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Surround Upmixing

SotF’s upmixer converts stereo audio into surround sound by decomposing the stereo signal into direct (phantom center) and ambient (diffuse) components using FFT analysis, then distributing them across surround channels using VBAP (Vector Base Amplitude Panning).

What you need

Section titled “What you need”
  • SotF (Terminal or Desktop)
  • A surround speaker setup (5.0, 5.1, or 7.1)
  • Your audio device configured for the correct number of output channels

Walkthrough

Section titled “Walkthrough”

  1. Set your output to surround

    In the Devices screen (or Settings), select your surround audio device. Verify it’s configured for 5.0, 5.1, or 7.1 channels in your OS audio settings.

  2. Add the Upmixer plugin

    Go to the plugin rack and add an Upmixer plugin.

  3. Configure output layout

    Set the target channel count to match your speaker layout:

    • 5 channels (5.0): L, R, C, LS, RS
    • 6 channels (5.1): L, R, C, LFE, LS, RS
    • 8 channels (7.1): L, R, C, LFE, LS, RS, LB, RB

  4. Adjust the spatial parameters

    ParameterWhat it doesSuggested range
    Center gainLevel of the extracted center channel-3 to 0 dB
    Surround gainLevel of the ambient/surround channels-6 to -3 dB
    LFE gainLow frequency effects channel level-10 to -6 dB
    FFT sizeAnalysis resolution (larger = better separation, more latency)4096 or 8192
    Ambient thresholdSensitivity of direct/ambient separation0.3 to 0.7

  5. Play stereo music

    The upmixer processes in real-time. Center vocals appear in the center channel, ambient/reverb content fills the surrounds.

How it works

Section titled “How it works”

The upmixer uses a three-stage process:

  1. FFT Analysis: The stereo signal is transformed to the frequency domain
  2. Direct/Ambient Decomposition: Each frequency bin is classified as “direct” (correlated between L and R, typically center-panned content) or “ambient” (decorrelated, typically reverb and spatial content)
  3. VBAP Rendering: Direct content is panned to the center speaker, ambient content is distributed to surround speakers, and the original L/R content is preserved in the front channels

Channel conflict handling

Section titled “Channel conflict handling”

The upmixer changes the channel count from 2 to 5+ channels. Any plugins after the upmixer must handle the new channel count. If a downstream plugin only supports stereo, SotF will warn you about the channel conflict.

Best practice: Place the upmixer as one of the last plugins in the chain. Apply EQ and dynamics processing on the stereo signal first, then upmix.

Tips

Section titled “Tips”
Section titled “Related”