Programmer Guide/SPU Reference/PPM

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) |}