1. A speech quality degradation estimation method for estimating the speech quality of a speech signal modified by a pitch-synchronous prosody modification method, the speech quality degradation estimation method comprising: extracting at least one source pitchmark from the speech signal; mapping the source pitchmark to at least one target pitchmark; and calculating at least one degradation measure based on the mapping between the source pitchmark and the target pitchmark, wherein the degradation measure includes at least one of the following duration-related mathematical functions: abs ⁡ ( 1 - DUR t / DUR s ) , { 1 N ⁢ ∑ i = 1 N ⁢ ⁢ [ pm_discount ⁢ ( i ) ] p } 1 / p , ⁢ ⁢ and ⁢ ⁢ max i ⁢ ( pm_discont ⁢ ( i ) ) , wherein abs( ) is absolute value function, max( ) is maximum value function, DUR s and DUR t are respectively the durations of the speech signal before and after being modified, N is the number of the jar pitchmarks, is a default positive integer, and pm_discont(i) is a default continuity function, which has different values based on whether the source pitchmarks mapped to the target pitchmarks are continuous.