5.6 Additional control parameters
26.4043GPPEnhanced aacPlus encoder Spectral Band Replication (SBR) partEnhanced aacPlus general audio codecGeneral audio codec audio processing functionsRelease 17TS
5.6.1 Introduction
In order to achieve optimal results, given the HF generator used in the decoder, several additional parameters apart from the spectral envelope are assessed. The noise floor is estimated for the current SBR frame. It is defined as the ratio between the energy of the noise that should be added to a particular frequency band, in order to obtain a similar tonal to noise ratio to that of the original signal, and the energy of the HF generated signal for that frequency band.
The noise floor is estimated once or twice per SBR frame dependent on the number of spectral envelopes estimated for the SBR frame (indicated by 
). The frequency resolution for the noise floor scalefactor is calculated according to the same algorithm subsequently used in the decoder and described in [1] subclause 4.6.18.3. The start and stop time borders of the different noise floors are given from the time grid.
The level of the inverse filtering applied in the decoder is estimated for different frequency ranges with the same frequency resolution as used for the noise floor scalefactor estimation. The estimation algorithm compares the tonality of the original and the tonality that will be attained after the HF generator in the decoder. The ratio between the two is mapped to four different inverse filtering levels, off, low, mid and high. These levels corresponds to different chirp factors in the HF generator as outlined in [1] subclause 4.6.18.5. Moreover, the encoder assesses where a strong tonal component will be missing after the HF generation in the decoder. This detection is done on the highest frequency resolution given by the high frequency resolution table, fTableHigh. The level of the tonal component is implicitly coded by the SBR envelope and the noise floor scalefactors, and thus only the frequency needs to be coded.