Linux Studio Plugins Project

# Begrenzer Stereo

Detailed: LSP Limiter Stereo - Begrenzer Stereo (B1S)

Formats: LADSPA, LV2, LinuxVST, JACK

Categories: Limiter

Developer: Vladimir Sadovnikov

Description:

This plugin implements a brick-wall limiter with flexible configuration. It prevents input monosignal from raising over the specified Threshold.

Attention: this plugin implements set of limiting modes, most of them are iterative. That means that CPU load may be not stable, in other words: the more work should be done, the more CPU resources will be used. Beware from extreme settings.

The short description how does the limiter work can be explained with this iterative algorithm:

1. Initializes gain reduction curve as all samples are reduced by 0 dB
2. Detects peaks of the input signal that was adjusted according to the gain reduction curve
3. If there are no peaks, it finally applies gain reduction curve to the signal and completes processing of the signal
4. Eliminates peaks by applying patch to the gain reduction curve
5. Repeats step 2

Simplified peak processing example is shown on the following picture:

Of course, the output signal does not repeat the envelope form of input signal because it's amplitude is changed smoothly, so actually the form of output signal is more complicated.

Currently there are three forms of patches applied to the gain curve - hermite (using cubic polynom for interpolation transients), exponential and linear. These forms can be explained with following picture:

Gain reduction patch affects not only the peak sample, but also surrounding samples. The position and form of this interpolation is related to the peak, so there are four different variants of patch envelope - thin, tail, duck and wide. All these forms related to the peak are shown on the following picture:

On this image, sloping lines mean the transision part of the patch. The flat cap in the middle before the peak is a half of attack time, the flat cap in the middle after the peak is a half of release time. Also it's obvious that different envelope forms differently affect dynamics of the signal.

Controls:

- Bypass - bypass switch, when turned on (led indicator is shining), the plugin bypasses signal.
- Pause - pauses any updates of the limiter graph.
- Clear - clears all graphs.
- Mode - the selected mode to use by limiter:

- Classic - limiter works like compressor with infinite ratio. May cause sound artifacts and clicking on extreme setup.
- Herm Thin, Herm Wide, Herm Tail, Herm Duck - hermite-interpolated cubic functions are used to apply gain reduction.
- Exp Thin, Exp Wide, Exp Tail, Exp Duck - exponent-interpolated functions are used to apply gain reduction.
- Line Thin, Line Wide, Line Tail, Line Duck - linear-interpolated functions are used to apply gain reduction.
- Mixed Herm, Mixed Exp, Mixed Line - classic mode with accurate overloaded peak removal by linear-interpolated, exponent-interpolated and linear-interpolated functions.

- O/S - oversampling mode:

- None - oversampling is not used.
- Half 2x(2L), Half 2x(3L) - 2x Lanczos oversampling of Sidechain signal with 2 or 3 lobes (L) in the kernel.
- Half 3x(2L), Half 3x(3L) - 3x Lanczos oversampling of Sidechain signal with 2 or 3 lobes (L) in the kernel.
- Half 4x(2L), Half 4x(3L) - 4x Lanczos oversampling of Sidechain signal with 2 or 3 lobes (L) in the kernel.
- Half 6x(2L), Half 6x(3L) - 6x Lanczos oversampling of Sidechain signal with 2 or 3 lobes (L) in the kernel.
- Half 8x(2L), Half 8x(3L) - 8x Lanczos oversampling of Sidechain signal with 2 or 3 lobes (L) in the kernel.
- Full 2x(2L), Full 2x(3L) - 2x Lanczos oversampling of Sidechain and Input signal with 2 or 3 lobes (L) in the kernel.
- Full 3x(2L), Full 3x(3L) - 3x Lanczos oversampling of Sidechain and Input signal with 2 or 3 lobes (L) in the kernel.
- Full 4x(2L), Full 4x(3L) - 4x Lanczos oversampling of Sidechain and Input signal with 2 or 3 lobes (L) in the kernel.
- Full 6x(2L), Full 6x(3L) - 6x Lanczos oversampling of Sidechain and Input signal with 2 or 3 lobes (L) in the kernel.
- Full 8x(2L), Full 8x(3L) - 8x Lanczos oversampling of Sidechain and Input signal with 2 or 3 lobes (L) in the kernel.

- Dither - allows to enable dithering for the specified sample bitness.
- SC - enables drawing of sidechain input graph and corresponding level meter.
- Gain - enables drawing of gain amplification line and corresponding amplification meter.
- In - enables drawing of limiter's input signal graph and corresponding level meter.
- Out - enables drawing of limiter's output signal graph and corresponding level meter.

'Limiter' section:

- SC Preamp - sidechain pre-amplification gain.
- Lookahead - the size of lookahead buffer in milliseconds. Forces the limiter to add the corresponding latency to output signal.
- Threshold - the maximum input level of the signal allowed by limiter.
- Boost - applies corresponding to the Threshold gain to the output signal.
- Attack - the attack time of the limiter. Can not be greater than Lookahead time (greater values are truncated) for some modes.
- Release - the attack time of the limiter. Can not be twice greater than Lookahead time (greater values are truncated) for some modes.
- Knee - additional knee applied to the gain reduction algorithm.

'Signal' section:

- Input - overall input gain.
- Output - overall output gain.