Bandwidth expansion
# Bandwidth expansion

Bandwidth expansion is a technique for widening the bandwidth or the resonances in an LPC filter. This is done by moving all the poles towards the origin by a constant factor $\gamma$. The bandwidth-expanded filter $A'(z)$ can be easily derived from the original filter $A(z)$ by:

    $A'(z)=A(z/\gamma )$

Let $A(z)$ be expressed as:

    $A(z)=\sum _{k=0}^{N}a_{k}z^{-k}$

The bandwidth-expanded filter can be expressed as:

    $A'(z)=\sum _{k=0}^{N}a_{k}\gamma ^{k}z^{-k}$

In other words, each coefficient $a_{k}$ in the original filter is simply multiplied by $\gamma ^{k}$ in the bandwidth-expanded filter. The simplicity of this transformation makes it attractive, especially in CELP coding of speech, where it is often used for the perceptual noise weighting and/or to stabilize the LPC analysis. However, when it comes to stabilizing the LPC analysis, lag windowing is often preferred to bandwidth expansion.

