TC Electronic DB4 MKII - Manual (Page 67)

ALC 5.1
English Manual 63
ALC 5.1
ALC 5.1 is an ITU-R BS.1770 Loudness Correc-
tion algorithm.
Introduction
Years of research and standardization work on
loudness and true-peak level has enabled TC
to design high resolution, low latency loudness
measurement and control equipment such as
this new Automatic Loudness Correction pro-
cessor, ALC5.1.
In broadcast, digitization is driving the number
of AV channels and platforms up, while the total
number of viewers remains roughly the same.
Using only a dialog-based level control principle
has led to ambiguous level management, more
“level jumps between programs”, and extra time
spent on audio production and management in
general. Non-dialog based level jumps are cur-
rently creating havoc in digital TV; and ALC5.1
helps correct that situation.
Fig 1
Target loudness for selected broadcast plat-
forms based on a consumer’s Dynamic Range
Tolerance, DRT.
When processing is centered around average
loudness, the –20 dB line, transparent platform
“trickle-down”, where the dynamic range can
be restricted step by step, is automatically en-
abled.
Note how different the broadcast requirements
are from those of Cinema. Several TC papers
are available about the subject. Visit the Tech
Library at the TC website for more details.
ALC5.1 is part of a universal approach to loud-
ness control, starting at the production or live
engineer with an easy-to-read loudness me-
ter and universal delivery specifications. When
downstream dynamic range is a known quantity
it can be adjusted during the production or in-
gest phases, requiring less processing at later
stages of a distribution chain. The chain ends
with the capability of quality controlling previous
stages by applying the same loudness measure
for logging purposes: A closed loop based on
the open standard ITU-R BS.1770.
The full leveling process needs not be put in
place all at once. Production engineers may
keep using VU, PPM or Dorrough meters with
which they are comfortable, as long as the av-
erage loudness normalization process and plat-
form ranging is known, and can be taken into
account.
Welcome to a new world of leveling, where dis-
torted and overly loud audio is unacceptable,
where program content with different dynamic
range may be broadcast back to back – without
abrupt level changes.
Automatic Loudness Correction
for Stereo and 5.1
ALC5.1 offers processing complementary to
ITU-R BS.1770, EBU R128 and ATSC A/85 based
normalization for use in broadcast ingest, link-
ing and transmission. ALC5.1 may fully or partly
correct level jumps within broadcast programs
and at transitions between them. The resolution
of ALC5.1 is sufficiently high that more than one
hundred processors may be cascaded without
degradation of sound quality.
ALC5.1 can be used to control level and improve
sound, not only in Dolby® AC3 based transmis-
sion and linking, but also on other broadcast
platforms, such as analog TV, mobile TV and
IPTV. The Engine uses the new ITU-R BS.1770
standard, which measures speech, music and
effects equally well, and can deal with mono,
stereo and 5.1 signals.
ALC5.1 makes life with Dolby AC3 easier for the
broadcaster by 1) limiting the amount of work
which has to be put into generating metadata,
2) making the end-listener experience more pre-
dictable, 3) reducing the amount of level jumps
between programming, and 4) improving the
overall DTV sound quality.