5910906 A abstract the invention is a method for performing tone detection on a digital signal processor that may be used in the presence of tone aliases caused by inadequate anti aliasing filtering the method involves processing an input signal at different sampling rates and can be used with any tone detection algorithm a tone is detected when it is present at each of the sampling rates the method allows tone detection to be performed without the use of a proper anti aliasing filter application_number US 68635396 A description referring to fig2 illustrated is an embodiment of a tone detector 10 in accordance with the present invention which is to detect a tone at frequency fo when present in analog input signal 12 and accordingly generate an output signal 14 indicative of such the tone detector 10 comprises in terms of functional blocks a low pass filter lpf 16 which receives the input signal 12 and which is connected to an analog to digital converter adc 18 the adc 18 samples the filtered input signal at two different predetermined rates f s1 and f s2 under the control of respective sampling clocks 20 and 22 signals from which are provided thereto in time division through multiplexing 24 and generates a corresponding digital data sample stream having the f s1 samples and the f s2 samples which respectively are provided in time division through multiplexing 26 to first and second tone detection processes 28 and 30 a multiplexing clock 32 with a time division window.