Getting CALMer, Bit by Bit
Some readers may recall that I
promised to write about subwoofers
this month. Well, that basically worthy topic (get it?) has
been put off for a month so I can share
with you a little more about the CALM
Several of the people involved with
the ATSC A/85 Recommended Practice
responded to my April column about
the CALM Act, and have shared more details about their
thinking and intentions in regard to the practices covered
by ATSC A/85.
Consequently, it seems appropriate to me that we go
a little further with the discussion, simply to clarify what
seems to be meant and desired in practice by A/85 and
The most important aspects of this have to do with
what the law actually requires and also with the notion
- • In fact, the CALM Act does not empower the
FCC to regulate loudness variations between
- • LKFS, as a term, needs to be thought of as "loudness"
(i.e. loudness, k-weighted, full scale).
- • –24 LKFS is not a mandated loudness, as I suggested,
but a recommended loudness to be
used in the absence of another agreed upon
loudness value, usually the metadata value provided
- • The "Golden Rule" of loudness is that the average
loudness measured over the duration of
an entire piece of content needs to equal the
displayed encoded metadata dialnorm value, or
"loudness = dialnorm."
This all merits some more detailed discussion.
To begin with, the CALM Act directs the FCC to
"prescribe... regulation that is limited to incorporating...
and making mandatory... the ''Recommended
Practice: Techniques for Establishing and Maintaining
Audio Loudness for Digital Television'' (A/85)... only
insofar as such recommended practice concerns the transmission of commercial advertisements
by a television broadcast station, cable operator,
or other multichannel video programming
I personally believe loudness changes
between channels will cease to be a
problem when such recommended practices
are implemented. This automatically
happens when loudness actually equals
the encoded dialnorm value for all ATSC
AC-3 based channels.
In fact, the whole desired outcome of
the CALM Act in its final form seems to be
to get dialnorm to work as planned. That
would be a very good thing.
It also needs to be noted here that
LKFS is a proxy for the subjective quality
of loudness. Although loudness and amplitude
differ in profound and significant
ways, it is the desire of the A/85 authors
to simplify and clarify the maintenance
of good loudness practices by the use of
For this reason, they wish to establish
a "loudness-as-LKFS" protocol that is independent
from previous reference levels
and amplitudes. They would like very
much for traditional level meters (VU and
PPM) to fall out of general use in television
broadcasting, replaced by loudnessas-LKFS meters as well as true-peak meters
(a part of this I haven't discussed).
This is a reasonable and good faith
goal, particularly for a production environment
(as opposed to a test-and-measurement
We all also need to understand the distinction
between "long form content" and
"short form content," which I'm sorry to
say I ignored in my April column.
"Long form" refers to broadcast program
material, typically more than 2–3
minutes long. "Short form" content is
commercial advertising (identified specifically
in the CALM Act), promotional
material and PSAs, etc. ("interstitials" less
than 2–3 minutes long).
In the case of "long form content"
we measure the loudness of the Anchor
Element, usually dialog, not whispered
or shouted. And it is that average dialog
level (only) that is the basis for the metadata
value that gets encoded for the "long
However, in the case of "short form
content" the loudness measured is the
loudness of the entire
piece of content,
meaning all audio elements, not just the
dialog. This means that the loudness of
"short form" commercials may be equivalent
to the loudness of dialog in the "long
The implication of this is that short
form content will never be louder than
the dialog in the long form content. The
reasoning behind this is that dialog loudness
in programs is typically what listeners
use to set their volume. If the loudness
of spots matches program dialog, listeners
will experience no need to reach for
the remote at the commercial break.
In all cases, the operator needs to actually
(emphasis mine) the loudness
in LKFS, and that measured loudness
needs to match the encoded dialnorm
value in the transmitted AC-3 bitstream.
If it doesn't, the resulting transmission is
not in compliance with either A/85 or the
And here is where –24 LKFS comes in.
In the instances where there is no metadata
available with the delivered content and no
other agreement about appropriate loudness,
the ATSC recommends –24 LKFS as
the target loudness. In my April column, I
gave this priority over delivering metadata
with the content, when in fact it appears
to be the fallback position. Without a doubt,
accurate metadata and measured loudness
(or the equivalents) are vital.
WHAT DOES IT ALL MEAN?
It is my fervent hope that the presence
of the CALM Act leads to widespread
compliance with dialnorm protocols.
This in turn, will result in much more
stable and viable audio loudness in television
broadcasting, both in terms of interstitials
and between channels.
Dialnorm is based on the assumption
that operators (stations, cable and satellite)
will maintain a uniform and consistent
loudness for their content as a matter
of intuitive course.
When this is accomplished for all programs
and interstitials on each channel
and this loudness equals the transmitted
dialnorm value, then all broadcasting
should be, in a very reasonable way, at the
same loudness. Let's hope!
A final comment: the ATSC Sub Group
on Audio Loudness (the group who actually
wrote A/85) has actually done a
great piece of work here. They deserve
our respect and thanks, and it is the fruits
of their efforts that are what make the
CALM Act viable. Thank you!
You can complain to Dave about
anything at his website, moultonlabs.com.