| 摘要 |
Reception of valid two-out-of-N incoming multifrequency signals is realized by employing a multifrequency detector (FIG. 1, 102) including a plurality of filters (FIG. 2, 202-1through 202-N) and a corresponding plurality of comparators (206-1 through 206-N). The comparators are jointly responsive to output signals from the filters and to a threshold level (RMS REF) dynamically generated (via 204) in response to the incoming signal for generating output pulse signals (103-1 through 103-N) representative of multifrequency tones which exceed the threshold level (RMS REF). The pulse signal outputs (103-1 through 103-N) from the multifrequency detector (102) are evaluated (by 105) to determine whether two and only two tones are present which meet a prescribed criteria. The criteria is that each tone be present for at least a predetermined minimum portion of a prescribed sampling interval and that both tones be present for at least a predetermined number of consecutive sampling intervals. Out-of-band signals are rejected while increasing receiver sensitivity to inband signals by maintaining the peak amplitude of outputs from the filters generated in response to a single tone incoming signal having a frequency midway between the center frequencies of adjacent ones of the filters (202-1 through 202-N) in prescribed relationship to the magnitude of the threshold level (RMS REF) generated in response to the single tone incoming signal (see FIG. 4). In one example, the peak amplitude of the filter outputs is substantially equal to the magnitude of the threshold level (RMS REF, FIG. 4). |
