The Sonnox Oxford Limiter for Pro Tools HD/HDX Systems has been developed from decades of professional audio experience to provide a very high degree of quality and facility in program loudness control and limiting functions. By employing highly accurate logarithmic side chain processing, along with innovative adaptive timing functionality using look-ahead signal acquisition, the limiter provides exemplary performance, whether one is seeking general transparent level control, program loudness maximization or heavily applied artistic sound effects.

Unique processing in the form of the Enhance function provides the sample value limiting needed to reliably avoid overloads in digital workstation environments and allows unprecedented volume and punch to be applied to program beyond that available from conventional limiting functions.

Comprehensive metering is provided which displays not only conventional peak sample value, but additionally allows the user to monitor the true validity of the program in order to avoid the generation of damaging reconstruction overloads in the target equipment which are often invisible during production (sometimes termed "intersample peaks").

A further function allows the user to dynamically correct for reconstruction overloads in real time, thereby achieving maximum possible modulation levels without the risks of producing illegal signals often associated with compression and limiting.

Comprehensive dithering functionality with selectable and variable depth noise shaping ensures first class mastering output quality in either 24-bit or 16-bit modes.

General
The limiter plug-in comprises of four cascaded processes in the order below:

¢ Peak program limiting section
¢ Program enhancement section
¢ Reconstruction-metering and compensation section
¢ Dithering and noise-shaping section


Pre-process Section
The Pre-process section is a musical program leveling function. Its primary purpose is to control program level over a wide range in order to provide optimum conditions for the following enhancement stage. When the enhancement is disabled in normal mode (at 0% with Safe mode deselected) the pre-process section can be used as a conventional leveling section in its own right.

Enhancement Section
The purpose of the enhancement process is to provide sample value limiting and overall program loudness improvement. The process follows the pre-process section in the signal path and is controlled by a separate fader from 0% (no action) to 125% (maximum action). In normal mode the range from 0% to 100% fades in the effect to full level, at which complete sample-value limiting occurs. Settings from 100% to 125% further modify the process to progressively increase loudness and program density at the expense of increasing potential distortion artifacts.

Safe mode is provided to allow absolute peak level control without excessive enhancer action, even when using slow attack settings. In Safe mode the enhance process is set to run permanently and the enhance slider modifies the action of the process (rather than the proportion of the effect). Setting ranges from 0% to 100% control the degree of program loudness boost generated by the enhancer. The control region from 100% to 125% works identically to normal mode. It should be noted that in safe mode signals at all levels are being processed permanently, therefore some minor changes to the program dynamics can occur even for a minimum setting of 0%.

The enhance process improves the perceived loudness and presence of the program by modifying the dynamic and harmonic content of the signal. Since the method used is different from the pre-processing section, it can further enhance the perceived volume of a previously processed signal whilst suppressing all signal overloads. As the limiting action does not involve conventional sample value clipping, harsh distortions are avoided and program detail and dynamic information is largely retained.

Attack timing
The addition of an attack timing control is a significant departure from conventional limiter applications.

The attack control provides a means of increasing the attack time to achieve a favorable improvement in the sonic qualities of the peak-reduction process by allowing peak program transient events to escape hard gain reduction. Since the plug-in has internal headroom these overshoot peaks are retained and not clipped.

Meter operation
When the Recon Meter is selected the meter is switched from conventional peak sample value mode into reconstruction mode. In this mode peak reconstruction levels will be displayed on the meter. Levels in the red overload range of the meter represent the presence of potential reconstruction errors.

Dither and Noise Shaping
The finite mathematical precision provided by digital audio systems and the effects of dither have been a source of confusion in the audio community for some years. When dither is applied, any signal-related error caused by finite word length limitation is turned into constant random noise with no relation to the signal itself, so such dithering provides complete removal of harmonic distortion due to precision limits. With the Oxford Limiter, Sonnox provides conventional 16- and 24-bit TPDF dither options.

In addition, the Oxford Limiter also provides several types of Noise-Shaping dither. Noise-shaped dithering is a mechanism that aims to reduce the perceived loudness of the noise of a dithered signal by either forcing the spectrum of the noise out of the audible range or placing it into frequency ranges to which we are less sensitive. In this way the noise at very low levels may be reduced and even lost entirely if it is at the limit of our hearing within ambient noise conditions.



NOTE: Sonnox plug-ins require a 2nd Generation iLok for authorization.