Recording Consoles 767BS4297:1968) defi ned the rise time of a BBC style PPM in terms of reading relative to
5-kHz tone-burst durations such that, for a steady tone adjusted to read scale 6, bursts of
various values should be within the limits given here:
Table 27.1 Burst duration and the respective meter reading
Burst duration Meter reading (relative to 6)
Continuous 0 dB
100 ms 0 0.5 dB
1 0 m s 2.5 0.5 dB
5 m s 4.0 0.5 dB
1. 5 m s 9.0 1. 0 d BThis defi nition has the merit of being testable.
27.2.3.2 VU Meter
The VU meter is essentially a milliammeter with a 200-mA FSD fed from a full-wave
rectifi er installed within the case with a series resistor chosen such that the application of
a sine wave of 1.228 V RMS (i.e., 4 dB above that required to give 1 mW in 600 R) causes
a defl ection of 0 VU. Technically, this makes a VU an rms reading volt meter. Of course,
for a sine wave the relationship between peak and rms value is known (3 dB or 1/ 2), but
no simple relationship exists between rms and pk for real-world audio signals.
In frequency response terms, the response of the VU is essentially fl at (0.2 dB limits)
between 35 Hz and 10 kHz. The dynamic characteristics are such that when a sudden sine
wave type signal is applied, suffi cient to give a defl ection at the 0 VU point, the pointer shall
reach the required value within 0.3 s and shall not overshoot by more than 1.5% (0.15 dB).
27.2.4 Opto-Electronic Level Indication
Electronic level indicators range from professional bargraph displays, which are designed
to mimic VU or PPM alignments and ballistics, through the various peak-reading displays
common on consumer and prosumer goods (often bewilderingly calibrated), to simple
peak-indicating LEDs. The latter, can actually work surprisingly well—and actually
facilitate a degree of precision alignment, which belies their extreme simplicity.
In fact, the difference between monitoring using VUs and PPMs is not as clear cut as
stated. Really, both meters refl ect a difference in emphasis: the VU meter indicates