Manual

LM2 (optional)
Loudness Pilot English Manual (2014-10-07) 111
the standard has subsequently come into ef-
fect. It has been debated if the loudness part
is robust enough, because it will obviously get
exploited where possible. However, with a va-
riety of program material, Leq(RLB) has been
verified in independent studies to be a relatively
accurate measure, and correlate well with hu-
man test panels. It therefore seems justified to
use Leq(RLB) as a baseline measure for loud-
ness, 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. In-
consistent peak meter readings, unexpected
overloads, distortion in data reduced delivery
and conversion etc. have been extensively de-
scribed, so in liaison with AES SC-02-01, an
over-sampled true-peak level measure was in-
cluded with BS.1770.
In conclusion, BS.1770 is an honorable attempt
at specifying loudness and peak level separate-
ly, instead of the simplistic (sample peak) and
mixed up measures (quasi-peak) in use today.
The loudness and peak level measurement en-
gine of LM2 follows the standard precisely.
Possible updates to the ITU standard may be re-
leased as LM2 updates, provided that process-
ing requirements 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 tcelectronic.
com/tech-library/ for details.
Meter Calibration
Because of the frequency and channel weight-
ing, and of the way channels sum, only specific
tones and input channels should be used for
calibration.
The most transparent results are obtained us-
ing a 1 kHz sine tone for calibration. Other fre-
quencies 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 founda-
tion, 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 dig-
ital peaks at 0dBFS, is regarded a 0dBFS tone,
BS.1770 and LM2 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.