LCn English Manual

Appendix: Level versus loudness
LCn Manual 32
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 loud-
ness and peak level measurement engine of LCn
follows the standard precisely. Possible updates
to the ITU standard may be released as updates
to LCn (provided that processing 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.
For details, visit the TC Tech Library at
tcelectronic.com/tech-library/
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 digi-
tal peaks at 0dBFS is regarded a 0dBFS tone),
BS.1770 and LCn 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.
Display
LCn may use either the measurement unit of LU
(Loudness Units) or LUFS (Loudness Units
Full Scale).
LU and LUFS are measurements in dB, reflect-
ing the estimated gain offset to arrive at a certain
Reference Loudness (LU) or Maximum Loudness
(LUFS) as defined in BS.1770. Since a common
reference point for LU has not been agreed on
at the time of writing, LUFS (or “LKFS”, point-
ing specifically to the Leq(R2LB) weighting of
BS.1770), might be favored initially to avoid am-
biguous use of the term LU.
The effectiveness of any loudness meter de-
pends on both the graphical appearance and dy-
namic behavior of its display, as well as on its un-
derlying measurement algorithms. A short-term
loudness meter also relies on the measurement
algorithm’s ability to output pertinent loudness
information using different analysis windows, for
instance, 200-800 ms for running real-time up-
dates. It should be noted how the optimum size
of this window varies from study to study, pos-
sibly because the objective of a running display
hasn’t been fully agreed upon.
Formal evaluation of a visualization system is
challenging: First of all, one or more metrics
must be defined by which the display should
be evaluated. The correspondence between the
sound heard and the picture seen is one aspect
to be evaluated. Another metric could character-
ize the speed of reading the meter reliably.