6.3 Envelope time borders
26.4023GPPAdditional decoder toolsEnhanced aacPlus general audio codecGeneral audio codec audio processing functionsTS
The merging of the time envelope grid is explained by using 
which contains all start/stop borders for all SBR envelopes in the current SBR frame, and the creation of 
from the bitstream is explained in [1].
Merging the two 
into one is done in the following sequence
1. The start border value of the merged 
is set to the largest of the start border values for the left and right channels.
2. The union of all borders for left and right channel values larger than the merged start border is added to the merged 
3. All envelopes smaller than two time slots are removed from the merged 
. This is achieved by starting from the beginning of 
and remove all stop borders that are closer than 2 from the start border.
4. If there are more than 5 envelopes the number of envelopes are reduced. This is achieved by starting from the end of 
and search towards the beginning of 
for a envelope smaller than 4 and remove the start border of that envelope, this continues until there are 5 envelopes left.
The index 
defined in Table 4.119 in [1] has to be merged into one from two. The merging algorithm uses the following recursion:
