VU and PPM meters
A level meter does not simply show the voltage. It shows the voltage through a specific kind of averaging, and different meters average differently. That is why two meters can disagree about the same signal.
VU meters
The volume unit meter was developed in the 1930s for broadcasting in the United States. Its ballistics are tuned to give a reading related to loudness.
- It responds to the average of the rectified signal (not RMS, but similar for most purposes).
- Its needle takes about 300 ms to reach full reading (the integration time), so brief peaks are not shown.
- Reference: 0 VU corresponds to +4 dBu on professional equipment (some broadcast systems use +8 dBu).
A VU meter gives a good impression of the average level but under-reads the peaks of music by 10 to 15 dB.
Peak programme meters (PPM)
The peak programme meter was designed in Europe to catch peaks that could overload transmission equipment.
- Fast attack: integration time of a few milliseconds (5 to 10 ms depending on type).
- Slow return: the display falls back slowly, over 1.5 to 3 seconds, so that the eye can read it.
- Different countries use different scales: DIN (ARD/IRT), EBU, BBC, Nordic.
A typical reference is the test level on the scale. The meter scales tool shows how the PPM and VU scales relate to dBu and dBFS.
Sample peak and true peak
Digital meters show the largest sample value. But when a digital signal is converted to analogue (or to a lossy format), the waveform between the samples can exceed the sample values. These inter-sample peaks can reach 1 to 3 dB above the highest sample. True-peak meters estimate them by oversampling, and broadcast standards specify limits in dBTP, such as −1 dBTP.
Loudness meters
Neither VU nor PPM measures loudness well. LUFS meters, defined in ITU-R BS.1770, apply a weighting similar to the ear's and average over time, giving a value that tracks loudness. They show momentary (400 ms), short-term (3 s) and integrated (whole programme) values. See Loudness and dynamics.
Which meter when
| Task | Meter |
|---|---|
| Setting recording gain, avoiding clipping | peak (sample or true peak) |
| Judging balance and average level | VU or RMS |
| Meeting a broadcast or streaming target | LUFS and true peak |
| Checking stereo compatibility | correlation meter, see Mono compatibility |
Calibration
Meters are aligned with a 1 kHz sine tone. A VU meter reads 0 VU for +4 dBu. A digital meter reads the alignment level of the system. The tone generator can supply a test tone, though it is capped for safety.
Draft: written by an AI assistant, awaiting technical review.
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