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# Long description

This figure is a block diagram showing the four stages that a signal goes through during digital filtering.

The signal starts as a continuous input signal. This has the overall shape of a sine wave but is more jagged, since it is subject to noise that causes it to constantly deviate by small amounts from the smooth curve of the sine wave. The midpoint of the sine wave crosses the horizontal axis.

The input signal enters an analogue-to-digital converter. The output is a sampled discrete signal, which consists of a series of evenly spaced vertical lines along the horizontal axis – some extending above the axis and some below. These represent the samples. Each line has a small filled circle at the end furthest from the axis. The height of each line is equal to the distance, at that point in time, from the horizontal axis to the input signal curve. Therefore if you were to join the filled circles, the resulting shape would be approximately the same as that of the input signal.

The sampled signal enters a processor. The output is a digitally filtered signal, which again consists of a series of evenly spaced vertical lines at the same points along the horizontal axis as for the previous signal. Once again, each line has a small filled circle at the end furthest from the axis. However, the heights of the vertical lines have now been adjusted so that if you were to join the filled circles, the resulting shape would be a sine wave.

The digitally filtered signal enters a digital-to-analogue converter. The output is a filtered continuous signal that has the shape of the same sine wave as the original input signal, but unaffected by noise.