Digitale Signalverarbeitung für Lautsprecher
Müller, Swen; Vorländer, Michael (Thesis advisor); Hill, Bernhard (Thesis advisor)
Dissertation / PhD Thesis
Zugl.: Aachen, Techn. Hochsch., Diss., 1999
This dissertation deals with digital signal processing for actively controlled multi-way loudspeakers. In addition to examining AD- and DA-conversion principles and methods to increase their dynamic range, the main focus is targeted on filter technology. IIR filters are only used as a fully parametric equaliser bank that precedes the main crossover-and linearisation FIR-filter section. To obtain satisfactory resolution in the lower frequency bands with FIR filters, a downsampling scheme is used. The resulting process delay can be reduced by using minimum-phase low-pass filters in the down- and upsampling stages. Regarding the resulting frequency- and impulse responses of the equalised loudspeakers, FIR filters can give considerably better results compared to IIR filters and are able to equalise frequency responses to almost perfect performance with regard to amplitude and phase. However, compared with elaborate analogue solutions, these improvements do not lead to a considerable enhancement of perceived audio quality, at least if directivity and the influence of room acoustics are not considered. Efficient protection against overloading is an indispensable task in the area of large-scale sound reinforcement and also very useful in home hi-fi applications to avoid permanent damage to loudspeakers. The non-linear signal treatment needed to realise the appropriate limiter stages is discussed. A look-ahead peak limiter with zero overshoot and very low distortion is presented, along with a two staged thermo limiter to protect voice coils from overheating. The applied measurement technology for evaluating linear and time-invariant systems is presented by a brief overview of different methods, old and new. Practical aspects are also taken into account, such as weak non-linearities always present in loudspeaker measurements as well as polar response. The generation of efficient FIR bandpass filters featuring high stop-band attenuation along with very good equalisation properties in the pass-band is presented in detail. Additionally, various pre-processing tools to deviate deliberately from flat on-axis response are explained. Listening tests were conducted to further investigate why different loudspeakers still sound different, even when all equalised to the same on-axis frequency responses. It is shown that these speakers still produce quite different ear signals (as may be demonstrated by dummy head measurements), probably caused by different radiation properties and presumably also by different wave front patterns. Once different loudspeakers are equalised to produce the same in-ear frequency response, they become virtually indistinguishable. Hence, phase errors and non-linear distortion seem to have only negligible effect on the perceived differences between loudspeakers.