TC Electronic DB4 MKII - Manual (Page 93)

LM6
English Manual 89
built into the standard. The final BS.1770 stan-
dard included a multichannel annex with a re-
vised weighting filter, R2 LB – 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. has 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 LM6 follows the standard precisely. Pos-
sible updates to the ITU standard may be re-
leased as LM6 updates, provided that process-
ing 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.tcelec-
tronic.com/techlibrary.asp 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 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.
Display
LM6 may use either the measurement unit of
LU (Loudness Units) or LUFS (Loudness Units
Full Scale). LU and LUFS are measurements in
dB, reflecting the estimated gain offset to arrive
at a certain Reference Loudness (LU) or Maxi-
mum 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”, pointing specifically to the Leq(R2 LB)
weighting of BS.1770), might be favored initially
to avoid ambiguous 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 to 800 ms for running realtime 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.
In TC Electronic LM2, LM5 and LM6, short-term,
mid-term and long-term of loudness measure-
ments are tied together coherently, and dis-
played in novel ways (angular reading and radar)
that were preferred in its development and test
phases. However, we remain open to sugges-
tions for further improvement of the visualization
of loudness.