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Synthesis quality issues for multi channels at low bit rate [13.2 to 32 kbps]

While looking into #817 (closed), I realise the synthesis is of a really bad quality (with af8cb9bd). Looking a bit more into it, the synthesis after ivas_sce_dec() is pretty much fine while the final synthesis suffers from framing like artefact plus wrong gains/tilts on all channels.

In the figure below, from top to bottom:

  • Floating point synthesis at the end of ivas_sce_dec()
  • Fixed point synthesis at the end of ivas_sce_dec()
  • Difference between float and fixed (± 60 of amplitude, compared to ±7000 for the synthesis on these frame)
  • spectrum of float, fix and difference

Except of a slightly high noise floor for the HF, nothing out of ordinary there

image

In the next figure, again from top to bottom, the 5.1 configuration :

  • Floating point left channel synthesis
  • Fixed point left channel synthesis
  • Difference between float and fixed (± 3000 of amplitude, compared to ±5000 for the synthesis on these frame)
  • spectrum of float, fix and difference

As you can see, the difference includes a lot of "spikes" which probably correspond to the "framing like" artifact heard. Also the spectrum is showing the different tilt. The fixed point LF being systematically too high while HF being systematically too low

image

The problem is illustrated for the left channel but present in all channel but the LFE

The bitstream used is the following 817_MC51_13200_48.bit

Command:
./IVAS_dec 5_1 48 817_MC51_13200_48.bit ltv48_MC51.wav_MC51_13200_48-48_5_1.wav