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Basic Processing and Loudness Control
Chapter 8 |
43
Load and Save
The Load button (B) is used to recall factory profiles, user profiles, or to import a profile previously saved to your computer.
Any changes or modifications to the current profile can be saved by using the cryptically-named Save button (C).
Bypass
APTO can be bypassed so that the input audio is passed through to the output without being processed by clicking on the
Bypass control (D). This is useful for a quick comparison between the unprocessed and processed audio. When bypass is
engaged, the button will turn red.
Reset
Clicking on the Reset button (E) resets the loudness measurements as well as the gain buffers APTO uses in the normalization
stage. Resetting at the start of each individual program element provides accurate per-segment loudness measurements, aids
in achieving overall compliance, and ensures that adaptive processing decisions are made based upon the current program
dynamics. A GPIO input may be used to trigger the reset automatically.
Target Loudness
The Target Loudness control (H) sets the desired average loudness level of the output signal in either LUFS or LKFS,
depending upon the profile. Some profiles, such as EBU R128, will measure loudness according to overall program levels while
others, such as ATSC A/85, will do so based on dialog and gated speech measurements.
To change the target loudness value, click in the Target Loudness field, type in the desired value, and click on the green check
mark to save your change (or the red "X" to exit without saving).
Adaptation
The Adaptation control (G) determines how much processing is applied to the incoming signal in the Dynamic Range
processing stage, and, in combination with various individual controls, determines dynamic range of the output audio.
The ideal amount of Adaptation depends upon both the source content and on the destination platform. Content that has been
pre-analyzed for loudness, scaled, and normalized in the file domain will require less realtime processing than, say, live sports,
which can have rather unpredictable audio levels. Programming streamed to a mobile device or expected to be heard on lower
quality earbuds will benefit from more Adaptation than the same content destined for a home cinema presentation.
Ideally, you want enough Adaptation to achieve compliance and keep levels within the viewer's comfort zone and at a stable
average level, but not so much that the audio sounds unnatural or over-processed. APTO is based on a psychoacoustic model
that takes into account human hearing and perceived loudness and remains very natural-sounding even when extensive
processing is applied, but it is generally advisable to keep the amount of Adaptation under 50% when possible. If incoming
content is so poorly controlled as to require higher values, it may be necessary to adjust some of the individual controls and
create a custom profile to address this scenario.
Maximum True Peak Limiter and Limiter Threshold
The True Peak Limiter control (J) enables and disables the True Peak Limiter, which is the final processing stage just ahead of
the final output. The Maximum True Peak value (F) sets the level beyond which the True Peak limiter engages and attenuates
the processed audio so as not to exceed the set level. These controls comply with the True Peak measurement as outlined in
ITU-R BS.177-4 Annex 2.
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