Multiplierless sigma-delta modulation beam forming for ultrasound nondestructive testing

Beamforming techniques based on sigma-delta modulation (SDM) have been proposed to reduce the hardware complexity of conventional multi-bit digital delay-sum beamformer, in which beamforming operation is performed upon over-sampled single bit sequences obtained by applying SDM to the received signals from each array elements. In conventional single bit SDM beamforming, the single bit sequence is altered in accordance with the associated dynamic focusing delays. The sum of all these altered sequences are passed to a demodulator, which also serves as a decimator, to obtain the aimed focused signal. Due to this signal distortion phenomenon, conventional single bit dynamic focusing produces low signal to noise ratio(SNR) signals. In this paper, a multiplierless single bit dynamic focusing method is presented, which can completely eliminate the demodulation error. In the proposed scheme, the low pass filter(LPF) produces the demodulated signals for successive receive focal points at the rate of Nyquist rate, N F , which are stored in a two-port memory. Next, the focused signal is obtained by simply adding these samples to those from other array elements. Since the demodulation filter length, L, is generally larger than the over sampling ratio, M, however, the proposed method requires using  ) / ( M L K = demodulation filters. To reduce the hardware complexity, we also propose an efficient scheme to implement the demodulation filter with a simple accumulator. Compared to a traditional delay-sum beamformer using 8-bit 40[MHz] analog-digital conterts (ADCs), the overall beamformer complexity is reduced by about 80[%] in gate counts by using the proposed scheme, whereas both scheme provide almost the same image quality. Introduction Traditional delay-sum digital beamformer uses multi-bit analog-digital converts(ADCs) to digitize the received signals from each array elements, with the sampling rate, 0 4 f Fs ≥ ( : 0 f center frequency of input signal). The data rate is increased during beamforming process by a factor of at least four to obtain the required delay resolution, for which the delay-sum beamformer requires a data interpolator per each channel. Since one output sample is needed for each imaging point(i.e., for each input data interval), the data interpolation is performed by passing the input data sequence through a multi-tap LPF, of which the coefficients can change dynamically. In summary, the traditional delay-sum beamformer need to have one ADC and one interpolator per each channel, each with a high degree of hardware complexity. Therefore, it is inadequate to be used for small-scale ultrasound scanners or multi-dimensional array systems with a large number of active channels[1-4]. To reduce the hardware complexity of the traditional digital beamformer, a new approach was developed. In the SDM beamformer, the received data from each array element is converted into an over-sampled 1-bit digital data through SDM. Focusing delays are applied to the 1-bit SDM data, yielding multi-bit focused SDM data. The focused SDM data are then demodulated with a LPF. Since the SDM process can be implemented with a simpler hardware than conventional multi-bit ADCs and only one demodulation filter is needed regardless of the number of active channels, the entire beamformer complexity can be reduced greatly[5,6]. Key Engineering Materials Vols. 270-273 (2004) pp 215-220 online at http://www.scientific.net © (2004) Trans Tech Publications, Switzerland Online available since 2004/Aug/15 All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of the publisher: Trans Tech Publications Ltd, Switzerland, www.ttp.net. (ID: 130.203.133.33-16/04/08,11:12:38) Title of Publication (to be inserted by the publisher) In conventional single bit dynamic focusing beamformer, dynamic focusing is carried out at the oversampling rate, i.e., for every bit of SDM data. This implies that the 1-bit SDM data sequence is altered because focusing delays vary with the sample position. This signal distortion causes demodulation error, resulting in degradation in image quality. It has been known that SNR is severely degraded due to the signal distortion. Moreover, the signal distortion may prevent SDM beamforming scheme from being used with pulse compression techniques. Moreover, the demodulation filter is a LPF that requires a few hundred multiplers in most cases[7,8]. In this paper, a multiplierless block-based single bit dynamic focusing SDM beamformer is presented, which can completely eliminate signal distortion with a reduced hardware complexity. In the proposed method, dynamic focusing is performed at the Nyquist rate, N F , for input data. For each imaging point, SDM samples to be added together are selected from all active channels according to the corresponding focusing delays. Then, for each channel, a block of data centered at the selected SDM sample is defined. Next, bit-wise block addition is performed to obtain the sum of all the blocks. Finally, the output of the block addition is fed to the demodulation filter. In consequence, if the block length is equal to the demodulation filter length, one can obtain the focused signal without any signal distortion. Direct realization of the proposed method requires   M L / demodulation filters, where L is demodulation filter length and M is over-sampling ratio, which diminishes the advantage of the SDM beamformer in hardware complexity. To implement the proposed method with as small hardware as possible, we also propose an efficient implementation method, in which the demodulation filters are replaced with simple accumulators. A block-based single bit dynamic focusing SDM beamformer Method. Fig. 1 shows a conceptual block diagram of the proposed SDM beamforming method. And Fig. 2 shows a detail block diagram of Fig. 1.