Telephone answering device with low cost dual tone multi-frequency

Abstract

A telephone answering device (TAD) includes a remote access mode which is operative responsive to dual tone multi-frequency (DTMF) signals. At least three bandpass filters each receive an input signal from a telephone line interface provided in the TAD. A software controlled sampling section receives outputs from all the bandpass filters for sampling outputs from the bandpass filters, the sampling section including a multiplexer which receives the outputs from all of the bandpass filters, and a peak hold circuit coupled to an output of the multiplexer. An analog-to-digital converter receives an output from the peak hold circuit, and a digital controller receives the output of the analog-to-digital converter for controlling operations of the TAD in a remote access mode responsive to the detected DTMF signals. First and second ones of the bandpass filters have respective center frequencies corresponding to frequencies of respective DTMF signals and a third bandpass filter has a different center frequency. The absence of an output from the third filter confirms that there is no noise or other undesirable signals that would indicate that an incoming signal is not a DTMF signal. At least one of the bandpass filters preferably is a switched capacitor filter having a center frequency which is switchable between at least two different center frequencies. At least the bandpass filters, multiplexer, peak hold circuit and analog-to-digital converter are all included in a single application specific integrated circuit (ASIC).