ITU-R - International Telecommunication Union/ITU Radiocommunication Sector
Scope
Introduction Digital television systems operate in very different ways from analogue systems with the consequence that a quite different set of picture impairments may be introduced. Impairments may occur both from the conversions to and from the digital domain (which include filtering@ sampling and quantization processes@ see Reports 629@ 962 and Recommendation 601) and by degradations of the digital signal itself (such as individual digit errors@ timing jitter or loss of frame synchronization). In the conversion processes@ the impairments may be picture dependent@ while errors in the digital domain may be bit-sequence dependent. In the digital domain@ an increase in noise or distortion above a certain threshold level can result in a rapid increase in the number of digit errors. Before that level is reached@ the error performance can be improved significantly by the use of error correction techniques. Picture impairments may thus arise from several sources: a) distortions in the conversion processes from analogue to digital form and from digital to analogue form; b) errors in the digital channel; c) distortions introduced by digital signal processing. Test and measurement methods for the digital television system must therefore include consideration of these separate factors and of the need for both offline (acceptance and maintenance testing) and on-line (monitoring and diagnostic) requirements. In the case of picture impairments@ either subjective or objective tests may be used. Section 2.1 of this report contains information on subjective test methods and section 2.2 considers objective testing. In either case@ care must be taken to consider the effect of two- or three-dimensional signal processing techniques which render many current analogue test signals of little use (in that case because of their line-repetitive nature). Section 2.3 of this report considers the testing of signals in digitally encoded form. In this case@ the emphasis is on the measurement of residual bit errors (after any correction has taken place)@ error distributions and their relation to picture or other information.Objective tests and specialized test equipment may be required. In digital equipment@ advantage can often be taken of the self-test capability of many digital integrated circuits and the ability of the digitalcircuits to generatewell-defined waveforms to test the analogue parts of the conversion circuits. There is a great deal of work remaining to create adequate measurement methods fordigital television equipment and administrations are thus invited to makecontributions on this subject.