The EDCF Guide to ALTERNATIVE CONTENT in Digital Cinema

Audio Interfacing
Global standardisation and interchange
Aside from the technology-offering, the vital ingredient of the
Dolby film programme was the extensive research carried out
in order to improve and to standardise many aspects of the
audio process from the mixing stage through to reproduction
within exhibition. Of relevance to this paper are the standards
that specify precise electro-acoustic level and frequency
response characteristics for each replay channel. These standards
are published and maintained within the Society of
Motion Picture and Television Engineers (SMPTE) and at the
International Organisation for Standardization (ISO) and have
been adopted, for all current cinema sound formats, by studios
and cinemas globally.
This tight standardisation provides the director and post-production
sound crew an audio system capable of high dynamic
range, high unification to the picture, and high accuracy of
tonal and level reproduction; and the capability for the crew to
base their detailed creative decisions (over typically hundreds of
sound elements) in an environment that matches what will be
heard by the final audience. The whole family of cinema standards
have enabled total interchange of cinematic materials
around the world—and this is set to continue as the industry
embraces and standardises Digital Cinema.
Members of the ISO TC36 Standards Committee at a meeting in
Seoul. The author can be seen far right.
The standards of interest are ISO2969 (1), which provides frequency
response characteristics, and ISO22234 (2), which provides
reference sound pressure levels for each loudspeaker
channel. The key element is that these standards describe reference
pink noise stimulus and its measurement in the acoustic
listening area of the studio or cinema itself. The target frequency
response achieved from the reference pink noise stimulus is
often referred to as the X-curve. The target sound pressure levels
achieved from the reference pink noise stimulus is 85
dBC(slow) for the full-range screen channels and 82 dBC(slow)
for each discrete surround channel array. The LFE / Sub-woofer
is set +10 dB relative to the 85 dBC(slow) achieved by the
screen channels and, due to its limited frequency response (20
– 125 Hz) is typically set with a Real Time Analyser for absolute
precision regardless of absolute bandwidth.
Interfacing non-cinema audio
The art
As discussed in greater detail in John Emmett’s paper in this
publication, sound mixes prepared specifically for DVD or television
broadcast, whilst potentially 5.1, are not generally prepared
in an environment set to the electro-acoustical standards
for cinema. Such mixes can give variable and often undesirable
results if replayed directly in the cinema without any adjustment.
Typical problems to look out for include:
• poor suitability of 2-ch stereo content;
• stereo width compatibility;
• the miss-use of phantom centre between Left and Right rather
than the proper use of the Centre channel;
• loudness uncertainty;
• surround level differences (level and delay settings);
• intelligibility and tonal issues due to the large auditorium size,
listening area, typical use of horn systems, and increased reverberation
time / characteristics;
• audio-visual synchronisation; and
• channel routing issues.
Key features to exploit include:
• high dynamic range available;
• certainty of replay level calibration;
• loudspeakers situated behind the screen for good image unification;
• centre channel for vocals, dialogue, and other key sound elements;
• enveloping surround ambiance for crowd or audience emulation;
and
• the presence of a discrete LFE channel with no risk of base
management intervention.
Transmission
Many projects utilise HDTV broadcast transmission encoding
and receiving products, although a higher data rate may be
used than for normal HD television services. Specifications for
the digital transmission equipment widely used in Europe have
been set by the DVB group; the standards document ETSI
TS101 154 (3) outlines the various audio coding formats which
are allowable. To date, HD DVB television services have widely
utilised the AC3 audio format, also known as Dolby Digital, for
multichannel broadcasts. The launch of new European HDTV
services utilising MPEG4 video is also resulting in the introduction
of new multichannel-capable audio codecs: both HE-AAC
(also known as aacPlus) and E-AC-3 (Dolby Digital Plus) formats
are expected to be used and may therefore also have
application in alternative content broadcasts in the future.
Care should be taken in the implementation of the broadcast
transmission system with regard to encoder reference synchronisation
and latency management (lip-sync). Alternative content
transmission projects to date have typically utilised separate
video and audio hardware encoders. In this configuration, the
audio encoder must be referenced to a correctly derived timing
signal from the video encoder to avoid data errors resulting in
the final transport stream. To maintain lip-sync, it is necessary
to quantify and manage the video and audio delays introduced
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