DirAC/SPAR concealment produces close-to-zero signal

While reviewing the P.800-4 (HOA2) listening test items, it was found that the IVAS decoder running at 13.2–32 kbps produces an almost zero-energy concealment segment over 8 frames. This concealment is triggered by a burst of erasures in the background-noise portion. The artifact is perceptually annoying but not a major issue, as effects like this are expected at 5% FER.

Interestingly, for bitrates ≥ 48 kbps the artifact does not appear, and the concealment yields a signal with sufficient energy. The observed “gap” seems related to DirAC/SPAR concealment, because the synthesized output from the SCE decoder does not contain it. See the illustration below:

Steps to reproduce:

./IVAS_cod -dtx -sba +2 16400 48 20ms_aligned_files_concatenated.wav bit
eid-xor -vbr -fer bit error_pattern.192 bit.fer
./IVAS_dec BINAURAL 48 bit.fer syn.fer.wav

The problem is observed in both FLOAT and BASOP frameworks.

All relevant files are uploaded here: ..\Box\_EXTERNAL_IVAS_BASOP_VERIFICATION\issues\float-issue-1404

Note: This may not necessarily indicate a defect in the codec, but it would be good to confirm.