766 Chapter 27
The peak program meter of the type used by the BBC is illustrated in Figure 27.4. Note
the scale marked 1 to 7, with each increment representing 4 dB (except between 1 and 2,
which represents 6 dB). This constant defl ection per decade is realized by the logarithmic
amplifi er. The line-up tone is set to PPM4 and signals are balanced so that peaks reach
PPM6, which is 8 dB above the reference level. (The BBC practice is that the peak studio
level is 8 dB above the alignment level.) BBC research has shown that the true peaks are
actually about 3 dB higher than those indicated on a BBC PPM and that operator errors
cause the signal to swing occasionally 3 dB above the indicated permitted maximum, that
is, a total of 14 dB above alignment level.
27.2.3 PPM Dynamic Performance
27.2.3.1 PPM
In BS55428 Part 9, the PPM is stated as requiring, “ an integration time of 12 ms
and a decay time of 2.8 s for a decay from 7 to 1 on the scale. ” This isn’t entirely
straightforward to understand. However, an earlier standard (British Standard
12(^3456)
7
(a)
Audio
input
Figure 27.3 : Peak reading meters.
12
(^3456)
7
(b)
Audio
input
logarithmic
amplifier
1
2
3 4 5
6
7
Figure 27.4 : BBC style PPM.