An MP3 “granule” (according to web search) is 576 points long. so L3_imdct_gr, L3_imdct36, and L3_imdct12 are going to be inverse modified discrete cosine trans

An MP3 “granule” (according to web search) is 576 points long. so L3_imdct_gr, L3_imdct36, and L3_imdct12 are going to be inverse modified discrete cosine transforms operating on 576, 36, and 12 coefficients, respectively. Modified discrete cosine transform means a DCT type 4 in which adjacent blocks are overlapped. See https://en.wikipedia.org/wiki/Modified_discrete_cosine_transform https://en.wikipedia.org/wiki/Modified_discrete_cosine_trans...