Introduction to Video CODECs
A raw digital video signal takes enormous amount of room. It is not practical to transmit or store digital video in its original form because pictures contain too much data. A reasonable solution is to compress video before transmission or storage, and to decompress it when needed. Since a video signal contains natural redundancy like static areas, repeating patterns, regions of similar texture and cyclic motion, such an approach can be quite effective.
The task of compression is handled by encoder – this is a device that analyzes the incoming video sequence, finds redundancies and produces a stream of binary codes, which describes the sequence according to some mathematical video coding model. When decompressed video is required, decoder takes these binary codes and reconstructs the sequence according to the same model. The pair encoder-decoder is commonly called CODEC, which is an abbreviation of coder-decoder.
Depending on the complexity of the model utilized, encoder can use fewer codes to describe the original sequence, and thus provide a higher level of compression. However, a complex model needs more computational power both for encoder to fit the original sequence to the model, and for decoder to reconstruct video. Since a human eye is not capable of catching every aspect of a visual signal, the encoder algorithm may decide not to code some details of the original sequence based on some psychovisual model.
A sequence reconstructed by decoder will not perfectly match the original sequence, but either the difference will be visually indistinguishable, or the decoded stream will contain small artifacts, which are acceptable. Sometimes the decoded stream can contain annoying artifacts, but they can be accepted, too, taking into account constraints of the transmission channel or storage media. With a lower video quality encoder produces a higher compression level, and decoder has less bits to decode, and thus consumes less resources.
Fitting a video coding model by encoder is a sophisticated optimization problem. An accurate solution for this problem is not always feasible in a reasonable amount of time, and encoder uses suboptimal or heuristic algorithms to fit the model. The more accurate algorithms used, the higher visual quality of the reconstructed sequence at the same compression level is possible. As a result, there is always a trade off between computational power, compression level and video quality.
As far as computer systems evolve, the more complex models come into use for video compression. The Motion Picture Expert Group (MPEG) is working to generate the specifications for audiovisual coding under International Organization for Standardization (ISO) and International Electrotechnical Commission (IEC). What is commonly referred to as “MPEG video” consists of MPEG-1, MPEG-2, and MPEG-4 standards, which were developed during the past 10 years in response to growing industry demand for video and audio compression and rapidly increasing computing power of electronic devices. H.264/AVC is the latest ratified video coding standard. It emerged as the result of joint development of International Telecommunication Union Video Coding Experts Group (ITU VCEG) and MPEG ISO. This standard is known as H.264 (ITU-T name), or MPEG-4 part 10 (ISO/IEC 14496-10), or MPEG-4 Advanced Video Coding (AVC).
When judging the quality of a CODEC, one must examine the engineering
and algorithms used to expand upon the video CODEC standards. Efficient
engineering and powerful algorithms result in a CODEC that provides
excellent video quality at a relatively low bit rate.
|Achieving plausible video and audio recording and transmission at approx. 1.5Mbps.||VHS quality at 1.5Mbps.|
Typical resolution SIF.
|Nearly every computer supports
MPEG-1 files. |
Typically used for lower resolution video, but can be used for any resolution. Progressive only.
|High bit rate and broader generic applications, including TV, Broadcast, consumer electronics.||High quality typically Full
D1 resolution (Broadcast and DVD are MPEG-2)|
at 8 Mbps in the early stages and 2-4Mbps today
|Application: Digital TV, HDTV,
DVD, Digital cable, Satellite and Terrestrial.|
The most common standard.
|Very low bit-rate CODEC, enabling low frame rate, low resolution.||Typically moderate quality.|
In full-frame rate, moderate / high quality requires similar bit-rate to MPEG-2.
|Delivery of digital content
video over Internet and wireless devices.|
Video conference using H.263 that is based on this standard.
Emerging International Standard
|All video requirements from High quality HDTV to low bit rate for cellular networks.||Variety of bit rates; e.g.
Full D1 30fps DVD quality at less than 1Mbps.
CIF at 256Kbps.
|The most advanced standard.|
Hardware or Software CODECs?
Initially, CODECs were primarily hardware devices due to the fact that the process of decompression demanded high levels of CPU usage that were out of reach. Now, with the advent of faster computers and their continuously evolving speed, companies are beginning to move towards software CODECs. The new software CODECs break the myth that software CODECs are only able to provide low quality results. Today software CODECs provide inexpensive solutions with extremely high quality results.