ITU-R REPORT BT.1206 PDF

Introduction

With the increasing application of digital coding and bit-rate reduced transmission, the assessment of coding impairments are of critical importance. An understanding of these assessment methods is relevant not only to the performance of new coding equipment, but also to an interpretation of measurements made on such equipments and to specifications for target performance. Moreover digital codecs as with all adaptive or non-linear digital processes, cannot be fully characterized with traditional television test signals or patterns.

Studies in connection with Question 3/11 and study Programme 3B/11 indicate the desirability of establishing relationships between objective measurements of signals impaired by digital coding, and the subjective assessments of the quality of the picture thus obtained. This Report gives progress towards this end, which is proving more difficult to achieve as codec complexity increases.

Subjective methods for conventional resolution television picture quality and impairment assessment are given in Recommendation 500 and for HDTV, are given in Recommendation 710. —The application of these methods to television codec assessment is considered in this report.

Recently, considerable experience has been gained in the Assessment of the performance of high quality codecs for 4:2:2 component television at 34, 45 and 140 Mbit/s [CCIR, 1986-90a]. In these trials, codec performance was examined in terms of basic decoded picture quality, quality after studio post-processes (chromakey and slow motion) applied to the decoded pictures, and the decoded picture impairment associated with the presence of a range of channel error rates. Parts of this report draw upon these experiences.

For distribution applications quality specifications can be expressed in terms of the Subjective judgement of observers. Such codecs can in theory therefore be assessed subjectively or objectively against these specifications. The quality of a codec designed for contribution applications however, could in theory be specified in terms of objective performance parameters because its ouput is destined not for immediate viewing, but for studio post-processing, storing and/or coding for further transmission. Because of the difficulty of defining this performance for a variety of post-processing operations, the approach preferred has been to specify the performance of a chain of equipment, including a post-processing function, which is thought to be representative of a practical contribution application. This chain might typically consist of a codec, followed by a studio post-processing function (or another codec in the case of basic contribution quality assessment), followed by yet another codec before the signal reaches the observer. Adoption of this strategy for the specification of codecs for contribution applications means that the measurement procedures given in this Report can also be used to assess them.

Throughout this Report the importance of choosing critical test picture sequences, mostly of natural scenes, is stressed and some guidelines on how such sequences may be generated or chosen is given.