Digital video compression techniques have played an important role in the world of telecommunication and multimedia systems where bandwidth is still a valuable commodity. Hence, video compression techniques are of prime importance for reducing the amount of information needed for picture sequence without losing much of its quality, judged by human viewers. Modern compression techniques involve very complex electronic circuits and the cost of these can only be kept to an acceptable level by high volume production of LSI chips (for a short introduction to how LSI chips are manufactured, visit Jim's site here). This means that we have to standardize the techniques of video compression.

The history of compression begins in the 1960s. An analogue videophone system had been tried out in the 1960s, but it required a wide bandwidth and the postcard-size black-and-white pictures produced did not add appreciably to voice communication! In the 1970s, it was realized that visual speaker identification could substantially improve a multiparty discussion and videoconference services were considered. Interest increased with improvements in picture quality and digital coding.

With the available technology in the 1980s, the COST211 video codec (Encoder/Decoder), based on differential pulse code modulation, DPCM (Pulse Code Modulation is still used in CD audio files, so they are called PCM/.wav files), was standardized by CCITT, under the H.120 standard. For more information on the history of conferencing, follow this link. This codec's target bitrate was 2 Mbit/s for Europe and 1.544 Mbit/s for North America, suitable for their respective first levels of digital heirarchy. However, the image quality, although having very good spatial resolution (due to the nature of DPCM working on a pixel-by-pixel basis), had a very poor temporal quality. It was soon realized that in order to improve the image quality, without exceeding the target bitrate, less than one bit should be used to code each pixel. This was only possible if a group of pixels (a ``block'') were coded together, such that the bit per pixel is fractional. This led to the design of so-called block-based codecs.

During the late 1980s study period, of the 15 block based videoconferencing proposals submitted to the ITU-T (formerly the CCITT), 14 were based on the Discrete Cosine Transform (DCT) and only one on Vector Quantization (VQ). The subjective quality of video sequences presented to the panel showed hardly any significant differences between the two coding techniques. In parallel to ITU-T's investigation during 1984-88, the Joint Photographic Experts Group (JPEG) was also interested in compression of static images. They chose the DCT as the main unit of compression, mainly due to the possibility of progressive image transmission. JPEG's decision undoubtedly influenced the ITU-T in favouring DCT over VQ. By now there was a worldwide activity in implementing the DCT in chips and on DSPs.

In the late 1980s it was clear that the recommended ITU-T videoconferencing codec would use a combination of interframe DPCM for minimum coding delay and the DCT. The codec showed greatly improved picture quality over H.120. In fact, the image quality for videoconferencing applications was found reasonable at 384 kbits/s or higher and good quality was possible at significantly higher bitrates if around 1 Mbit/s. This effort was later extended to systems based on multiples of 64 kbit/s (upto 30 multiples of this value). The standard definition was completed in late 1989 and is officially called the H.261 standard, and the coding method is referred to as the `p x 64' method (p is an integer between 1 and 30).

In the early 1990s, the Motion Picture Experts Group (MPEG) started investigating coding techniques for storage of video, such as CD-ROMs. The aim was to develop a video codec capable of compressing highly active video such as movies, on hard disks, with a performance comparable to that of VHS quality. In fact, the basic framework of the H.261 generation of MPEG, called the MPEG-1 standard, was capable of accomplishing this task at 1.5 Mbit/s. Since for the storage of video, encoding and decoding delays are not a major constraint, one can trade delay for compression efficiency. For example in the temporal domain a DCT might be used rather than DPCM, or DPCM used but with much improved motion estimation,  such that the motion compensation removes temporal correlation. This later option was adopted with MPEG-1.

These days, MPEG-1 decoders/players are becoming commonplace for multimedia on computers. MPEG-1 decoder plug-in hardware boads (e.g. MPEG magic cards) have been around for a while, and now software MPEG-1 decoders are available with the release of operating systems or multimedia extensions for PC and Mac platforms. Since in all standard video codecs, only the decoders have to comply with proper syntax, software based encoding has added the extra flexibility that might even improve the performance of MPEG-1 in the future.

MPEG-1 was originally optimized for typical applications using non-interlaced video of 25 frames per second (fps), in European format and 29.9 fps in North American format, in the range of 1.2 to 1.5 Mbits/s for image quality comparable to home VCRs, it can certainly be used at higher bitrates and resolutions. Early versions of MPEG-1 for interlaced video, such as those used in broadcase, were called MPEG1+. A new generation of MPEG, called MPEG-2 was soon adopted by broadcasters (who were initially reluctant to use any compression on video!). MPEG-2 codes for interlaced video at bit rates 4-9 Mbits/s, and is now well on its way to making a significant impact in a range of applications such as digital terrestrial broadcasting, digital satellite TV, digital cable TV, digital versatile disc (DVD) and many others. Television broadcasters have started using MPEG-2 coded digital forms since the late 90s.

A slightly improved version of MPEG-2, called MPEG-3, was to be used for coding of High Definition (HD) TV, but since MPEG-2 could itself achieve this, MPEG-3 standards were folded into MPEG-2.

It is foreseen that by 2014, the existing transmission of NTSC North America format will cease and instead HDTV with MPEG-2 compression will be used in terrestrial broadcasting.
 
 
 
 
 

Go to the next page, or use one of the following quick links:

Main Page
Motivation For Compression - Some True Stories
A Brief History of Compression (heheh - brief, compress... get it?)
Requirements From Any Compression Algorithm
Data Compression Fundamentals
Some Compression Techniques
Video Compression Techniques: The MPEG-1 Standard
The Future: MPEG-4 And MPEG-7
Related Links
 
 
 

Contact Me: sundar@pha.jhu.edu
Copyright © Sundar Srinivasan 2002