Hi,
I am a noob in voice coding. Now I want to do a project about Codec2, so I went
over some
theories of voice coding, LPC etc. Also I downloaded the reference C-code and
David's Thesis.
There are some points I don't understand, I hope you guys can help me on these.
1. In the decoder block, I almost get the idea of how it works (thank to
David's PhD thesis)
However, I don't understand why after we get LPC coefficients from
LSP2LPC process, we
have to use FFT (aks_to_M2 function to get ak[i]). As long as I
know, LPC coefficients can
synthesize speech data by all-pole filter from excitation, I wonder
why this algorithm use
FFT directly to the ak[i] to get Aw[]. How do LPC coefficients
relates to the speech power
spectrum. If you guys know any good reference, please let me know.
2. How important is the lpc_post_filter() ? I look at the code, it is
pretty high computation
process with FFT, and power estimation. I tried to decode with and
without post filter at
1200 bps, I don't hear any difference much. what is the criteria to
use/not use the post
filter ?
Thank you for your helps in advance.
Sumek.
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