Ring tone detection for a telephone system

A method of processing signals received from a telephone network, having a voice band at a telephone apparatus said telephone apparatus having at least an on-hook state and an off-hook state comprising the steps of:

• inputting, while said apparatus is on-hook, said received signals into a digital signal processor;
• digitally processing said signals from a first side of a transformer to detect information transmitted outside the voice band of the telephone network while said apparatus is on-hook;
• switching said apparatus to an off-hook state;
• and
• digitally processing, in the processing means, said signals from a second side of the transformer to detect information within the voice band Of the telephone network after said step of switching.

A method of digitally estimating the frequency of a signal received from a telephone networks, said signal having an auto-correlation function, a frequency, and a period, and of activating a ring tone detect logic circuit, the method comprising the steps of:

• digitally calculating the auto-correlation function of said received signal at multiple time lags to obtain a numerator;
• digitally calculating the auto-correlation function of said received signal at a single time lag to obtain a denominator;
• dividing the numerator by the denominator to obtain a result;
• obtaining an estimate of said frequency, from said result, after no more than one half of said period;
• and
• asserting a logic signal to a predetermined state if said frequency estimate is within a predetermined range to thereby activate said logic circuit.

Apparatus for detecting ring tone in a telephone network having a voice bandwidth said apparatus comprising:

• filter means having a passband and arranged to receive said ring tone from said telephone network, said filter means being arranged such that said ring tone is outside said passband;
• an analog to digital (A/D) convertor having an input and an output and being connected to the output of said filter means for digitizing said filtered ring tone signal;
• a digital signal processor coupled to said output of said A/D convertor for digitally processing samples output by said A/D convertor and for detecting when ring tone is present;
• and
• means for detecting whether said apparatus is in an on hook or off hook state, and for connecting the input of said A/D convertor to a different electrical point based upon said on hook or off hook state.

Apparatus for detecting ring tone in a telephone network having a voice bandwidth said apparatus comprising:

• filter means having a passband and arranged to receive said ring tone from said telephone network, said filter means being arranged such that said ring tone is outside said passband;
• an analog to digital (A/D) convertor having an input and an output and being connected to the output of said filter means for digitizing said filtered ring tone signal;
• a digital signal processor coupled to said output of said A/D convertor for digitally processing samples output by said A/D convertor and for detecting when ring tone is present;
• and
• means for detecting whether said apparatus is in an on hook or off hook state, and for connecting the input of said A/D convertor to process information either inside or outside the voice bandwidth based upon said on hook or off hook state.