Abstract - Previous work has indicated that a limitation on the performance of a circular microphone array for holographic sound field recording at low frequencies is phase mismatch between the microphones in the array. At low frequencies these variations become more significant than at mid-range and high frequencies because the high order phase mode responses at low frequencies are lower in amplitude. This paper demonstrates the feasibility of a “self-calibration” method. The basis of the calibration is to estimate the location of one or more wide-band sources using mid-range frequencies and to use this source location information to perform correction to the array at low frequencies. In its simplest form the calibration must be performed in an anechoic environment, since multipath effects at widely differing frequencies are uncorrelated. The approach is first demonstrated in such an environment using recordings from an array of high quality microphones. The technique is then extended to an adaptive calibration that can be used in an environment that is somewhat reverberant. The validity of the adaptive approach is demonstrated using recordings from an array of inexpensive microphones.
Abstract - Sub-Nyquist bandpass sampling offers an attractive approach to reducing the complexity of multi-channel sonar receivers also reducing costly duplication of analogue hardware. Digital frequency down-conversion following high-rate sampling is an established method for economic multiple-channel receiver design. This paper describes an architecture, design procedure and prototype multi-channel receiver where direct sub-Nyquist sampling replaces the separate digital down-conversion stage. The prototype design discussed combines multiplexing of multiple channels to fewer sub-Nyquist samplers to achieve second-order quadrature bandpass sampling while maintaining a simple uniform overall sample rate. Design equations for the sampling-rate reduction enabling the decrease in complexity are included. In more traditional jargon, the prototype implements uniform quadrature second-order integer-positioned bandpass sampling combined with a multiplexer to provide multiple channels.
Abstract - Traditional multi-channel sonar receivers require costly replication of analogue hardware before digitisation and processing of the signals. This paper describes the design process and architecture used to develop a multi-channel bandpass sampling receiver capable of directly and sequentially sampling each channel signal in quadrature such that the overall sampling rate remains uniform. The capability of the traditional multi-channel systems is retained while hardware complexity and cost are dramatically reduced.