2-BIT CORRELATION - AN ADAPTIVE TIME-DELAY ESTIMATION

Time delay estimation is a very important operation in ultrasound time -domain flow mapping and correction of phase aberration of an array tr ansducer. As the interest increases in the application of one and a ha lf-dimensional (1.5-D) and two-dimensional (2-D) array transducers to improving image quality and three-dimensional (3-D) imaging [1], [2], the need of simple, fast, and sufficiently accurate algorithms for rea l-time time delay estimation becomes exceedingly crucial. In this pape r, we present an adaptive time-delay estimation algorithm which minimi zes the problem of noise sensitivity associated with the one bit corre lation [3] while retaining simplicity in implementation. This algorith m converts each sample datum into a two bit representation including t he sign of the sample and an adaptively selected threshold. A bit patt ern correlation operation is applied to find the time delay between tw o engaged signals. By using the criterion of misregistration as an ind icator, we are able to show that the proposed algorithm is better than one bit correlation in susceptibility to noise level. Analytical resu lts show that the improvement in reducing misregistration of the two b it correlation over its counterpart is consistent over a wide range of noise level. This is achieved by an adaptive adjustment of the thresh old to accommodate signal corruption due to noise. The analytical resu lts are corroborated by results from simulating the blood as a random distribution of red blood cells. Finally, we also present a memory-bas ed architecture to implement the two bit correlation algorithm whose c omputation time does not depend upon the time delay of the signals to be correlated.