A group of three students developed a five-band, adjustable digital audio equalizer. The Simulink block diagram for real-time implementation is shown in Figure 4. The blocks labeled 750 Hz, 1.5 kHz, 2.5 kHz, 3.5 kHz, and 4.2 kHz are adjustable gains for the frequency bands. As these gains are changed in the block diagram, the real-time algorithm is updated on-the-fly and the effect is observed in the output signal. The system was demonstrated with real music waveforms.
This project contained several interesting design issues. First, the digital filter type, order, and frequency range had to be chosen. Then, analog filters were designed for anti-aliasing (at the dSPACE input) and reconstruction (at the dSPACE output). Students were somewhat surprised that the first- and second-order analog filters in their textbook were not sufficient for this application, so they designed higher-order filters. The students also characterized the frequency response of their overall system using a computer-controlled GPIB frequency response measurement system that they developed earlier in the semester. Interesting effects were observed in the overall response at frequencies where the equalizer bands overlap. The phase response of each filter is important in these overlap regions, and the challenge of designing of an overall ``flat'' response for the equalizer was appreciated.
