The Auditory front-end framework has been designed in such a way that it can be easily upgraded. To add a new processor, write its class definition in a new .m file and add it to the /src/Processors folder. If correctly written, the processor should be automatically detected by the framework and be ready to use. This section documents in details how to correctly write the class definition of a new processor. It is highly recommended to look into the definition of existing processors to get a grasp of how classes are defined and written in Matlab. In the following, we will sometimes refer to a particular existing processor to illustrate some aspects of the implementation.

Note

• The following descriptions are exhaustive, and adding a processor to the framework is actually easier than the length of this page suggests!
• This tutorial is written assuming limited knowledge about object- oriented programming using Matlab. Hence most OOP concepts involved are briefly explained.
• You can base your implementation on the available templateProc.m file which contains a pre-populated list of properties and methods. Simply copy the file, rename it to your processor name, and follow the instructions.

## Check-list for adding a new processor¶

To write the class definition for a new processor such that it will be recognised and properly integrated, one has to follow these steps:

## Getting started and setting up processor properties¶

The properties of an object are a way to store data used by the object. There are two types of properties for processors, those which:

• store all the parameters needed to integrate the processor into the framework (e.g., the sampling frequency on which it operates, the number of inputs/outputs, ...)
• store parameter values which are used in the actual processing

When writing the class definition for a new processor, it is only necessary to implement the latter: parameters which are needed in the computation. All parameters needed for the integration of the processor in the framework are already defined in the parent Processor class. Your new processor should inherit this parent class in order to automatically have access to the properties and methods of the parent class. Inheritance in Matlab is indicated by the command < nameOfParentClass following the name of your new class in the first line of its definition.

The new processor class definition should be saved in a .m file that has the same name as the defined class. In the example below, that would be myNewProcessor.m.

There are usually two categories of properties to be implemented for a new processor: external (user-controlled) parameters and internal parameters necessary for the processor but which do not need to be known to the “outside world”.

Note

Only the two types of properties below have been used so far in every processor implementation. However, it is fine to add more if needed for your new processor.

### External parameters controllable by the user¶

External parameters are directly related to the parameters the user can access and change. The actual values for these are stored in a specific object accessible via the .parameters property of the processor. Defining them as individual properties seems redundant, and is therefore optional. However it can be very convenient in order to simplify the access to the parameter value and to make your code more readable.

Instead of storing an actual value, the corresponding processor property should only point to a value in the .parameters object. This will avoid having two different values for the same parameter. To do this, external parameters should be defined as a set of dependent properties. This is indicated by the Dependent = true property attribute. If a property is set to Dependent, then a corresponding “getter” method has to be implemented for it. This will be developed in a following section. For example, if your new processor has two parameters, parA and parB, you can define these as properties as follow:

classdef myNewProcessor < Processor

properties (Dependent = true)
parA;
parB;
end

%...

end


This will allow easier access to these values in your code. For example, myNewProcessor.parA will always give the same output as myNewProcessor.parameters.map('xx_nameTagOfParameterA'), even if the parameter value changes due to feedback. This simplifies greatly the code, particularly when many parameters are involved.

### Internal parameters¶

Internal parameters are sometimes (not always) needed for the functioning of the processor. They are typically used to store internal states of the processor (e.g., to allow continuity in block-based processing), filter instances (if your processor involves filtering), or just intermediate parameter values used to make code more readable.

Because they are “internal” to the processor, these parameters are usually stored as a set of private properties by using the GetAccess = private property attributes. This will virtually make the property invisible and inaccessible to all other objects.

## Implementing static methods¶

Static methods are methods that can be called without an existing instance of an object. In the implementation of processors, they are used to store all the hard-coded information. This can be for example the processor name, the type of signal it accepts as input, or the names and default values of its external parameters. A static method is implemented by defining it in a method block with the (Static) method attribute:

classdef myNewProcessor < Processor

% ... Properties and other methods definition

methods (Static)

function out = myStaticMethod_1(in)
%...
end

function out = myStaticMethod_2(in)
%...
end

end

end


Static methods share the same structure and names across processors, so they can easily be copy/pasted from an existing processor and then modified to reflect your new processor. The following three methods have to be implemented.

• .getDependency(): Returns the type of input signal by its user request name
• .getParameterInfo(): Returns names, default values, and descriptions of external parameters
• .getProcessorInfo(): Returns information about the processor as a Matlab structure

As they are used to hard-code and return information, none of these methods accept input arguments.

### getDependency¶

This method returns the type of input signal your processor should accept:

function name = getDependency()
name = 'requestNameOfInputSignal';
end


where 'requestNameOfInputSignal' is the request name of the signal that should be used as input. “Request name” corresponds to the request a user would place in order to obtain a particular signal. For example, the inner hair-cell envelope processor requires as input the output of e.g., a gammatone filterbank. The request name for this signal is 'filterbank' which should therefore be the output of the static method ihcProc.getDependency(). You can also check the list of currently valid request names by typing requestList in Matlab’s command window.

If you are unsure about which name should be used, consider which processor would come directly before your new processor in a processing chain (i.e., the processor your new processor depends on). Say it is named dependentProc. Then typing:

dependentProc.getProcessorInfo.requestName


in Matlab’s command window will return the corresponding request name you should output in your getDependency method.

### getParameterInfo¶

This method hard-codes all information regarding the (external) parameters used by your processor, i.e., lists of their names, default values, and description. These are used to populate the output of the helper script parameterHelper and to give a default value to parameters when your processor is instantiated.

The lists are returned as cell arrays of strings (or any other type for the default parameter values). They should follow the same order, such that the n-th member of each of the three lists relate to the same parameter.

Parameter names need not be the same as the parameter property name you defined earlier. This will become apparent in the next section. In fact, names should be changed to at least include a two or three letters prefix that is unique to your new processor. You can make sure it is not already in use by browsing through the output of the parameterHelper script.

The method should look something like this:

function [names,defValues,description] = getParameterInfo()

names = {'xx_par1','xx_par2','xx_par3'};

defValues = {0.5, ...
[1 2 3 4], ...
'someStringValue'};

description = {'Tuning factor of dummy example (s)',...
'Vector of unused frequencies (Hz)',...
'Model name (''someStringValue'' or ''anotherValue'')'}

end


This dummy example illustrates the following important points:

• Use a unique prefix in the name of the parameters (xx_ above) that abbreviates the name or task of the processor.
• Find a short, but self-explanatory parameter name (not like parX above). If it makes sense, you can re-use the same name as a parameter involved in another processor. The prefix will make the name unique.
• Default values can be of any type (e.g., float number, array, strings,...)
• Descriptions should be as short as possible while still explanatory. Mention if applicable the units or the different alternatives.

### getProcessorInfo¶

This method stores the properties of the processor that are needed to integrate it in the framework. It outputs a structure with the following fields:

• .name: A short, self-explanatory name for the processor
• .label: A name for the processor that is used as a label. It can the same as .name if that is sufficient, or a bit longer if needed.
• .requestName: The name tag of the request that a user should input when calling the .addProcessor method of the manager. This has to be a valid Matlab name (e.g., it cannot include spaces).
• .requestLabel: A longer name for the signal this processor produces, used e.g., as plot labels.
• outputType: The type of signal object (name of the class) this processor produces. If none of the existing signals in the framework are suitable, you will need to implement a new one.
• isBinaural: Set to 0 if your processor operates on a single channel (e.g., an auditory filterbank) or to 1 if it needs a binaural input (e.g., the inter-aural level differences processor). If your processor can operate on both mono and stereo signals (such as the pre-processor preProc.m), set it to 2.

Your method should initialise the structure that will be returned as output and give a value to all of the above-mentioned fields:

%...

function pInfo = getProcessorInfo

pInfo = struct;

pInfo.name = 'MyProcessor';
pInfo.label = 'Processor doing things';
% etc...

end


## Implementing parameters “getter” methods¶

As described in an earlier section, external parameters of the processor, i.e., those that can be modified by the user, are implemented as Dependent properties of your processor class. For your implementation to be valid, a “getter” method needs to be implemented for each of these parameters. If not, Matlab will generate an error when trying to access that parameter value. If a property is set as Dependent, then its getter method will be called whenever the program tries to access that property. In general, this can be useful for a property that depends on others and that need to be recomputed whenever accessed. In the present case, we will set the getter method to read the corresponding parameter value in the parameter object associated with your processor. If the value of the parameter has changed throughout the processing (e.g., in response to feedback), then we are sure to always get the updated value.

“Getter” methods for parameters are implemented without any method attribute and always follow the same structure. Hence they can easily be copy/pasted and adjusted:

methods

function value = get.parName(pObj)
value = pObj.parameters.map('xx_parNameTag')
end

% ... implement one get. method for each parameter

end


In the above example, parName is the name of the parameter as a dependent property of your processor class, and xx_parNameTag is the name of the parameter defined in the static .getParameterInfo method. pObj represents an instance of your processor class, it does not need to be changed across methods.

## Implement the processor constructor¶

For any possible application, every class should implement a very specific method: a class constructor. A class constructor is a function that has the exact same name as your class. It can take any combination of input arguments but can return only a single output: an “instance” of your class.

In the Auditory front-end architecture however, the input arguments to the constructor of all processors have been standardised, such that all processor constructors can be called using the exact same arguments. The input arguments should be (in this order) the sampling frequency of the input signal to the processor and an instance of a parameter object returned e.g. by the script genParStruct.m. The constructor’s role is then to create an object of the class, and often to initialise all its properties. Most of this initialisation step is the same across all processors (e.g., setting input/output sampling frequencies, indicating the type of processor, ...). Hence all processor constructors rely heavily on the constructor of their parent class (or super-constructor), Processor(...) which defines these across-processors operations. This allows to have all this code in one place which reduces the code you have to write for your processor, as well as reducing chances for bugs and increasing maintainability. This concept of “inheritance” will be discussed in a further section.

In practice, this means that the constructor for your processor will be very short:

  1 2 3 4 5 6 7 8 9 10 11 12 13 function pObj = myNewProcessor(fs,parObj) %myNewProcessor ... Provide some help here ... if nargin<2||isempty(parObj); parObj = Parameters; end if nargin<1; fs = []; end % Call super-constructor pObj = pObj@Processor(fs, fsOut,'myNewProcessor',parObj); % Additional code depending on your processor % ... end 

Note

The constructor method should be placed in a “method” block with no method attributes.

Let us break down the constructor structure line by line:

• Line 1: As stated earlier, all processor constructors take two input and return a single output, your processor instance pObj. Matlab restricts all constructors to return a single output. If for any reason you need additional outputs, you would have to place them in a property of your processor instead of a regular output. Input arguments are the input sampling frequency, i.e., the sampling frequency of the signal at the input of the processor, and a parameter object parObj.
• Line 2: This is where you will place help regarding how to call this constructor. Because they have a generic form across all processors, you can easily copy/paste it from another processor.
• Lines 4 and 5: An important aspect in this implementation is that the constructor should be called with no input argument and still return a valid instance of the processor, without any error. Hence these two lines define default values for inputs if none were specified.
• Line 8: This line generates a processor instance by calling the class super-constructor. The super-constructor takes four inputs:
• the input sampling frequency fs
• the output sampling frequency. If your processor does not modify the sampling rate, then you can replace fsOut with fs. If the output sampling rate of your processor if fixed, i.e., not depending on external parameters, then you can specify it here, in place of fsOut. Lastly, if the output sampling rate depends on some external parameters (i.e., susceptible to change via feedback from the user), then you should leave the fsOut field empty: []. The output sampling rate will be defined in another method that is called every time feedback is involved.
• the name of the children processor, here myNewProcessor.
• the parameter object parObj already provided as input.
• Line 11: Your processor might need additional initialisation. All extra code should go there. To ensure that no error is generated when calling the constructor with no arguments (which Matlab sometimes does implicitly), the code should be embedded in a if nargin > 0 ... end block. Here you can for example initialise buffers or internal properties.

Warning

The initialisation of anything that depends on external parameters (e.g., filters, framing windows, ...) is not performed here on line 11. When parameters change due to feedback, these properties need to be re-initialised. Hence their initialisation is performed in another method that will be described in a following section.

## Preliminary testing¶

At this stage of the implementation, your processor should be correctly instantiated and recognised by the framework. In some cases (e.g., your processor is a simple single input / single output processor), it might even be correctly integrated and routed to other processors. In any case, now is a good time to take a break from writing code and do some preliminary testing. We will go through a few example tests you can run, describe which problems could arise and suggest how to solve them. Try to run these tests in the order they are listed below, as this will help troubleshooting. They should run as expected before you go further in your implementation.

Note

You will not be able to instantiate your processor before you have written a concrete implementation to Processor abstract methods. To carry out the tests below, just write empty processChunk and reset methods. In this way, Matlab will not complain about trying to instantiate a class that contains abstract methods. The actual implementation of these methods will be described in later sections.

### Default instantiation¶

As mentioned when implementing the constructor, you should be able to get a valid instance of your processor by calling its constructor without any input arguments:

>> p = myNewProcessor


If this line returns an error, then you have to revise your implementation of the constructor. The error message should indicate where the problem is located, so that you can easily correct it. If your processor cannot be instantiated with no arguments, then it will not be listed as a valid processor.

If on the other hand this line executed without error, then there are two things you should control:

1. The line above (if not ended by a semicolon) should display the visible, public properties of the processor. Check that this list corresponds to the properties you defined in your implementation. The property values should be the default values you have defined in your getParameterInfo static method. If a property is missing, then you forgot to list it in the beginning of your class definition (or you defined it as Hidden or Private). If a value is incorrect, or empty, then it is a mistake in your getParameterInfo method. In addition, the Type property should refer to the name field returned by getProcessorInfo static method.
2. Inspect the external parameters of the processor by typing p.parameters. This should return a list of all external parameters. Control that all parameters are there and that their default value is correct.

To test that your external properties are indeed dependent, you can change the value of one or more of them directly in your parameter processor property and see if that change is reflected in the dependent property. For example if you type:

p.parameters.map('xx_par1') = someRandomValue


then this should be reflected in the property associated with that parameter.

Note

The input and output frequency properties of your processor, FsHzIn and FsHzOut are probably incorrect, but that is normal as you did not specify the sampling frequency when calling the constructor with no arguments.

### Is it a valid processor?¶

To test whether your processor is recognised as a valid processor, run the requestList script. The signal request name corresponding to your processor should appear in the list (i.e., the name defined in getProcessorInfo.requestName). If not (and the previous test did work), then maybe your class definition file is not located in the correct folder. Move it to the src/Processors folder. Another possibility is that you made your processor hidden (which should not happen if you followed these instructions). Setting explicitly the bHidden property of your processor to 1 will hide it from the framework. This is used in order to allow “sub-processors” in the framework, but it is probably not the case for you here so you should not enable this option.

### Are parameters correctly described?¶

If your processor is properly recognised, then you can call the parameterHelper script from the command window. There you should see a new category corresponding to your processor. Clicking on it will display a list of user-controllable parameters for your processor, as well as their descriptions. Feel free to adjust your getParameterInfo static method to have a more suitable description.

## Implementing the core processing method¶

At this stage, and if the previous tests were successfully passed, your processor should be correctly detected by the Auditory front-end framework. However, there is still some work to do. In particular, the core of your processor has to be implemented, which performs the processing of the input signal and returns a corresponding output.

This section will provide guidelines as to how to implement that method. However, this task is very dependent on the functionality of a particular processor. You can get insights as to how to perform the signal processing task by looking at the code of the .processChunk methods of existing processors.

Note

Some of the challenges in implementing the processing method were already presented in a section of the technical description. It is recommended at that stage to go back and read that section again.

### Input and output arguments¶

The processing method should be called processChunk and be placed in a block of methods with no attributes (e.g., following the class constructor). The function takes a single effective input argument, a chunk of input signal and returns a single output argument, the corresponding chunk of output signal. Because it is a non-static method of the processor, an instance of the processor is passed as first input argument. Hence the method definition looks something like this for a monaural single-output processor:

function out = processChunk(pObj,in)

% The signal processing to obtain "out" from "in" is written here
%
% ...

end


Or, for a binaural single-output processor (such as ildProc):

function out = processChunk(pObj,in_left,in_right)

% The signal processing to obtain "out" from "in" is written here
%
% ...

end


If your processor is not of one of the two kinds described above, then you are free to use a different signature for your processChunk method (i.e., different number of input or output arguments). However, you will then have to override the initiateProcessing method.

Given an instance of your processor, say p, this allows you to call this method (and in general all methods taking an object instance as first argument) in two different ways:

• processChunk(p,in)
• p.processChunk(in)

The two calls will of course return the same output.

Note

Having an instance of the processor as an argument means that you can access all of its properties to carry out the processing. In particular, the external and internal parameter properties you have defined earlier. For example, the processing method of a simple “gain” processor could read as out = in * p.gain

The arguments in and out are arrays containing “pure” data. Although signal-related data is stored as specific signal objects in the Auditory front-end, only the data is passed around when it comes to processing. It is done internally to avoid unnecessary copies. So it is not something that has to be addressed in the implementation of your processing method. Your input is an array whose dimensionality depends on the type of signal. Dimensions are ordered in the same way as in the data-storing buffer of the signal object. For example, the input in in the gammatoneProc.processChunk is a one-dimensional array indexing time. Similarly, the output should be arranged in the same way than in its corresponding output signal object. For example, the output out of modulationProc.processChunk is a three-dimensional array where the first dimension indexes time, the second refers to audio frequency and the third corresponds to modulation frequency. Just like the way data is stored in the modulationSignal.Data buffer.

Note

The first dimension for all signals used in the Auditory front-end is always indexing time.

### Chunk-based and signal-based processing¶

As the name of the method processChunk suggests, you should implement the processing method such that it can process consecutive chunks of input signal, as opposed to the entire signal at once. This enables “online” processing, and eventually “real-time” processing once the software has been sufficiently optimised. This has two fundamental consequences on your implementation:

1. The input data to the processing method can be of arbitrary duration.
2. The processing method needs to maintain continuity between input chunks. In other words, when concatenating the outputs obtained by processing individual consecutive chunks of input, one need to obtain the same output as if all the consecutive input were concatenated and processed at once.

Point 1. above implies that depending on the type of processing you are carrying out, it might be necessary to buffer the input signal. For example, processors involving framing of the signal, such as ratemapProc or ildProc, need to put the segment of the input signal that went out of bound of the framing operation in a buffer. This buffer is then appended to the beginning of the next input chunk. This is illustrated in a section of the technical description of the framework. This also means that for some processor (those which lower the sampling rate in general), an input that is too short in time might produce an empty output. But this input will still be considered in the next chunk.

Point 2. is the most challenging one because it very much depends on the processing carried out by the processor. Hence there are no general guidelines. However, the Auditory front-end comes with some building blocks to help with this task. It features for instance filter objects that can be used for processing. All filters manage their internal states themselves, such that output continuity is ensured. For an example on how to use filters, see e.g. gammatoneProc.processChunk. Sometimes however, one need more than simple filtering operations. One can often find a workaround by using some sort of “overlap-save” method using smart buffering of the input or output as described in the technical description. A good example of using buffering for output continuity can be found in e.g., ildProc.processChunk.

### Reset method¶

To ensure continuity between output chunks, your new processor might include “internal states” (e.g., built-in filter objects or internal buffers). Normally, incoming chunks of input are assumed to be consecutive segments of a same signal. However, the user can decide to process an entirely new signal as input at any time. In this case, your processor should be able to reset its internal states.

This is performed by the reset method. This method should be implemented in a method block with no method attributes, just like the constructor. It should simply reset the filters (if any) by calling all the filters reset methods, and/or empty all internal buffers.

If your processor does not need any internal state storage, then the reset method should still be implemented (as it is an abstract method of the parent class) but can be left empty (see, e.g., itdProc.reset).

## Override parent methods¶

The Auditory front-end framework was developed to maximise code reusing. Many of the existing processors, although they carry out different processing tasks, have common attributes in terms of e.g., number of inputs, number of outputs, how to call their processing methods, ... Hence all aspects of initialisation (and re- initialisation following a response to feedback) and input/output routing have been implemented for common-cases as methods of the parent Processor class. If your processor does not behave similarly to others in one of these regards, then this approach allows you to redefine the specific method in your new children processor class definition. In the object oriented jargon, this procedure is called method overriding.

In the following, we list the methods that might need overriding and how to do so. Subsections for each methods will start with a description of what the method does and a note explaining in which cases the method needs to be overridden, such that you can quickly identify if this is necessary for your processor. Some examples of existing processors that override a given method will also be given so they can be used as examples. Note that all non- static methods from the parent Processor class can be overridden if necessary. The following list only concerns methods that were written with overriding in mind to deal with particular cases.

Note

Overridden methods need to have the same method attribute(s) as the parent method they are overriding.

### Initialisation methods¶

#### verifyParameters¶

This method is called at the end of the Processor super-constructor. It ensures that user-provided parameters are valid. The current implementation of the Auditory front-end relies on the user being responsible and aware of which type or values are suitable for a given parameter. Therefore, we do not perform a systematic check of all parameters. Sometimes though, you might want to verify that user- provided parameters are correct in order to avoid Matlab returning an error at a later stage. For example, ihcProc.verifyParameters will check that the inner hair-cell model name entered by the user is part of the list of valid names.

Another use for the verifyParameters method is to solve conflicts between parameters. For example, the auditory filterbank in gammatoneProc can be instantiated in three different ways (e.g., by providing a range of frequency and a number of channels, or directly a vector of centre frequencies). The user- provided parameters for this processor are therefore potentially “over- determining” the position of centre frequencies. To make sure that there is no conflict, some priority rules are defined in gammatoneProc.verifyParameters to ensure that a unique and non-ambiguous vector of centre frequencies is generated.

Note

This method does nothing by default. Override it if you need to perform specific checks on external parameters (i.e., the user-provided parameters extended by the default values) before instantiating your processor.

To override this method, place it in a methods block with the Access=protected attribute. The method takes only an instance of the processor object (say, pObj) as input argument, and does not return any output.

If you are checking that parameters have valid values, replace those which are invalid with their default value in pObj.parameters.map (see e.g., ihcProc.verifyParameters). It is a good practice here to inform the user by returning a warning, so that he/she knows that the default value is used instead.

If you are solving conflicts between parameters, set up a priority rule and only retain user-provided parameters that have higher priority according to this rule (see e.g., gammatoneProc.verifyParameters). Mention explicitly this rule in the help line of your processor constructor.

#### prepareForProcessing¶

This method performs the remaining initialisation steps that we purposely did not include in the constructor as they initialise properties that are susceptible to change when receiving feedback. It also includes initialisation steps that can be performed only once processors have been linked together in a “processing tree”. For example, ildProc needs to know the original sampling frequency of the signal before its cross-correlation was computed to provide lag values in seconds. But to access the cross-correlation processor and request that value, the two processors need to be linked together already, which does not happen at the level of instantiation but later. Hence this method will be called for each processors once they all have been inter-linked, but also whenever feedback is received.

Note

Override this method if your processor has properties or internal parameters that can be changed via user feedback or that comes directly from preceding processors in the processing tree.

This method should have the Hidden=true method attribute. Hidden methods are sometimes used in the Auditory front-end when we need public access to it (i.e., other objects than the processor itself should be able to call the method) but when it is not deemed necessary to have the user call it. The user can still call the method by explicitly writing its name, but the method will not appear in the list of methods returned by Matlab script methods(.) nor by Matlab’s automatic completion.

The method only takes an instance of the processor as input argument and does not return outputs. In the method, you should initialise all internal parameters that are susceptible to changes from user feedback. Note that this includes the processor’s output sampling frequency FsHzOut if this frequency depends on the processor parameters. A good example is ratemapProc.prepareForProcessing, which initialises internal parameters (framing windows), the output sampling frequency and some filters.

#### instantiateOutput¶

This method is called just after a processor has been instantiated to create a signal object that will contain the output of this new processor and add the signal to the data object.

Note

Override this method if your output signal object constructor needs additional input arguments (e.g., for a FeatureSignal), if your processor generates more than one type of output, or if your processor can generate either mono or stereo output (e.g., the current preProc). There is no processor in the current implementation that generates two different outputs. However, the pre- processor can generate either mono or stereo outputs depending on the number of channels in the input signal (see preProc.instantiateOutput for an example).

This method should have the Hidden=true method attribute. It takes as input an instance of your processor and a instance of a data object to add the signal to. It returns the output signal object(s) as a cell array with the usual convention that first column is left channel (or mono) and right column is right channel. Different lines are for different types of signals.

Warning

Because there is no such processor at the moment, creating a new processor that returns two different types of output (and not just left/right channels) might involve additional changes. This is left to the developers responsibility to test and adjust existing code.

### Input/output routing methods¶

When the manager creates a processing “tree”, it also populates the Input and Output properties of each processors with handles to their respective input and output signal objects. The methods defined in the parent Processor should cover most cases already, and it is unlikely that you will have to override them for your own processor. For these two methods, it is important to remember the internal convention when storing multiple signals in a cell array: columns are for audio channels (first column is left or mono and second column is right). Different lines are for different types of signals.

The way Input and Output properties are routed should be in accordance with how they are used in the initiateProcessing method, which will be described in the next subsection.

#### addInput¶

This method takes an instance of the processor and a cell array of handles to dependent processors (i.e., processors one level below in the processing tree) and does not return any arguments. Instead, it will populate the Input property of your processor with a cell array of handles to the signals that are outputs to the dependent processors. The current implementation of Processor.addInput works for three cases, which overall cover all currently existing processors in the Auditory front-end:

• There is a single dependent processor which has a single output.
• There are two dependent processors each with single output corresponding to the left and right channels of a same input signal.
• There is a single dependent processor which produces two outputs: a left and a right channel (such as preProc for stereo signals).

Note

Override this method if your processor input signals are related to its dependent processors in a different way than the three scenarios listed above.

This method should have the Hidden=true attribute. You should just route the output of your dependent processors to the input of your new processor adequately. Again, it was not necessary thus far to override this method, hence no examples can be provided here. Additionally, this functionality has not been tested, so it might imply some minor reworking of other code components.

#### addOutput¶

This method adds a signal object (or a cell array of signals) to the Output property of your processor.

Note

Override this method if your processor has multiple outputs of different types. If your processor returns two outputs as the left and right channel of a same representation, it is not necessary to override this method.

This method should have the Hidden=true method attribute. It takes as input an instance of the processor and a single or a cell array of signal objects.

### Processing method¶

#### initiateProcessing¶

This method is closely linked to the addInput, addOutput and processChunk methods. It is a wrapper to the actual processing method that routes elements of the cell arrays Input and Output to actual inputs and outputs of the processChunk method and call that method. It also appends the new chunk(s) of output to the corresponding output signal(s).

The parent implementation considers two cases: monaural and binaural (i.e., a “left” and a “right” inputs) which produce single outputs.

Note

Override this method if your processor is not part of the two cases above or if your implementation of the processChunk has a different signature than the standard.

A good example of an overridden initiateProcessing method can be found in preProc.initiateProcessing, as the processing method of the pre-processor does not have a standard signature as it returns two outputs (left and right channels).

## Allowing alternative processing options¶

Sometimes, two different processors (implemented as two different classes) can perform the same operation. The choice between such alternative processors is made depending on a given user-provided (or default) request parameter value. This is the case for example for the auditory filterbank, which can be performed by either a Gammatone filterbank (gammatoneProc.m) or a dual-resonance non- linear filterbank (drnlProc.m).

As can be seen when browsing parameterHelper, the two processors should be listed under the same request name, and one of the parameters ('fb_type' in the example above) should allow to switch between the two (or more) alternatives. When the manager instantiates the processors and notices that a given representation has alternative ways of being computed, it will call the methods isSuitableForRequest of each alternatives to know which one should be used.

Therefore, if your processor represents an alternative way of carrying out a given operation, you should implement its isSuitableForRequest method, as well as for its alternative, if it was not already existing.

This method takes as unique input an instance of a processor and will look into its parameters property to determine if it is the suitable alternative. It will return a boolean indicating if it is suitable (true) or not (false). Note that this method is called internally, not from an actual processor instance that would be used afterwards, but from a dummy, empty processor generated using the user-provided request and parameters.

See as examples gammatoneProc.isSuitableForRequest and drnlProc.isSuitableForRequest.

## Implement a new signal type¶

The Auditory front-end supports already a wide range of signal types:

• TimeDomainSignal: used for single-dimensional signal
• TimeFrequencySignal: used for two-dimension signals (time and frequency)
• CorrelationSignal: used for three-dimension signals (time, frequency and lags)
• ModulationSignal: used for three-dimension signals (time, audio frequency and modulation frequency).
• FeatureSignal: used for a labelled collection of time-domain signals
• BinaryMask: used for two-dimensional (time and frequency) binary signals (0 or 1).

If your new processor generates a new type of signal that is not currently supported, you might have to add your own implementation of a new signal. This tutorial will not go in details on how to implement new signal types. However, the following aspects should be considered:

• Your signal class should inherit the parent Signal class.
• It should implement the abstract plot method. If there is no practical way of plotting your signal, this method could be left empty.
• Its constructor should take as argument a handle to your new processor (that generates this signal as output), a buffer size in seconds, and a vector of size across the other dimensions ([size_dim2, size_dim3,...]). If more arguments are needed (as is the case for FeatureSignal), then this signature can be changed, but the instantiateOutput of your processor should also be overridden.

## Recommendations for final testing¶

Now the implementation of your new processor should be finalised, and it is important to test it thoroughly. Below are some recommendations with regard to testing:

• Make sure that all aspects of your implementation work. Test for mono as well as stereo input signals, vary your processor parameters and check that the change is reflected accordingly in the output.
• If you have based your implementation on another existing implementation (even better, one that is documented in the literature), then compare your new implementation with the reference implementation and control that both provide the same output up to a reasonable error. A reasonable error, for a processor that does not involve stochastic processes should be around quantisation error, assuming that your new implementation is exactly as the reference.
• Test the online capability of your processor (i.e., maintaining the continuity of its output) by processing a whole signal and the same signal cut into chunks. Both runs should provide the same output (up to a “reasonable error”). You can use the test script test_onlineVSoffline to perform that task.