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Whereas the VU meter has fairly equal attack and release times, the PPM
is characterized by having a very slow fall-back time, taking over 1.5
seconds to fall back 20dB (the specifications vary slightly for Type I and II
meters). The reasoning for the slow fall-back was to reduce eye-fatigue
and make the peak indication easier to assimilate. The specifications of all
types of PPM are detailed in IEC 60268-10 (1991), and the scale used by
the BBC comprises the numbers 1-7 in white on a black background.
There are 4dB between each mark, and PPM 4 is the reference level
(0dBu). EBU, DIN and Nordic variants of the PPM exist with different
scales. The EBU version replaces the BBC numbers with the equivalent
dBu values, while both the Nordic and DIN versions accommodate a
much wider dynamic range.
