A signal processing apparatus has a multi-bit quantizer and a processing circuit. The multi-bit quantizer determines and outputs code segments of a multi-bit output code sequentially. The code segments include a first code segment and a second code segment. The processing circuit generates digital outputs according to the code segments, respectively. The digital outputs include a first digital output derived from a first code segment and a second digital output derived from a second code segment. A first transfer function between the first digital output and the first code segment is different from a second transfer function between the second digital output and the second code segment.