Francis Rumsey

On the relative importance of spatial and timbral fidelities in judgments of degraded multichannel audio quality

Mean opinion score ratings of reproduced sound quality typically pool all contributing perceptual factors into a single rating of basic audio quality. In order to improve understanding of the trade-offs between selected sound quality degradations that might arise in systems for the delivery of high quality multichannel audio, it was necessary to evaluate the influence of timbral and spatial fidelity changes on basic audio quality grades. The relationship between listener ratings of degraded multichannel audio quality on one timbral and two spatial fidelity scales was exploited to predict basic audio quality ratings of the same material using a regression model. It was found that timbral fidelity ratings dominated but that spatial fidelity predicted a substantial proportion of the basic audio quality.

Frequency dependency of the relationship between perceived auditory source width and the interaural cross-correlation coefficient for time-invariant stimuli.

Previous research has indicated that the relationship between the interaural cross-correlation coefficient (IACC) of a narrow-band sound and its perceived auditory source width is dependent on its frequency. However, this dependency has not been investigated in sufficient detail for researchers to be able to properly model it in order to produce a perceptually relevant IACC-based model of auditory source width. A series of experiments has therefore been conducted to investigate this frequency dependency in a controlled manner, and to derive an appropriate model. Three main factors were discovered in the course of these experiments. First, the nature of the frequency dependency of the perceived auditory source width of stimuli with an IACC of 1 was determined, and an appropriate mathematical model was derived. Second, the loss of perceived temporal detail at high frequencies, caused by the breakdown of phase locking in the ear, was found to be relevant, and the model was modified accordingly using rectification and a low-pass filter. Finally, it was found that there was a further frequency dependency at low frequencies, and a method for modeling this was derived. The final model was shown to predict the experimental data well.

Relationships between experienced listener ratings of multichannel audio quality and naïve listener preferences.

The preferences of a large number of naïve listeners were elicited in response to a selection of multichannel audio items that had been degraded in quality by using band-limiting and down-mixing algorithms. Relationships were sought between these preference ratings and the quality judgements of experienced listeners in an attempt to determine whether one could be predicted from the other. Results suggest that a simple regression model can be used to do this with adequate results, but that a better prediction can be successfully based on experienced listener ratings of timbral and spatial fidelity. There is a difference between naïve and experienced listeners in the weightings of the fidelities and their relationship to overall quality.

Feature Extraction for the Prediction of Multichannel Spatial Audio Fidelity

This paper seeks to present an algorithm for the prediction of frontal spatial fidelity and surround spatial fidelity of multichannel audio, which are two attributes of the subjective parameter called basic audio quality. A number of features chosen to represent spectral and spatial changes were extracted from a set of recordings and used in a regression model as independent variables for the prediction of spatial fidelities. The calibration of the model was done by ridge regression using a database of scores obtained from a series of formal listening tests. The statistically significant features based on interaural cross correlation and spectral features found from an initial model were employed to build a simplified model and these selected features were validated. The results obtained from the validation experiment were highly correlated with the listening test scores and had a low standard error comparable to that encountered in typical listening tests. The applicability of the developed algorithm is limited to predicting the basic audio quality of low-pass filtered and down-mixed recordings (as obtained in listening tests based on a multistimulus test paradigm with reference and two anchors: a 3.5-kHz low-pass filtered signal and a mono signal)

An auditory onset detection algorithm for improved automatic source localization

An algorithm is described which detects auditory onsets quickly in arbitrary binaural audio streams. Aspects of the precedence effect are implemented to speed up computation, and to increase the usability of the output. The onset detector is tested with a number of binaural signals. Onsets that are suitable for spatial auditory processing are found reliably. This will allow spatial feature extraction to be performed.

Measuring perceived spatial quality changes in surround sound reproduction

The spatial quality of audio content delivery systems is becoming increasingly important as service providers attempt to deliver enhanced experiences of spatial immersion and naturalness in audio-visual applications. Examples are virtual reality, telepresence, home cinema, games and communications products. The QESTRAL project is developing an artificial listener that will compare the perceived quality of a spatial audio reproduction to a reference reproduction. The model is calibrated using data from listening tests, and utilises a range of metrics to predict the resulting spatial sound quality ratings. Potential application areas for the model are outlined, together with exemplary results obtained from some of its component parts.

Prediction of spatial perceptual attributes of reproduced sound across the listening area

Audio systems and recordings are optimized for listening at the “sweet spot,” but how well do they work elsewhere? An acoustic‐perceptual model has been developed that simulates sound reproduction in a variety of formats, including mono, two‐channel stereo, five‐channel surround and wavefield synthesis. A virtual listener placed anywhere in the listening area is used to extract binaural signals, and hence interaural cues to the spatial attributes of the soundfield. Using subjectively‐validated models of spatial sound perception, we can predict the way that human listeners would perceive these attributes, such as the direction (azimuth) and width of a phantom source. Results will be presented across the listening area for different source signals, sound scenes and reproduction systems, illustrating their spatial fidelity in perceptual terms. Future work investigates the effects of typical reproduction degradations.

Evaluation of a model of auditory source width based on the interaural cross‐correlation coefficient

A model of human perception of auditory source width has been developed, based on the interaural cross‐correlation coefficient (IACC). The following factors differentiate it from more commonly used IACC‐based models: the use of a running analysis to quantify variations in width over time; half‐wave rectification and low pass filtering of the input signal to mimic the breakdown of phase locking in the ear; compensation for the frequency and loudness dependency of perceived width; combination of a model of perceived location with a model of perceived width; and conversion of the results to an intuitive scale. Objective and subjective methods have been used to evaluate the accuracy and limitations of the resulting model.