| 摘要 |
<p>An ultrasonic ranging system for a camera includes an ultrasonic transducer responsive to a keying pulse for transmitting a frequency modulated burst of ultrasonic energy toward a subject. The leading half of the burst is a chirp whose frequency varies from a higher to a lower value; during the trailing half of the burst, the frequency remains substantially constant. The system also includes a receiver for processing an echo signal produced by the transducer on receipt of an echo within a predetermined time interval following the burst, and producing a range signal with a characteristic linearly related to the distance of a subject being photographed from the camera. A variable Q filter in the receiver filters echo signals produced by the transducer eliminates the need for a matched filter. The filter Q has a relatively low value during the initial portion of said predetermined time interval causing the bandwidth of the filter to be wide enough to pass all the frequencies of the chirp returned from a subject relatively close to the camera. During the remaining portion of the interval, the Q increases narrowing the filter bandwidth and optimizing it, with regard to the signal-to-noise ratio, for the constant frequency of the burst returned from a relatively remote subject. This arrangement reduces sensitivity of the receiver to echo cancellation due to interference between waves reflected from a nearby subject without adversely affecting the signalto-noise ratio for returns from remote subjects. An accompanying change in filter admittance reduces angular sensitivity of the receiver to off-axis echoes.</p> |
