Maintaining Audio Quality in the Broadcast Facility 2011 - Orban

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Maintaining Audio Quality
The Jones & Torick algorithm improves upon the original loudness measurement
algorithm developed by CBS researchers in the late 1960s. Its foundation is psychoacoustic
studies done at CBS Laboratories over a two year period by Torick and the
late Benjamin Bauer. After surveying existing equal-loudness contour curves and
finding them inapplicable to measuring the loudness of broadcasts, Torick and
Bauer organized listening tests that resulted in a new set of equal-loudness curves
based on octave-wide noise reproduced by calibrated loudspeakers in a semireverberant
16 x 14 x 8 room, which is representative of a room in which broadcasts are
normally heard. They published this work in “Researches in Loudness Measurement,”
IEEE Transactions on Audio and Electroacoustics, Volume AU-14, Number 3,
September 1966, pp. 141-151. This paper also presented results from other tests
whose goal was to model the loudness integration time constants of human hearing.
BS.1770 Loudness Meter
Developed by G.A. Soulodre, the BS.1770 loudness meter uses a frequency-weighted
r.m.s. measurement intended to be integrated over several seconds—perhaps as
long as an entire program segment. As such, it is considered a “long-term” loudness
measurement because it does not take into account the loudness integration time
constants of human hearing, as does the CBS meter.
Orban’s BS.1770 loudness meter uses the Leq(RLB2) algorithm as specified in the
Recommendation. This applies frequency weighting before the r.m.s. integrator. The
frequency weighting is a series connection of pre-filter and RLB weighting curves.
The Orban meter precisely implements equations (1) and (2) in this document by using
a rolling integrator whose integration time is user-adjustable from one to ten
seconds. In an AES convention preprint, Soulodre proposed using a three second integration
time when the BS.1770 meter was used to adjust program levels in approximately
real time. However, the published BS.1770 standard does not specify a
specific integration time.
The 2010 version of the BS.1770 meter has been shown to be inaccurate when the
program material contains considerable energy at very low frequencies, which is
true of many action movie soundtracks 3 . Work is ongoing to improve the standard
in this area.
Because the measurement of loudness per BS.1770 is an integrated measurement,
quiet passages (like pauses in dialog) tend to inappropriately decrease the measured
value. To avoid this, the integration may be paused during quiet passages. Automatic
triggering, hold and resume, generally called “gating”, was incorporated in
the EBU Recommendation R 128 4 loudness meter, which is based on BS.1770. While
the EBU has expressed the desire to incorporate the R 128 improvements into a revision
of BS.1770, this has not yet occurred as of this writing.
3 Norcross, Scott G; Lavoie, Michel C.,”Investigations on the Inclusion of the LFE
Channel in the ITU-R BS.1770-1 Loudness Algorithm,” AES Convention Paper 7829,
127 th
AES Convention, New York 2009.
4 EBU Recommendation R 128: “Loudness normalisation and permitted maximum
level of audio signals,” Geneva, 30 August 2010. http://tech.ebu.ch/docs/r/r128.pdf