Closing the Loop: PID Feedback Control for Interpretable Activation Steering in Symbolic Music Generation

Activation steering controls generation at inference without retraining. In symbolic music, Sparse Activation Steering via Sparse Autoencoders enables interpretable single-layer attribute control but fails in temporal smoothing: fractional steering magnitudes fall below the Top-K sparsity threshold, zeroing the intervention. We propose PID Steering in two forms: Spatial PID validates control-theoretic steering in the generative music domain, while Temporal PID dynamically adjusts λ at each autoregressive step via a closed-loop controller whose integral term accumulates error to overcome the Top-K barrier. Experiments on the Multitrack Music Transformer show that Temporal PID overcomes the Top-K threshold failure for pitch and duration steering, enabling smooth transitions with less intervention strength and 5% lower Frechet Music Distance versus static SAS.