Language Directed Mono-to-Stereo Music Upmixing

The same mono song can become many valid stereo mixes. A producer may keep the kick centered, spread the claps across the speakers, or widen a vocal only in the chorus; none of these choices is determined by the mono recording alone. We introduce Width as a Command, a language-directed system that makes such choices explicit. It turns a user's request into an editable plan for the width of individual musical elements, grounds that plan in the recording, and generates a stereo mix while preserving the original mono content. Our key representation describes width through rapid left–right motion, independent of an element's overall position. A generative model then realizes a plausible motion for each requested width without requiring the user to specify an exact stereo trajectory. Experiments on public music examine both automatic upmixing and authored width control; ablations indicate that left–right motion provides a more useful width description than side-channel energy. Making stereo imaging describable and editable offers an interface for music production and a path toward spatially annotated data for future music generation.