Video compression become an hot topic of the month for a long time yet,. We originally produced an article on compression and recording in the 2007, when it really was very much in headlines of DVR and Security camera product topics.We thought video recording compression in CCTV would develop at a rate comparable with the PC and video industry but this has not happened. The PC industry and processors have increased tenfold in power and speed at significantly lower prices. One area of great interest to digital video images is the capacity of hard disc drives.
There was a lot of progress has been behind the scene with developments of DVR and Surveillance system with various compression techniques to, create more efficient surveillance system with smaller file sizes. It may be worth revising some of the techniques involved in video compression for surveillance systems. The following is a brief review of principles and Practice of CCTV.
Principles of Video Compression.
In Video compression technology each field is divided in to an array of individual points or pixels. At each of these points, converters convert voltages representing the color and brightness at that point to a binary digital number. This array of binary digital numbers can then be stored digitally in memory with a name cross referenced against time and date. A single frame of monochrome video use about 450Kb (Kilobytes) of space for storage and single frame of color needs about 650Kb. This is the uncompressed frame size that would be needed for storage on hard disc..
For example to store a video length of a videotape, we will need a total storage capacity of about 300GB (Gigabytes) would be needed for one camera. This is considerably very large hard discs for a camera for 2 hours of recording. The technique of reducing the amount of space required is generally referred to as video compression.
The video picture frame contains a large amount of redundant information in surveillance DVR that can be eliminated without a great loss in perceived picture quality. A common types of compression used are MPEG, MPEG2000, MPEG4, H-264. Most compression methods are effective up to a certain point, before image quality quickly degrades.
To reduce the amount of size required to store the video signal can be represented in a form known as YUV. The YUV format consists of the Y (luminance) and UV (color difference) signals. The advantage of using YUV format is that fewer bytes are needed to digitize the video. Normally, recording all of the color base components; red, green, blue would need three bytes, one byte for each color. If we are using YUV format the luminance can be digitized as one byte and the color difference signal as one byte. Which mean that we need only two bytes rather than three, a saving of one third of the storage space required? This technique can be used together with compression to minimize the hard drive usage.
Type of Video Surveillance Compression.
The most known used standard compression for Security DVR and surveillance systems are, JPEG, JPEG2000, MPEG4, H264. Using JPEG compression can compress about 15:1 compression. Another more recent compression standard was devised by the Motion Picture Expert Group specifically for the digitization 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. This type of compression contains all of the video information required to make a complete picture. Predicted frames are generated by previous I-frames or P-frames and are used to generate future P-frames. Bi-directional Predicted 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 and allows compression ratios of 100:1.
MPEG-2 is the most common format used in the latest DVD technology, which can store about 90 minutes of VHS quality video and audio on to only 600Mb of disk space, such as a CD-ROM. MPEG has 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.
MPEG-4 is the most popular compression for security DVR and surveillance systems; it uses media objects to represent aural, visual or audiovisual content. Media objects can be synthetic like in interactive graphics like in digital television. These media objects can be combined to form compound media objects. MPEG-4 multiplexes and synchronizes the media objects before transmission to provide QOS (quality of service) and it allows interaction with the constructed scene at DVR machine.
Compression organizes the picture frames in a hierarchical fashion where the lowest level has primitive media objects like still images, video objects, and audio objects. MPEG-4 has a number of primitive media objects which can be used to represent 2 or 3-dimensional media objects. MPEG-4 also defines a coded representation of objects for text, graphics, synthetic sound, talking synthetic heads.
Visual part of the MPEG-4 standard describes methods for compression of images and video, compression of textures for texture mapping of 2-D and 3-D meshes, compression of implicit 2-D meshes, and compression of time-varying geometry streams that animate meshes. It also provides algorithms for random access to all types of visual objects as well as algorithms for spatial, temporal and quality scalability, content-based scalability of textures, images and video. Algorithms for error robustness and resilience in error prone environments are also part of the standard.
For synthetic objects MPEG-4 has parametric descriptions of human face and body, parametric descriptions for animation streams of the face and body. MPEG-4 also describes static and dynamic mesh coding with texture mapping, texture coding with view dependent applications.