  Neural upmixing, the task of generating immersive music with an increased
number of channels from fewer input channels, has been an active research area,
with mono-to-stereo and stereo-to-surround upmixing treated as separate
problems. In this paper, we propose a unified approach to neural upmixing by
formulating it as spherical harmonics - more specifically, Ambisonic
generation. We explicitly formulate mono upmixing as unconditional generation
and stereo upmixing as conditional generation, where the stereo signals serve
as conditions. We provide evidence that our proposed methodology, when decoded
to stereo, matches a strong commercial stereo widener in subjective ratings.
Overall, our work presents direct upmixing to Ambisonic format as a strong and
promising approach to neural upmixing. A discussion on limitations is also
provided.
