Analog Cameras
Analogue Cameras on a Digital Video Recorder
(DVR) Compressed Storage
Compression
Compression: Currently, there are no standards over which compression standard should be used for video. The main ones were all originally designed for internet transmission of graphics. Computer Graphics, Video is a whole different ball-game. The cinema industry has developed DVD which is currently MPEG 2 the next generation will be MPEG 4 H265. The current quality DVR’s use MPEG 4 H264. Video is made up of two half pictures at 50 fields / 25 frames per second, while graphics are a single grid of pixels at fixed ratios 4/3 (i.e.; 640 X 480, 800 X 600, 1024 X 768, 1280 X 960). With Video, we count by lines of resolution ... how many lines paint the picture on the screen. The more lines the better the detail of the image. With Graphics, the more pixels crammed onto a screen, the better the resolution or detail of the image. To compress video, we must first turn it into a graphic image. This by itself can cost as much as 25% of the initial video image's resolution. Then we take whatever we end up with and compress it into a smaller file to take up less space. How we compress or what engine we use will determine the quality of the product when we go to use it later.
Imagine a picture that is 200mm by 150mm. It is an excellent picture with a high resolution (good detail). This picture is made up of dots that are arranged in rows and columns. We have 800 columns and 600 rows of dots. Now, if we need to blow the picture up at this point, we can make it twice as large and still have good detail. However, let's compress it first. We will make it 25mm by 19mm. The only way to do this is to cut down the number of columns and rows of dots. Now we have 100 columns and 75 rows of dots. OK, we store the image in a much smaller area. We can even view it as a 25mm by 19mm image and it appears to be very sharp. However, now we want to make it 2 times bigger than 200mm X 150mm again... about the only thing we see is large dots spread across our screen or paper.
Based upon the above example, the type of compression and the amount of compression that you use is very important. Which one is better ... who knows, everyone claims to be using the best? I do know that any compression of 20% or more in JPEG will cause too much detail loss to be of use. The biggest key to compression engines is which one will be around in two or three years and which ones will fall by the way side.
Resolution of the image
Based upon the above, you can see what compression can do to resolution. Consequently, the resolution of the final image becomes important as based upon several different items.
1) The size of the initial image. The larger the object of concern (in the image) the less resolution will be needed to reproduce it. The smaller the object of concern, the more resolution will be needed. Consequently, if you are looking at a person at the end of a football field and that person appears to be 20mm tall in a 20cm screen, you better have (and keep) as much resolution as possible if you intend to be able to identify the person at a later date. Bottom line, either do a better job with the design of the camera/lens or do not use a low quality compression storage method. On the other hand, if you are using an image of a person that was only a few feet from the camera and their face and/or body takes up the majority of the screen, the resolution of the beginning and/or the final product is not as much of a concern.
2) The resolution will be dependant upon the purpose or need of the final image. If you are using the stored image for the purpose on counting cars that pass through a gate over a specific period of time, the compression rate will be of no consequence. All you are interested in is how many cars pass through the gate ... count the shadows. On the other hand, if you intend to or could end up in a situation where someone's welfare, safety, and/or conviction relies upon the detail of an image, you must be very care how much compression is used to store the image.
Images Per Second
These are directly related to the National power grid standards i.e. 50Hz can have 50 frames per second. We have determined that a minimum of 25 full frames of video per second will constitute real time or constant motion imagery. In the US, and other areas with 60Hz power, they work with 30 pictures per second. For years, we have stored these images onto magnetic video tape ... all 25 per second. However, because of the amount of room that these images take up on a disk or magnetic tape or chip or whatever, we are now being told that we don't need 25 pictures per second ... we only need 2 or 5 or what ever. The facts are that each job is different and each job can have several different aspects to it. If I am trying to count the number of people passing through a gate, 2 pictures per second may not be enough ... I might miss a person. On the other hand, do I need 25 pictures, per second, of an empty room? Of course not. Since the introduction of digital storage systems, we have been forced to design our systems more conservatively, more engineering skills in advance. Since we are limited on space for storage, we must be conservative, but realistic on how we use our space. Think the application through and apply each camera to its own circumstances. Everything can be tested on paper prior to being put in place.
Access Security Limited has presentations available for how different video formats, compression, resolution and frame rates accurately reflect in the cost of storage.