Voice Quality Testing (VQT) Software (POLQA, PESQ)
POLQA® is a registered trademark of OPTICOM. GL is one of the Test & Measurement manufacturers that has adopted POLQA/P.863 in its Voice Quality Test solution, by obtaining the essential rights to use POLQA® standard, and hereby acknowledge that the images or text references to POLQA used in this document originally copyrights with Opticom.
The optional POLQA v3 (latest version of the POLQA algorithm) supports Full Band Audio Analysis which provides improved scoring for mobile based VoLTE, 5G and OTT applications using EVS and OPUS codecs. This latest POLQA v3 includes analysis which is more sensitive to distortions across the entire audio spectrum. In addition, POLQA v3 supports less harsh analysis of micropauses within the speech, reacts with less sensitivity to linear frequency distortions, and includes a significantly improved and streamlined perceptual model.
Perceptual Objective Listening Quality Analysis (POLQA), the successor of PESQ (ITU-T P.862) analysis, is the next generation voice quality testing standard for fixed, mobile and IP-based networks. Based on ITU standard ITU-T P.863, POLQA supports the latest HD-quality speech coding, and higher-bandwidth audio signals suitable for Fixed, Wireless (3G, 4G/LTE) and VoIP networks. The POLQA algorithm handles the higher bandwidth audio signals. POLQA supports measurements in the narrow band (NB, 300-3400 Hz), and significant new capabilities for wideband (WB, 100 – 7000 Hz), and super-wideband (SWB, 50-14000 Hz), commonly found in VoIP and next generation mobile networks.
The ITU-T P.863 algorithm can operate in two modes, narrowband mode, and super wideband mode. In the narrowband mode, both the reference and degraded signals are pre-filtered with an IRS receive filter representing a listening situation in which subjects judge the quality of the speech signals over an IRS receive handset in monotic mode or over an IRS receive headset in monotic mode. In the super wideband mode, both the reference and degraded signals are not filtered, representing a listening situation in which subjects judge the quality of the speech signals over a diffuse field equalized headset in dichotic mode.
The ITU-T P.863 algorithm is designed to take into account the impact of the play back level for the perceived quality prediction in super wideband mode; the playback level is calculated relative to a nominal level of –26 dBov, which represents 73 dB(A) SPL in dichotic presentation.
Even if the sample rate determination cannot be made with perfect accuracy, e.g., in case of signals with additional variable delay, the detected sample rate ratio is still accurate enough to bring the signals back to the safe operating range of the temporal alignment.
If the detected sample rate ratio is larger than 0.01, the signal with the highest sample rate will be down sampled and the entire processing starts from the beginning. This happens at most one time to avoid excessive looping in case of signals for which the sample rate ratio cannot be determined in a reliable manner.
The detection of this effect as implemented in the ITU-T P.863 algorithm is based on the delay per frame vector and the detected active sections of the speech signals, as determined by the temporal alignment.
The sample rate ratio detection is required to compensate for perceptually irrelevant differences in the play-out speed of both, the reference and the degraded signal.
Results Provided by POLQA
Perceptual Results
MOS-LQO
The most eminent result of POLQA is the MOS-LQO. It directly expresses the voice quality on the MOS scale. It is important to understand and consider the two different operational modes supported by the ITU-T P.863 algorithm:
- super wideband mode for listening over super wideband headphones;
- narrow-band mode for listening over loosely coupled IRS type handsets.
In the super wideband mode the impact of play back level is modeled and the default calibration factor (C) of 2.8 has to be used in combination with the standard –26 dBov scaling for play back levels of 73 dB(A) SPL (dichotic). Play back levels down to 53 dB(A) SPL and up to 78 dB(A) SPL may be used and MOS-LQO scores should be reported in the format MOS-LQOsw (dB level). In narrowband mode only the play back level of 79dB(A) SPL (monotic) is supported. Narrowband mode MOS scores are referred to as MOS-LQO.
The maximum ITU-T P.863 MOS-LQO score is 4.5 in narrow-band while in super wideband mode this point lies at 4.75. Under some circumstances, when the reference signal contains noise or when the voice timbre is distorted, a transparent chain will not provide the maximum MOS score of 4.5 in narrowband mode or 4.75 in super wideband mode.
Below table compares PESQ and POLQA MOS scores:
Mode
P.862.1/2
MOSmin
P.862.1/2
MOSmax
POLQA
MOSmin
POLQA
MOSmax
NB
1
4.5
1
4.5
WB
1
4.5
SWB
1
4.75
G.107 R-Factor / Ie Value
POLQA also provides a mapping of the MOS-LQO score to the scale used by G.107 (e-model). The resulting parameter is equivalent to an Ie – Value. Many people also refer to it as an R-factor. The scale ranges from 0 (bad) up to 100 (best). All values below 60 indicate unacceptable quality.
Non-Perceptual Results
Attenuation
Especially all analog equipment modifies the level of the speech signal. A high attenuation generally leads to a worse perception of voice. In contrast to PESQ, POLQA does weight this as degradation of the signal. Knowing the value of the attenuation is also important for optimizing the overall system design. Attention should be paid to signals which show either a negative attenuation, or attenuations larger than approximately 10dB. In the first case, the signal was amplified instead of attenuated. This may eventually lead to level clipping during the transmission. In the second case, the quantization noise may become an important source of degradation, if low level analog signals are converted to the digital domain and are subsequently amplified in the digital domain. Depending on the test setup, both cases may be ok and intended, but this has to be decided on a case by case basis.
In order to calculate the attenuation, POLQA computes P.56 like active speech levels of the reference as well as the degraded signal in dB. The level of the degraded signal minus the level of the reference signal is then used as the attenuation.
Level and Background Noise Measurements
In transmission systems it is frequently important to know the exact levels of the signals. Especially for VoIP systems and voice activity detection (VAD) it becomes also important to know the signal level during the silent intervals as well as during active speech. It is important, that the received background noise does not exceed a certain limit. Levels can be measured in dB if you want to relate the level directly to a sound pressure or electrical level, or as loudness levels.
Signal to Noise Ratio (SNR)
POLQA calculates the SNR for the reference and the degraded signal independently. The noise as well as the signal level is calculated by the VAD which POLQA uses for the temporal alignment.
Active Speech Ratio (ASR)
ASR is calculated by POLQA based on the information calculated by the Voice Activity Detection (VAD) which is part of the temporal alignment. The ASR defines the ratio between active speech and the overall signal length.
