This code path is 2% faster.

10% speedup.  Strange, huh?

For some reason, this speeds up the library by 2%.

Obtained from the Mercurial repository at
https://code.google.com/p/dsd2pcm/

Instead of doing run-time initialisation, use a constant lookup table.

Fixes gcc warnings.

[this is the code from dsd2pcm_src.zip, published on a forum by
Sebastian Gesemann.  Upon request, he has given permission to
redistribute and modify his code, without referring to a specific
license. - mk]