Decimation is a clever technique in digital signal processing that allows multi-rate processing of digital signals. A high sampling rate signal can be down sampled in such a manner that data is not lost in the process and yet a communication link is established. For example , a long wave communication channel for communicating with submarines. To further our understanding of decimation lets consider a band limited signal sampled at 300 Khz.

Assume the highest frequency component in the signal is at 100 Khz. If I want to reduce the sampling rate down to 100 Khz, then I must make sure there are no components of the input signal above 50 kHz. However, as mentioned above the signal has some components at 100 Khz. So if we really want a low sampling rate we will have to band limit the signal to 50 Khz. We must filter the signal before applying it to a down sampler. This technique and the combination of filter and down sampler is called DECIMATION.