Digital storage systems: What you need to know
 
Digital storage systems: What you need to know

Sep 1, 1998 12:00 PM
CHARLIE PIERCE

Whether upgrading, improving space requirements or designing anew, educate yourself about digital. Over the past few months, we have looked at the advances and changes that digital is making in the CCTV industry. In this article, we will discuss digital storage systems. To review, analog CCTV systems are what we have been accustomed to for 50 or so years. Digital CCTV is a more precise way to recreate video images with many options and advantages not afforded by analog. The first point of confusion about digital storage is there are two ways to achieve it: the digital recorder and the personal computer. Each method transforms video information produced by acamera into a digital format and stores the information accordingly. A digital recorder still uses magnetic tape to capture and maintain images (much like a standard VHS recorder) but is more compact. A PC or computer interface works with a hard drive or disk. Which system will work for you will be based on your application and budget. Beware however, that both systems have one common thread: format compatibility. Just because you own or use a digital storage system does not mean you suddenly have the ability to play back or interact your video information on someone else's digital video system. This might mean you will need to review your evidence and transfer it to a standard video tape format for review at a remote site. Provided your information is always available in its original digital format, the potential for having to make standard VHS copies is more an inconvenience than a problem of net result.

Why digital storage? There are several good reasons for using digital storage in place of good old-fashioned analog and an equal number of good reasons to be careful: Advantages and disadvantages include: - simultaneous, multiple camera recording capabilities; - event recording and post-event evidence; - image quality and enhancement capabilities/storage size or space; - information recall and organization; and - long-term maintenance costs.

Simultaneous, multiple-camera recording The first problem with standard VHS tape recorders is that they have only the ability to record a single input at a time. This means we must work through video switchers, multiplexers or quad units to record multiple video images. With digital storage systems however, our options increase tenfold. Digital recorders may come with built-in capabilities to handle up to 16 individual camera inputs. Through the use of advanced multiplexing techniques, the user can program cameras individually for 24-hour time-lapse, real-time, or event-driven surveillance. Some models even include built-in video motion detection. Buyer beware, however. Although more flexible than their VHS counterparts, most digital recorders still have the problem of "all camera programing." That is, what one camera is set for, all are set for. PC-driven systems, on the other hand, offer complete control of each camera individually. So, if you want camera one to be in the 24-hour mode while camera two is event-driven and camera three is tied to a dry-contact alarm point, no problem! Additionally, you have the ability to grow your system well beyond the 16-camera limitation of a digital recorder. With PC storage, how much information or how many cameras you can manage will be based on the size of your hard drive or disk storage system. There is nothing that says you cannot expand your digital storage system as your needs grow, but system expansion should be investigated from the outset. Nothing is worse than buying a system today, only to find out tomorrow it is obsolete or restricted to a size below your potential needs. Another problem with system expansion is format. If you need to upgrade your existing system, for whatever reason, you must be able to match your existing storage format. If your new, larger or more sophisticated system does not match the compression format of your existing system, all previously stored video information will become unaccessible - unless you are willing to maintain your original system on the side.

Event recording and post-event evidence Event video recorders surfaced about 15 years ago. The purpose of event recording was to save tape, cut down on tape review time, initiate a more immediate way to record information, and provide video evidence based on actual actions or events. The primary problem with event recorders, however, is lost time and visual evidence just prior to an actual event. With digital storage systems, we are now able to record cameras in true-event mode. Through advances of digital storage, you can easily store and recall information relating to the first few nano-seconds, full seconds, or minutes prior to, during and afteran alarm. But although event recording is a wonderful advantage, it requires serious forethought and some form of alarm interface.

Image quality and enhancement capabilities When recording with standard VHS or SVHS tape units, the quality of playback depends on the ability of the recorder to maintain the original resolution of the video image as recorded. With digital, however, it depends on the format and compression rate of video information dropped to tape or disk. Joint Photographic Expert Group (JPEG) format is currently the most common format for digital recording applications. But if your system has a JPEG storage system, it will not play back or display on a system using a different format. Pay careful attention to such details to avoid problems. As for compression of the video image, this is where it gets sketchy, depending on who you talk to. I do not pretend to be computer literate and so have sorted out some pieces of information to consider. The first is that the video image comprises 30 full frames (NTSC) or pictures per second. Each full frame of video information requires around 450k (kilobyte - black-and-white) or 650k (color) of storage space if left uncompressed. Simple math indicates it would require a huge amount of space to store this information if left alone. Hence, compression methods are required. In layman's terms, compression refers to taking large forms of information (such as 650k images) and squeezing them into a very small space through the use of mathematical equations called algorithms. But whenever a video image is compressed, certain key elements will be lost - little things such as resolution. We know that a compression rate of 30:1 in JPEG is obtainable, but the final resolution of the playback would be greatly reduced. Therefore, the current ratio of compression of video is about 15:1. This means that a full-frame video image (450k black-and-white) would be reduced to a smaller image requiring 30k of space for storage (43.33k color). This also means that we can store up to 15 frames of video information on a disk or tape in the same space it would normally take to store a single frame. Insofar as computer compression rates and technology change daily, it is easy to believe that within the next year or two, we will achieve even more than our current ratios suggest. There are currently several methods of digital storage: - We can store up to 20 hours of real-time video on a computer disk. On the same disk, we can store as much as a month's information at one frame per second. Once full, the disk is stored in the same manner as a video cassette, but without the space requirements. - Digital tapes are also more than acceptable. Much smaller than their VHS counterparts, they hold huge amounts of information for recall. - Read/write CDs store up to 650mb (megabyte) of video information and can be reused again and again. Since they are optical, there is not the wear and tear associated with magnetic tapes. - Of course, digital video disks (DVD) are coming at us full blast. With this improvement, it is projected that we will have as much as 10 times the amount of storage space on a single disk. Time will tell. - Last but not least, we have computer hard drives and zip drives. It is nothing to find a 30g (gigabyte) hard drive or several such hard drives tandemed together in a digital video storage system. With this much space available, we can record several hundred cameras, in a wide array of modes, simultaneously. To review, look for three things in your digital storage unit, be it magnetic tape or computer disk: - The speed of the unit - how fast can it capture, compress and record video information? This will determine how much information you will be able to record. - The compression rate of the system - how much resolution or information are you willing to or able to give up for the sake of space and ease of playback? - The size of your storage medium - how large a disk drive does your application require to ensure you do not run out of space too soon? As for enhancement capabilities, this is where digital outruns standard VHS. Using digital, the end-user is able to enlarge, crop, scan, lighten, darken and even add or take away information from an original video image. This is also where the debate of courtroom admissibility comes into play. Since I now have an image that I can do just about anything to, what keeps me from enhancing someone right into the image? Encryption codes are used to ensure digital information integrity. Encryption says that my unit recorded the original image and only my unit can decipher it for playback. Therefore, the original information can be copied for use in a courtroom while remaining intact and providing proof of originality - something that is important to consider while looking for a good digital storage system. Additionally, individual cameras from anywhere in the system can be displayed on the screen in split, quad or more for viewing. Through digital options, we can review specific information while the system continues to record.

Information recall and organization If you have ever had the pleasure of reviewing a videotape that was recorded in conjunction with a standard sequential switcher, then you have stepped into the nightmare of video surveillance. Because it can literally take hours to find a single frame of usable video information off a standard VHS tape, the cost of security can become very high. Additionally, through the use of VHS recording methods, we can end up with noise, lines, skips, jumps and more in our image. Digital storage via disk or hard drive gives us the ability to search through hours, days or even weeks of recorded video information in a matter of minutes. Not only can we search according to a specific date or day, we can narrow the search down to the hour, minute or even second. If you know what frame of information you want, even this can be done. The key is to understand that when designing a major digital storage system, you will need to forget or throw away half of what you know about designing a video recording system and learn anew. Since each camera in the system can now be programmed independently, and we can gather huge numbers of cameras to a single source, it will become increasingly important to think through the initial design or flow of the system completely prior to buying or installing equipment. Consider your digital storage system a 16-drawer filing cabinet. Each drawer has a master label on the outside to bring you to the major subject. Inside each drawer there are 10 dividers. Between each divider there are 50 file folders. Inside each file folder there is one year's worth of information pertaining to a specific sub-subject. Now, link it all together in a master card catalog file, such as at a library. Need to find information? Look under day, date, name, author, camera number, incident type, etc. It may seem somewhat overwhelming, and it should. This is a massive amount of information that is continuously being sorted out. Never before in the history of industrial CCTV have we had to put so much effort into designing an information filing system. At the same time, however, we have never before had so much information available or the means with which to organize it so completely off-the-shelf.

Long-term maintenance costs Cost plays a part in every system design. For the most part, digital will be out of reach for the average mom- and-pop store for another three years or so. But consider that the average cost of ongoing maintenance of a video recorder is about $300 per unit every 18 months. Add the cost of videotapes, space requirements for several recorders, lost information due to inadequate switching capabilities, lost resolution at playback, the cost of sitting and searching for information, of enhancing a video image from a videotape, and you end up realizing that current methods of video recording are really very expensive. So should you run out today and look to digital as your newfound video storage savior? Not necessarily. But if you are looking to upgrade an existing system, improve space requirements, or if you are designing a system that will be installed in the next year or two, you should educate yourself about digital storage, look hard at the options available today, and plan for tomorrow.

 
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