US Patent 7199738 Sigma delta beamformer and method with reduced artifact

Dynamic focusing is performed at Nyquist rate FN for the received signal. For each imaging point, a single-bit SDM sample is selected from each active array channel, according to the corresponding focusing delay. A block of data, centered at the selected SDM sample, is defined for each channel. Next, bit-wise block addition is performed to obtain the sum of all the blocks. The block addition output is fed to the demodulation filter. K(=┌L/M┐) demodulation filters are used. In another approach, the demodulation filters are placed just behind sigma-delta modulators. In each channel, K filters produce successively the demodulated signals for K consecutive imaging points. By taking the sum of samples retrieved from the same memory locations, dynamic focusing is achieved. Since each demodulation filter takes a single-bit SDM sequence as an input data, a simple accumulator calculates the sum of filter coefficients to be multiplied by input samples.