9.3 Extensions of H.263

In late 1990s the Video Coding Experts Group (VCEG) of the ITU-Telecommunications standardisation sector set up two activities. The aim was to develop very low bit rate video coding at bit rates less than 64 kbit/s and more specifically at less than 24 kbit/s. One activity is looking at the video coding for very low bit rates, under the name of H.263+ [5]. Recently work of this activity has continued under the name of H.263++, indicating further improvements on H.263+ [6]. The other activity, which has more in common with MPEG-4, is work on advanced low bit rate video coding, under the name of H.26L [7].

The H.263+/H.263++ development effort is intended for short term standardisation of enhancements of the H.263 video coding algorithm for real-time telecommunication and related nonconversational services. The H.26L development effort is aimed at identifying new video coding technology beyond the capabilities of enhancements to H.263 by the H.263+/H.263++ coding algorithms.

These two subgroups also have a close cooperation in the development of their codecs, since the core codec is still H.263. They also work closely with the other bodies of ITU. For example, the collaboration between H.263+ and the mobile group has led to consideration for greater video error resilience capability. The back channel error resilience in H.263+ is especially designed to address the needs of mobile video and other such unreliable bit stream transport environments. The H.26L group work very closely with the MPEG-4 group, as this group has the mandate of developing advanced video coding for storage and broadcasting applications [7].

One of the key features of the H.263+, H.263++ and H.26L is the real-time audio-visual conversational services. In a real-time application, information is simultaneously acquired, processed and transmitted and is usually used immediately at the receiver. This feature implies critical delay and complexity constraints on the codec algorithm.

An important component in any application is the transmission media over which it will need to operate. The transmission media for H.263+/H.26L applications include PSTN, ISDN (1B), dial-up switched-56/64 kbit/s service, LANs, mobile networks (including GSM, DECT, UMTS, FLMPTS, NADC, PCS etc.), microwave and satellite networks, digital storage media (i.e. for immediate recording) and concatenation of the above media. Due to the large number of likely transmission media and the wide variations in the media error and channel characteristics, error resiliency and recovery are critical requirements for this application class.

9.3.1 Scope and goals of H.263+

The expected enhancements of H.263+ over H.263 fall into two basic categories:

enhancing quality within existing applications
broadening the current range of applications.

A few examples of the enhancements are:

improving perceptual compression efficiency
reducing video coding delay
providing greater resilience to bit errors and data losses.

Note that H.263+ has all the features of H.263, and further tools are added to this codec to increase its coding efficiency and its robustness to errors. This is an ongoing process and more tools are added every year. Recently this codec has been designated H.263++, to emphasise the ongoing improvement in the coding efficiency [6].

9.3.2 Scopes and goals of H.26L

The long-term objective of the ITU-U video experts group, under the Advanced Video Coding project, is to provide a video coding recommendation which at very low bit rates can perform substantially better than that achievable with the existing standards (e.g. H.263+). The adopted technology should provide for:

enhanced visual quality at very low bit rates and particularly at PSTN rates (e.g. at rates below 24 kbit/s)
enhanced error robustness in order to accommodate the higher error rates experienced when operating for example over mobile links
low complexity appropriate for small, relatively inexpensive, audio-visual terminals
low end-to-end delay as required in bidirectional personal communications.

In addition, the group is closely working with the MPEG-4 experts group, to include new coding methods and promote interoperability. Advances in this direction will be discussed in detail at the end of this Chapter.

9.3.3 Optional modes of H.263

In the course of development of H.263 numerous optional modes have been added as annexes to the main specifications to improve the visual communication efficiency of this codec. Some of the annexes were introduced along with the introduction of the core H.263, and many more were added gradually under H.263+ and H.263++. It is not the intention to introduce here all these annexes, nor to specify which annex belongs to which generation of the codec. Instead, we try to classify them into groups, without specifying in which generation they were introduced, rather to give a better appreciation of these optional modes in improving coding efficiency. As we will see, since the H.263 video codec is primarily aimed for mobile video communications, and UHF channels are particularly prone to channel errors, the majority of the annexes deal with the protection of visual data against channel errors.