Welcome back to our series! If your memory is good, you will recall that in the end of my last column, I mentioned that we would take a look at video compression and the differences between video surveillance and video monitoring. So, without any extra dilly dally let's get started.
The first question most people won't ask about compression is "Why bother?", as in "If compression messes up my picture, why bother?"
Most everyone that I speak with thinks that he or she understands compression factors and such, but in the end most are reciting words or processes that they have read on the Internet and/or have heard around the water cooler at the office. OK, this sounds a bit over the top or conceited, but it is true. The worst part about it is that most of these folks read my words on the Internet or were listening to me around the water cooler, so I'm the one to blame for what they're just "repeating". So let's take a good look at the problem, do the math and then move into the solutions ... if there are any.
Problem number one is that the average analog video image, once digitized into pixels, will require between 1 megabyte and 1.5 megabytes of storage space on a hard drive. That's just one image. Now, multiply that by 30 and you have one second of required hard drive space as based upon good old NTSC standards. Multiply that by 60 (one minute), take the result and multiply again by 60 (one hour) and that times 24 (hours in a day) and you have one day. If we did the math, we would find that one camera, uncompressed, over a day's time would require a whole lot of storage space, not to mention transmission bandwidth. If we averaged 1.2 megabytes per frame from this 30 fps camera, it would mean we'd need to have about 3 terabytes of storage from that camera by the end of the day.
This being the fact, we are actually faced with two dilemmas. The first is that storage space is sold by the byte. I actually remember, because it was not so long ago when a hard drive could be priced at $100 per megabyte. Obviously, it's not that bad today, but if you consider that we are using terabytes (approximately 1 trillion bytes) and pedabytes (equal to 1,024 terabytes) the cost potential is still there. The second half of the consideration is the actual size and space required by the storage unit(s). The more memory you require, the more general space for the equipment is required. So the idea of compression is to reduce cost and space. So we "compress".
As discussed in many of the articles that I have written in the past, there are several ways to compress information. The first (and sometimes the best) method of compression is to reduce the actual number of images that you record in a second. Do you really require 30 images per second or would one or five per second fit your needs? "Real" time, after all, is what you "really need" as it applies to your application. Look back a column or two and you will find this discussion (see earlier article).
In the world of computers, we have two types of compression, "Lossless" and "Lossy", and the latter is often referred to as lousy. Lossless compression is a format that is primarily seen in the medical industry. Just as the name infers, there is very little or no information or detail lost when using this format of compression. This is very good for X-rays and such, but not very cost effective for long-term video storage. Lossy compression is used where the playback or restoration of information can be "good enough" when compared to the original information. Lossy compression allows us to have maximum storage capability on a minimal usage of disk space. Again this is very good for space and cost savings, but not necessarily good for reproduction.