Programmer Guide/SPU Reference/PPM

Level meter (PPM). This atom is used for the level display in the recorder application.

[SPU PPM X TYPE T UNIT REF SIZE OUT Y M P]

input	description	data type	value type	default value
X	input signal	number, vector	variable
TYPE	measurement method (HOLD, EXPONENTIAL, RMS)	number (int.), string	constant	0 (= HOLD)
T	measurement time constant in samples	number	constant	100
UNIT	output value and unit (VALUE, SQUARE, RATIO, DB)	number (int.), string	constant	0 (=VALUE)
REF	reference value, used for UNIT values RATIO and DB	number	constant	0
SIZE	length of bar (graph) used for ppm display	number	constant	100
output	description	data type	value type	comment
Y	ppm value	number	variable
M	maximum Y	number	variable
P	ppm bar position, always computed for the ppm value in dB (independent of UNIT)	number	variable

Description

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
1 or SQUARE	v2
2 or RATIO	v / REF
3 or DB	20 * log10(v / REF)

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

The output P is set to a value in range 0 to SIZE and can be used to display a level bar. The levels -140dB to 0dB are mapped to the values 0 to SIZE using a non-linear mapping function.

See also

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