Improving the separation of a signal into source and filter 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?

Cheers,
Ben
Have you considered using a higher order filter for your LPC analysis?  LPC analysis is known to be sensitive to the presence of noise, say additive, so it may be useful to preprocess/denoise the signal to have the best/cleanest signal going in. Are you using a microphone on a mobile device?

If those don't work, you should look into research on spectral envelope extraction, or alternative algorithms for voice-morphing.