Chapter 6: Coding for Videoconferencing (H.261)

Overview

The H.261 standard defines the video coding and decoding methods for digital transmission over ISDN at rates of p × 64 kbit/s, where p is in the range of 1–30 [1]. The video bit rates will lie between approximately 64 kbit/s and 1920 kbit/s. The recommendation is aimed at meeting projected customer demand for videophone, videoconferencing and other audio-visual services. It was ratified in December 1990.

The coding structure of H.261 is very similar to that of the generic codec of Chapter 3 (Figure 3.18). That is, it is an interframe DCT-based coding technique. Interframe prediction is first carried out in the pixel domain. The prediction error is then transformed into the frequency domain, where the quantisation for bandwidth reduction takes place. Motion compensation can be included in the prediction stage, although it is optional. Thus the coding technique removes temporal redundancy by interframe prediction and spatial redundancy by transform coding. Techniques have been devised to make the codec more efficient, and at the same time suitable for telecommunications.

It should be noted that any recommendation only specifies what is expected for a decoder; it does not give information on how to design it. Even less information is given about the encoder. Therefore the design of the encoder and the decoder is at the discretion of the manufacturer, provided they comply with the syntax bit stream. Since the aim of this book is an introduction to the fundamentals of video coding standards, rather than giving instructions on the details of a specific codec, we concentrate on the reference model (RM) codec. The reference model is a software-based codec, which is devised to be used in laboratories to study the core elements as a basis for the design of flexible hardware specifications.

During the development of H.261, from May 1988 to May 1989, the reference model underwent eight refinement cycles. The last version, known as reference model eight (RM8) [2], is in fact the basis of the current H.261. However, the two may not be exactly identical (although very similar), and the manufacturers may decide on a different approach for better optimisation of their codecs. Herein we interchangeably use RM8 for H.261. Before describing this codec, we will first look at the picture format, and spatio-temporal resolutions of the images to be coded with H.261.