Audio Multiband Stereo Width

A mastering-style multiband imager. The stereo signal is split into low / mid / high bands by 4th-order Linkwitz-Riley crossovers (each boundary is two cascaded 2-pole sections — a single Butterworth pair was measured to sum with deep notches at the crossover frequency, while the cascaded LR4 sum is transparent to within 0.05 dB). Each band then runs through an exact mid/side matrix L′ = (1+G)/2·L + (1−G)/2·R, R′ = (1−G)/2·L + (1+G)/2·R, where G is the band's width control: 0% collapses the band to mono (the default for the low band — bass stays phase-safe for vinyl and club systems), 100% leaves it untouched, and up to 300% stretches it wide. Because the FFmpeg pan filter silently renormalizes channel coefficient sums, the matrix is applied with aeval expressions per channel, verified bit-exact at G = 1. The three bands are summed at unity (amix normalize=0) and a limiter guards the widened output. Before and after the render the tool measures the side-channel energy of the high band and the low band plus the mid-channel energy between the crossovers, so the report shows the width change where you asked for it and the collateral damage where you did not.