LPCToLSF

Converts LP coefficients to LSF coefficients.

Syntax

IppStatus ippsLPCToLSF_G723_16s (const Ipp16s* pSrcLPC, const Ipp16s* pSrcPrevLSF, Ipp16s* pDstLSF);

Parameters

pSrcLPC	Pointer to the LPC input vector [10]: a ₁,...,a ₁₀ in Q13.
pSrcPrevLSF	Pointer to the previous normalized LSF coefficients vector [10], in Q15.
pDstLSF	Pointer to the normalized LSF coefficients vector [10], in Q15.

Description

The function ippsLPCToLSF_G723 is declared in the ippsc.h file. This function converts a set of 10th order LP coefficients to LSF coefficients by implementing the following operations:

1. Apply the 7.5Hz bandwidth expansion.

2. Calculate the polynomial coefficients of F₁(z) and F₂(z), using the following recursion:

f₁(i+1) = a_i+1 + a_10-i - f₁(i+1),

f₂(i+1) = a_i+1 + a_10-i - f₂(i+1)

i = 0...4 ,

where f₁(0) = f₂(0) =1.0

3. Use Chebyshev polynomials to find the roots of F₁(z) and F₂(z), and obtain the LSF coefficients:

c₁(ω) = cos (5ω) + f₁(1) cos (4ω) + f₁(2) cos (3ω) + f₁(3) cos (2ω) + f₁(4) cos (ω) + f₁(5) / 2

c₂(ω) = cos (5ω) + f₂(1) cos (4ω) + f₂(2) cos (3ω) + f₂(3) cos (2ω) + f₂(4) cos (ω) + f₂(5) / 2

4. If all 10 roots needed to determine LSF coefficients are not found, use the set of previous LSF coefficients.

Return Values

ippStsNoErr	Indicates no error.
ippStsNullPtrErr	Indicates an error when one of the specified pointers is NULL.