a 36Gb disc would provide 6.67 days of continuous recording, (26M images). The options therefore would to accept an archive period of just less than 7 days or transfer the full disc to another removable medium for longer archiving. The medium could be another HDD or a DAT.

HDDs can be removable slot-in devices, therefore it would be practicable to remove a full disc and replace it with a blank pre-formatted disc to continue recording, just as is done with VCR tapes. One example of this is digital recording in trains where it is not practical to review incidents on the train. The hard disc is replaced with a blank disc and the original taken back to a central control for reviewing.

A major advantage of hard disc recording is that any part of the disc can be reviewed without interrupting the continuous recording.

DAT drives are miniature audiocassettes incorporating magnetic tape similar to a VCR and can have capacities up to 50GB. (A 50GB tape costs in the order of £40.00). One common use of DAT drives would be to download from a HDD when it is full for archiving. As many tapes as necessary could be used to provide the total storage time required. Rewind time would be about 3 minutes for a full tape. Although search parameters may be similar to a HDD, the seek time could be comparable to a VCR. If involved searches are required, the DAT could be downloaded to a HDD for faster output. It should be noted that transfer rates of data can be quite slow, from 1 to 12 Mb/sec. At the best rate, transferring 50Mb could take over one hour or up to four hours at the slower rates. Similar comments apply to a DAT as to a VCR cassette; there is a thin magnetic tape being drawn across read/write heads. Again similar to VCRs, because the cassette is a fixed size, greater capacity is achieved by using thinner tape.

This used to be known as Digital Video Disc, but is now used for all types of data storage. It utilises the same principle as a CD in that indentations are burned on to the disc by a laser. The same laser reads these indentations. These drives are now readily available as read/write devices at modest prices and can be used exactly the same as a HDD. Capacities of discs are quoted as being 2.6 or 5.2Gb, the latter uses both sides. A 5.2Gb single sided disc will be available shortly. DVD drives can read a range of devises such as, CD-ROM, CD-RW, PD format, DVD-RAM and DVD-ROM. Beware though of quoted capacities because data protocols use quite a lot of space. A DVD formatted as FAT 16, which is the international standard for CD-ROMs, reduces the capacity to 2GB. Another format is used for long continuous files of video is known as UDF, in this case capacity is limited to 2.32GB. At the previously noted files sizes and number of cameras this would equate to nearly 9 hours of continuous recording, (11M images). This medium could be useful for downloading excerpts from a HDD drive for evidence or distribution. If formatted to FAT 16 it could be replayed on any PC. Most DVD drives include MPEG1 compression software so that recordings could be made directly from a composite or S VHS input and replayed on a PC with MPEG1 decoding. (Most have this). DVD discs cost about £13.00 for 2.6Gb or £19.00 for 5.2GB.

CD writer drives are now available for under £200.00, with CD-R discs less than £1.00 each. The capacity though is limited to 640MB with several caveats. This is nearly 3 hours (.3M images) of recording on the previous basis. CD-RW discs are written to ISO9660 standards so any CD device may read them. Now for the caveats, and these apply to your CD writer that you use for every day applications. The header information requires 27Mb, so this leaves only 627Mb for data. If you write several separate sessions, then 5 sessions needs 79Mb for headers leaving 561Mb for data, and 10 sessions leaves only 490Mb for data. Writing speed is up to 900Kb/sec, so 600Kb of data would be read in about 11 seconds. As with the DVD, this would be a inexpensive medium for transferring data.

As with most systems, there are no common standards for video data storage in the CCTV industry. The various systems on the market incorporate most of the types of compression mentioned earlier. Add to this many methods of encryption and watermarking and there are the makings of a massive problem of the use of digitally recorded video. Life was simple when we had VHS, there were even problems when S VHS was introduced and that was only two standards, although they are internationally agreed.

There is no doubt that digital recording is now a potent force in the CCTV armoury and will prove to be the most effective and efficient method of video recording and archiving. It is still a case of ‘caveat emptor’, be suspicious of the specification that states 8 video inputs and offers continuous 24 hour recording with an 8Gb hard disc. You will probably find that this is only for one camera with a 15Kb file size and 5 frames per second.