Patent ID: 6560331
Filing Date: 2003-05-06
Classification: H04Q

Abstract:
In a communication network, a method of detecting tones using a modified discrete Fourier transform (DFT) for dual-tone modulated frequency (DTMF) detection, said method comprising the steps of:receiving a signal from across said communication network; sampling the signal at a sampling frequency that is at least twice a maximum frequency in the signal; sending the signal sample into a series of signal data blocks stored chronologically in a buffer; recursively computing the modified DFT of the signal data blocks in the buffer, said modified DFT containing a phase correction component to reduce leakage from each signal data block; measuring energy levels along a preset frequency spectrum; upon detection of energy levels above a preset threshold level at a detected frequency, measuring the second harmonic energy levels associated with the detected frequency; upon detection of energy levels above a preset threshold level at a primary frequency and energy levels at an associated second harmonic frequency below a preset secondary threshold, indicating the detection of a tone at the primary frequency.