A Primer On Mpeg Video Compression

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A Primer on MPEG Video Compression By Diego Villa Created Mar 4 2009 - 15:11

What is video compression? Video compression is a technology that refers to the reduction of the amount of data that represents encoded video signal. Encoding is the process of converting analog video to digital. Video compression devices or software usually do both encoding and compression in the same box or application. Why is video compression needed? Video compression technology was developed because of the need to send video signals through available transmission paths with limited bandwidth capacities. Uncompressed Standard Definition (SD) digital video is typically encoded at 270 Mbps while uncompressed High Definition (HD) video is at 1.5Gbps. Terrestrial TV channel can only handle around 19 Mbps while typical satellite TV channel is usually available at 3 Mbps. DSL connection at home in most countries is offered at 2 Mbps while typical office LAN networks have capacities of either 10 Mbps or 100 Mbps. Take note that it is impossible to fit 1.5 Gbps or even 270 Mbps signal data streams in any of these transmission paths! History You may not be aware of it but compression was already being used for a very long time in analog TV. Interlacing is a technique wherein analog video is scanned and displayed alternately (odd and even fields) to effectively reduce the required bandwidth to one half. But the advent of digital TV necessitated the development of digital video compression technology. In 1988, the Moving Pictures Experts Group (MPEG) was formed by the International Organization for Standardization (ISO) to set the standards for audio and video compression and transmission. The first compression technique they developed was MPEG-1, which was the technology used for the Video Compact Disc or VCD. Then came MPEG-2 which was then utilized for DVD and the Digital Video Broadcast (DVB) standard for satellite, cable and terrestrial transmission. This was also later adopted in the American ATSC and Japanese ISDB-T terrestrial transmission standards.

The need to put more content to existing and developing media like IPTV also saw the development of MPEG-4, with its most popular variant H.264, offering up to 50% savings in bandwidth compared to MPEG2. This is now widely used by telco operators worldwide in deploying their IPTV systems and slowly gaining ground in professional broadcast applications. Group of Pictures (GOP) ? Basis of MPEG Compression

The basic principle in data compression is to check for redundancy in the reference signal and just repeat this information in the coded signal. This way, lesser processing power and data bits are used because only the non-redundant information is processed. The equivalent of these reference and coded signals in MPEG compression is the GOP or Group of Pictures that consist of the following: 1. Intra Pictures (I-Pictures) 2. Predicted Pictures (P-Pictures) 3. Bidirectional Pictures (B-Pictures) GOP is the basis of MPEG video compression technology. The GOP begins with an I picture (the reference picture) and then has P pictures (coded pictures) spaced throughout. The remaining pictures are B pictures (other coded pictures). The GOP ends at the last picture just before the next I picture.

I pictures are coded using the information present in the picture itself and serve as the reference for the succeeding picture frames. P pictures are coded with respect to the previous I pictures or P pictures. This scheme is called forward prediction. P pictures require roughly half the data of an I picture. B pictures on the other hand use bidirectional prediction by using both past and future pictures as reference. B pictures more or less require one quarter the data of an I picture but use the most processing power. MPEG-1 The initial work of the MPEG committee was to study techniques for the storage of video in available media then such as the CD-ROM. They come up with a codec termed MPEG-1 that can compress video with picture quality comparable to that of the Video Home System (VHS) analog video tape format. This is accomplished at encoding rates at around 1.5 Mbps that can also be increased to improve quality if the available bandwidth permits. MPEG-2 and MPEG-3 Broadcasters initially were reluctant to adapt MPEG-1 for their applications because analog video then still offered far better picture quality. To address this issue, MPEG-2 was developed, which offered comparable if not better broadcast quality picture at encoding rates from 4 to 9 Mbps. This became the de facto standard in a range of applications, such as digital terrestrial broadcasting, digital satellite TV, digital cableTV and the optical medium DVD. There were efforts to develop MPEG-3 for High Definition TV (HDTV), but since MPEG-2 itself was capable of achieving this, the MPEG-3 standards were just made part of MPEG-2. MPEG-4 The MPEG-4 standard was developed to address the emerging developments in the Internet. This is the reason why the specified encoding rate is from less than 100 kbps to the Mbps range depending on the Internet connection. The MPEG-4 standard family incorporated many of the MPEG-1 and MPEG-2 features, adding new ones such as support for ?le handling, digital rights management and many other interactive applications. H.264 or AVC

In 2001, the ISO formed the Joint Video Team (JVT), which came out with the standard MPEG-4 Part 10, also known as H. 264 codec or Advanced Video Coding (AVC). The standard specified video coding layer and network adaptation layer for transmission of the video data stream in IP networks. The basic functional elements of H.264 are also similar to those of MPEG-1, MPEG-2 and MPEG-4. The important advancement is in the improvements in the implementation of these functional elements that resulted to more bandwidth-usage efficiency. Future Trends Video compression technology has improved dramatically from the early years when MPEG-1 was introduced. MPEG-2 is now a mature technology and is used extensively in TV signal transmission through cable, satellite and terrestrial broadcast. It is also the technology of choice for popular optical disc media like the HD-DVD and Blu-ray. It may still be around for a couple of years more with the more bandwidthefficient H.264 gradually taking its place. H.264 is to date the most efficient video compression technology that can deliver HDTV and multimedia content through scarce and valuable media bandwidth. Technology mpeg video compression Trademarks

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