# Hardware Noise in Microphone Input: Overview

The explanation on this page relates to TWL hardware behavior.

## Description

It has been confirmed that slight hardware noise at approximately 8 kHz will be mixed with microphone input on the TWL system. To remove this hardware noise and achieve clearer microphone input, use the `SNDEX_SetIirFilter[Async]` functions to implement an 8 kHz BEF filter. This type of filter will damp a specific frequency component.

## Implementation

We recommend that you pass the following parameters (`SNDEXIirFilterParam` structure) to one of the `SNDEX_SetIirFilter[Async]` functions.

- For a Sampling Frequency of 32.73 kHz

- n0: 6B36
- n1: FC2D
- n2: 6B36
- d1: 03D3
- d2: A994

- For a Sampling Frequency of 47.61 kHz

- n0: 7108
- n1: C7D7
- n2: 7108
- d1: 3829
- d2: 9DF0

## Notes

This filter can only be implemented in CODEC-TWL mode. Implementing the BEF filter will also damp normal 8 kHz components that are not noise.

## Revision History

2008/09/01 Initial version.

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