Omnia Audio Omnia.ONE - Manual (Page 110)

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Final Limit
The Omnia.ONE Multicast employs a final look-ahead limiter to provide absolute precision peak control. This
limiter has been designed to minimize processing side-effects like IM distortion, which are usually associated with
limiters of this type. Using an innovative design that cancels intermodulation products before they develop allows
this limiter to sound extremely transparent.
This type of peak controller is used instead of a clipper because it does not generate the same high levels of
harmonic distortion as a Clipper does. This distortion would cause added difficulties in a coded audio system
because the harmonics generated from the clipping action create added artifacts and workload in the encoder. These
are especially annoying at high frequencies.
There are tradeoffs in how each of these peak controllers sound when they are set to produce added loudness. When
a clipper is pushed, the audio may sound edgier. This is from the added harmonic content. In contrast, the look-
ahead limiter will sound busier, or denser, when more limiting is applied.
Drive
The Drive control controls the depth of final limiting. The amount of gain-reduction occurring in
the final limiter can be monitored on the Process metering bargraph screen. Note, however, that
this meter cannot show extremely fast action in the limiter, so your ear must be the final judge.
LPF Freq (Low Pass Filter Frequency)
This is a key control and is used for matching the audio bandwidth of the processed audio to the
bitrate of a following codec. Setting this control to the proper bandwidth for the codec’s settings
will ensure minimal side effects and aliasing distortion products from the codec process. The
available settings are: 4kHz, 6kHz, 8kHz, 10kHz, 13kHz, 16kHz and 22kHz. These are the
frequencies at which the low pass filter reaches a minimum of 100dB of attenuation.
One of the most important features that an audio processor for codec provisioning must have is a
high quality, low overshoot low pass filter after the final peak control stage. Omnia.ONE
Multicast offers this feature as a low pass output filter that can be set to one of seven stop band
frequencies. Each of the filters possess minimal passband ripple and are phase linear below cutoff.
Response in the filter stop bands is greater than -100dB.
The following chart recommends low pass filter settings that, in our experience, best complement
the AAC+SBR, MP3, and Windows Media (WMA) codecs. They were derived through thousands
of hours of critical listening and we feel they are very good starting points for achieving the best
overall sound although you may use other settings that you feel are more appropriate for your
particular codec application.
AAC/SBR Output LPF MP3 Output LPF WMA Output LPF
24kbps 8kHz 24kbps 6kHz 24kbps 5kHz - 6kHz
32kbps 10kHz 32kbps 6kHz 32kbps 8kHz
48kbps 13kHz – 16kHz 48kbps 8kHz – 10kHz 48kbps 10kHz – 13kHz
64kbps 16kHz 64kbps 10kHz – 16kHz 64kbps 13kHz – 16kHz
!!!--- IMPORTANT NOTE ---!!!
Always set the low pass filter frequency so that it is at or below half the sampling frequency of the encoder.
This must be done in order to comply with the Nyquist Theorem
1 . Doing so will achieve the highest perceived
audio quality.