Long description

This figure is a graph of gain G against normalised frequency. The ideal brick-wall response is shown as a horizontal line at gain 1 to the left of the cut-off frequency, then a vertical line at the cut-off frequency, with the gain above the cut-off frequency being zero. Three graph lines are shown in comparison to this ideal, representing first-order, second-order and eighth-order filters. Each line starts at a gain of 1 for low frequencies, and curves down first gradually and then more steeply to reach a gain of 1 over root 2 at the cut-off frequency. After this, each line continues to descend steeply and then more gradually, until at high frequencies it approaches zero. The higher the order of the filter, the steeper the descent and the closer the graph line comes to the ideal.