TC Electronic LM2 Plug-in - Manual (Page 20)

20 / LM6 Radar Loudness Meter
in independent studies to be a relatively accurate measure, and correlate well
with human test panels. It therefore seems justified to use Leq(RLB) as a baseline
measure for loudness, especially because room for improvement is also built into the
standard. The final BS.1770 standard included a multichannel annex with a revised
weighting filter, R2LB – now known as “K” weighting - and a channel weighting
scheme. These two later additions have been less verified than the basic Leq(RLB)
frequency weighting.
The other aspect of BS.1770, the algorithm to measure true-peak, is built on solid
ground. Inconsistent peak meter readings, unexpected overloads, distortion in data
reduced delivery and conversion etc. has been extensively described, so in liaison
with AES SC-02-01, an over-sampled true-peak level measure was included with
BS.1770.
In conclusion, BS.1770 is an honorable attempt at specifying loudness and peak
level separately, instead of the simplistic (sample peak) and mixed up measures
(quasi-peak) in use today. The loudness and peak level measurement engine of LM6
follows the standard precisely. Possible updates to the ITU standard may be released
as LM6 updates, provided that processing requirments doesn’t exhaust the system.
Technical papers from AES, SMPTE, NAB and DAFX conferences with more
information about loudness measurement, evaluation of loudness models, true-peak
detection, consequences of 0 dBFS+ signals etc., are available from the TC website.
Visit the Tech Library at www.tcelectronic.com/techlibrary.asp for details.
Meter Calibration
Because of the frequency and channel weighting, and of the way channels sum, only
specific tones and input channels should be used for calibration.
The most transparent results are obtained using a 1 kHz sine tone for calibration.
Other frequencies or types of signal may be used (square wave, noise etc.), but don’t
expect similar results. The beauty of the system lies in its RMS foundation, so this is
a feature, not an error. The same feature enables the loudness measure to identify
overly hot CDs or commercials, and to take out of phase signals into account just as
much as signals that are in phase.
If we stick to standard methods for measuring peak audio level in a digital system,
where a sine wave (asynchronous of the sample rate) with digital peaks at 0 dBFS, is
regarded a 0 dBFS tone, BS.1770 and LM6 output these results:
One front channel fed with a –20 dBFS, 1 kHz sine tone => Reading of –23,0 LUFS.
Two front channels fed with a –20 dBFS, 1 kHz sine tone => Reading of –20,0 LUFS.
All 5.1 channels fed with a –20 dBFS, 1 kHz sine tone => Reading of –15,4 LUFS.