Detection of a call-waiting tone by a modem

An apparatus comprising:

• a signal sampler coupled to receive a signal from a communications medium and to form a sampled sequence of values corresponding to said signal;
• and
• a call-waiting tone detector coupled to operate on a stored representation of a first sequence corresponding to said sampled sequence, said call-waiting tone detector including:
• a first correlator having access to said stored representation to calculate a power value of said signal;
• a second correlator having access to said stored representation to cross-correlate said stored representation with a first temporal displacement thereof to produce a first cross-correlation value;
• a third correlator having access to said stored representation to cross-correlate said stored representation with a second temporal displacement thereof to produce a second cross-correlation value;
• and
• decision logic responsive to said first, said second, and said third correlators, said decision logic signaling detection of a call waiting tone:
• if said power value exceeds a threshold power level, and
• if said first cross-correlation value exceeds a first threshold percentage of said power value, and,
• if the additive inverse of said second cross-correlation value exceeds a second threshold percentage of said power value;
• wherein said first and said second temporal displacements are each by integer numbers of samples, wherein said first temporal displacement corresponds to an integer multiple of the period of a call-waiting tone, and wherein said second temporal displacement corresponds to a half-integer multiple of the period of said call-waiting tone.

A method comprising:

• establishing a connection between a first modem and a second modem;
• communicating data between the first modem and the second modem using a signal;
• detecting a tone in the signal received at the first modem by:
• sampling the signal to form a first sequence, the sequence including a plurality of signal samples;
• cross-correlating the first sequence with a second sequence to form a first cross-correlation value;
• cross-correlating the first sequence with a third sequence to form a second cross-correlation value;
• and
• identifying the tone by comparing the first cross-correlation value with a first threshold value and comparing the second cross-correlation value with a second threshold value;
• and
• indicating that the tone is detected.

A modem comprising:

• transmit path structures including an encoder and a modulator;
• and
• receive path structures including a decoder, a demodulator, and a call-waiting tone detector having:
• a correlation structure capable of correlating a received signal with itself, with a first derived signal, and with a second derived signal to produce a plurality of correlation values, the first and second derived signals being derived from the received signal;
• and
• a comparator for comparing at one of the plurality of correlation values with a first threshold value and for comparing another of the plurality of correlation values with a second threshold value.