# 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.