Add Reverberation Using Freeverb Algorithm

This example shows how to apply reverberation to audio by using the Freeverb reverberation algorithm. The reverberation can be tuned using a user interface (UI) in MATLAB or through a MIDI controller. This example illustrates MATLAB® and Simulink® implementations.

Introduction

Reverberators are used to add the effect of multiple decaying echoes, or reverbs, to audio signals. A common use of reverberation is to simulate music played in a closed room. Most digital audio workstations (DAWs) have options to add such effects to the sound track.

In this example, you add reverberation to audio through the Freeverb algorithm. Freeverb is a popular implementation of the Schroeder reverberator. A high-level model of the Freeverb algorithm is shown below:

Example Architecture

The reverberator is implemented in the System object audioexample.FreeverbReverberator. The object has five properties that can be tuned while the simulation is running: RoomSize, StereoWidth, WetDryMix, Balance, and Volume. RoomSize affects the feedback gain of the comb filters. StereoWidth and WetDryMix both take part in the mixing stage that happens after filtering is complete. The default values of the StereoSpread, CombDelayLength, and AllpassDelayLength properties are taken from the Freeverb specifications.

MATLAB Simulation

To use the reverberator on an audio signal, run audioFreeverbReverberationExampleApp.

audioFreeverbReverberationExampleApp

The audioFreeverbReverberationExampleApp command first sets up the audio source and player. It then iteratively calls the audioexample.FreeverbReverberator System object with the audio input, providing addition of reverberation in a streaming fashion. The output of the object is played back so you can hear the effect added to the audio.

The simulation opens a UI to interact with audioexample.FreeverbReverberator while the simulation is running. The UI allows you to tune parameters and the results are reflected in the simulation instantly. For example, moving the slider Room size to the left while the simulation is running decreases the reflectivity of the walls of the room being simulated.

There are also three buttons on the UI - the Reset button will reset the states of the comb and allpass sections in reverberator to their initial values and the Pause Simulation button will hold the simulation until you click on it again. The simulation may be terminated by either closing the UI or by clicking on the Stop simulation button. If you have a MIDI controller, it is possible to synchronize it with the UI. You can do this by choosing a MIDI control in the dialog that is opened when you right-click on the sliders or buttons and select "Synchronize" from the context menu. The chosen MIDI control then works in accordance with the slider or button so that operating one control is tracked by the other.

If you see a lot of queue underrun warnings, you will need to adjust the buffer and queue size of audio player used in audioFreeverbReverberationExampleApp. More information on this can be found at the documentation page for audioDeviceWriter. The audio source in this example is an audio file, but you can replace it with an audio input device (through audioDeviceReader) to add reverberation to live audio. For ways to reduce latency while not having any overruns/underruns, you can follow the example Measure Audio Latency.

Using a Generated MEX File

Using MATLAB Coder™, you can generate a MEX file for the main processing algorithm by executing the HelperFreeverbCodeGeneration command. You can use the generated MEX file by executing the audioFreeverbReverberationExampleApp command with true as an argument.

audioFreeverbReverberationExampleApp(true)

Simulink Version

audiofreeverbreverberation is a Simulink model that implements the same Freeverb reverberation example highlighted in the previous sections.

In this model, the addition of reverberation is modeled using the audioexample.FreeverbReverberator System object used inside a MATLAB System block. Using the MATLAB System block saves you the effort of reimplementing a MATLAB algorithm in Simulink. You can open the UI to tune Freeverb parameters by clicking the 'Launch Parameter Tuning UI' link on the model.

The model generates code when it is simulated. Therefore, it must be executed from a folder with write permissions.

Acknowledgement

The algorithm in this example is based on the public domain 'Freeverb' model written by Jezar at Dreampoint (June 2000).

Reference

Smith, J.O. "Freeverb", in "Physical Audio Signal Processing", https://ccrma.stanford.edu/~jos/pasp/Freeverb.html, online book, 2010 edition, accessed April 24, 2014.