Figure 11
From: An Overview of the Coding Standard MPEG-4 Audio Amendments 1 and 2: HE-AAC, SSC, and HE-AAC v2
Envelope adjustment in a real-valued QMF filter bank. In the top panel, sinusoids are displayed within QMF subbands. The QMF subband responses are stylistically drawn and at a higher frequency resolution than that of the filter bank, in order to illustrate where within the QMF subband the sinusoids are located. The second panel illustrates the gain vector as calculated by the envelope adjustment module, where the gain values are given for every subband. The third panel illustrates the output after envelope adjustment. Here it is evident that, for example, the sinusoid located between subbands 16 and 17, where also the gain values differ between the subbands, will produce an aliasing component that is not part of the signal in the top panel.