TC Electronic DB6, Manual

Page: 64 / 76

LM6
60 DB6 Single / Multi / AES – Firmware version 1.30
Quasi-peak level meters have this effect. They
tell little about loudness and also require a head-
room in order to stay clear of distortion. Using
IEC 268-18 meters, the required headroom is
typically 8-9 dB.
Sample-based meters are also widely used, but
tell even less about loudness. Max sample de-
tection is the general rule in digital mixers and
DAWs. The side effect of using such a simplistic
measure has become clear over the last decade,
and CD music production stands as a monu-
ment over its deficiency. In numerous TC papers,
it has been demonstrated how sample-based
peak meters require a headroom of at least 3 dB
in order to prevent distortion and listener fatigue.
The only type of standard level instrument that
does not display some sort of peak level is the
VU meter. Although it has been developed for
another era, this kind of meter is arguably bet-
ter at presenting an audio segment’s center of
gravity.
However, a VU meter is not perceptually opti-
mized, or ideal for looking at audio with markedly
different dynamic range signatures.
Unlike electrical level, loudness is
subjective,
and listeners weigh its most important aspects
(SPL, frequency contents and duration) different-
ly. In search of an “objective” loudness measure,
a certain
Between Listener Variability (BLV)
and Within Listener Variability (WLV) must be
accepted – meaning that even loudness assess-
ments by the same person are only consistent to
some extent, and depend on the time of day, the
listener’s mood etc. BLV adds ambiguity as cul-
ture, age and other parameters are introduced
as variables.
Because of the variations, a generic loudness
measure is only meaningful when it is based on
large subjective reference tests and solid statis-
tics. Together with McGill University in Montreal,
TC Electronic has undertaken extensive loud-
ness model investigation and evaluation.
The results denounce a couple of Leq measures,
namely A and M weighted, as generic loudness
measures. In fact, a quasi-peak meter showed
better judgement of loudness than Leq(A) or
Leq(M). Even when used just for speech, Leq(A)
is a poor choice, and it performs worse on music
and effects.
An appropriate choice for a low complexity, ge-
neric measurement algorithm, which works for
listening levels used domestically, has been
known as Leq(RLB).
Combined loudness and peak level meters exist
already, for instance the ones from Dorroughs,
but BS.1770 now offers a standardized way of
measuring these parameters.
In 2006, ITU-R Working Party 6J drafted a new
loudness and peak level measure, BS.1770, and
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 LM6 follows the standard precisely.
Possible updates to the ITU standard may be re-
leased as LM6 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 www.tcelec-
tronic.com/tech-library/ for details.