MATLAB: How to visualize filter coefficients after fixed point conversion using FVTOOL

Filter Design Toolbox

I have an FIR filter with a single coefficient "pi" and I perform the following steps :

1. Open a new Simulink model.

2. Use the Digital Filter block from the Signal Processing Blockset to represent this filter in a three-bit word and two-bit fraction length fixed-point representation.

The mask option "View Filter Response" opens FVTOOL to show the filter response but the Magnitude squared value of the FVTOOL plot shows pi^2 instead of the saturated value 0.75^2 after fixed point conversion.

Best Answer

The ability to view filter coefficients in fixed-point format is not available in the Filter Design Toolbox when using the Filter Visualization Tool (FVTool).

As a workaround you can use the Filter Design and Analysis Tool (FDATool) to view fixed-point filters.

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MATLAB: What is the difference between the Signal Processing Toolbox and the Filter Design Toolbox

The Filter Design Toolbox builds upon the Signal Processing Toolbox by adding advanced filter design algorithms, fixed-point filter analysis

and simulation, multirate filters, and adaptive filters. Some of the

key features for the Filter Design Toolbox are:

- Design and analysis of filters from the FDATool GUI or from the command line

- Advanced FIR filter design methods: generalized Remez, halfband, Nyquist, interpolated FIR, CIC compensators, inverse-sinc, minimum-phase, constrained-ripple, sloped stopband, least Pth-norm, and perfect reconstruction 2 channel filter banks.

- Advanced IIR filter design methods: arbitrary group-delay equalization, comb filters, peaking/notching filters, arbitrary magnitude, constrained radius.

- IIR frequency transformations to convert lowpass filters into bandpass, multiband and complex filters among others.

- A suite of multirate filtering efficient polyphase structures for interpolation, fractional interpolation, decimation, fractional decimation, and sampling-rate conversion.

- Support for CIC-interpolation and CIC-decimation.

- An extensive suite of adaptive filtering algorithms including steepest-descent type, least-squares type, frequency-domain and block adaptive filters.

- Fixed-point implementations of various filter structures including, direct-form FIR and its transposed version, symmetric/antisymmetric FIR, direct form I and II second-order sections, and polyphase interpolators/decimators/sample-rate converters.

- Scaling of second-order section IIR filters for fixed-point implementation.

- Estimation of the power spectral density estimation for round-off noise in fixed-point digital filters.

- Limit cycle analysis for fixed-point IIR filters

For complete product descriptions, please refer to the product pages:

http://www.mathworks.com/products/filterdesign/

http://www.mathworks.com/products/signal/

MATLAB: Does the IIR filter become unstable when I change Filter arithmetic to Fixed-point using FDATOOL in Filter Design Toolbox 4.3 (R2008a)

Converting an IIR filter from floating-point to fixed-point can be difficult, especially when the poles of the filter are close to the unit circle. The coefficients quantization error can make the filter unstable (as it is in this case). So it will not be possible to directly create a fixed-point filter with the same frequency preferences. However, depending on the application, one can use multirate/multistage FIR designs as they can be an efficient alternative to IIR designs for highly selective filters. FIR filters are much easier to convert to fixed-point.

Another alternative might be to use Wave Digital (also known as IIR polyphase) filters. A demo on this topic can be found by executing the following statement on the MATLAB command prompt (in R2008a):

web([matlabroot '/toolbox/filterdesign/filtdesdemos/html/iirallpassdemo.html'], '-helpbrowser')

Best Answer

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MATLAB: What is the difference between the Signal Processing Toolbox and the Filter Design Toolbox

MATLAB: Does the IIR filter become unstable when I change Filter arithmetic to Fixed-point using FDATOOL in Filter Design Toolbox 4.3 (R2008a)

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