The Evolution of MOS

MOS has been considered the Gold standard, commonly used to measure the overall voice call quality for decades. It was standardized by the International Telecommunications Union (ITU-T), and generally, it referred to numerical measures of the quality of human voice and video sessions. Its main purpose was to provide a subjective analysis of audio signals for codec evaluation based on network metrics.

Although scores were derived from detailed surveys of experts, with the advancement in technology and evolved codecs, MOS evolved into PESQ (Perceptual Evaluation Of Speech Quality). With time, PESQ then evolved into POLQA.

PESQ: The Evolved Version Of MOS

PESQ is the evolved version of MOS and its main objective is to predict subjective MOS scores. Over the last 2 decades, a series of algorithms have been developed to predict the outcomes of subjective listening tests. PESQ supports modern codecs and provides more realistic, objective results. 

Spearline utilizes this algorithm in its global in-country number testing. It is an industry-standard audio quality testing technique used to measure the following parameters:

  • Audio sharpness
  • Call volume
  • Background noise
  • Variable latency or lag in audio  
  • Clipping
  • Audio interference

Let’s discuss a few pointers and understand why MOS is good but PESQ is a better alternative.

The Benefits of PESQ over MOS

  • Codecs have evolved in the last 2 decades and the PESQ technique is finely tuned to the behavior of these new codecs.
  • PESQ captures the actual audio signal and analyzes the entire call path. It also produces an objective audio quality score.
  • It offers a quick and repeatable way of predicting effective results with better insights.
  • PESQ was designed to provide audio analysis for audio in the narrowband and wideband frequency ranges (up to 7,000Hz). 

The Limitations of PESQ

PESQ is the successor of MOS but it comes with a few limitations. 

  • PESQ cannot evaluate speech signals above 7,000Hz.
  • The listening conditions for the narrowband and wideband scales are not the same and the results that come from these two tests are not comparable.
  • PESQ cannot accurately predict scores for audio that has the dynamic time-warping technique applied. This can be found in the latest HD voice codecs.

POLQA: The Evolved Version Of PESQ

POLQA is the evolved version of PESQ that supports the evaluation of speech signals above the 7,000Hz audio frequency range. This includes wideband and super wideband audio signal testing. POLQA provides more opportunity for testing emerging technologies and modern architectures.

Spearline uses the POLQA algorithm when analyzing HD voice, VoHSPA, and VoLTE as more and more customers switch to wideband and super wideband.

Benefits of POLQA over PESQ and MOS

  1. POLQA has a narrowband scale and a universal super wideband scale. It is used to compare the performance of narrow-band, wideband and super-wideband signals. This enables a valid and strong comparison of signals falling between the narrowband and the super wide band scales. 
  2. POLQA can accurately track the time-warping behavior of new codecs and gives more realistic scores.
  3. POLQA can accurately predict audio quality results for emerging voice technologies such as VoLTE, 3G, VoIP and VoHSPA.

Table of Comparison:

How is it calculated? It is calculated using network metrics. PESQ/POLQA is calculated using a reference audio file for comparison.
Support for Wideband audio signals above the 3,400Hz range It does not accurately assess wideband telephone networks and speech codecs above the 3,400Hz range PESQ supports wideband signal testing up to 7,000Hz. POLQA supports complete wideband signals.
Complete analysis of the call path MOS does not consider the entire call path. It will only consider the last leg of the call on the internal network. PESQ/POLQA support end-to-end analysis of the call path from the external PSTN to the internal network.
Support for newer codecs and analysis of their traits:

  • Time warping
  • Speech delay 
MOS does not support the analysis of any modern codes and their traits, including time-warping, and speech delay. PESQ supports speech delay but does not support time warping.
POLQA supports all the newer traits of modern codecs.

List of Abbreviations

Abbreviation Definition
MOS Mean Opinion Score
PESQ Perceptual Evaluation of Speech Quality
POLQA Perceptual Objective Listening Quality Analysis
NB Narrowband
WB Wideband
SWB Super Wideband
HD High Definition
VoLTE Voice over Long-Term Evolution
VoHSPA Voice over High Speed Packet Access
VoIP  Voice over Internet Protocol
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