5.5 Envelope estimation

26.4043GPPEnhanced aacPlus encoder Spectral Band Replication (SBR) partEnhanced aacPlus general audio codecGeneral audio codec audio processing functionsRelease 17TS

By using the time/frequency grid created by the framing generator and the transient information from the transient detector, the QMF bank subband matrix is grouped in time and frequency into envelope scalefactorbands. For each scalefactorband the squared average energy is calculated and stored in the energy matrix according to the recursion below.

If a missing harmonic has been detected in a certain scalefactorband the squared energy for that scalefatorband is calculated as the maximum energy instead of average energy. Since the missing harmonics detection and signalling always operate using the recursion shown below.

For stereo with no channel coupling, the energy for every channel is calculated as in the mono case shown above. In the case of stereo and coupling the energy is calculated according to: