What dashed lines in magnitude response mean?

Hi all,

A question of curiosity: sometimes when I am using the Filter Visualization Tool with Magnitude Response I am seeing dashed red - orange vertical lines. They are not really connected to particular filter design and the created filters are marked as 'stable'. They are also not (Basic Filter Design Process Help):

"If you have the DSP System Toolbox installed, the Filter Visualization tool produces the following figure with the dashed red lines indicating the transition bands and unity gain (0 in dB) over the passband. If you do not have the DSP System Toolbox, the figure appears without the dashed red lines."

since there can be so many of them that the blue line of the response cannot be seen any more and while I cannot see the ends of most of them, some are stopping within 'seeing' range. Also altering the particular filter design is affecting them.

And here is an illustration:

http://img220.imageshack.us/img220/3886/magresponse.jpg

In advance thank you for your time :)

Margaret

Answer 1

[Wayne King]

If you have the DSP system toolbox, you automatically get a "mask" that shows how close your designed filter comes to your specifications.

For example, the following lowpass filter specification has a passband of 100 Hz, a stopband frequency of 110 Hz, a passband ripple of 0.5 dB for data sampled at 1 kHz.

d = fdesign.lowpass('Fp,Fst,Ap,Ast',100,110,0.5,50,1000);
Hd = design(d,'equiripple');
fvtool(Hd)

If you zoom in on the passband, you will see the +/- 0.5 dB lines and see that the ripple stays there. Similarly, you will see vertical lines at the passband and stopband frequencies (100 and 110) Hz.

So the mask lets me see how close my actual filter is to what I specified.

If you want to turn the design mask off, you can with:

   fvtool(Hd,'DesignMask','off')

[Eric van Duin]

Thank you for the answer. Somehow I mis-read the information in the Help file, that's why I didn't connet it with my, rather unusual, filter design. So the option you have given, 'DesignMask', pointed to the answer straight away. Regards