Design and synthesis of audio equalizers by digital signal processing techniques
This thesis proposes methods to design and synthesize audio equalizers with a practical equalizer order. In order to prevent the equalizers from boosting the signals in some frequency regions, we clip the peak portions of the equalizer’s amplitude response and re-synthesize the equalizer based on the artificial spectrum. In this research, four different techniques are used to synthesize the equalizer, but only two of them — the REMEZ equalizer and the FSF (Frequency Sampling Filtering) equalizer — have acceptable performance. The REMEZ equalizer is an FIR equalizer with linear phase, while the FSF equalizer is a structure of the HR filter bank. Both the REMEZ and the FSF equalizers can reduce the reverberation effect with a smaller equalizer order. A useful criterion — howling margin - is used to evaluate the equalizer’s performance and to calculate the optimum threshold for clipping the amplitude response of the original equalizer. In a real-time experiment, the REMEZ equalize, which is implemented on the Analog Device SHARC DSP board, is shown to have additional howling margin and to be able to reduce the reverberation effect in an auditorium.