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Unlike electrical level, loudness is subjective, and listeners weigh its most important
factors - SPL, Frequency contents and Duration - differently. 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 assessments
by the same person are only consistent 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 statistics. Together with McGill
University in Montreal, TC Electronic has undertaken extensive loudness 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 used just for speech, Leq(A) is a poor pick, and it
performs worse on music and effects. An appropriate choice for a low complexity,
generic 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 effect. It has been debated if
the loudness part is robust enough, because it will obviously get exploited where
possible. However, with a variety of program material, Leq(RLB) has been verified in
independent studies to be a relatively accurate 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 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. Inconsistent peak meter readings, unexpected overloads, distortion in data
reduced delivery and conversion etc. has been extensively described, so in liaison with
AES SC-02-01, an over-sampled true-peak level measure was included with BS.1770.
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 loudness and peak level measurement engine of LM5 follows
the standard precisely. Possible updates to the ITU standard may be released as LM5
updates, 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.
Visit the Tech Library at www
.tcelectronic.com/techlibrary.asp for details.
ITU-R BS.1770 PRIMER
ITU-R BS.1770 standardizes the measurement of long-term loudness and of true-peak
level. LM5 displays both parameters, and additionally offers a consistent way of
breaking up long-term loudness into a coherent, running measure of instant loudness
and of loudness history.
A block diagram of the loudness detection part of the BS.1770 measure is shown in
Fig 10. The way channels are summed (means square), to some extent mimic s
speaker summing in a the real world. Therefore, BS.1770 gives a meaningful
indication of loudness, regardless if the input format is mono, stereo or 5.1.
Fig 10, ITU-R BS.1770 Loudness Measure Block Diagram.
Note how an extra filter has been added in front of the original RLB filter, and that
the LFE component is disregarded completely in the measure.
The three front channels - L, C, R - employ the same channel weighting, while the
measure is 1.5 dB more sensitive to surround channel signals. The reading of a signal
entered into a surround channel is therefore 1.5 dB higher than if the same signal
was entered into a front channel. The LFE channel is completely disregarded by the
BS.1770 loudness measure.