Training-Free Instruction TTS Gender Bias Calibration Using Model-Adaptive Steering
Instruction-based text-to-speech (ITTS) systems exhibit implicit gender biases that are difficult to calibrate without retraining. This work proposes a model-adaptive steering method that directly adjusts post-encoder representations for training-free bias correction through group bias vectors, coarse-to-fine intensity search, and deterministic lexical gating. To our knowledge, this mechanism is the first to operate without retraining while generalizing across heterogeneous architectures, eliminating implicit biases while strictly preserving explicit prompt semantics. Experimental results demonstrate that the proposed approach reduces aggregate calibration error by 0.8 to 5.9 percentage points across four mainstream ITTS models, with negligible degradation in audio quality and intelligibility.