the single pulse generated by the transmitting transducer d it is therefore important to identify each individual echo since these echoes may emanate from totally different directions i e different objects once the true start time and amplitude of the first echo has been determined it can be subtracted from the digitized signal utilizing a single echo pulse train captured during the calibration cycle the pulse train is scaled down prior to storage to u00bd the maximum amplitude of the a d converter 16 and then scaled up to the identified echo prior to subtraction once subtracted the start of the second echo is revealed and the process is continued the sample rate of the a d converter 16 is ideally two hundred fifty six times the transmission frequency therefore the time between samples is 360 u00b0 u00f7 256 1 40625 u00b0 if the frequency of transmission is approximately 68 5 khz then the time between samples is 1 u00f7 68500 u00f7 256 57 nanoseconds this degree of resolution is desired to minimize the possibility of a blind spot directly below the transducer assembly this is due to the small difference in travel time between objects and the receiving transducers referring to fig4 the true start of the echo is determined as the difference between the u201c offset u201d and the time for the pulse to reach the first sample y which is the first sample above zero value the offset may in turn be derived from the following.