LCn English Manual

Appendix: Level versus loudness
LCn Manual 31
The path to BS.1770
When level normalization in audio distribution is
based on a peak level measures, it favors low
dynamic range signatures. This is what has hap-
pened to the CD format.
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 headroom needed is
typically 8-9 dB.
Sample based meters are also widely used,
but tell even less about loudness. Max sample
detection 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. Though developed for another era,
this kind of meter is arguably better at presenting
an audio segment’s center of gravity. However, a
VU meter is not perceptually optimized, or ideal
for looking at audio with markedly different dy-
namic range signatures.
Unlike electrical level, loudness is subjective,
and listeners weigh its most important factors
– SPL, frequency contents and duration – dif-
ferently. In search of an “objective” loudness
measure, a certain Between Listener Variabil-
ity (BLV) and Within Listener Variability (WLV)
must be accepted – meaning that even loudness
assessments by the same person are only con-
sistent to some extent, and depends on the time
of day, her mood etc. BLV adds further to the
blur, when sex, culture, age etc. 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 pick, and it performs worse on mu-
sic and effects. An appropriate choice for a
low-complexity, generic measurement algorithm
which works for listening levels used domesti-
cally 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 ex-
ploited where possible. However, with a variety
of program material, Leq(RLB) has been veri-
fied in independent studies to be a relatively ac-
curate 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 stan-
dard included a multichannel annex with a re-
vised 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.