This does depend on how your DCT has been defined. The essentialOriginally Posted by malibupete
connection is if you assume that the real signal is symmetric about 0, and
extend it to include the negative time components (at large positive
times as the signal is assumed periodic) then the imaginary components
of the DFT will be zero, and the real components will contain the DCT.
The Wikipedia article on the DCT discusses this WikiDCT