Hi Andrew,
Sounds like you're very close. The lsp_to_lpc() function is actually
much simpler than the encode side. IIRC it's just a big polynomial
multiplication, but this routine uses a method that clocks an impulse
thru 2nd order filters formed by the odd and even LSP pairs.
The output LPCs are very sensitive to quantisation error, especially at
higher orders. So one thing to try is doubles instead of floats.
I can't recall exactly, but maybe it only work on even LPC orders.
Cheers,
David
On 04/10/17 18:59, Andrew Fyfe wrote:
Hi Everyone,
I'm working on a speech vocoder and have so far been able to use a
combination of SPTK and codec2 functions to achieve most of this.
However I'm confused as I want to be able to control the order of the
lpc coefficients in real-time as a parameter but my LSP_to_LPC seems to
have an upper limit to the order it can convert from LSPs to LPCs (I'm
converting to LSPs to do some interpolation and then converting back). I
have found the highest order to be 25 but can push the LPC order to 512
for example if I bypass the LPC to LSP/LSP to LPC conversion stage and
just filter with the initially derived LPC coefficients.
It looks like the LPC to LSP function is working fine and the problem is
in the conversion back to LPC...
Does anyone know why there would be a limit to the order in this
conversion. Is there an element in the theory of LSPs that explains
this? I'm thinking it is to do with precision error but I'm not sure?
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