Efficient AAC-to AACplus transcoding
Publication Date: 2004-May-28
The IP.com Prior Art Database
Efficient AAC-to-aacPlus transcoding
Spectral Band Replication (SBR) can be used to increase the coding efficiency of existing audio coders. For example, SBR is used in mp3PRO (enhanced MP3) and in MPEG-4 High Efficiency AAC (also called aacPlus) which is an enhanced MPEG-4 AAC. When SBR is used, the core coder (e.g. MP3 or AAC) typically runs at half the output sampling frequency. This makes it difficult to transcode MP3 data to mp3PRO or to transcode AAC data to aacPlus as the core signal needs to be re-encoded/requantized.
In the aacPlus standard, a downsampled mode is defined which allows the core AAC coder to run at the same sampling frequency as the output sampling frequency. In this case, efficient transcoding is possible using AAC core transcoding by just throwing away all spectral data above the bandwidth covered by the core coder. This bandwidth is known from the SBR parameters. The AAC core side information has to be adapted accordingly.