A filter alters the frequency spectrum of a sound coming out of an oscillator. Common filters are low-pass (meaning low frequencies are let through), high-pass (allowing high frequencies through), notch (blocking a narrow range of frequencies), and band-pass (allowing only a narrow range of frequencies). Filters, even in the digital world, cannot be truly linear, i.e. the graph of the filter's response over frequency shows a curved "knee" dropping rather than a vertical line. This means that a low-pass filter set to 100Hz will let some frequencies lower than that through, although the lower the frequency, the more it will be cut. The steepness of the drop in a filter is called it's Q parameter. Historically, some analog filters also introduced "resonance" or amplification of frequencies around the "knee" point in the filter. This effect has been used in much electronic music and is included in LMMS