  More recently, a direct method of the time-frequency approach, called signal separation operation (SSO), was introduced for multicomponent signal separation.
<meaning-changed>   More recently, a direct method of the time-frequency approach, called signal separation operation (SSO), was introduced for multicomponent signal separation.
 The reconstruction accuracy for a component depends heavily on the accuracy of the IFs estimation carried out in the first step. More recently, a direct method of the time-frequency approach, called signal separation operation (SSO), was introduced for multicomponent signal separation.