FIR filter bandpass filter from lowpass filter prototype

I want to make bandpass filter from lowpass filter prototype.

first i made lowpass filter and frequency translated using 
cosine function and i saw the freq. response of translated coefficient.
but it differs from lowpass filter response.

b  = firceqrip(368, 4.3/(122.88/2), [0.1, 0.0018], 'slope', 0);
fvtool(b);

n=0:length(b)-1;
cos_b = b.*(cos(((3*pi)/8)*n));
fvtool(cos_b)

> lowpass filter : attenuation is 55dB
> bandpass fitler : attenuation is about 50dB

and filter shape is different, especially filter edge part.
am i wrong in implement bandpass filter from lowpass protytype ?






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On Tue, 25 Aug 2015 20:37:45 -0500, tommy_kwon wrote:

> I want to make bandpass filter from lowpass filter prototype.
> 
> first i made lowpass filter and frequency translated using cosine
> function and i saw the freq. response of translated coefficient.
> but it differs from lowpass filter response.
> 
> b  = firceqrip(368, 4.3/(122.88/2), [0.1, 0.0018], 'slope', 0);
> fvtool(b);
> 
> n=0:length(b)-1;
> cos_b = b.*(cos(((3*pi)/8)*n));
> fvtool(cos_b)
> 
>> lowpass filter : attenuation is 55dB bandpass fitler : attenuation is
>> about 50dB
> 
> and filter shape is different, especially filter edge part.
> am i wrong in implement bandpass filter from lowpass protytype ?

Why are you asking the exact same question twice?  Did you read the 
answers that you already got?  If those weren't sufficient, why didn't 
you reply to one or both of them with follow-on questions?

The regulars on this group (the ones that you get useful answers from, by 
and large) aren't a computer that's been on too long -- we're a bunch of 
(mostly) cranky, (mostly) middle-aged, (mostly) men who respond well to 
polite respect, but less so to being treated like faulty computing 
hardware.

-- 

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com