  A converse method is developed for the source broadcast problem.
Specifically, it is shown that the separation architecture is optimal for a
variant of the source broadcast problem and the associated source-channel
separation theorem can be leveraged, via a reduction argument, to establish a
necessary condition for the original problem, which unifies several existing
results in the literature. Somewhat surprisingly, this method, albeit based on
the source-channel separation theorem, can be used to prove the optimality of
non-separation based schemes and determine the performance limits in certain
scenarios where the separation architecture is suboptimal.
