One important issue that must be considered when IIR filters are implemented on a fixed-point processor is that the filter coefficients that are actually used are quantized from the "exact" (high-precision floating point) values computed by MATLAB. Although quantization was not a concern when we worked with FIR filters, it can cause significant deviations from the expected response of an IIR filter.
By default, MATLAB uses 64-bit floating point numbers in all of its computation. These floating point numbers can typically represent 15-16 digits of precision, far more than the DSP can represent internally. For this reason, when creating filters in MATLAB, we can generally regard the precision as "infinite," because it is high enough for any reasonable task.
Note:
For this section exercise, you will examine how this
difference in precision affects a notch filter
generated using the butter command:
[B,A] = butter(2,[0.07 0.10],'stop').
Quantizing coefficients in MATLAB
It is not difficult to use MATLAB to quantize
the filter coefficients to the 16-bit precision used on the
DSP. To do this, first take each vector of filter
coefficients (that is, the
Next, quantize the resulting vectors to 16 bits of precision
by first multiplying them by
round), and then dividing the resulting vectors
by 32768. Then multiply the resulting numbers, which will be
in the range of -1 to 1, back by the power of two that you
divided out.
Effects of quantization
Explore the effects of quantization by quantizing the filter
coefficients for the notch filter. Use the
freqz command to compare the response of the
unquantized filter with two quantized versions: first,
quantize the entire fourth-order filter at once, and second,
quantize the second-order ("bi-quad") sections separately
and recombine the resulting quantized sections using the
conv function. Compare the
response of the unquantized filter and the two quantized
versions. Which one is "better?" Why do we always
implement IIR filters using second-order sections instead of
implementing fourth (or higher) order filters directly?
Be sure to create graphs showing the difference between the filter responses of the unquantized notch filter, the notch filter quantized as a single fourth-order section, and the notch filter quantized as two second-order sections. Save the MATLAB code you use to generate these graphs, and be prepared to reproduce and explain the graphs as part of your quiz. Make sure that in your comparisons, you rescale the resulting filters to ensure that the response is unity (one) at frequencies far outside the notch.
