Discrete Cosine Transform

The DCT is similar to the Discrete Fourier Transform: It transforms a signal from the time domain into the frequency domain. Just as the fourier transform uses sine and cosine waves to represent a signal, the DCT only uses cosine waves. The DCT I and II, is mostly used in image compression, while MDCT (DCT-IV) is used in audio encoding. The DCT is an invertible, discrete orthogonal transformation.