Psychoacoustic Models for Perceptual Audio Coding—A Tutorial Review

Psychoacoustic models of human auditory perception have found an important application in the realm of perceptual audio coding, where exploiting the limitations of perception and removal of irrelevance is key to achieving a significant reduction in bitrate while preserving subjective audio quality. To this end, psychoacoustic models do not need to be perfect to satisfy their purpose, and in fact the commonly employed models only represent a small subset of the known properties and abilities of the human auditory system. This paper provides a tutorial introduction of the most commonly used psychoacoustic models for low bitrate perceptual audio coding.

Detection of Montage in Lossy Compressed Digital Audio Recordings

This paper addresses the problem of tampering detection and discusses methods used for authenticity analysis of digital audio recordings. Presented approach is based on frame offset measurement in audio files compressed and decoded by using perceptual audio coding algorithms which employ modified discrete cosine transform. The minimum values of total number of active MDCT coefficients occur for frame shifts equal to multiplications of applied window length. Any modification of audio file, including cutting off or pasting a part of audio recording causes a disturbance within this regularity. In this study the algorithm based on checking frame offset previously described in the literature is expanded by using each of four types of analysis windows commonly applied in the majority of MDCT based encoders. To enhance the robustness of the method additional histogram analysis is performed by detecting the presence of small value spectral components. Moreover, computation of maximum values of nonzero spectral coefficients is employed, which creates a gating function for the results obtained based on previous algorithm. This solution radically minimizes a number of false detections of forgeries. The influence of compression algorithms' parameters on detection of forgeries is presented by applying AAC and Ogg Vorbis encoders as examples. The effectiveness of tampering detection algorithms proposed in this paper is tested on a predefined music database and compared graphically using ROC-like curves.