Systems from Snell & Wilcox for testing, estimation and checking of MPEG streams

By Andrey Medvedev

Review contents:
MPEG-2 and measurements TV Network Monitoring Equipment of the the IMOS Company Going digital with program insertion — challenges and solutions Innovative T&M system for MPEG-2 monitoring and analysis Systems from Snell & Wilcox for testing, estimation and checking of MPEG streams Video test and measurement equipment

With issue of new digital standard of broadcasting TV it is appeared a need to transfer the more data through relatively narrow channels: satellite, terrestrial, cable and so on. This can be the delivery of the finished programs, of content for the following editing or TV bridge compositing. At present under transmission of the TV signal the algorithms of the MPEG compression are often used that, in turn, superimpose some restrictions on parameters of the source signal.

Accordingly new standards require new methods of the checking and measurements. Unfortunately, there are miscellaneous opinions about the methods of the execution of the correct and full measurement of the MPEG system. At least this occurs so because it is no any algorithm of measurement approved (or standardized) internationally. There is study of several thousands of the MPEG system typical failures. As the number of related MPEG devices increases the corresponding experience and failure statistics will be accumulated. Certainly, the revision of the specifications and parameter correction of the concrete manufacturing devices will be continued. During many decennials of exploitation of analog TV equipment the sufficient experience is accumulated and the conditions for stable round-the-clock functioning of worldwide TV networks are created. The MPEG systems exist not so long ago, but this systems are principally differed from their own predecessors on way of media delivery for wide auditorium.

For full control of the compression system in program of the measurements two components are usually included:

• Influence on the stream (test MPEG stream);
• Monitoring of the system status.

The decisions designed by Snell & Wilcox are encompassed both components. The library of MPEG test stream Test Card M (TCM) provides the test influence on transmission path and MPEG decoder; the program (Mosalina) and hardware (IQMPAR) analyzers of the stream provide an analysis and checking of MPEG-2 stream quality and tables of service information DVB SI.

The transmission path can be any in the following range: from the cable cut, connecting coder with decoder, to satellite delivery system, including several levels. In general on input of the path a precision repeated digital stream must be fed, and signal on output of the path must be checked on absence of system level error, as well as be subjected to checking decoding.

Under testing of the broadcasting path, terrestrial or satellite channels to the path input through corresponding modulator the compressed digital stream of the test table is fed. This variant is more preferable than the feed of usual video signal of the test table to the compression coder input since he guarantees a certainty of the test stream with accuracy to each separate bit and resolve the problem about compatibility of coder with tested path and decoder.

The Snell & Wilcox company is at the head of the project Test Card M, within the framework of which the efforts of Great Britain leading broadcasting companies and Independent TV Commission (ITC) have been joined. The digital test table, used in this project, takes into account the particularities of digital data transmission and compression under conservation of the using simplicity, peculiar to traditional analog test pictures.

The Library contains the set of specially developed digital streams of changing complexity with strictly determined parameters. Each stream includes one or several streams of compressed video, audio and additional data. By them it is possible to test the devices of the signal processing (for instance, remultiplexer) and receivers-decoders. TCM does not check the quality of the compressed devices functioning.

All types of test streams are divided into four categories: video (VID), audio (AUD), data (DAT), multiplexing (MUX). Depending on the stream complexity four levels of the stress for processing are used: a — low; b — average; c — high; x — inadmissible. The main difference of proposed test signals from traditional statistical signals is a moving picture with smoothly moving elements, not breaking on borders of the cycles. This allows recognizing such defects of the decoding, as “scattering” of the frame, “freezing”, repeat of the frames and so on.

A frame from the “video” test streams library

As example at the figure it is shown one of the TCM pictures of VID category. The picture contains several sort of levels for corresponding evaluation of test device functioning. The general configuration and size of the picture, as well as in analog test table, allow to value the correctness of the format reproducing, and separate segments contain some special movable pictures for checking of any functions of a decoder. The table can be loaded and reproduced on equipment of many leading manufacturers: Adherent, Rohde & Schwarz, Snell & Wilcox, Tektronix and others. Detailed description of the methods of the use by program is kept in accompanying documentation.

The second direction in checking and system measurement of the compression system is the analyzers of the transport stream. They research the transport stream, realize monitoring of its main parameters and display any possible errors. For objective quality evaluation of compressed picture a new measuring method was developed by Snell & Wilcox company; this method was named as PAR (Picture Appraisal Rating). The systems with PAR technology are worked with input stream and check the artifacts, created by MPEG-2 compression. They present the simple numeric evaluation of the picture distortions, contributed by MPEG coding, like the school assessment system, applicable as well as under subjective quality evaluation according to Recommendation ITU-R. Herewith it is taken into account several factors at once: selected scales of the quantization, visibility of the block edges and so on.

It is very important that PAR allows separating the MPEG distortions from the other picture defects such as grain of the film or cross hindrances of composite decoding. The numerical value of PAR is not a general quality of the picture, but it is a loss of quality, caused by MPEG coding defects.

Mosalina software MPEG stream analyzer

With use growing of the multimedia technology such as DVD and digital broadcasting, it is desirable to get the “qualitative” MPEG, falling within available range of the stream rates. Snell & Wilcox have developed the program multifunctional analyzer of the transport stream Mosalina, working both outside of, and in real time, allowing to evaluate the compression quality of the SD and HD pictures according to MPEG-2 standard and is vastly more suitable, than frame-by-frame analysis of the material. Also, Mosalina displays the following parameters: the digital stream rate and the quantization scale code (QSC). Built-in program decoder allows the compressed video streams to be viewed.

Alongside with the program analyzers by Snell & Wilcox Company have developed the stream analyzer ASI for popular module system IQ (IQMPAR). The module allows simultaneous to analyze up to three video streams of general rate up to 14 Mb/sec. As well as Mosalina, the IQMPAR is functioned according to PAR technology and allows assigning a threshold value, at excess of which a signal of the error is generated. Monitoring is realized through RollCall network that is to say centrally from single control room, intended for control of the whole complex.

Thereby, the systems of the MPEG stream quality checking by Snell & Wilcox Company allow to find and to localize the errors of the transmission confidently. However it does not follow to forget about quality of the source picture. Even low noises, being present in compressed picture, sharply enlarge the possibility of the appearance stressful situation in coder, but this, as a rule, brings to appearance of artifacts after decoding. The simplest and the cheapest decision in this case is an using of the preprocessing of the picture before compression. The preprocessing contributes a great contribution to decision of permanent compromise which is “speed of the stream — a quality”.

Material is provided by Snell & Wilcox Company