Perceptual Evaluation of Speech Quality

Perceptual Evaluation of Speech Quality (PESQ) is a family of standards comprising a test methodology for automated assessment of the speech quality as experienced by a user of a telephony system. It is standardized as ITU-T recommendation P.862 (02/01). Today, PESQ[1] is a worldwide applied industry standard for objective voice quality testing used by phone manufacturers, network equipment vendors and telecom operators. Its usage requires a license.

Measurement scope

PESQ was particularly developed to model subjective tests commonly used in telecommunications (e.g. ITU-T P.800) to assess the voice quality by human beings. Consequently, PESQ[1] employs true voice samples as test signals. In order to characterize the listening quality as perceived by users, it is of paramount importance to load modern telecom equipment with speech-like signals. Many systems are optimized for speech and would respond in an unpredictable way to non-speech signals (e.g. tones, noise). Guidelines for proper applications of voice test samples are defined in the PESQ application guide ITU-T P.862.3.

ITU-T’s family of full reference objective voice quality measurements started in 1997 with P.861 (PSQM), which was superseded by P.862 (PESQ)[1] in 2001. P.862 was later complemented with the recommendations P.862.1[2] (mapping of PESQ scores to a MOS scale), P.862.2[3] (wideband measurements) and P.862.3[4] (application guide). Since 2011 P.863 (POLQA)[5] is in force. Two additional implementer’s guides for P.863 have been consented by ITU-T Study Group 12 in November 2011. In addition to the above listed full reference methods, the list of ITU-T’s objective voice quality measurement standards also includes P.563[6] (no-reference algorithm).

Testing typology

Depending on the information that is made available to an algorithm, voice-quality test algorithms can be divided into two main categories:

  • A "full reference" (FR) algorithm has access to and makes use of the original reference signal for a comparison (i.e. a difference analysis). It can compare each sample of the reference signal (talker side) to each corresponding sample of the degraded signal (listener side). FR measurements deliver the highest accuracy and repeatability but can only be applied for dedicated tests in live networks (e.g. drive test tools for mobile network benchmarks).
  • A "no reference" (NR) algorithm only uses the degraded signal for the quality estimation and has no information of the original reference signal. NR algorithms (like e.g. P.563[6]) are low-accuracy estimates only, as the originating voice characteristics (e.g. male or female talker, background noise, non-voice) of the source reference is completely unknown. A common variant of NR algorithms doesn't even analyze the decoded audio signal but works on an analysis of the digital bit stream on an IP packet level. The measurement is consequently limited to a transport-stream analysis.

PESQ is a full-reference algorithm and analyzes the speech signal sample-by-sample after a temporal alignment of corresponding excerpts of reference and test signal. PESQ[1] can be applied to provide an end-to-end (E2E) quality assessment for a network, or characterize individual network components.

PESQ results principally model mean opinion scores (MOS) that cover a scale from 1 (bad) to 5 (excellent). A mapping function to MOS-LQO is outlined under P.862.1.[2]

See also

References
  1. http://www.itu.int/rec/T-REC-P.862/en ITU-T Recommendation P.862: Perceptual evaluation of speech quality (PESQ): An objective method for end-to-end speech quality assessment of narrow-band telephone networks and speech codecs
  2. http://www.itu.int/rec/T-REC-P.862.1/en ITU-T Recommendation P.862.1: Mapping function for transforming P.862 raw result scores to MOS-LQO
  3. http://www.itu.int/rec/T-REC-P.862.2/en ITU-T Recommendation P.862.2: Wideband extension to Recommendation P.862 for the assessment of wideband telephone networks and speech codecs
  4. http://www.itu.int/rec/T-REC-P.862.3/en ITU-T Recommendation P.862.3 Application guide for objective quality measurement based on Recommendations P.862, P.862.1 and P.862.2
  5. http://www.itu.int/rec/T-REC-P.863/en ITU-T Recommendation P.863: Perceptual objective listening quality assessment
  6. http://www.itu.int/rec/T-REC-P.563/en ITU-T Recommendation P.563: Single-ended method for objective speech quality assessment in narrow-band telephony applications

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