### Length FIR Loop Filter Controlled by ``Brightness'' and ``Sustain''

Another convenient parametrization of the second-order symmetric FIR
case is when the dc normalization is relaxed so that two degrees of
freedom are retained. It is then convenient to control them
as *brightness* and *sustain* according to the
formulas

where is the period in seconds (total loop delay), is the desired sustain time in seconds, and is the brightness parameter in the interval . The sustain parameter is defined here as the time to decay by dB (or time-constants) when brightness is maximum () in which case the loop gain is at all frequencies, or . As the brightness is lowered, the dc gain remains fixed at while higher frequencies decay faster. At the minimum brightness, the gain at half the sampling rate reaches zero, and the loop-filter amplitude-response assumes the form

**Next Section:**

One-Zero Loop Filter

**Previous Section:**

Length Three FIR Loop Filter