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 output M is the maximum of all Y values since the spu creation.

M = max(Y₀, Y₁, ..)

See also

<SP-atoms>

PPM - signal peak measurement

Usage:

PPM X TYPE T UNIT REF

Inputs:

X	signal (sample or frame)
TYPE	type of measurement
T	measurement interval, integration time (in samples)
UNIT	output unit/mode
REF	reference value for relative output units/modes

Outputs:

Y	current peak value
M	maximum peak value

Function:

This function implements a set of magnitude measurement methods. If input X is a number, a peak value for each sample is computed. If input X is a vector, only one peak value for the vector is computed.

The computed peak value is converted to the selected unit/mode before it is written to the output. For the relative output modes (UNIT equal 2 or 3) the value of input REF (>0) is used as the reference value.

The output M is set to the maximum of all computed Y values (M(t) = max(Y(0), Y(1), .., Y(t)).

Table: Peak magnitude measurement methods selected by input TYPE{| |- |value of TYPE |method (P = peak value) |- |0 or HOLD |The absolute peak magnitude is measured using a sample/hold circuit:if |X(t)| > P(t-1) or holdtime > T:P(t) = |X(t)|; holdtime is set to zerootherwise:P(t) = P(t-1); holdtime is incremented |- |1 or EXPONENTIAL |Use an exponential decay for peak measurement:P(t) = max(k . P(t-1), |X(t)|)The factor k is set to k=0.51/T . This means the ppm value falls to 50% after T samples, if input values are lower than the ppm output. |- |2 or RMS |Pseudo ppm measurement based on running rms magnitude (just for test purposes):(1) r(t) = ((T-1).r(t-1) + X2(t)) / T(2) P(t) = (2.r(t))1/2 |}

Table: Peak magnitude output formats selected by input UNIT{| |- |value of UNIT |conversion |- |0 or VALUE |Y(t) = P(t) |- |1 or SQUARE |Y(t) = P(t)2 |- |2 or RATIO |Y(t) = P(t) / REF |- |3 or DB |Y(t) = 20.log10(P(t) / REF) |}