Reverberator

Add reverberation to audio signal

• Library:

• Audio Toolbox / Effects

• 

Description

The Reverberator block adds reverberation to mono or stereo audio signals. You can tune parameters of the Reverberator block to mimic different acoustic environments.

Ports

Input

x — Input signal

matrix | 1-D vector

• Matrix input –– Each column of the input is treated as an independent channel.

• 1-D vector input –– The input is treated as a single channel.

This port is unnamed unless you specify additional input ports.

Data Types: single | double

Delay — Pre-delay for reverberation (s)

scalar in the range [0,1]

Dependencies

To enable this port, select Specify from input port for the Pre-delay (s) parameter.

Data Types: single | double

HighCut — Lowpass filter cutoff

positive scalar in the range [0, (Sample Rate)/2]

Dependencies

To enable this port, select Specify from input port for the Highcut frequency (Hz) parameter.

Data Types: single | double

Diffusion — Density of reverb tail

scalar in the range [0,1]

Dependencies

To enable this port, select Specify from input port for the Diffusion parameter.

Data Types: single | double

Decay — Decay factor of reverb tail

scalar in the range [0,1]

Dependencies

To enable this port, select Specify from input port for the Decay factor parameter.

Data Types: single | double

Damping — High-frequency damping

scalar in the range [0,1]

Dependencies

To enable this port, select Specify from input port for the High frequency damping parameter.

Data Types: single | double

WetDry — Ratio of wet (reverberated) signal to dry (original) signal

scalar in the range [0,1]

Dependencies

To enable this port, select Specify from input port for the Wet/dry mix parameter.

Data Types: single | double

Output

Port_1 — Output signal

matrix

The Reverberator block outputs a signal with the same data type as the input signal. The size of the output depends on the size of the input:

• Matrix input –– The block outputs a matrix of the same size and data type as the input signal.

• 1-D vector input –– The block outputs an N-by-1 matrix (column vector), where N is the number of elements in the 1-D vector.

Data Types: single | double

Parameters

If a parameter is listed as tunable, then you can change its value during simulation.

Pre-delay (s) — Pre-delay for reverberation

0 (default) | scalar in the range [0, 1]

Pre-delay for reverberation is the time between hearing direct sound and the first early reflection. The value of Pre-delay (s) is proportional to the size of the room being modeled.

To specify Pre-delay (s) from an input port, select Specify from input port for the parameter.

Tunable: Yes

Highcut frequency (Hz) — Lowpass filter cutoff

20000 (default) | scalar in the range [0, (Sample Rate)/2]

Lowpass filter cutoff is the –3 dB cutoff frequency for the single-pole lowpass filter at the front of the reverberator structure. It prevents the application of reverberation to high-frequency components of the input.

To specify Highcut frequency (Hz) from an input port, select Specify from input port for the parameter.

Tunable: Yes

Diffusion — Density of reverb tail

0.50 (default) | scalar in the range [0, 1]

Diffusion is proportional to the rate at which the reverb tail builds in density. Increasing Diffusion pushes the reflections closer together, thickening the sound. Reducing Diffusion creates more discrete echoes.

To specify Diffusion from an input port, select Specify from input port for the parameter.

Tunable: Yes

Decay factor — Decay factor of reverb tail

0.50 (default) | scalar in the range [0, 1]

Decay factor is proportional to the time it takes for reflections to run out of energy. To model a large room, use a long reverb tail (low decay factor). To model a small room, use a short reverb tail (high decay factor).

To specify Decay factor from an input port, select Specify from input port for the parameter.

Tunable: Yes

High frequency damping — High-frequency damping

0.0005 (default) | scalar in the range [0, 1]

High frequency damping is proportional to the attenuation of high frequencies in the reverberation output. Setting High frequency damping to a large value makes high-frequency reflections decay faster than low-frequency reflections.

To specify High frequency damping from an input port, select Specify from input port for the parameter.

Tunable: Yes

Wet/dry mix — Ratio of wet (reverberated) signal to dry (original) signal

0.3 (default) | scalar in the range [0, 1]

Wet/dry mix is the ratio of wet (reverberated) signal to dry (original) signal that your Reverberator block outputs.

To specify Wet/dry mix from an input port, select Specify from input port for the parameter.

Tunable: Yes

Inherit sample rate from input — Specify source of input sample rate

on (default) | off

When you select this parameter, the block inherits its sample rate from the input signal. When you clear this parameter, you specify the sample rate in Input sample rate (Hz).

Input sample rate (Hz) — Sample rate of input

44100 (default) | positive scalar

Tunable: Yes

Dependencies

To enable this parameter, clear the Inherit sample rate from input parameter.

Simulate using — Specify type of simulation to run

Interpreted execution (default) | Code generation

• Interpreted execution – Simulate the model using the MATLAB^® interpreter. This option reduces startup time and the simulation speed is comparable to Code generation. In this mode, you can debug the source code of the block.

• Code generation – Simulate the model using generated C code. The first time you run a simulation, Simulink^® generates C code for the block. The C code is reused for subsequent simulations, as long as the model does not change. This option requires additional startup time, but the speed of the subsequent simulations is comparable to Interpreted execution.

Model Examples

Mimic Acoustic Environments

Examine the Reverberator block in a Simulink® model and tune parameters. The reverberation parameters in this model mimic a large room with hard walls, such as a gymnasium.

Trigger Reverberation Parameters

Examine the Reverberator block in a Simulink® model where the reverberation parameters are triggered by time.

Block Characteristics

Data Types	double | single
Direct Feedthrough	no
Multidimensional Signals	no
Variable-Size Signals	yes
Zero-Crossing Detection	no

Algorithms

The algorithm to add reverberation follows the plate-class reverberation topology described in [1] and is based on a 29,761 Hz sample rate.

The algorithm has five stages.

The description for the algorithm that follows is for a stereo input. A mono input is a simplified case.

Stereo-to-Mono

A stereo signal is converted to a mono signal: $$x[n]=0.5\times \left(x_R[n]+x_L[n]\right)$$.

Preconditioning

A delay followed by a lowpass filter preconditions the mono signal.

• The pre-delay output is determined as $$x_p[n]=x[n-k]$$, where the Pre-delay (s) parameter determines the value of k.

• The signal is fed through a single-pole lowpass filter with transfer function

  $$LP(z)=\frac{1-\alpha }{1-\alpha z^{-1}},$$

  where

  $$\alpha =\exp \left(-2\pi \times \frac{f_c}{f_s}\right).$$

  • f_c is the cutoff frequency specified by the Pre-delay (s) parameter.

  • f_s is the sampling frequency specified by the Inherit sample rate from input parameter or the Input sample rate (Hz) parameter.

Decorrelation

The signal is decorrelated by passing through a series of four allpass filters.

The allpass filters are of the form

$$AP(z)=\frac{\beta +z^{-k}}{1+\beta z^{-k}},$$

where β is the coefficient specified by the Diffusion property and k is the delay as follows:

• For AP₁, k = 142.

• For AP₂, k = 107.

• For AP₃, k = 379.

• For AP₄, k = 277.

Tank

The signal is fed into the tank, where it circulates to simulate the decay of a reverberation tail.

The following description tracks the signal as it progresses through the top of the tank. The signal progression through the bottom of the tank follows the same pattern, with different delay specifications.

1. The new signal enters the top of the tank and is added to the circulated signal from the bottom of the tank.

2. The signal passes through a modulated allpass filter:

   $$ModulatedA{P}_1(z)=\frac{-\beta +z^{-k}}{1-\beta z^{-k}}$$

   • β is the coefficient specified by the Diffusion parameter.

   • k is the variable delay specified by a 1 Hz sinusoid with amplitude = (8/29761) × (sample rate). To account for fractional delay resulting from the modulating k, allpass interpolation is used [2].

3. The signal is delayed again, and then passes through a lowpass filter:

   $$L{P}_2(z)=\frac{1-\phi }{1-\phi z^{-1}}$$

   • φ is the coefficient specified by the High frequency damping parameter.

4. The signal is multiplied by a gain specified by the Decay factor parameter. The signal then passes through an allpass filter:

   $$A{P}_5(z)=\frac{\beta +z^{-k}}{1+\beta z^{-k}}.$$

   • β is the coefficient specified by the Diffusion parameter.

   • k is set to 1800 for the top of the tank and 2656 for the bottom of the tank.

5. The signal is delayed again and then circulated to the bottom half of the tank for the next iteration.

A similar pattern is executed in parallel for the bottom half of the tank. The output of the tank is calculated as the signed sum of delay lines picked off at various points from the tank. The summed output is multiplied by 0.6.

Wet/Dry Mix

The wet (processed) signal is then added to the dry (original) signal:

$$y_R[n]=\left(1-\kappa \right)x_R[n]+\kappa x_{3R}[n],$$

$$y_L[n]=\left(1-\kappa \right)x_L[n]+\kappa x_{3L}[n],$$

where the Wet/dry mix parameter determines κ.

References

[1] Dattorro, Jon. "Effect Design, Part 1: Reverberator and Other Filters." Journal of the Audio Engineering Society. Vol. 45, Issue 9, 1997, pp. 660–684.

[2] Dattorro, Jon. "Effect Design, Part 2: Delay-Line Modulation and Chorus." Journal of the Audio Engineering Society. Vol. 45, Issue 10, 1997, pp. 764–788.

Extended Capabilities

C/C++ Code Generation
Generate C and C++ code using Simulink® Coder™.