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Hardware compression vs software compression

Since the beginning of digital CCTV there has been a division between the embedded and the PC-based systems. Both parties have valid arguments as to what qualities make theirs best.

One of the key differences between these systems is how they compress the images they capture. PC-based systems use software processes to compress the images captured. This is known as software compression (SWC). Embedded systems employ a process known as hardware compression (HWC). A PC-based (SWC) DVR consists of a PC motherboard, RAM and CPU, as well as a video capture card, or cards, which plug into one or more PCI slots on the motherboard. All of this equipment is known as hardware. The capture card has one or more chipsets on it responsible for capturing frames from each video signal connected to the card. The number of chips on the card will determine the speed of the card or how many frames per second in total the card can capture. So for example, a 16 channel capture card that captures 100 fps in total captures 6,25 fps per channel, or a 16-channel card capturing 400 fps in total captures 25 fps per channel. These images or frames are captured by hardware and then sent to the CPU where, using the appropriate algorithms in the software, these images are compressed and stored to the hard drive as a video file.

With hardware compression these images are captured and compressed entirely by the hardware. This process is not dependent on any software environment. Hardware compression has usually only been used by embedded DVRs. However, in the constant quest to keep improving and evolving their products, both PC-based capture card manufacturers and embedded manufacturers have been finding new ways to add features and stability to their products. For instance embedded systems have been introducing Linux embedded operating systems in order to add some of the flexibility of the PC-based systems. At the same time the PC-based systems have been chasing after the stability boasted of by the embedded system.

One of the problems that PC-based systems struggle with is 'CPU loading'. CPU loading is when the central processing unit (CPU) in the PC is over working. This is usually not a problem on systems that run a slower frame rate. However, with faster realtime systems where the processor has 400 images to compress every second, the CPU is running at near maximum capacity. Why is this a problem? Well for several reasons.

Firstly, not all frames are the same size, or they do not all require the same amount of work to process, which means the work load varies constantly irrespective of the type of compression codec being used. Also, the CPU needs to process things like GUI (graphic user interface) changes, as well as handle playback functions and other functions such as motion detection, network operation etc. With the CPU already near saturation point should the CPU suddenly receive several large frames successively or need to run several other processes simultaneously, it can quite easily be taken over saturation point.

This is not normally a problem in your normal everyday PCs because tasks simply just wait their turn for time with the CPU, but in a DVR it is a problem, a big problem! Frames cannot wait to be processed, because at 400 frames every second they start piling up pretty quickly and when that happens you get 'frame drops'. This means that frames are simply ignored and they are lost. Sometimes your natural eye cannot see this and so it goes unnoticed. But in severe cases the CPU simply says: 'I have had enough' and stops working altogether. That is what happens when the PC stops responding and hangs or crashes. This is a problem that embedded systems are not normally troubled by, but every PC-based system is, to some extent, affected by this.

Up to now no one has successfully merged hardware compression with the software environment of a PC. At Securex 2005, Sentient Technologies launched the arrival of SkyView DVRs in South Africa, boasting its top-of-the-range Hardware Compression DVR Capture Card. The SZT-MP4 is the perfect marriage of all of the strongest features of both embedded and PC-based DVRs, but at the same time eliminates the problems that have plagued so many other PC-based systems. And this is not all, because of its hardware compression it can record true 25 fps from each channel, up 32 channels of video and audio at 704 x 576 (PAL) resolution without losing one single frame. It is hardly surprising, that with its superb features SkyView has becomes Taiwan's top selling DVR and fast stealing the hearts of many SA clients.

Rodney Siems

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