David Hermann

Window Based Prototype Filter Design for Highly Oversampled Filter Banks in Audio Applications

This paper describes a window based method for designing near perfect-reconstruction prototype filters for highly oversampled, complex modulated filter banks. The design method extends some well-known simple methods for critically sampled filter banks to the over-sampled case and to the case of different length analysis and synthesis filters. This design method is simple and effective for designing a large range of filter bank configurations. The design method is particularly useful in developing audio applications using oversampled filter banks where the target systems requirements are highly variable. The simplicity and flexibility of the design method means that this one method can be used to generate multiple prototype filters as the application requirements change

An Ultra-Low Power Subband-Based Electronic Stethoscope

Electronic stethoscopes are able to offer signal amplification and other benefits over traditional stethoscopes. However, many electronic stethoscopes rely on a personal computer for their signal processing, which reduces portability and requires a relatively large amount of power. This paper presents a low power, portable electronic stethoscope system that is based on a signal processing approach using an over-sampled filterbank. This system is implemented on an ultra-low resource DSP system. The stethoscope incorporates multiple filtering modes as well as audio record and playback (full and half speed) functionality

Low-power implementation of the Bluetooth subband audio codec

This paper describes a low-power implementation of the Bluetooth subband codec (SBC) for high-fidelity wireless audio. The design uses a configurable weighted overlap-add (WOLA) filterbank coprocessor to implement the analysis and synthesis filterbanks. A new method to convert the two-times over-sampled, complex WOLA subband signals to equivalent critically sampled, real-valued SBC subband signals is presented. The WOLA coprocessor allows for an efficient parallel implementation of the filterbank and quantization portions of the SBC algorithm. Details of the overall system design are also presented, including measurements of power consumption and resource requirements. The final real-time, fixed-point implementation is compared to an off-line floating-point reference and found to produce no audible difference in decoded signal quality.

Joint filterbank structures for integrating audio coding into hearing aid applications

To help develop ultra-low power wireless hearing aid products, we investigate the integration of subband audio coding with hearing aid applications. Both the audio coding and the hearing aid application use subband processing, but their requirements for the filterbanks are totally different. The hearing aid application typically uses an oversampled filterbank to reduce the aliasing in each subband, whereas the audio codec needs a critically sampled filterbank for maximal coding efficiency. A joint filterbank structure is proposed in this paper to satisfy these contradictive filterbank requirements. With this structure, the two filterbanks are combined into a single stereo filterbank operation, which can be efficiently implemented on a filterbank coprocessor. This structure substantially reduces the computation complexity, power consumption and memory usage.

Oversampled filter bank evaluation for joint subband audio processing and coding

This paper presents an experimental evaluation of oversampled, modulated filter banks for joint subband audio processing and coding applications. Joint subband processing and coding may be useful in some wireless audio devices such as advanced wireless digital hearing aids. We examine the use of oversampled GDFT and cosine modulated filter banks and propose using single sideband (SSB) real-valued filter banks as a compromise which is ideal for this application. The SSB filter bank provides real-valued signals for audio coding which are free from any aliasing cancellation constraints and hence are also suitable for audio processing such as subband gain adjustment. We support this conclusion with an experimental analysis of various filter bank designs for subband gain adjustment and subband audio coding.