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H.264 Rocks The CCTV World

What do Apple iTunes and YouTube videos share in common with the video surveillance industry? H.264, the latest standard for video compressor/decompressors, or codecs.

Developed by the ITU-T Video Coding Experts Group and ISO/IEC Moving Picture Experts Group, H.264 follows the highly successful MPEG2 and MPEG4 video standards and offers improvements in both video quality and compression.

Video codecs compress digital video in order to reduce the amount of bandwidth required to transmit and store the images. Compression is necessary because the raw data rate of uncompressed, digitally-encoded analog CCTV video at 30 fps is more than 158 megabits per second -- 300 times the capacity of a 512 kilobit-per-second asynchronous digital subscriber line (ADSL) connection. In terms of storage, a one-hour recording would fill an 80 gigabyte hard disk.

Scaling the video to lower resolution and compressing it with standard utilities such as WinZip or gzip could achieve 10:1 compression. However, at least 300:1 compression is needed to stream live video over an ADSL connection and to achieve 300 hours recording to an 80GB hard disk. This level of compression can be achieved with H.264.

An example of the bandwidth savings that can be achieved from a typical traffic surveillance camera, which must alternately transmit images of moving and non-moving traffic, is demonstrated in the following graph. In this example, the same 24-hour video sequence has been encoded using four different encoders: the IndigoVision 8000 MPEG4, the IndigoVision 9000 H.264, an MPEG-4 encoder with no motion estimation, and an MJPEG encoder.All were encoded at 25fps (with the exception of MJPEG at 5fps) to the same subjective video quality.

The graph shows that compared to MPEG4, H.264 can achieve savings of typically between 20 percent and 25 percent in bandwidth usage and in excess of 50 percent during periods of scene inactivity -- i.e., when there is no moving traffic. Not only does this reduce the overall bandwidth requirements of the IP video system but more importantly, it can significantly reduce the amount of storage required for recording the video, often one of the most expensive items in the system.

Implementing The Standard

It is important, when looking at H.264, to understand the difference between comparing a standard versus an implementation of a standard. The two are very different.

Thus when people say H.264 provides better video quality than MPEG2, it is a little misleading.

As a compression standard, H.264 defines the syntax of an encoded bitstream— in other words, the underlying language and grammar of the code. Naturally, a H.264-compliant decoder must conform exactly and be able to implement all the necessary tools defined by the standard in order to decode the bitstream.

An H.264 encoder, conversely, can implement a subset of the syntax defined by the standard, something akin to a language dialect, providing it produces a compliant bitstream. So it is more appropriate to say that H.264 provides a richer syntax and toolset than MPEG2. In addition, various implementations and algorithms within the encoder are also not defined by the standard, so manufacturers are free to innovate with design and functionality. As such, H.264 encoders from different vendors will compress streams at the same bit rate, but at differing quality, or generate the same quality video at a much lower bit rate.

H.264 has caught on. Apple uses it for all iTunes video and by the end of this year, YouTube hopes to have all of its video encoded in the standard. Its application in video surveillance will be no less significant.

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