In this paper, we present a fast algorithm for bi-directional conversion between discrete cosine transform (DCT)
coefficients of a signal block and its sub-blocks. Specifically, we show that a DCT of a block of size N x N can be
efficiently obtained from the N/2 x N/2 DCT coefficients of its sub-blocks, and vice versa. The algorithms are
obtained by formulating each unidirectional conversion as a relatively simple combination of N/2 x N/2 DCT
calculations. For typical DCT block sizes our algorithms result in a 20% savings in multiplications over the
fastest existing methods. Simulation results also suggest high accuracy using the proposed conversion scheme.