Convolution reverb
Encyclopedia
In audio signal processing, convolution reverb is a process for digitally simulating the reverberation
of a physical or virtual space. It is based on the mathematical convolution
operation, and uses a pre-recorded audio sample of the impulse response
of the space being modelled. To apply the reverberation effect, the impulse-response recording is first stored in a digital signal-processing
system. This is then convolved
with the incoming audio signal to be processed. The process of convolution multiplies each sample of the audio to be processed (reverberated) with the samples in the impulse response file.
is a recording of the reverberation that is caused by an acoustic space when an ideal impulse
is played. However, an ideal impulse is a mathematical construct, and cannot exist in reality, as it would have to be infinitesimal
ly narrow in time. Therefore, approximations have to be used: the sound of an electric spark, starter pistol shot or the bursting of a balloon, for instance. A recording of this approximated ideal impulse may be used directly as an impulse response. Techniques involving starter pistols and balloons are sometimes referred to as transient methods, and the response is contained at the beginning of the recording in an impulse.
Another technique, referred to as the sine sweep method, covers the entire audible frequency range, which can result in a broader-range, and higher-quality, impulse response. This involves the use of a longer sound to excite a space (typically a sine sweep), which is then put through a process of deconvolution
to produce an impulse response. This approach has the advantage that such sounds are less susceptible to distortion; however, it requires more sophisticated processing to produce a usable impulse response.
A third approach involves using maximum-length sequences
, but this is difficult in practice because such sequences are highly susceptible to distortion.
The impulse response of a system is equal to the inverse Fourier Transform
of the cross-correlation of the output of the system with the auto-correlation of the input to the system. For example, to sample the acoustic properties of a larger space such as a small church or cathedral, the space can simply be excited using white noise, with the result recorded both near the source, and somewhere else in the space. The coefficients of a finite impulse response
can then be generated using the mathematical approach mentioned above.
example of capturing an impulse response would be to set up a microphone
in a concert hall and to place the microphone in the centre of the auditorium. Next, produce a very brief pulse (often an electric spark) of sound, and record everything that the microphone picks up, which includes both the original sound and the response of the room to it. The recorded take would then be cleanly edited and loaded into the convolution processor. This convolution can be applied as part of a signal processing chain.
Reverberation
Reverberation is the persistence of sound in a particular space after the original sound is removed. A reverberation, or reverb, is created when a sound is produced in an enclosed space causing a large number of echoes to build up and then slowly decay as the sound is absorbed by the walls and air...
of a physical or virtual space. It is based on the mathematical convolution
Convolution
In mathematics and, in particular, functional analysis, convolution is a mathematical operation on two functions f and g, producing a third function that is typically viewed as a modified version of one of the original functions. Convolution is similar to cross-correlation...
operation, and uses a pre-recorded audio sample of the impulse response
Impulse response
In signal processing, the impulse response, or impulse response function , of a dynamic system is its output when presented with a brief input signal, called an impulse. More generally, an impulse response refers to the reaction of any dynamic system in response to some external change...
of the space being modelled. To apply the reverberation effect, the impulse-response recording is first stored in a digital signal-processing
Digital signal processor
A digital signal processor is a specialized microprocessor with an architecture optimized for the fast operational needs of digital signal processing.-Typical characteristics:...
system. This is then convolved
Convolution
In mathematics and, in particular, functional analysis, convolution is a mathematical operation on two functions f and g, producing a third function that is typically viewed as a modified version of one of the original functions. Convolution is similar to cross-correlation...
with the incoming audio signal to be processed. The process of convolution multiplies each sample of the audio to be processed (reverberated) with the samples in the impulse response file.
Creation of impulse responses
An impulse responseImpulse response
In signal processing, the impulse response, or impulse response function , of a dynamic system is its output when presented with a brief input signal, called an impulse. More generally, an impulse response refers to the reaction of any dynamic system in response to some external change...
is a recording of the reverberation that is caused by an acoustic space when an ideal impulse
Dirac delta function
The Dirac delta function, or δ function, is a generalized function depending on a real parameter such that it is zero for all values of the parameter except when the parameter is zero, and its integral over the parameter from −∞ to ∞ is equal to one. It was introduced by theoretical...
is played. However, an ideal impulse is a mathematical construct, and cannot exist in reality, as it would have to be infinitesimal
Infinitesimal
Infinitesimals have been used to express the idea of objects so small that there is no way to see them or to measure them. The word infinitesimal comes from a 17th century Modern Latin coinage infinitesimus, which originally referred to the "infinite-th" item in a series.In common speech, an...
ly narrow in time. Therefore, approximations have to be used: the sound of an electric spark, starter pistol shot or the bursting of a balloon, for instance. A recording of this approximated ideal impulse may be used directly as an impulse response. Techniques involving starter pistols and balloons are sometimes referred to as transient methods, and the response is contained at the beginning of the recording in an impulse.
Another technique, referred to as the sine sweep method, covers the entire audible frequency range, which can result in a broader-range, and higher-quality, impulse response. This involves the use of a longer sound to excite a space (typically a sine sweep), which is then put through a process of deconvolution
Deconvolution
In mathematics, deconvolution is an algorithm-based process used to reverse the effects of convolution on recorded data. The concept of deconvolution is widely used in the techniques of signal processing and image processing...
to produce an impulse response. This approach has the advantage that such sounds are less susceptible to distortion; however, it requires more sophisticated processing to produce a usable impulse response.
A third approach involves using maximum-length sequences
Maximum length sequence
A maximum length sequence is a type of pseudorandom binary sequence.They are bit sequences generated using maximal linear feedback shift registers and are so called because they are periodic and reproduce every binary sequence that can be reproduced by the shift registers...
, but this is difficult in practice because such sequences are highly susceptible to distortion.
The impulse response of a system is equal to the inverse Fourier Transform
Fourier transform
In mathematics, Fourier analysis is a subject area which grew from the study of Fourier series. The subject began with the study of the way general functions may be represented by sums of simpler trigonometric functions...
of the cross-correlation of the output of the system with the auto-correlation of the input to the system. For example, to sample the acoustic properties of a larger space such as a small church or cathedral, the space can simply be excited using white noise, with the result recorded both near the source, and somewhere else in the space. The coefficients of a finite impulse response
Finite impulse response
A finite impulse response filter is a type of a signal processing filter whose impulse response is of finite duration, because it settles to zero in finite time. This is in contrast to infinite impulse response filters, which have internal feedback and may continue to respond indefinitely...
can then be generated using the mathematical approach mentioned above.
Real space simulation
The primary goal of a convolution reverb is to sample real spaces, in order to simulate the acoustics of the sampled space. A straightforward and simple monoMonaural
Monaural or monophonic sound reproduction is single-channel. Typically there is only one microphone, one loudspeaker, or channels are fed from a common signal path...
example of capturing an impulse response would be to set up a microphone
Microphone
A microphone is an acoustic-to-electric transducer or sensor that converts sound into an electrical signal. In 1877, Emile Berliner invented the first microphone used as a telephone voice transmitter...
in a concert hall and to place the microphone in the centre of the auditorium. Next, produce a very brief pulse (often an electric spark) of sound, and record everything that the microphone picks up, which includes both the original sound and the response of the room to it. The recorded take would then be cleanly edited and loaded into the convolution processor. This convolution can be applied as part of a signal processing chain.
Machine simulation
It is also possible to sample the impulse response of a reverberation unit, instead of sampling a real space. Thus, it is possible to use a convolution reverb in place of a hardware machine. The techniques used to sample a reverberation unit are the same as the ones used to sample real spaces.External links
- Acoustical Impulse Response Measurement with ALIKI: Includes comparison of methods for creating impulse responses
- Examples of sampled impulse responses to be used in convolution reverbs (Fokke Van Saane)
- Examples of impulse responses synthesized from oscillator spectra, to be used in convolution reverbs (Emmanuel Deruty)
- Impulse Response Library Open repository for free impulse responses covering everything from rooms to speaker cabs
- Freeverb3 Impulse Response Processor: Opensource zero latency impulse response processor with VST plugins
- A database of measured room impulse responses to generate realistic reverberation effects
- Free reverb impulse responses (Samplicity's Bricasti M7 library)
- Vienna MIR - Multi Impulse Response Mixing and Reverberation Engine by Vienna Symphonic Library (VSL): Three-dimensional virtual halls based on multi-sampled impulse responses
- Xenium Audio XR-1 - VST zero latency convolution processor: With Impulse Response Library obtained by 3D auralization technique.