Omnia-audio Omnia.ONE User Manual download pdf (Page 110)

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

. Doing so will achieve the highest perceived

audio quality.

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Omnia-audio Omnia.ONE User Manual Page 110

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