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One important issue that must be considered when IIR filters are implemented on a fixed-point processor is that the filtercoefficients that are actually used are quantized from the "exact" (high-precision floating point) values computed byMATLAB. Although quantization was not a concern when we worked with FIR filters, it can cause significant deviationsfrom 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 typicallyrepresent 15-16 digits of precision, far more than the DSP can represent internally. For this reason, when creating filtersin MATLAB, we can generally regard the precision as "infinite," because it is high enough for any reasonable task.
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')
.
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 filtercoefficients (that is, the $A$ and $B$ vectors) and divide by the smallest power of two such that the resulting absolute value of the largest filtercoefficient is less than or equal to one. This is an easy but fairly reasonable approximation of how numbers outsidethe range of -1 to 1 are actually handled on the DSP.
Next, quantize the resulting vectors to 16 bits of precision
by first multiplying them by
$2^{15}=32768$ , rounding to the nearest integer (use
round
), and then dividing the resulting vectors
by 32768. Then multiply the resulting numbers, which will bein the range of -1 to 1, back by the power of two that you
divided out.
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 separatelyand recombine the resulting quantized sections using the
conv
function. Compare the
response of the unquantized filter and the two quantizedversions. Which one is "better?" Why do we always
implement IIR filters using second-order sections instead ofimplementing fourth (or higher) order filters directly?
Be sure to create graphs showing the difference between the filter responses of the unquantized notch filter, the notchfilter quantized as a single fourth-order section, and the notch filter quantized as two second-order sections. Savethe MATLAB code you use to generate these graphs, and be prepared to reproduce and explain the graphs as part of yourquiz. 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.
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