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Audio: Part II
Surviving the loudness
rollercoaster
Peter Poers outlines ways of solving audio level problems,
which have intensified with the migration to HD
rogramme supplies have long
been aware that surprise level
changes in broadcast audio
is annoying for viewers, especially if they
have to keep adjusting the volume.
While differences in loudness are
most common between separate channels
and between TV programming and
advertisements, the problem isn't just
confined to the ad breaks or the channel
surfer. Notable loudness differences can
also exist in the same programme, for
example during a movie, talk show or a
sports programme (where studio
commentary is mixed with a live feed from
an outside broadcast vehicle).
Having to deal with a wide variety
of audio sources is one reason why
loudness is so difficult to control, but
equally relevant are the number of different
formats - for example, mono, stereo or 5.1
surround sound. The migration to HDTV
services has intensified the problem as the
audio for most digital HD services is
transmitted in multichannel coded format.
The most common standard used is the AC3, also known as Dolby Digital. For HDTV
broadcasters, audio levels are a huge issue
because there is often a remarkable
difference in loudness between PCM stereo
tracks and related 5.1 coded AC-3 streams.
Many people, including some involved
in the broadcast industry, don't understand
that there is a difference between audio
level and loudness. Measuring and
controlling one doesn't necessarily measure
or control the other, which is why this issue
has been so difficult to address.
Historically, broadcasters have
concentrated their efforts on measuring
audio levels, which is relatively easy to do
using PPM and QPPM metering. But if you
P
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want to give your viewers a more pleasant
listening experience then you have to
measure and control loudness as well,
because it is differences in loudness that
are likely to have them reaching for the
volume control.
Dealing with complexity
To deal with loudness broadcasters need to
perform some kind of gain control to level
the volume between the various audio
sources. Yet to make this acceptable to the
viewer, it is important that the gain control
is inaudible. To understand this, one has to
look at the psychoacoustic effects.
The human ear does not recognise
changes in the intensity of an acoustical
event provided the transition time is long
enough. If a gain change is performed at
1dB in three to four seconds, the human
ear is physically incapable of noticing that
change. This means that if broadcasters
perform gain changes at a speed that is no
faster than 1dB/3sec, the manipulation will
not be perceptible to the listener. Therefore,
it is possible to achieve inaudible automated
gain control so long as whatever equipment
is being used doesn't try to perform gain
control too quickly.
There is another psychoacoustic effect
that is related to immediate audio level
changes. Listening to dynamic sound isn't
something people find irritating. In fact,
viewers need to percept different audio
levels in order to appreciate the dramatic
content of a programme. In the right
acoustical environment, listening to audio
material with a dynamic range of 60 dB it
is not a problem, but if the audio level
drops or jumps more than 12 to 14 dB
from one moment to another it becomes
very difficult for one's senses to
comprehend, and for the listener these
unexpected and immediate loudness
changes can be very unpleasant indeed.
When taking into account the
psychoacoustic effects, it becomes clear
that dealing with loudness is complicated.
Tackling loudness requires today's
broadcasters to move away from purely
dBFS level measure and control methods
and start to introduce 'Loudness Units' as
well. However, because loudness is a
subjective interpretation that can't simply be
measured using a 'peak' approach, there is
also a need for the industry to agree and
adopt a common rule or standard so that
the problems it currently faces get better
and not worse.
Which broadcasters are being most
affected by loudness - and why?
Surprise level changes are an issue for all
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Audio: Part II
broadcasters whether they are terrestrial,
satellite, cable or IPTV providers. However,
for re-broadcasters the situation is even
harder to address because generally they
have no control over the average operating
levels of the content they are being asked
to re-broadcast.
Broadcast playout centres receive radio
and TV programmes from an incoming
source and then distribute them on to the
viewer. Sometimes, prior to analogue
transmission, the sources are modulated
and the frequency converted into different
channels. In the digital environment, sources
are received and decoded before new
encoding and multiplexing is carried out
prior to further distribution.
With broadcast contribution systems the
situation is very different because there are
many more sources to deal with. These
often include direct lines from studios, as
well as sources from worldwide satellite
networks or from fibre link-based
distribution. The loudness level of the audio
content may vary significantly depending on
the source, but added to that there is also
the issue of the wide range of technical
hardware coming together in the
transmission path. For most broadcasters,
this means dealing with a range of
professional and semi-professional receiver
Cable & Satellite International
boxes. Audio interfaces can also be very
different in format and specifications,
particularly when you are combining
or moving between analogue and digital
audio interfacing.
The problem has grown to the extent that
governments are getting involved by
bringing in guidelines to control audio
loudness, specifically between programming
and commercials.
Good quality audio can also help service
providers differentiate: those that don't
provide proper and stable images, audio
and connectivity will lose customers. And as
viewers install home cinema systems with
5.1 surround sound loudspeakers they
demand better quality sound and pictures.
How is the industry resolving this?
The broadcast industry has recognised the
need to agree and adopt a common rule to
measure loudness and both the ATSC in the
US and the EBU in Europe are currently
working on projects that will set a proper
norm to help all broadcasters follow
industry standards.
Also involved is the ITU-R, which in July
2006 published a new recommended
algorithm for estimating program loudness
levels for use in digital broadcasting: ITU-R
Rec BS1770 - Algorithms to Measure Audio
Programme Loudness and True-Peak Audio
Level. Either Leq(A) or the new ITU-R
measurement method will, on average, yield
the same results. On both sides of
the Atlantic some kind of
agreement seems to be
moving towards the ITU-R
Rec. BS.1770 standard.
The advantage of the
standard is that it
allows for the
control of audio
content to a given
Dialnorm value,
which is part of the
metadata that
comes as standard
with AC-3, and is used
to normalise the audio
output of the receiver/ decoder
in the viewer's home in order to
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equalise dialogue loudness. By being able
to control audio content to a given Dialnorm
value, the industry now has a real
opportunity to improve the consistency of
coded 5.1 audio content because it can
optimise the audio level so that it conforms
to the Dialnorm value used during
production or transmission.
Another advantage is that the
accompanying PCM stereo track can also
be controlled in the same way, with the
result that both audio programmes will have
the same loudness impression. This
significantly improves quality for consumers
who no longer need to change their volume
settings when changing channel or
switching from 5.1 to stereo programming.
To understand how this works it is worth
looking at the differences between PCM and
5.1 coded signals. PCM stereo is the
standard audio for TV and is transmitted as
it has been recorded, which means there
shouldn't be any level or loudness change
during the transport process. The
implication is that loudness issues can be
handled before playout because transport
of the signal won't alter the content. With
surround sound it is a different story
because the six channels (5+1) have
to be compressed and coded so they can
be transmitted over a stereo audio channel.
Apart from the ATSC standard, no special
coding methods are specified as standard,
although Dolby has sold an estimated three
billion Dolby Digital 5.1 decoder chips
worldwide. Added to this, there is a need to
handle transitions between stereo (PCM)
and surround (AC3) signals - and here
again the broadcaster and
re-broadcaster faces significant loudness
issues because the digital coding process
sets certain parameters that, through the
metadata, enable the receiver box to set the
appropriate values. Dolby targets a
reference value for loudness at -31 dBFS,
while a figure of around 20dBFS is being
discussed by the EBU's PLOUD group.
Whatever standard is adopted, it is in
broadcasters’ own interest to do something
to tackle the issue. CSI
Peter Poers is managing director at Jünger
Audio
www.csimagazine.com
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