Extract magnitude response in dB using fvtool(_) or freqz(_)

Hi, I have some filter coefficients 'h' and I am plotting the frequency response using fvtool(h) or freq(z).

Now, I would like to know the magnitude response (Y-axis) values of any given Normalized frequency (X-axis) shown in the figure (attached).

I have to manually place a cursor on Normalized frequency (X-axis) and drag it to a point to know its corresponding Y-axis value as shown below.

Example: Y-axis value (Magnitude) = 0.3523 for a given X-axis value (Normalized frequency) 0.2427.

This is quite uncomfortable when I want to know the Y-axis values for 100s of X-axis values. Is there any alternative way or MATLAB command to get the Y-axis values for any given X axis value?

Answer 1

[Pere Garau Burguera]

You can still use freqz, and in h you get the frequency response.

[h,w] = freqz(z);

This returns an array of 8192 elements, you can change this number with

[h,w] = freqz(z,n);

Then

freqz(z)

without assigning it to a variable is what you have been doing, and it automatically plot the response for you (which actually uses fvtool), as indicated in freqz.

Where n is the number of samples of the frequency response. Check freqz for more info.

Since h is complex you can then get the magnitude with

h_mag = abs(h);

And the phase with

h_ph = angle(h);

[Shannon Cherry]

Thanks. This helps.

h_mag lies between 0 to 1, but I need it in dB as shown in plot. Any idea how this can be done?

In the end, if I plot h_mag vs w, will I get the plot shown above.

[Pere Garau Burguera]

To convert from linear to logarithmic, you can use

h_db = 10*log10(h_mag);

[Shannon Cherry]

Thanks. This works.

Answer 2

[Chunru]

fvtool visualizes filter frequency response and it has no return value. freqz returns the frequency response and has the syntax such as "[h,f] = freqz(___,n,fs)". You should use freqz to get the response h vs f.