Production Technology Seminar 2009 - EBU Technical

Some extreme examples [13]: movies with very low level of average loudness (e.g. -28 dBFS) and a big
difference between the average loudness and the maximum peak level – and on the other hand
commercials with very high average loudness level (e.g. -13 dBFS) and very small dynamic range. And
we cannot transmit audio unaltered with this 15 dB difference - that would be unacceptable for listeners.
What did we do up to now? We normalized to the peaks with PPM (Peak Programme Meter) [14] (to
'quasi-peaks', taking into account the 10 ms reaction time meters), making the situation of average
loudness even worse with an even larger difference between movies and commercials! So to broadcast
it, we compress the audio signal [15]. We still have the same peaks, but we push up the low-level details.
Of course we sacrifice the dynamic range. So, this solution is already a compromise.
The ideal solution would be to normalise to loudness instead of peaks [16]. The -31 dB figure in the
"Line mode" stems from the Dolby system (it is the lowest possible value of their loudness metadata
parameter 'DIALNORM'). Everything, from the line level signal to the decoder of the set-top box, is
aligned to the same -31 dBFS level, loudness normalised, and we have a totally varying peak value with
the constant loudness value. That would already be a fantastic solution, the consumer at home not being
forced anymore to adjust the volume with his/her remote control. An interesting area is this huge amount
of headroom for the programmes that used to be highly compressed, especially the commercials that
could be produced again in a transparent dynamic way. Compression would only be used for artistic
reasons and not for the only purpose of sounding louder and louder! A problem might be that the
dynamic range may be too big for the living room.
But we live in a non-homogeneous coding world (PCM, Dolby, MPEG…). If we switch to a channel with
MPEG-1 Audio Layer II, usually the loudness is in the range of –20 dB [17], and so there is a gap of
11 dB between programmes normalised at -31 dB loudness and programmes transmitted as they are.
Therefore there is a second mode in the system called "RF mode" normalising the loudness to -20 dB,
more comparable to the legacy MPEG Audio programmes found in other channels. We have again
loudness normalisation, but we have nevertheless to apply some compression to avoid overshooting, for
the action movies, for example.
Therefore we are looking at the Recommendation developed by the ITU Working group 6G,
normalising everything to -23 dB, suggesting a new maximum true peak level of -2 dB [18].The ultimate
goal is that the ITU Rec. will be the same as the EBU Rec.
Another recommendation from the Utrecht school of Music Technology [19] suggests -21 dB as the
target level (but it is only 2 dB apart ITU Rec., even less in fact) and a maximum peak level of -5 dB, the
reason for this value being the concern for the analog re-broadcasters.
As far as measurement is concerned the ITU Group issued Recommendation BS.1770, which is now
the basis for the implementation of most loudness meters. It is a very simple measurement, easy to
implement. Starting from the well-known weighting curves [21]: A (very low level signals) … D (for noise
measurement). The revised low-frequency B-curve is a very easy to implement high pass filter, and the
B-curve has been modified for surround sound to include a high-frequency weighting filter [22], and this
is the basis for the ITU measurement. This 2 nd Revised Low Frequency B-curve is named R2LB, or Kweighting.
If you measure Loudness, you then speak of LKFS (Loudness, K-Weighting and Full Scale),
for example "-23 LKFS" (no need to say "dB LKFS"…), and if you substitute R2LB to K it becomes
LR2LBFS [26]!
One of the issue is on which signal type do we base our measurement? There are strong contenders
for voice (Dolby) but there are others for music (concerts), sound effects (commercials - there are very
short pieces where an algorithm for detecting dialog and speech has not enough time). The ultimate goal
is to find a basis as broad as possible, and we certainly will recognise and recommend all three types of
signals [28].
Gating is also very important. For example, during a golf sport transmission, nothing is happening in
audio for a long time (very little atmosphere, presenter saying something, then half a minute silence…).
You don't want the measurement to be just too low because most of your transmission is very low level.
So we are thinking about a threshold level below which the measurement is paused. This must be the
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