| 摘要 |
In a telephone signalling detector of the cross-correlation type, samples from a selected communications channel are multiplied with sine and cosine coefficients of different frequencies. The resulting sine and cosine products for each frequency are separately accumulated over a period of time after which the accumulated results are each squared and then summed for the individual frequencies. A series of sums of squares is compared in accordance with predetermined tone detection parameters to determine the presence or absence of a signalling tone defined by the coefficients. In a telephone signalling detector intended to detect any of a large family of specified signalling tones in any of a plurality of channels, the square root value of each of the sums of squares is typically provided by digital circuitry for subsequent use in a processor which is operated to complete the tone detection function in accordance with program instructions. A cost reduced square root circuit provides the square root values from a look-up ROM (read-only-memory) on a prompt and regular basis. In the square root circuit a large sum of squares is shifted toward lesser significance and a smaller sum of squares is shifted toward greater significance by means of a logic controlled shift register. Thereafter only a central portion of the shift register content is used along with a corresponding shift record to periodically address the look-up ROM.
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