Q: What is video compression and why is it used?
A: It’s a way of electronically compressing the amount of video data produced by a CCTV camera so that it needs less hard disk space to record it. Compression is used principally to cut the cost of storing CCTV images. Even basic compression methods can reduce the amount of storage needed by a factor of at least 20 without noticeably affecting image quality. Much better results can be achieved with the latest compression techniques, or if some loss of quality can be accepted.
Q: Why are there so many different types of compression?
A: Partly for historical reasons. New techniques are constantly being introduced, usually to provide higher compression with less degradation in image quality. In addition, all compression techniques have benefits and drawbacks, so it’s useful to be able to choose the best for each job.
Q: What are the main types of compression used in CCTV, and how do they compare?
A: MJPEG compresses each frame of the video individually as if it were a still photograph. For some time this compression method has been the choice of the legal system as it does not employ conditional refresh. Down sides are high packet size and bandwidth required if transmitting over IP.
MPEG4 is a true video compression system and gives good results at high frame rates. Its general efficiency is reduced when recording at very low frame rates or when there’s a lot of fast movement in the image. Digital artefacts can occur in these circumstances.
MPEG4+, which is also known as H264, is a development of MPEG4 that provides even better performance. Its drawback is that it needs a lot of processing power, which can lead to latency when monitoring live images that have been compressed. This can, for example, make it difficult to control PTZ cameras.
MJPEG2000, which is also known as wavelet compression, is like MJPEG in that each frame is compressed individually. The compression method is, however, very different and even at high compression rates the results still look good. Like MPEG4+, this method needs a lot of processing power, which can lead to time delays.
Q: Which method should I use?
A: That really depends on the application but, at the present time, MPEG4 would be a good choice for most installations, as it’s efficient, effective and well supported. Nevertheless, H264 is also rapidly growing in popularity and video compression is a fast-moving area, so it’s always worth talking to your CCTV supplier to find out about the latest developments.
- Jon Hill is Video Systems Specialist at Siemens Building Technologies, Security Products Limited (formerly Bewator). The company is a leading supplier of innovative security products and systems, encompassing access control, CCTV, intruder and integrated security management systems. Siemens Security Products serves the UK and Republic of Ireland for pre-qualification, supply, training and after-sales support for all security products bearing the Bewator and Siemens brands.
- Contact 0871 386 0835 or visit www.buildingtechnologies.siemens.co.uk/securityproducts/.