Adaptive echo canceller with sparse dynamically positioned taps

An echo canceller for a transmission line, comprising:

• shift register means having an input for receiving an input signal from a signal source and having a plurality of output taps for providing a plurality of equally spaced delayed signals;
• tap coefficient storage means for storing a plurality of tap coefficients;
• multiplying means for multiplying certain of said delayed signals by certain of said tap coefficients to produce multiplier outputs;
• tap allocation means for determining how many and which of said taps are to be turned ON so that its corresponding tap coefficient is multiplied by said multiplying means and for assigning an ON status to said turned ON taps, a STANDBY status to taps which are not turned ON but which are more likely to be needed than others in the event of changes in transmission line characteristics, and an OFF status to any remaining taps;
• tap status memory means for storing said ON, OFF or STANDBY status for each of said taps;
• and
• canceling means for canceling echoes by subtracting said multiplier outputs from signals received on said transmission line.