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).
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.