Reverb Algorithm

Question

I'm looking for a simple or commented reverb algorithm, even in pseudocode would help a lot.

I've found a couple, but the code tends to be rather esoteric and hard to follow.

Answer 1

Here is a very simple implementation of a "delay line" which will produce a reverb effect in an existing array (C#, buffer is short[]):

int delayMilliseconds = 500; // half a second
int delaySamples = 
    (int)((float)delayMilliseconds * 44.1f); // assumes 44100 Hz sample rate
float decay = 0.5f;
for (int i = 0; i < buffer.length - delaySamples; i++)
{
    // WARNING: overflow potential
    buffer[i + delaySamples] += (short)((float)buffer[i] * decay);
}

Basically, you take the value of each sample, multiply it by the decay parameter and add the result to the value in the buffer delaySamples away.

This will produce a true "reverb" effect, as each sound will be heard multiple times with declining amplitude. To get a simpler echo effect (where each sound is repeated only once) you use basically the same code, only run the for loop in reverse.

Update: the word "reverb" in this context has two common usages. My code sample above produces a classic reverb effect common in cartoons, whereas in a musical application the term is used to mean reverberation, or more generally the creation of artificial spatial effects.

A big reason the literature on reverberation is so difficult to understand is that creating a good spatial effect requires much more complicated algorithms than my sample method here. However, most electronic spatial effects are built up using multiple delay lines, so this sample hopefully illustrates the basics of what's going on. To produce a really good effect, you can (or should) also muddy the reverb's output using FFT or even simple blurring.

Update 2: Here are a few tips for multiple-delay-line reverb design:

• Choose delay values that won't positively interfere with each other (in the wave sense). For example, if you have one delay at 500ms and a second at 250ms, there will be many spots that have echoes from both lines, producing an unrealistic effect. It's common to multiply a base delay by different prime numbers in order to help ensure that this overlap doesn't happen.

• In a large room (in the real world), when you make a noise you will tend to hear a few immediate (a few milliseconds) sharp echoes that are relatively undistorted, followed by a larger, fainter "cloud" of echoes. You can achieve this effect cheaply by using a few backwards-running delay lines to create the initial echoes and a few full reverb lines plus some blurring to create the "cloud".

• The absolute best trick (and I almost feel like I don't want to give this one up, but what the hell) only works if your audio is stereo. If you slightly vary the parameters of your delay lines between the left and right channels (e.g. 490ms for the left channel and 513ms for the right, or .273 decay for the left and .2631 for the right), you'll produce a much more realistic-sounding reverb.

Answer 2

Digital reverbs generally come in two flavors.

• Convolution Reverbs convolve an impulse response and a input signal. The impulse response is often a recording of a real room or other reverberation source. The character of the reverb is defined by the impulse response. As such, convolution reverbs usually provide limited means of adjusting the reverb character.

• Algorithmic Reverbs mimic reverb with a network of delays, filters and feedback. Different schemes will combine these basic building blocks in different ways. Much of the art is in knowing how to tune the network. Algorithmic reverbs usually expose several parameters to the end user so the reverb character can be adjusted to suit.

The A Bit About Reverb post at EarLevel is a great introduction to the subject. It explains the differences between convolution and algorithmic reverbs and shows some details on how each might be implemented.

Physical Audio Signal Processing by Julius O. Smith has a chapter on reverb algorithms, including a section dedicated to the Freeverb algorithm. Skimming over that might help when searching for some source code examples.

Sean Costello's Valhalla blog is full of interesting reverb tidbits.

Answer 3

What you need is the impulse response of the room or reverb chamber which you want to model or simulate. The full impulse response will include all the multiple and multi-path echos. The length of the impulse response will be roughly equal to the length of time (in samples) it takes for an impulse sound to completely decay below audible threshold or given noise floor.

Given an impulse vector of length N, you could produce an audio output sample by vector multiplication of the input vector (made up of the current audio input sample concatenated with the previous N-1 input samples) by the impulse vector, with appropriate scaling.

Some people simplify this by assuming most taps (down to all but 1) in the impulse response are zero, and just using a few scaled delay lines for the remaining echos which are then added into the output.

For even more realistic reverb, you might want to use different impulse responses for each ear, and have the response vary a bit with head position. A head movement of as little as a quarter inch might vary the position of peaks in the impulse response by 1 sample (at 44.1k rates).

Answer 4

You can use GVerb. Get the code from here.GVerb is a LADSPA plug-in, you can go here if you want to know something about LADSPA.

Here is the wiki for GVerb , including explaining of the parameters and some instant reverb settings.

Also we can use it directly in Objc:

ty_gverb        *_verb;
_verb = gverb_new(16000.f, 41.f, 40.0f, 7.0f, 0.5f, 0.5f, 0.5f, 0.5f, 0.5f);
AudioSampleType *samples = (AudioSampleType*)dataBuffer.mBuffers[0].mData;//Audio Data from AudioUnit Render or ExtAuidoFileReader
float lval,rval;
for (int i = 0; i< fileLengthFrames; i++) {
     float value = (float)samples[i] / 32768.f;//from SInt16 to float
     gverb_do(_verb, value, &lval, &rval);
     samples[i] =  (SInt16)(lval * 32767.f);//float to SInt16
}

GVerb is a mono effect but if you want a stereo effect you could run each channel through the effect separately and then pan and mix the processed signals with the dry signals as required