convolution in matlab

Hi all,
I am writing a user defined convolution program in matlab. I have some
indexing problems ...please correct and help me with my code:

function Out = userconv(a,b)

L =length(a);%input
M=length(b); %filter
t=0;
c=zeros(1,L+M-1); % initialize the output data buffer
w=zeros(1,M);

for i=1 : L+M-1
  c(i)=0;
  for j=1 :M
  if (i>j) && if ( i-j)<L
          c(i)=c(i)+b(i-j)         
      end
  end
      
Thanks ,

Robert.

"robert_dn" <robert_dn2002@yahoo.com> wrote in message
news:3c74b7a.0308051001.77cc97c4@posting.google.com...
> Hi all,
> I am writing a user defined convolution program in matlab. I have some
> indexing problems ...please correct and help me with my code:
>
> function Out = userconv(a,b)
>
> L =length(a);%input
> M=length(b); %filter
> t=0;
> c=zeros(1,L+M-1); % initialize the output data buffer
> w=zeros(1,M);
>
> for i=1 : L+M-1
>   c(i)=0;
>   for j=1 :M
>   if (i>j) && if ( i-j)<L
>           c(i)=c(i)+b(i-j)
>       end
>   end

your "w" isn't used anywhere.
nor t.
a is an important input but isn't used anywhere after finding its length.

I wouldn't use conditions on the indices as you've done - rather make the
indices what you intend them to be.  Maybe have conditional code segments.

Doesn't Out have to be assigned a value now and then?

Fred

robert_dn2002@yahoo.com (robert_dn) wrote in message news:<3c74b7a.0308051001.77cc97c4@posting.google.com>...
> Hi all,
> I am writing a user defined convolution program in matlab. I have some
> indexing problems ...please correct and help me with my code:
> 
> function Out = userconv(a,b)
> 
> L =length(a);%input
> M=length(b); %filter
> t=0;
> c=zeros(1,L+M-1); % initialize the output data buffer
> w=zeros(1,M);
> 
> for i=1 : L+M-1
>   c(i)=0;
>   for j=1 :M
>   if (i>j) && if ( i-j)<L
>           c(i)=c(i)+b(i-j)         
>       end
>   end
>       
> Thanks ,
> 
> Robert.

Please define your UserConv.

1. Your if statement,  U need a single &,
2. Same statement another "if"??
3. b(i-j)  ... MISTAKE...  size of b is limited to M you will be
having an indexing problem if the i-j > M.

The procedure for computing the convolution is wrong, you are trying
to convolute a and b. In your program the sequence a does not come
into picture any where in the code.
The convolution of any 2 sequences is given by 

c(n)=sum (k= -inf to k= +inf) { x(k) * h(n-k)}

in your code assume a to be x(n) and b to be h(n),

length of a and b be m,n respectively

then length of the convoluted sequence is n+m-1;

So modify your code

c(i)=c(i)+ b(i) * a(i-j);
and put a condition to check h(i-j) and b(i) is defined or not, this
should work.

hi..
here i m sending correct convolution code for
matlab...%*************************************************************************
%                       Convolution of Two signals
%**************************************************************************
%DKV
clc;
close all;
clear all;
x=[1,2,3,4]; %first signal
h=[1,2,3,4,5]; %second signal
N1=length(x);
N2=length(h);
X=[x,zeros(1,N2)];% padding of N2 zeros
H=[h,zeros(1,N1)];% padding of N1 zeros
    for i=1:N1+N2-1
        y(i)=0;
      for j=1:N1
          if(i-j+1>0)
        y(i)=y(i)+X(j)*H(i-j+1);
          else
          end
      end
    end
    stem(y);
    ylabel('y[n]');
    xlabel('----->n');
    title('Convolution of Two Signal');