If you want to remove DC from your 100hz signal why do you expect the output after filtering to be1?
You can filter out DC component by simply doing:
y = x-mean(x);

i.e. for a signal of length N
y(n) = x(n) - {x(n)+x(n-1)=..x(0)}/N

mean(x) is your dc component, in case you need that.

-Sachin

________________________________
From: T.E Ravi
To: m...
Sent: Tuesday, February 17, 2009 8:53:46 PM
Subject: [matlab] Re: how to design LPF given i/p signal its sampling freq, reqiured o/p, in MATLAB
hai every one,
First of all thanks for all your quick response

t=0:0.05:5;X= 1+sin(pi* t); plot(t,X)
Since ,the required o/p is 1(in time domain).
I thought if i should get the o/p as 1, the LP filter coefficients had to be designed
in such a way so as to get the reqiured o/p as 1.
i multiplied with x(t) and y(t) with u(t) [step function]
x(t)=[1+sin( pi*t)]u(t) ; y(t)=u(t)
I took Laplace transform to get Transfer function
to obtain the transfer function in MATLAB
>> H=tf([1 0 pi.*pi],[1 pi pi.*pi])

Transfer function:
s^2 + 9.87
------------ --------
s^2 + 3.142 s + 9.87

>> [B,A]=tfdata( H,'v')
B 1.0000 0 9.8696

A 1.0000 3.1416 9.8696
>> Y=filter(B,A, X);plot(t, Y)
i didn't get the required o/p which is 1. instead Y is obtained a constant 0 and at the near end of 5 some fluctuations are also seen.
Can anybody give any hint and correct me if i am wrong.
Thanks in advance.

--- On Tue, 17/2/09, stevezsuzska wrote:

From: stevezsuzska
Subject: [matlab] Re: how to design LPF given i/p signal its sampling freq, reqiured o/p, in MATLAB
To: matlab@yahoogroups. com
Date: Tuesday, 17 February, 2009, 10:30 AM

--- In matlab@yahoogroups. com, "T.E Ravi" wrote:
>
> Hai everyone,
> I have an i/p signal x = 1 +0.5* sin(pi*t) where t = [0:0.01:5],
basically a signal sampled at 100 Hz. I need to filter out the d.c.
component i.e. get 1 as a constant output so for
> which I have to design a low pass filter.
>
> can any one tel me how to design low pass filter in MATLAB
>
> Thank you
>
Here is what I found with a quick Google. At
http://www.physics. uq.edu.au

% Low pass filter with fir1
%
nP; % Order 50 !!
Wn=0.4; % Cut-off at normalized frequency 0.4 (digital frequency
0.4*pi)
b=fir1(n,Wn) % defaults to lowpass filter
[H,q]=freqz( b,1,512); % Note a=[1]
mags(H); % Magnitude response
plot(q,mag,' .');
title(' fir1 filter, orderP, Wn=0.4*pi');
xlabel('Digital Frequency (rad/sample) ');
ylabel(' Magnitude Response');

The coefficient vector b then needs to be convolved with the input
vector to get the filtered output.

Regards,

Steve

hai every one,
First of all thanks for all your quick response

t=0:0.05:5;X=1+sin(pi*t); plot(t,X)
Since ,the required o/p is 1(in time domain).
I thought if i should get the o/p as 1, the LP filter coefficients had to be designed
in such a way so as to get the reqiured o/p as 1.
i multiplied with x(t) and y(t) with u(t) [step function]
x(t)=[1+sin(pi*t)]u(t) ; y(t)=u(t)
I took Laplace transform to get Transfer function
to obtain the transfer function in MATLAB
>> H=tf([1 0 pi.*pi],[1 pi pi.*pi])

Transfer function:
     s^2 + 9.87
--------------------
s^2 + 3.142 s + 9.87

>> [B,A]=tfdata(H,'v')
B     1.0000         0    9.8696

A     1.0000    3.1416    9.8696
>> Y=filter(B,A,X);plot(t,Y)
i didn't get the required o/p which is 1. instead Y is obtained a constant 0 and at the near end of 5 some fluctuations are also seen.
Can anybody give any hint and correct me if i am wrong.
Thanks in advance.

--- On Tue, 17/2/09, stevezsuzska wrote:

From: stevezsuzska
Subject: [matlab] Re: how to design LPF given i/p signal its sampling freq, reqiured o/p, in MATLAB
To: m...
Date: Tuesday, 17 February, 2009, 10:30 AM

--- In matlab@yahoogroups. com, "T.E Ravi" wrote:
>
> Hai everyone,
> I have an i/p signal x = 1 +0.5* sin(pi*t) where t = [0:0.01:5],
basically a signal sampled at 100 Hz. I need to filter out the d.c.
component i.e. get 1 as a constant output so for
> which I have to design a low pass filter.
> Â
> can any one tel me how to design low pass filter in MATLAB
> Â
> Thank you
>
Here is what I found with a quick Google. At
http://www.physics. uq.edu.au

% Low pass filter with fir1
%
nP; % Order 50 !!
Wn=0.4; % Cut-off at normalized frequency 0.4 (digital frequency
0.4*pi)
b=fir1(n,Wn) % defaults to lowpass filter
[H,q]=freqz( b,1,512); % Note a=[1]
mags(H); % Magnitude response
plot(q,mag,' .');
title(' fir1 filter, orderP, Wn=0.4*pi');
xlabel('Digital Frequency (rad/sample) ');
ylabel(' Magnitude Response');

The coefficient vector b then needs to be convolved with the input
vector to get the filtered output.

Regards,

Steve

--- In m..., "T.E Ravi" wrote:
>
> Hai everyone,
> I have an i/p signal x = 1 +0.5* sin(pi*t) where t = [0:0.01:5],
basically a signal sampled at 100 Hz. I need to filter out the d.c.
component i.e. get 1 as a constant output so for
> which I have to design a low pass filter.
>
> can any one tel me how to design low pass filter in MATLAB
>
> Thank you
>
Here is what I found with a quick Google. At
http://www.physics.uq.edu.au

% Low pass filter with fir1
%
nP; % Order 50 !!
Wn=0.4; % Cut-off at normalized frequency 0.4 (digital frequency
0.4*pi)
b=fir1(n,Wn) % defaults to lowpass filter
[H,q]=freqz(b,1,512); % Note a=[1]
mags(H); % Magnitude response
plot(q,mag,'.');
title(' fir1 filter, orderP, Wn=0.4*pi');
xlabel('Digital Frequency (rad/sample)');
ylabel(' Magnitude Response');

The coefficient vector b then needs to be convolved with the input
vector to get the filtered output.

Regards,

Steve