An Adaptive Digital Dynamic Range Controller

Abstract

High fidelity digital audio sources are capable of reproducing a much wider dynamic range than most conventional consumer media (e.g. AM/FM radio and audio cassettes). The research presented here addresses the problem of matching this wide dynamic range to that of a device ( or channel) with lower dynamic range capabilities using a Dynamic Range Controller (DRC). Currently available digital signal processing hardware allows the implementation of entirely Digital DRC's (DDRC's) that interface directly to digital sources and eliminate unnecessary data (analog +--> digital) conversions. The DDRC design presented in this thesis uses an adaptive level measurement scheme and an adaptive recovery time to improve performance. The perceived distortion introduced by rapid gain reductions (attack) is lessened by allowing attacks only at the zero crossing preceding a transient. A single-channel version of the Adaptive DDRC has been implemented for real-time operation on a DSP56000 evaluation board. Tests showed that the Adaptive DDRC has insignificant total harmonic distortion. Intermodulation distortion measurements compare favourably with a previous DDRC design [11] that was reported as having good subjective performance. The results of our listening tests show great promise for the Adaptive DDRC. Listeners rated the average sound quality of an Adaptive DDRC configuration higher than a conventional design (with peak level gain control). However, since other Adaptive DDRC configurations (i.e. different parameter sets) did not perform as well, further testing is required to optimize the Adaptive DDRC parameter set.