# Basic Filter Question

Started by September 16, 2003
```Hi,

The order of a digital filter is the number of previous inputs (stored
in the processor's memory) used to calculate the current output.

My doubt is, how can one estimate the current output based on the
previous inputs ?

E.g in Adaptive Noise cancellation, if FIR filter is used, then the
output of the filter is,

y[n] = x[n]*h[0] + x[n-1]*h[1] + ... + x[0]*h[n]

If x is the noise source,then how do u estimate the current output
based on the past inputs of x ?

+---------+        s[n]   (+)+--+
Speech ------>| Channel |----------------->|  |-----+--> e[n]
Source        +---------+                  |__|     |
^(-)   |
+---------+       +----/----+  |      |
Noise ------>| Channel |------>| Adaptive|--+      |
Source        +---------+   x[n]| Filter  | y[n]    |feedback
+-/------ +         |
/                  |
+-------------------+

e[n] = s[n] - y[n]

y[n] has all the components of previous noise inputs.
When u obtain e[n], does it not affect the output?
If not, then how do the charecteristics of previous inputs
influence the present output (i.e cancelling the noise) ?

--Sandeep
```
```"Sandeep Chikkerur" <sandeep_mc81@yahoo.com> wrote in message
> Hi,
>
> The order of a digital filter is the number of previous inputs (stored
> in the processor's memory) used to calculate the current output.
>
> My doubt is, how can one estimate the current output based on the
> previous inputs ?
>
> E.g in Adaptive Noise cancellation, if FIR filter is used, then the
> output of the filter is,
>
> y[n] = x[n]*h[0] + x[n-1]*h[1] + ... + x[0]*h[n]
>
> If x is the noise source,then how do u estimate the current output
> based on the past inputs of x ?
>
>                +---------+        s[n]   (+)+--+
>  Speech ------>| Channel |----------------->|  |-----+--> e[n]
>  Source        +---------+                  |__|     |
>                                               ^(-)   |
>                +---------+       +----/----+  |      |
>   Noise ------>| Channel |------>| Adaptive|--+      |
>  Source        +---------+   x[n]| Filter  | y[n]    |feedback
>                                  +-/------ +         |
>                                   /                  |
>                                  +-------------------+
>
>
>      e[n] = s[n] - y[n]
>
>
>     y[n] has all the components of previous noise inputs.
>     When u obtain e[n], does it not affect the output?
>     If not, then how do the charecteristics of previous inputs
>     influence the present output (i.e cancelling the noise) ?

Sandeep,