Patent US 5,828,811

Speech coding uses a periodic excitation signal source, a non-periodic signal excitation source, a synthesizing unit, a filter, and an error evaluating unit. The synthesizing unit synthesizes a periodic excitation signal and a non-periodic excitation signal output from the above-mentioned sources to generate an excitation sound source signal. The filter regenerates an input speech signal from the excitation sound signal. The error evaluating unit controls the periodic excitation signal source and the non-periodic excitation signal source so that the above-mentioned units output periodic and non-periodic excitation signals which minimize a difference of the regenerated signal from an input speech signal. When the error is minimized, the above-mentioned sound source is supplied to a feedback excitation signal modifying unit. The feedback excitation modifying unit reduces a non-periodic component in the above-mentioned supplied signal so that the non-periodic component is reduced more when the relative amount of the periodic component is greater. The modified signal is fed back to the periodic excitation signal source unit to modify the content thereof.