Programmer Guide/SPU Reference/LIMITER

Apply limiter function to a signal.

[SPU LIMITER X TYPE MAX LIM OUT Y Q]

input	description	data type	value type	default value
X	input signal	number, vector	variable
TYPE	limiter function (RECTANGLE, ATAN, EXPONENTIAL)	number (int.), string	constant	0 (= RECTANGLE)
MAX	maximum value; 0 < MAX	number	constant	1
LIM	limiter start value; 0 < LIM ≤ MAX	number	constant	1
output	description	data type	value type	comment
Y	limited signal	same type as A	variable
Q	overload ratio	number	variable

Description

This SP-atom applies a non-linear magnitude weighting (= limiter function) to the signal. The limiter function is only applied if the absolute value of the signal magnitude is higher than the specified limiter start magnitude LIM. For the limiter function, the following algorithm is used: Failed to parse (unknown function "\leslant"): {\displaystyle y_i = \begin{cases} x_i & \mbox{if }|x_i|\leslant LIM \\ 3n+1, & \mbox{if }n\mbox{ is odd} \end{cases} }

The level meter value v_t (t = sample index) is computed for each value x_t of the input signal X. The last v_t of an evaluation cycle is used to update the outputs Y and P.

TYPE	computation of vt
0 or HOLD	v t = max ( | x t | , | x t − 1 | , . . , | x t − T + 1 | ) {\displaystyle v_{t}=\max(|x_{t}|,|x_{t-1}|,..,|x_{t-T+1}|)\!} $v_{t}=\max(|x_{t}|,|x_{t-1}|,..,|x_{t-T+1}|)\!$
1 or EXPONENTIAL	v t = m a x ( | x t | , v t − 1 . k ) , with:  k = 1 2 T {\displaystyle v_{t}=max(|x_{t}|,v_{t-1}.k){\mbox{, with: }}k={\frac {1}{\sqrt[{T}]{2}}}} $v_{t}=max(|x_{t}|,v_{t-1}.k){\mbox{, with: }}k={\frac {1}{\sqrt[{T}]{2}}}$
2 or RMS	r t = ( r t − 1 . ( T − 1 ) + x 2 ) T {\displaystyle r_{t}={\frac {(r_{t-1}.(T-1)+x^{2})}{T}}} $r_{t}={\frac {(r_{t-1}.(T-1)+x^{2})}{T}}$ v t = 2. r t {\displaystyle v_{t}={\sqrt {2.r_{t}}}} $v_{t}={\sqrt {2.r_{t}}}$

The last value v_t of an evaluation cycle is converted to the selected UNIT and stored in the output Y.

UNIT	Y
0 or VALUE	v t {\displaystyle v_{t}\!} $v_{t}\!$
1 or SQUARE	v t 2 {\displaystyle v_{t}^{2}} $v_{t}^{2}$
2 or RATIO	v t R E F {\displaystyle {\frac {v_{t}}{REF}}} ${\frac {v_{t}}{REF}}$
3 or DB	20. log 10 ⁡ v t R E F {\displaystyle 20.\log _{10}{\frac {v_{t}}{REF}}} $20.\log _{10}{\frac {v_{t}}{REF}}$

The output P is set to a value in range 0 to SIZE and can be used to display a level bar. For P always the logarithmic value (level in dB) is used. The level interval -140dB to 0dB is mapped to the position interval 0 to SIZE using a non-linear mapping function.

The output M is the maximum of all computed v_t values converted to the selected output unit.

LIMITER - amplitude limiter (non-linear)

with:

x, y	input and output signal sample
LIM, MAX	limiter start amplitude and maximum signal amplitude
f()	limiter function selected by input TYPE

Table: Limiter functions{| |- |The value of TYPE |The limiter function f(z) |- |0 or RECTANGLE |File:1241.png ('cut-off') |- |1 or ATAN |File:1242.png |- |2 or EXPONENTIAL |File:1243.png |}

with:

The output Q is set to the relative number of processed samples:

Q = number of changed samples / number of processed samples

Usage:

LIMITER X TYPE MAX LIM

Inputs:

X

The signal vector or sample.

TYPE

The type of limiter function (see table above).

MAX

The maximum magnitude (full attenuation).

LIM

The limiter start magnitude. This value must be less than MAX.

Outputs:

Y

The limited signal (the same type as X).

Q

The relative number of limited (changed) samples.

See also

<SP-atoms>