This, in a nutshell, explains Watts’ pursuit of unprecedentedly long interpolation filters, which he designs using what’s known as a windowed-sinc technique. As the previous paragraph suggests, this involves extracting a chunk from the centre of the sinc function, but for optimum results this process needs to be more subtle than a simple ‘lift’ of the sinc function values and truncation of the remainder. Better results are obtained if the excerpted sinc function is windowed, ie shaped, to avoid sudden truncation at either end. Watt’s WTA (Watts Time Alignment) windowing algorithm is a closely guarded secret, and it has had to be refined as filter lengths have increased, but its name indicates Watts’ principal design criterion: the maintenance of accurate transient timing. So far as I’m aware, no other designer has followed in Watts’ footsteps. And you have to suppose it would be a daunting task to do so, given the decades that Watts has been treading his lonely path.