Speech Quality Improvement of Commercial Flat Screen TV-Sets

Abstract

This work deals with the improvement of perceived speech quality of flat screen TV-sets on the various broadcasting platforms of the Italian radio and TV broadcasting company Rai. It is well known that the reduced thickness of flat TVs implied a degradation of the audio quality due to the adoption of miniaturised and cheap loudspeakers compared to the ones in the former Cathode Ray Tube televisions. The research activity gave rise to a Transfer Function (TF), which modifies in real-time the frequency spectrum of the audio signal from the TV station before the transmission to the broadcasting tower. In this way, the final users receive the processed audio signal without the need of additional hardware. A Digital Audio Optimizer dynamically equalizes the sound level, boosting the audio signal towards a flat frequency response without any increase in the loudness levels. Given that the majority of the broadcasted audio signals have speech or singing contents, the TF boosts the speech level in the frequency range that is more important for speech intelligibility, i.e., between 1 kHz and 4 kHz. Subjective evaluations of the proposed TF have been carried out in a laboratory in compliance with the standard ITU-R BS.1116. Three different commercial TV-set models, 30 audio excerpts from video tracks divided into the three genres Speech, Sing and Music, and Sport, and 72 subjects aged between 21 and 53 years, were part of the study. Overall, the perceived improvement in the audio quality compared to the non-processed signal was 25.3% on average among the three TV-set models and the three genres. In order to estimate the perceived improvement directly from the audio signal, regression analyses have been performed, which allow the estimation of subjective outcomes from objective indexes based on intensity features and frequency content of the signal, with standard errors between 10% and 14%.