ATRAC2

The next generation of ATRAC after ATRAC1, ATRAC2 was developed (a single-chip stereo encoder/decoder LSI was developed) but never actually implemented in an MD music recorder product. MD-Data was chosen as the format for ATRAC2, due to the need for greater compression in MD-Data applications. Technically, ATRAC2 differs quite substantially from ATRAC1 on several points:

• twice the transform window size - 1024 samples (23.2ms) for ATRAC2 versus ATRAC1's 512 samples (11.6ms)
• encoding tone components seperately from other spectra
• splitting the input signal into 4 bands instead of 3
• using Huffman encoding on the final bit stream to reduce redundancy
• a substantially lower bit rate - 73-36kbps/channel, a compression ratio of approximately 10:1 to 20:1 versus ATRAC1's 146kbps/channel or approximately 5:1
• minimising pre-echo by employing gain control to simplify the spectral srtucture of the system - adaptively amplifying the signal preceeding an attack before performing the MDCT then restoring it to the original level after performing the Inverse MDCT in the decoder

Sources:
Minidisc.org MDLP FAQ - http://www.minidisc.org/mdlpfaq.html
Minidisc.org ATRAC2 Information Page - http://www.minidisc.org/atrac2.html