Digital Audio Processing in MP3 Compressed Domain and Evaluation of Perceived Audio Quality

The state of the art provides several digital audio signal processing in uncompressed domain (PCM Pulse Code Modulation). We can found several works in the literature which explain different methods to modify an audio signal both in the time and in the frequency domain, in order to normalize the intensity, or to apply filters, special effects and so on. Currently the MP3 format has not been deeply considered by literature. The most meaningfully works are related to MP1 and MP2 formats. There is not a exhaustive formalization of the digital audio signal processing in the MP3 compressed domain. Furthermore, there is not a software framework which allow to develop and implement every kind of processing algorithm in MP3 compressed domain. There are only some simple software which directly split and wrap MP3 files and process the volume in a very simple way. In this dissertation we define different approaches to develop every kind of algorithm for digital signal processing in the MP3 compressed domain. The contributions of this dissertation are the formalization the problem of MP3 direct processing defining different approaches (or levels), with respect to the various steps of the decoding/encoding phases, the development of algorithms for the MP3 format working as nearest as possible to the MP3 domain, and the improvement and the customization of methods and protocol described in the recommendation of the International Telecommunication Union (ITU-R) to evaluate the objective and subjective perceived audio quality. We define three different domain where it is possible to manage MP3-coded audio information. We develope algorithms to moving the frame, control the gain by RMS, filter and the channel selection. Filters and channel selection have been developed to downgrade MP3 files. For each algorithm we have chosen the best approach, finding the best trade-off among time consumption, perceived audio quality and problems related to unmasking and aliasing. This formalization represents the base concepts for the development of a software framework which allows the implementation of every kind of algorithm from the PCMdomain to the MP3-domain. Finally we improve and customize the methods and the protocol to evaluate the objective and subjective perceived audio quality, described in the recommendation of the International Telecommunication Union (ITU-R). We evaluate the objective performance of modern MP3 codec with respect to tandem coding. We study the level of reliable of objective tests, comparing them with the subjective. We compare the MP3-coded audio processed both following the traditional and the direct approach to editing.