# Downsampling

- Downsampling is used to reduce the sampling rate of data. Shannon's Sampling Theorem must be obeyed, i.e., before you sample, you must low-pass filter the data with a cut-off frequency less than half of the new sampling rate.

**Note:**If you omit the filtering, the data will be aliased and cannot be corrected.

- Low-pass filter the data with a cut-off frequency less than half of the new sampling rate by an integer factor.

- Decimate the filtered data by the desired factor.

**See also: **Aliasing, Decimation.

**Subjects: ** Noise & Vibration