I am implementing a voice morphing system, but I am having difficulties getting a good quality output signal. The method I am using is LPC analysis and synthesis. In theory, the LPC process should split the signal into a source/filter representation by calculating a residual and a set of filter coefficients.
However, in practice, I am finding that the residual contains data which I believe should be contained in the filter.
What techniques are available for better separating an input signal into source and filter?