# Infinite Impulse Response Filter (IIR Filter)

This operator implements an infinite impulse response filter.

It returns a weighted average of the current ping and the calculated previous ping.

Echoview accepts a single variable of the following types as input:

• Linear
• Multibeam magnitude
• Multibeam Sv
• Multibeam TS
• Multibeam unspecified dB
• Power dB
• Sv
• TS
• Unspecified dB

See: IIR algorithm.

## Settings

The Infinite Impulse Response Filter Variable Properties dialog box pages include (common) Variable Properties pages and these operator pages:

### IIR Filter page

 Setting Description Coefficient Enter the coefficient to be used to alter signal content.

## Algorithm

### IIR filter

The result for any given data point may be written as:

Result(0,d) = Operand(0,d)
Result
(p,d) = C * Operand(p,d) + (1-C) * Result(p-1,d)            where p>0

Where:

p identifies the ping
d
identifies the data point (each ping contains a number of data points)
C
is the coefficient entered on the IIR page of the Variable Properties dialog box.
Result
(p,d) is the value of the data point d in ping p in the resulting variable.
Operand
(p,d) is the value of the data point d in ping p in the operand variable.

Hence, if C = 1 the filter will make a copy of the operand variable, and if C = 0, the filter will make a copy of the operand variable with each ping containing the values of ping 0 from the operand variable.

Notes:

• This operation involves two pings, one from the operand variable another from the result variable. These pings must have matching ping geometry. If they do not, there will be no resulting ping. See Using multiple operands for further information.
• IIR calculations are carried out in the linear domain. dB values are converted to linear for calculations and the output is re-expressed in dB.