TC Electronic LM2 - Manual (Page 19)

19
EdIT PAgE
Target Level in these countries is -23 LUFS or -24 LUFS,
measurement gating at -10 LU.
United States. Page 11 of ATSC A/85 (May 25, 2011)
references ITU-R BS.1770-1, even though BS.1770-2
was in effect at that time. The same page also says that
“All referenced documents are subject to revision”. The
wording is ambiguous and it’s up to the reader to decide
whether or not a relative gate (the difference between
BS.1770-1 and BS.1770-2) is applied when measuring
Program Loudness. The “Leq(K)” setting in LM2 disables
the relative gate, while the setting “BS.1770-2” includes
a relative gate at -10 LU. The BS.1770-2 setting is better
across genres and for controlling loud commercials.
Check in at www.atsc.org to see if the CALM act has
forced ATSC to make up their mind.
Target Level in United States is -24 LUFS, measurement
gating not clearly defined.
Limiter
LM2 includes a true-peak limiter and 48 bit internal
precision. Limiter Threshold sets the max true-peak
level on LM2’s outputs. In case positive normalization is
invoked, the limiter comes into action when the peak level
could otherwise have created an overload.
Profiles: Dynamic, AC3 Codec, Universal, Loud, Voice.
The adaptive Profiles allow you to tell The Limiter what
your intentions are.
Several internal variables are dynamically updated based
on the Profile chosen, so distortion is perceived (and
measured) to be as low as possible for a given loudness,
and reduction in average/peak ratio. Maximum perceived
distortion tolerated is different from Profile to Profile. The
multi-dimensional parameter-space is fully continuos to
avoid situations where distortion could occur, or where
processing is suddenly altered without a detectable
cause. Acquaintance yourself with the different Profiles
to have them at hand when called for by a specific
application.
The Dynamic Profile offers gentle processing with
minimum static and dynamic distortion. It’s a first choice
for audiophile limiting, for instance with classical music,
choir etc.
The AC3 Codec profile has been optimized for protecting
the relatively sensitive AC3 codec, which was invented
before squeeze-box techniques in production had
proliferated. When used for stereo, there are no down-mix
issues with the codec, and the threshold of the true-peak
limiter can safely be set to -2 dBFS or -1 dBFS.
The Universal Profile has a higher tolerance for
perceived distortion than Dynamic. It is still magnitudes
better than analog designs, but may be on the aggressive
side for sensitive music. It is a good choice for starters,
and in production where you don’t know what to expect.
The Loud Profile again has a higher tolerance for
perceived distortion, and is therefore better suited for
production and mastering rather than live applications.
The Voice Profile is targeted human voice, and adapts
well to asymmetrical signals. Used on composite material,
perceivable distortion is to be expected with large ratios
of limiting.
Limiter Threshold
Range: -18 dB to off
LM2’s limiter employs a look-ahead delay of 0.6 ms and
complies with ITU-R BS.1770-2 true-peak specifications.
Because true-peak limiting is a better strategy from an
audio quality point of view than sample-peak limiting, and
because LM2 has an immense internal headroom, there
is generally no need to set the limiter threshold lower than
-1 dBFS. However, if the downstream path contains low
bit-rate codecs or emphasis/de-emphasis circuitry, the
limiter threshold may need to be set considerably lower.
To enable pristine audio quality, the limiter threshold
should not be set lower than needed for a certain
broadcast platform. Also notice how you may be able to
lift the limiter threshold over time as a distribution path
improves.