The Princen-Bradley condition is a criterion that must be satisfied when using overlapping windows in signal processing. It is also called the perfect reconstruction criteria since it ensures that the reconstructed signal after [[windowing]] and [[overlap-add]] processing is as close as possible to the original signal. The condition specifies that the sum of the squares of the window functions at adjacent frames must be equal to 1. $ \sum_{k=0}^{L/2-1} w^2\left( n+\frac{L}{2} \right) w^2(n) = 1 $ This results from the need to perform two windowing operations: one for the input signal and another before reconstructing the signal to reduce the amplitude of the samples closer to the frame boundaries. To satisfy this condition, it is important to use a window function that reduces the amplitude of samples near the frame boundaries. The [[sine window]] is an example of a window function that meets this requirement and follows the perfect reconstruction criteria.