Let's define the N-point DFT $Y(f)$ of a signal $y[n]$ as
$$\begin{align*} Y[f] &= \sum\limits_{n=0}^{N-1} y[n] e^{j2\pi f\,\frac{n}{N}},& f\in \{0,\dots,N-1 \} \end{align*}$$ The DCT-II (which is most probably what we're looking at; the others are mathematically useful, but less nice to implement) $\mathbf{Y}[f]$:
$$\begin{align*} \mathbf{Y}[f] &= \sum\limits_{n=0}^{N-1} y[n] \cos\left(\pi (k+\frac12 \frac nN \right) ,& f\in \{0,\dots,N-1\} \end{align*}$$