Method for Simultaneous Detection of Standard and Non-Standard Frequency Telephone Call Progress Signals
Original Publication Date: 1986-Feb-01
Included in the Prior Art Database: 2005-Mar-08
A method is described which detects telephone call progress signals composed of standard frequencies in a very short period of time, while detecting signals of non-standard frequencies in a longer period of time. This digital signal processing program low-pass filters input samples at a sampling frequency of 4800 Hz, with a filter whose cutoff frequency is 700 Hz. Each odd-numbered output sample of the low-pass filter is processed by resonators tuned to standard call progress signal frequencies. Each standard call progress signal is composed of two different frequencies. These frequencies are 350 Hz (Dial Tone), 440 Hz (Ringback and Dial Tone), 480 Hz (Ringback and Busy), and 620 Hz (Busy).