This is an audio limiter, a tool to restrict audio signals to a specific range (typically -1 to 1) while reducing perceived distortion.

This audio limiter works by filling a delay line with samples of the incoming signal. This delay line is searched for peaks that exceed the range even after being multiplied by the attenuation signal. If a peak is found, the attenuation signal is adjusted to make that peak lie within the range. The attenuation signal is the convolution of impulses placed some samples before the signal value to be attenuated and a signal consisting of half of a Hann window for the ramp up, and an exponential decay of an infinite impulse-response (IIR) filter for the ramp down. In this way, the attenuation signal can be updated efficiently for future values (simply by computing the output of the filter).

There is also a very simple noise gate. This simply makes the signal 0 by multiplying with an attenuation signal if the power as estimated over a local window is less than some threshold. There is some fading done to smooth the attenuation signal.

Some optimization has been done to use this on the ARM Cortex-M4 architecture. See the liblimiter_ir_af.a recipe in arm_cm4.mk for a command line that will compile the limiter into a library for this architecture.

You can also compile a simple program that will limit little-endian float-32 encoded files. See the bin/limit_mono_f32 recipe of tools/Makefile

To compile a simple program that applies a noise gate to a little-endian float-32 encoded file, see the bin/noise_gate_mono_f32 recipe of tools/Makefile.

All source code in this repository is Copyright (c) 2019 Nicholas Esterer.

All source code in this repository is covered under the GNU General Public License V3 (https://www.gnu.org/licenses/gpl-3.0.en.html).

Please contact nicholas dot esterer at gmail dot com for other licensing.

This software comes with absolutely no warranty.