The precision of plot filter design function is different than plot just coefficients of the same function of filter

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Johan Johan 2020-1-27

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评论： Star Strider 2020-1-27

I'm confused about plotting the design filter with the same precision ,for example i have this coefficients (i found it by using fir1(68,[.35,.65])):

bb= [0.0005

0

0.0010

0.0000

-0.0024

-0.0000

0.0020

-0.0000

0.0015

-0.0000

-0.0066

0.0000

0.0078

-0.0000

-0.0142

-0.0000

0.0220

-0.0000

-0.0093

0

-0.0232

-0.0000

0.0519

-0.0000

-0.0411

0.0000

-0.0305

-0.0000

0.1464

0.0000

-0.2551

0.0000

0.2995

0.0000

-0.2551

0.0000

0.1464

-0.0000

-0.0305

0.0000

-0.0411

-0.0000

0.0519

-0.0000

-0.0232

0

-0.0093

-0.0000

0.0220

-0.0000

-0.0142

-0.0000

0.0078

0.0000

-0.0066

-0.0000

0.0015

-0.0000

0.0020

-0.0000

-0.0024

0.0000

0.0010

0

0.0005];

%% plot
freqz(bb,1,512)

the design Fig01

if i use the same last funtion fir1 with same specification

b=fir1(68,[.35,.65])';
freqz(b,1,512)

the design in Fig02

Why the Fig1 don't like the Fig2 in stop band ,why don't find the same precision ?

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Answer 1

Star Strider 2020-1-27

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https://ww2.mathworks.cn/matlabcentral/answers/502153-the-precision-of-plot-filter-design-function-is-different-than-plot-just-coefficients-of-the-same#answer_412210

在 MATLAB Online 中打开

To reproduce the posted results, plot them as:

b = fir1(68,[.35,.65]);
bb = round(b, 4);
figure
freqz(bb,1,512);
figure
freqz(b',1,512)

In plotting ‘bb’, note that the coefficients have significantly reduced precision. Rounding (or truncating) the coefficients significantly affects the plotted filter frequency response.

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Johan Johan 2020-1-27

在 MATLAB Online 中打开

Thank you, but i found this error

Error using round
Too many input arguments.

Does it the same idea with

b = fir1(68,[.35,.65]);
n=4;
bb = round(b*10^n)/10^n;

Star Strider 2020-1-27

在 MATLAB Online 中打开

As always, my pleasure!

Yes.

That should work.

I created my own version a few years ago:

roundn = @(x,n) round(x .* 10.^n)./10.^n;   % Round ‘x’ To ‘n’ Digits, Emulates Latest ‘round’ Function

I do not now remember when the new round was introduced, although I believe it was R2016b, since many similar upgrades occurred them.

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The precision of plot filter design function is different than plot just coefficients of the same function of filter

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The precision of plot filter design function is different than plot just coefficients of the same function of filter

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