Thursday, December 1, 2005

PEAQ: Perceptual Evaluation Audio Quality

Audio quality is one of the key factors when designing a digital system for broadcasting. The rapid introduction of various bit-rate reduction schemes has led to significant efforts in establishing and refining procedures for subjective assessments, simply because formal listening tests have been the only relevant method for judging audio quality.
The experience gained was the foundation for Recommendation ITU-R BS.1116, which then became the basis for most listening tests of this type.
Since subjective quality assessments are both time consuming and expensive, it is desirable to develop an objective measurement method in order to produce an estimate of the audio quality.

Traditional objective measurement methods, like Signal-to-Noise-Ratio (S/N) or Total-Harmonic-Distortion (THD) have never really been shown to relate reliably to the perceived audio quality. The problems become even more evident when the methods are applied on modern codecs which are both non-linear and non-stationary.
A number of methods for making objective perceptual measurements of perceived audio quality have been introduced during the last decade. But none of the methods were thoroughly validated, and consequently neither standardized nor widely accepted. In 1994, ITU-R identified an urgent need to establish a standard in this area and the work was initiated.
An open call for proposals was issued and the following six candidates for measurement methods were received: Disturbance Index (DIX), Noise-to-Mask Ratio (NMR), Perceptual Audio Quality Measure (PAQM), Perceptual Evaluation (PERCEVAL), Perceptual Objective Measure (POM) and The Toolbox Approach.

The measurement method in this Recommendation is the result of a process where the performance of each of the above six methods was studied, and the most promising tools extracted and integrated into one single method. The recommended method has been carefully validated at a number of test sites. It has proven to generate both reliable and useful information for several applications. One must, however, keep in mind that the objective measurement method in this Recommendation is not generally a substitute for arranging a formal listening test.

How PEAQb works
The test items were selected from Database 3 (DB3), which was used for the validation of the models. To simplify testing, a subset of the 84 items of DB3 was selected.
This subset consists of 16 items. The major criterion for the selection was that the resulting MOVs and DI (Distortion Index) values cover a broad range.

The test items are available from the ITU-R as WAV-files (Microsoft RIFF format) on a CD-ROM. All items were sampled at 48 kHz, 16-bit PCM. The reference and test signals as provided by the ITU are already time and level adapted to each other, so that no additional gain or delay compensation is required.

The measurement algorithm must be adjusted to a listening level of 92 dB SPL.

In order to conform to the Recommendation, the calculated DI values must reproduce the values given in Tables 22 and 23, with a tolerance of less than 0.02 for all test items.
If an implementation does not produce results within this tolerance it does not conform to this Recommendation.


# ./peaqb
usage: ./peaqb

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