Long description

This figure is a graph of gain against frequency. An ideal low-pass filter line is shown that is horizontal at a gain of 1 for low frequency values, then falls vertically at the cut-off frequency and remains horizontal at a gain of 0 for frequency values above this point. Another line is shown representing the actual response of the digital filter. At low frequencies, it oscillates by a small amount around a gain of 1; this is known as the passband ripple. At a frequency slightly below the cut-off, it descends diagonally until it reaches a frequency slightly above the cut-off; the distance between these two frequency values is known as the transition width. At frequencies above this, the response oscillates by a small amount around a gain of 0; this is known as the stop-band ripple.