What is the basic method to implement FIR filter?

　　The FIR filter structure contains two things: one is the sampling point delay line, one is the coefficient. The FIR filter can be implemented by the following methods:

　　1. Insert the input sampling point into the delay line.

　　2. Multiply the numbers in the delay line with the corresponding coefficients and add them up.

　　3. Shift so that the next input sampling can enter the delay line.

　　4.2 How to implement FIR filter with C language?

　　In order to show the many methods and skills, here provides the C language implementation of FIR filter algorithm.

　　It includes the following functional modules:

　　1. fir_basic: implement the basic FIR filter

　　2. fir_circular: describes how circular buffers implement FIR.

　　3. fir_shuffle: Some TI processors use the shuffle down technique

　　4. fir_split: split the FIR filter into two pieces to avoid using circular cache.

　　5. fir_double_z: Use a double precision delay line to enable a flat buffer.

　　6. fir_double_h: Use double coefficients so that a flat buffer can be used.

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