  Fully exploiting ad-hoc microphone networks for distant speech recognition is
still an open issue. Empirical evidence shows that being able to select the
best microphone leads to significant improvements in recognition without any
additional effort on front-end processing. Current channel selection techniques
either rely on signal, decoder or posterior-based features. Signal-based
features are inexpensive to compute but do not always correlate with
recognition performance. Instead decoder and posterior-based features exhibit
better correlation but require substantial computational resources. In this
work, we tackle the channel selection problem by proposing MicRank, a learning
to rank framework where a neural network is trained to rank the available
channels using directly the recognition performance on the training set. The
proposed approach is agnostic with respect to the array geometry and type of
recognition back-end. We investigate different learning to rank strategies
using a synthetic dataset developed on purpose and the CHiME-6 data. Results
show that the proposed approach is able to considerably improve over previous
selection techniques, reaching comparable and in some instances better
performance than oracle signal-based measures.
