In most "lossy" compression schemes, we reduce the file size of an image by throwing away specific bits of information. The most common format of this is to eliminate repetition. If the background of an image doesn't change, why continuously store it? If I have 26 shades of red, can I compress them into five or six shades of red? The key is not to throw away the baby with the bath water. All compression schemes are accomplished by specified algorithms. An algorithm is nothing more than a fancy name for a mathematic formula. After all, we are only dealing with zeros and ones - the binary language of computers.
We refer to these algorithms as "codecs". A codec may be a device or a program. In essence, we first use an encoder to compress the information and then a decoder to put it back to the nearest possible reproduction of the original. Since this is a two-sided, proprietary process, we adopted the idea of calling our codecs as "engines". Each engine we give a name, something like JPEG, Wavelet, Mpeg, Mpeg4, H263, etc. The key here is that all compression engines are some form or variation of an original scheme.
Everyone has their own idea and method for compressing information, and of course, every new engine that comes along is "the best". If you don't believe me, just ask the manufacturer which compression engine is the best and you will promptly be told that theirs is. You may even be blessed with a 45-minute technical explanation about the benefits of the manufacturer's compression engine as opposed to all the others. In the end, however, all compression schemes are, one way or another, just another incorrect, problematic way to store an image. But it's a problem we deal with because we don't have the luxuries of unlimited storage and unlimited transmission bandwidth.
As I inferred earlier, each compression scheme approaches the problem of reducing information from a different angle. One engine may compress colors. That's not a problem unless you go overboard and your playback ends up looking like a child's drawing where the child only had four or five primary colors to work with. The next engine might record a single full frame of information and then only those things that change in the image from that point on. Again, this is not bad unless you get carried away by not refreshing the full frame often enough. A third image may attack the resolution or detail of the image. This is easy if you remember that all images are made up of dots or squares of color. If I throw out every eighth square of color, I have a 25 percent reduction. Again, this isn't a bad idea unless you get carried away.
The newest compression schemes are working from a truly digital format. They look at each individual pixel point and redesign the image into complex digital codes or equations and store them in a text format. Text format requires practically little or no storage space as compared to an actual image. I've probably got the technicalities off just a little, but the gist is correct for this emerging "text-based" compression scheme.
Still, the key with any compression factor is to remember five things:
1. Once compressed, there is no retrieving the "thrown out" information.
2. If it looks or sounds too good to be true, it probably is (spoken like a true pragmatist).
3. There is no such thing as a "best" compression scheme or engine.
4. There are no standards for compression in the CCTV industry so whatever you end up will probably be proprietary. Buyer beware. If you have multiple locations and you expect to be able to view all of your video from one central point, you must be careful to use only those compression schemes that are able to be decoded by your software at the central location. In other words, be careful how you mix and match.
5. Lastly and most importantly, the compression engine or scheme that is right for you is the one that reproduces the information that you need to view, in a way that is beneficial to your application. In other words, don't be afraid to ask for an on-site test for a week or two under your control, and not based upon a 15-minute demonstration by a sales guy or gal that knows how to strut their stuff.
In my next column, we will pull the most common known engines apart and see how they work. We will look at the benefits and the pitfalls. So until then, avoid tight places and try not to give up too many squares.