Algorithm Manual Revision 4

ALC 6
English Manual – Updated 2018-02-28 171
ALC6 – Limit page
The Limiter in ALC6 uses true-peak detection
exceeding the oversampling requirements of
BS.17 70.
Fig 5
Center Trim
Use Center Trim to apply static gain control to
the Center channel after the ALC section, but
before the output limiter.
The range of the Trim parameter is -18 to +18dB.
Because DB4 and DB8 use 48 bit processing, a
positive setting does not create overload, even if
the signal is already at full scale.
Use the Trim parameters for Target transcoding
e.g. to mobile platforms with 7-8dB of static gain
to bring the output up to -16 LUFS/LKFS.
Lateral Trim
Use Lateral Trim to apply static gain control to all
the Main channels, except for Center, after the
ALC section, but before the output limiter.
The range of the Trim parameter is -18 to +18dB.
Because DB4 and DB8 use 48 bit processing, a
positive setting does not create overload, even if
the signal is already at full scale.
Use the Trim parameters for Target transcoding
e.g. to mobile platforms with 7-8 dB of static gain
to bring the output up to -16 LUFS/LKFS.
LFE Trim
Use LFE Trim to apply static gain control to the
LFE channel after the ALC section, but before
the output limiter.
The range of the Trim parameter is -18 to +18dB.
Because DB4 and DB8 use 48 bit processing, a
positive setting does not create overload, even if
the signal is already at full scale.
Limit Threshold
Use Limit Threshold to set the true-peak Limit
Threshold for all limiters. True-peak detection
makes overload of downstream devices, such as
data reduction codecs, sample rate converters
and DA converters, less likely.
Though digital samples may go to full scale, it
is recommended to use a conservative Limit
Threshold, even in digital transmission. Reserve
the top of the digital scale for occasional peaks
in wide range material (feature films, classical
music), so don’t go above -6 dBFS in HDTV
for normal broadcast programming. This way,
down-mixing or bass management at the con-
sumer will also not generate unexpected distor-
tion.
The distance between the Target Level of the
ALC section and the Limit Threshold is an im-
portant audio quality defining factor, so-called
headroom. Analog TV only had around 10dB of
headroom, but DTV should have at least 15-16
dB of Headroom for a chance of conveying audio
of a decent quality.
Raising the headroom can be accomplished by
moving down the Target Level and/or raising
the Limit Threshold. The headroom in BS.1770-
based standards is typically 22 dB – significantly
more than broadcast ever had before.
Limiter Link
The Limit Link settings define which limiters work
together.
ALL
If a threshold is exceeded in any channel, all
channels are limited.
LCR, LFE
If a threshold is exceeded in one of the Main
channels, all Main channels are limited. If the