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Energy increase in higher frequencies for Mono outputs of stereo test vectors

There is an increased presence of higher frequencies for Mono output of 16kHz stereo input at bitrates lower than 48kbps.

To reproduce:

./IVAS_cod_ref.exe -stereo -dtx 24400 16 stvST16n.wav bit

./IVAS_dec.exe MONO 16 bit out.wav

The issue is present for both DTX and non-DTX cases.

Floating code output:

stvST16n_mono_24400_16-16_DTXfl

Fixed code output:

stvST16n_mono_24400_16-16_DTXfx

Frequency Analysis, where red line reflects to the floating code output and green line reflects to the fixed code output:

stvST16n_mono_24400_16-16_DTX_freq

Edited by Sumeyra Demir Kanik