  Source separation models either work on the spectrogram or waveform domain.
In this work, we show how to perform end-to-end hybrid source separation,
letting the model decide which domain is best suited for each source, and even
combining both. The proposed hybrid version of the Demucs architecture won the
Music Demixing Challenge 2021 organized by Sony. This architecture also comes
with additional improvements, such as compressed residual branches, local
attention or singular value regularization. Overall, a 1.4 dB improvement of
the Signal-To-Distortion (SDR) was observed across all sources as measured on
the MusDB HQ dataset, an improvement confirmed by human subjective evaluation,
with an overall quality rated at 2.83 out of 5 (2.36 for the non hybrid
Demucs), and absence of contamination at 3.04 (against 2.37 for the non hybrid
Demucs and 2.44 for the second ranking model submitted at the competition).
