graphicEQ
Standards-based graphic equalizer
Description
The graphicEQ
System object™ implements a graphic equalizer that can tune the gain on individual octave or
fractional octave bands. The object filters the data independently across each input channel
over time using the filter specifications. Center and edge frequencies of the bands are based
on the ANSI S1.11-2004 standard.
To equalize an audio signal:
Create the
graphicEQobject and set its properties.Call the object with arguments, as if it were a function.
To learn more about how System objects work, see What Are System Objects?.
Creation
Description
equalizer = graphicEQ
equalizer = graphicEQ(Name,Value)Name to the specified Value.
Unspecified properties have default values.
equalizer = graphicEQ('Structure','Parallel','EQOrder','1/3
octave') creates a System object, equalizer, which implements filtering using a parallel
structure and one-third octave filter bandwidth.Properties
Usage
Description
Input Arguments
Output Arguments
Object Functions
To use an object function, specify the
System object as the first input argument. For
example, to release system resources of a System object named obj, use
this syntax:
release(obj)
The createAudioPluginClass and configureMIDI
functions map tunable properties of the graphicEQ
System object to user-facing parameters:
| Property | Range | Mapping | Unit |
|---|---|---|---|
Gains | [–20, 20] | linear | dB |
Examples
Perform Graphic Equalization
Create objects to read from an audio file and write to your audio device. Use the sample rate of the reader as the sample rate of the writer.
frameLength = 512; reader = dsp.AudioFileReader('RockDrums-48-stereo-11secs.mp3','SamplesPerFrame',frameLength); player = audioDeviceWriter('SampleRate',reader.SampleRate);
In an audio stream loop, read audio from a file and play the audio through your audio device.
while ~isDone(reader) x = reader(); player(x); end release(reader) release(player)
Create a one-octave graphic equalizer implemented with a cascade structure. Use the sample rate of the reader as the sample rate of the equalizer.
equalizer = graphicEQ( ... 'Bandwidth','1 octave', ... 'Structure','Cascade', ... 'SampleRate',reader.SampleRate);
Specify to increase the gain on low frequencies and then visualize the equalizer.
equalizer.Gains = [5,5,5,5,0,0,0,0,0,0]; visualize(equalizer)
In an audio stream loop, read audio from a file, apply equalization, and then play the equalized audio through your audio device.
while ~isDone(reader) x = reader(); y = equalizer(x); player(y); end release(reader) release(player)
Tune Graphic EQ Parameters
Create a dsp.AudioFileReader to read in audio frame-by-frame. Create an audioDeviceWriter to write audio to your sound card. Create a graphicEQ to process the audio data. Call visualize to plot the frequency response of the graphic equalizer.
frameLength = 1024; fileReader = dsp.AudioFileReader('RockDrums-44p1-stereo-11secs.mp3','SamplesPerFrame',frameLength); deviceWriter = audioDeviceWriter('SampleRate',fileReader.SampleRate); equalizer = graphicEQ('SampleRate',fileReader.SampleRate,'Gains',[0,10,-10,5,-5,2,-2,1,-1,0]); visualize(equalizer)
Call parameterTuner to open a UI to tune parameters of the equalizer while streaming.
parameterTuner(equalizer)
In an audio stream loop:
Read in a frame of audio from the file.
Apply equalization.
Write the frame of audio to your audio device for listening.
While streaming, tune parameters of the equalizer and listen to the effect.
while ~isDone(fileReader) audioIn = fileReader(); audioOut = equalizer(audioIn); deviceWriter(audioOut); drawnow limitrate % required to update parameter end
As a best practice, release your objects once done.
release(deviceWriter) release(fileReader) release(equalizer)
Algorithms
The implementation of your graphic equalizer depends on the Structure
property. See Graphic Equalization for a discussion of the pros and cons of the parallel
and cascade implementations. Refer to the following sections to understand how these
algorithms are implemented in Audio Toolbox™.
Parallel Structure
The parallel implementation designs the individual equalizers using the octaveFilter design
method and spaces them on the spectrum according to the ANSI S1.11-2004
standard.
If you set the SampleRate
property so that the Nyquist frequency
(SampleRate/2
The final bandpass filter is the one whose upper bandpass edge is less than the Nyquist frequency.
The final filter is implemented as a highpass filter designed by the
designParamEQfunction.
The input signal is fed into a filterbank of M filters, where M depends on the specified
BandwidthandSampleRateproperties.Each branch of the filterbank is multiplied by the linear form of the corresponding element of the
Gainsproperty.The branches are summed and the output signal is returned.
Cascade Structure
The cascade implementation designs the graphic equalizer filter bank using the multibandParametricEQ
System object.
If the EQOrder
property is set to 2, then a gain correction is calculated according to [1]. The gain correction is
independent of the requested gains. The gain correction is recomputed during the real-time
processing only if the SampleRate property is modified.
If the EQOrder property is not set to 2, no gain correction is
applied, and the requested gains are passed on to the
multibandParametricEQ object.
The input signal is fed into a cascade of M biquad filters, where
M depends on the specified Bandwidth and
SampleRate properties.
References
[1] Oliver, Richard J., and Jean-Marc Jot. "Efficient Multi-Band Digital Audio Graphic Equalizer with Accurate Frequency Response Control." Presented at the 139th Convention of the AES, New York, October 2015.
[2] Acoustical Society of America. American National Standard Specification for Octave-Band and Fractional-Octave-Band Analog and Digital Filters. ANSI S1.11-2004. Melville, NY: Acoustical Society of America, 2009.
[3] International Organization for Standardization. Acoustics –– Preferred frequencies. ISO 266:1997(E). Second Edition. 1997.