改进的离散余弦变换（Modified Discrete Cosine Transform, MDCT）是一种与傅里叶变换相关的变换，以第四型离散余弦变换（DCT-IV）为基础，重叠性质如下：它是应用于处理较大的数据集合，当连续的数据区块中，当前的数据区块跟后续的数据区块有重叠到的情形；即当前数据区块的后半段与下一个数据区块的前半段为重叠的状态。这样的重叠情形，除了具有离散余弦变换的能量压缩特性外，也使这种变换在应用于信号压缩时更引人注目。因为它有助于避免由于数据区块边界所产生的多余数据。因此，这种变换可应用于MP3，AC-3，Ogg Vorbis，和AAC的音频压缩等方面。

The modified discrete cosine transform (MDCT) is a lapped transform based on the type-IV discrete cosine transform (DCT-IV), with the additional property of being lapped: it is designed to be performed on consecutive blocks of a larger dataset, where subsequent blocks are overlapped so that the last half of one block coincides with the first half of the next block. This overlapping, in addition to the energy-compaction qualities of the DCT, makes the MDCT especially attractive for signal compression applications, since it helps to avoid artifacts stemming from the block boundaries. As a result of these advantages, the MDCT is employed in most modern lossy audio formats, including MP3, AC-3, Vorbis, Windows Media Audio, ATRAC, Cook, AAC, and Opus.