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Static Scaling for Downmixes

As discussed on in https://forge.3gpp.org/rep/ivas-codec-pc/ivas-codec/-/wikis/Meetings/Reports/2024-08-22%20Meeting%20Report:

Try Option 1:

  • Follow an approach of downscaling by constant factor (per configuration) and application of limiter afterwards; apply the factor when calculating the DMX, inverse factor when calculating the Upmix
  • Implement first in floating-point code, verify that all operation modes still work
  • then port to fixed-point code, evaluate potential quality degradation there
  • Do different scaling factors introduce an issue for bitrate switching? If bitrate switching between different technologies/configurations triggers state reset, then it shouldn't be an issue. Needs to be checked (also for AGC).

Affected technologies:

  • McMASA
  • ParamMC
  • ParamISM
  • SBA
  • OSBA
  • OMASA

I would propose to implement the scaling factors where the DMX are created and revert them again where the Upmix happens.

Please use for the moment different switches for the different technologies - maybe we can harmonize them later!