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QC control and management

More and more audiovisual content needs to be produced for different distribution channels and platforms. IT-based production environments provide the basis, but present new challenges for quality assurance. In IT structures, video files are permanently moved without an implicit control by an operator. Traditional QC procedures are no longer sufficient to control the integrity of the content in the IT process.

IRT supports broadcasters in the introduction of quality control and quality management in the file-based production process. The experts carry out process analyzes, determine the control requirements, identify useful QC tools and define the QC results for operational decisions. In the production process, a management of all QC activities must take place. It is necessary to define responsibilities and to define effective and economic QC points for the examinations as well as to integrate them into the workflows.

Measuring audio quality as per internationally standardised test methods

Modern audio-coding techniques featuring data reduction typically function with variable bit rates. A reduced data rate is by definition associated with lower quality. Unfortunately, sufficiently reliable results cannot be provided by objective methods for determining the audio quality of special audio-coding techniques or cascade chains (repeated coding/decoding and use of different audio-coding techniques within a transmission chain). For this reason, experts must rely on psychoacoustic measurements, which are based on subjective quality evaluations generated by experimental means.

Two internationally standardised test methods for measuring the quality of audio-coding techniques and cascade chains have been used successfully for several years in the field of broadcasting:

Test method ITU-R BS.1116
In the context of the ITU-R BS.1116 test method, a listener hears three stimuli: A, B, and C. “A” is always the reference stimulus. The test signal which is to be evaluated and the hidden reference are presented in a random sequence as the stimuli B and C. The listener must first recognise the reference as well as the test signal which requires assessing, and then compare the test signal with A on a scale from 5 (imperceptible) to 1 (very annoying).

MUSHRA method
The MUSHRA method (MUlti-Stimulus test with Hidden Reference and Anchor, ITU-R BS.1534) makes it possible to select any test signal requiring evaluation from as many as twelve synchronous test signals. The chosen test signal is then reproduced via loudspeakers or headphones for assessment.

In addition to the test sequences themselves, MUSHRA features three further stimuli. These include a labelled reference (unmodified original signal which does not require evaluation) as well as a hidden reference and a hidden anchor (7 kHz low-pass filtered original), which are to be assessed as the other test sequences. Scores are assigned on the basis of a continuous scale of quality from 0 to 100 %, or in the form of attributes from “bad” to “excellent”. The “overall quality of audio” is evaluated in consideration of all perceptible forms of interference.

Studies on the picture quality of video-over-IP solutions

Conventional techniques for studying picture quality no longer lend themselves to digital signals, which are sometimes plagued by entirely different kinds of errors. Despite efforts to develop objective measuring methods, the results repeatedly fail to reflect quality as perceived by the human eye, which is uninterested in purely logical approaches. Subjective tests are therefore recommended for checking the quality of digital videos.

IRT plays an instrumental role in developing these subjective test methods and conducts its own tests of current video codecs.

SAMVIQ
SAMVIQ (Subjective Assessment Methodology for VIdeo Quality) was developed primarily for evaluating the quality of online video formats, yet can also be used for conventional TV formats.

Existing subjective test methods have been standardised in Recommendation ITU-R BT.500. These methods, however, were developed first and foremost for the TV sector. While these conventional methods are good tools for evaluating errors such as blurring and blocking, continuous picture streams are of the utmost importance for multimedia applications. The picture size in this context usually corresponds to a fourth (SIF) or 1/16 (QCIF) of standard resolution.

SAMVIQ, on the other hand, was custom-designed for multimedia purposes and permits the use of manufacturer-specific decoders (e.g. MPEG-4 Player, RealVideo, QuickTime, Windows Media) as well as various resolutions, including SDTV. Moreover, the optimised test procedure allows for more detailed findings and absolute quality assessments.

Loudness in audio-visual media

The problem of annoying loudness jumps between different TV and radio stations or programs, e. g. if zapping between various programs respectively transmission paths or at the transition between various program blocks within one program became generally worldwide known. The occurring loudness differences or jumps may amount up to 10 dB. That means doubling of the loudness impression, which naturally would extremely annoy.

Because of the numerous viewer complaints the broadcasters started already 10 year ago to modelling strategies in order to avoid the annoying loudness jumps. An important milestone was contributed by IRT by the development of a broadcast loudness meter. This loudness meter offering high correlation between the subjective and objective loudness have enabled broadcasters to measure the subjective parameter loudness objectively. The IRT loudness algorithm implementation - completed by data logging - by the manufacture PINGUIN have been used in the broadcast studios of ARD and ZDF since a couple of years for controlling loudness relationships.

Since publication of the recommendations ITU-R BS.1770 in 2006, a standardized tool is available to measure the loudness of broadcast programs. Because this algorithm does not differ significantly from the IRT algorithm, the changeover to the international standard can be done flowingly.

The actual conversion of the international loudness standard was established by the EBU project group P-LOUD during recent two years. P-LOUD elaborated a complete package of recommendations or rather technical documents, which exceeded recommendation ITU-R BS.1770.

In detail the elaborated recommendations by P-LOUD are:

Your contact

Dr. Markus Ludwig
Sachgebietsleiter
Head of Department
Tel. +49 (0)89 32399-321