ites="Foote2016 Ulyanov2016 Wyse2017">(Foote et al. 2016, Ulyanov &amp; Lebedev 2016, Wyse 2017)</span>, which are dealt with in <span class="citation" data-cites="Briot2017">(Briot et al. 2017)</span>, but in general, a gradient is used on an input and a target in order to synthesize a third, hybrid sound. In <span class="citation" data-cites="verma2018neural">(Verma &amp; Smith 2018)</span>, Verma and Smith treat audio synthesis as a <em>style transfer</em> problem, and they use back-propagation to optimi