Cascaded long term prediction for coding polyphonic audio signals
The long term prediction (LTP) tool is used in audio compression systems to exploit periodicity in signals. This tool capitalizes on the periodic component of the waveform by selecting a past segment as the basis for prediction of the current frame. However, most audio signals are polyphonic in nature, consisting of a mixture of periodic signals. This renders the LTP suboptimal, as the mixture's period equals the least common multiple of its individual component periods, which typically extends far beyond the duration over which the signal is stationary. Instead of seeking a past segment that represents a “compromise” for incompatible component periods, we propose a more complex filter that caters to the individual signal components. This proposed technique predicts every periodic component of the signal from its immediate history, and this is achieved by cascading LTP filters, each corresponding to individual periodic component. We also propose a recursive “divide and conquer” technique to estimate the parameters of all the LTP filters. For a real world evaluation, we employ this technique within the Bluetooth Sub-band Codec. Considerable gains achieved on a variety of polyphonic signals demonstrate the effectiveness of the proposal.