In the scheme above, the phase of each depends only on the previous value for the same channel. The phase relationships between neighboring channels are left to chance. This is adequate in some situations, but sometimes the incoherence of neighboring channels gives rise to an unintended chorus effect. We would ideally like for and to have the same phase relationship as for and , but also for the phase relationship between and to be the same as between and .
These equations for phases in general will have no solution, but we can alter the equation for above so that whenever there happens to be a solution to the over-constrained system of equations, the reconstruction algorithm homes in on the solution. This approach is called phase locking [Puc95], and has the virtue of simplicity although more sophisticated techniques are available [DL97]).
The condition for an exact solution to exist at stage is that: