SWB TBE target missing in TCX->ACELP switch DFT Stereo
When running DFT (Unified) Stereo @ 13.2 kbps with 48 kHz input sampling rate, the SWB TBE target signal shb_speech in swb_pre_proc.c is not generated. Instead, it is zeroed. Due to the delay compensation of the SWB TBE processing, the previous frame is included when processing the current frame. During a switch from TCX to ACELP, the beginning of the SWB TBE buffer will be zero. At low bit rates the ACELP is used for handling transients. The missing SWB TBE target may lead to energy loss in the transient onset and holes in the spectrum in the BWE region.
At 32 kbps, a low complex SWB TBE target generation is provided. However, the generated target band spans 8-16 kHz instead of 6-14 kHz for 12.8 kHz ACELP core and 7.5-15.5 kHz for 16 kHz ACELP core. If the spectral tilt is strong, the shift of target band may also lead to an energy drop.
A description of the issue and a proposed fix is described in the presentation below (shown at Fürth F2F)