MPEG-4 Part 2
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MPEG-4 is a suite of standards which has many "parts", where each part standardizes various entities related to multimedia, like audio (part 3), video (parts 2 and 10), file formats (parts 12, 14 and 15). To know more about various parts and what they mean, please see the entry for MPEG-4.
MPEG-4 Part 2 is a video compression technology developed by MPEG, and now many documents simply refer this as MPEG-4 video. The other common video codec defined as part of the MPEG-4 Standard, MPEG-4 part 10 is commonly refered to as H.264 (jointly developed by ITU-T and MPEG). It belongs to the MPEG-4 ISO/IEC standard (ISO/IEC 14496-2).
Like the Audio components of MPEG-4, the video components are divided into several profiles that are aimed for use in several different standards. Most of the video compression schemes standardize the bitstream (and thus the decoder) leaving the encoder design to the individual implementations.
To address various applications starting with low quality, low bit rate and low resolution surveillance cameras (or mobile streaming) to high definition TV broadcasting and DVDs, many video standards group features into profiles and levels. MPEG-4 Part 2 has nearly 21 profiles, commonly used being simple, advanced simple, main, core, advanced coding efficiency, advanced real time simple, etc. The most commonly deployed profiles are Advanced Simple and Simple, which is a subset of Advanced Simple.
Simple Profile
Simple Profile is mostly aimed for use in situations where low bit rate and low resolution are mandated by other conditions of the applications, like network bandwidth, device size etc etc. Examples are cell phones, some low end video conferencing systems, surveillance systems etc.Advanced Simple Profile
Advanced Simple Profile (ASP) is a "profile" defined in the digital compression codec standard MPEG-4 Part 2 (Visual).MPEG-4 Visual is essentially still a Discrete Cosine Transform (DCT) compression standard, similar to previous standards such as MPEG-1 and MPEG-2.
Notable technical features of ASP relative to the Simple Profile, which is roughly similar to H.263, include:
- Support for mpeg quantization
- Support for interlaced video
- Support for B pictures
- Quarter-sample motion compensation (QPel)
- Global motion compensation (GMC)
The interlace support is designed in a basically similar way to the way it is found in MPEG-2 Part 2.
The B picture support is designed in a basically similar way to the way it is found in MPEG-2 Part 2 and H.263v2.
The optional quarter-pixel motion compensation feature of ASP was innovative, and was later also included (in somewhat different forms) in MPEG-4 Part 10 and VC-1. Many implementations omit support for this feature, because it has a significant harmful effect on speed, and it often does not help quality.
The global motion compensation feature is not actually supported in most implementations although the standard officially requires decoders to support it. Most encoders do not support it either, and some experts say that it does not ordinarily provide any compression benefit. When used, ASP's global motion compensation has a large unfavorable impact on speed, and adds considerable complexity to the implementation.
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