Dynamic range reduction of audio signals using multiple allpass filters on a GPU accelerator

Maximising loudness of audio signals by restricting their dynamic range has become an important issue in audio signal processing. Previous works indicate that an allpass filter chain can reduce the peak amplitude of an audio signal, without introducing the distortion associated with traditional non-linear techniques. Because of large search space and the consequential demand of the computational needs, the previous work selected randomly the delay-line lengths and fixed the filter coefficient values. In this work, we run on a GPU accelerator multiple allpass filter chains in parallel that cover all relevant delay-line lengths and perform a wide search on possible coefficient values in order to get closer to the optimal choice. Our most exhaustive method, which tests about 29 million parameter combinations, reduced the amplitude of test signals by 23% to 31%, whereas the previous work could only achieve a reduction of 23% at best.