procedure 400 is illustrated in fig4 here the frame is divided into two subframes a small subframe of l number of samples and large subframe of n u2212 l number of samples which could be for example 20 and 82 samples respectively the frame power for each of the two subframes is then taken at steps 401 and 402 where the frame power for each subframe is calculated as where x i is the amplitude of the unfiltered samples and i 1 to l for the small subframe and i l 1 to n for the large subframe at step 403 the ratio of energies e large e small is compared with the sample ratio n u2212 l l if the ratio e large e small is close within a selected tolerance to the ratio n u2212 l l then the frame is declared valid step 404 on the other hand if e large e small gt gt n u2212 l l then the frame is declared a transient frame step 405 and is not processed further after frame validation the samples are passed to a bank of 8 goertzel filters 303 one for each of the eight possible dtmf tones in the illustrated system two n sample frames indexed frames i and j respectively where j i 1 are used for each detection with the initial conditions.