  We explore frame-level audio feature learning for chord recognition using
artificial neural networks. We present the argument that chroma vectors
potentially hold enough information to model harmonic content of audio for
chord recognition, but that standard chroma extractors compute too noisy
features. This leads us to propose a learned chroma feature extractor based on
artificial neural networks. It is trained to compute chroma features that
encode harmonic information important for chord recognition, while being robust
to irrelevant interferences. We achieve this by feeding the network an audio
spectrum with context instead of a single frame as input. This way, the network
can learn to selectively compensate noise and resolve harmonic ambiguities.
  We compare the resulting features to hand-crafted ones by using a simple
linear frame-wise classifier for chord recognition on various data sets. The
results show that the learned feature extractor produces superior chroma
vectors for chord recognition.
