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Hi Shehryar, The choice of N is mainly driven by the frequency resolution needed, which sets a lower boundary. N also is chosen so that (k/N)fs most accurately coincides with the actual DTMF frequencies assuming k are integer values and fs is a sampling frequency of 8 ksps. The DTMF detection procedure is usually done (as you have chosen) by using the classical Goertzel algorithm, and sends detected digit to the master CPU. It works on an 8 kHz, 8 samples current received frame. The detection itself is done by calling two procedures, as a working (very well) example : * goertzel() executes 16 Goertzel algorithms to detect the 8 DTMF frequencies and their second harmonics. This procedure must be called XXX consecutive times, i.e. detection is done with an equivalent DFT order = XXX * frame duration = N samples. This function also includes a gain control function to protect accumulators from overflow. * dtmfchecks() analyses the results of the Goertzel algorithms. Once the spectral information (squared magnitude for the row and column frequencies and their second harmonics) is collected, a series of tests need to be executed to determine the validity of tone and digit results. A first check makes sure the signal strength of the possible DTMF tone pair is sufficient. The sum of the squared magnitudes of the peak spectral row component and the peak spectral column component needs to be above a certain threshold. Tone twists are investigated in a separate check to make sure the twist ratio specifications are met. The spectral information can reflect the types of twists. The more likely one, called "reverse twist", assumes the row peak to be larger than the column peak. The detector, therefore, computes a reverse twist ratio and checks if it is above a threshold. The other twist, called "standard twist", occurs when the row peak is smaller than the column peak. Similarly, a "standard twist ratio" is computed and its threshold checked. The program makes a comparison of spectral components within the row group as well as within the column group. The strongest component must stand out (in terms of squared amplitude) from its proximity tones within its group by more than a certain threshold ratio. Finally, the program checks on the strength of the second harmonics in order to be able to discriminate DTMF tones from possible speech or music. This second harmonics check makes sure that the ratio of the second harmonics component and the fundamental frequency component is below a certain threshold. If the DTMF signal pair passes all these checks, we say a valid tone pair, which corresponds to a digit, is present. Then, the tone-pair is mapped to its corresponding digit and comparing it with the previously detected digit. Only if the last digit was preceded by a pause do we accept the current digit as a valid digit. The detector is then forced into a state where it waits for a pause (no signal) before being able to accept a new digit. This last step is necessary to ensure the discrimination of identical keystrokes succeeding one another. As you understood, you can perfectly meet the ITU-T specification by doing more than one DFT for a same digit detection) + waiting for the necessary time before indicating any detection. Hope that helps. Regards, Franck Hocquet Field Application Engineer EBV ELEKTRONIK , Paris -----Original Message----- From: Shehryar Amir [mailto:] Sent: samedi 16 frier 2002 13:51 To: ; Subject: [speechcoding] DTMF Detection Dear All, I am new to this group. I want to implement DTMF Detection algorithm in C/C++. I have read some material on web and have chosen to start with NonUniform Discrete Fourier Transform i:e Goertzel Algoritm. How can I meet the ITU-T specifiaction ? How can I choose a suitable value of N that meets both Signal Duration and Frequency Tolerance.? Regards Sheheyr _____________________________________ Note: If you do a simple "reply" with your email client, only the author of this message will receive your answer. You need to do a "reply all" if you want your answer to be distributed to the entire group. _____________________________________ About this discussion group: To Join: To Post: To Leave: Archives: http://www.yahoogroups.com/group/speechcoding Other DSP-Related Groups: http://www.dsprelated.com ">http://docs.yahoo.com/info/terms/ |
RE: [speechcoding] DTMF Detection
Started by ●February 18, 2002
