| 摘要 |
<p>Method and apparatus for detection of acoustic signals whereby errors due to "noise", "waveform stretch" and "cycle skipping" may be minimized. Detection of corresponding sequential half-cycle arrivals of each acoustic signal wave from transmitter at two spaced receivers activates zero crossing detectors to provide times of zero crossings A' and A in first half-cycles of waves from respective receivers and of corresponding zero crossings B' and B of succeeding half-cycles of waves from respective receivers. Difference of A'-A is used as most accurate indication of travel time if proved valid by tests such as evaluation against a predetermined reference time. Otherwise difference of B'-B may be used instead. Alternatively, B'-B may be subjected to similar tests for validity before using and, if found invalid, discarded in favor of the last previously used or recorded valid time difference.</p> |
