Home > Pro Audio Software >

Sonnox Oxford Limiter HD
Sonnox Oxford Limiter HD
 
Alternative Views:


Our Price: $500.00

Product Code: SONNOX OXFORD LIMITER HD
call to purchase

Description
 
The Oxford Limiter has been developed from decades of professional audio experience to provide a very high degree of quality and facility in programme 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, programme loudness maximisation 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 programme 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 programme in order to avoid the generation of damaging reconstruction overloads in the target equipment which are often invisible during production (sometimes termed 'inter sample 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.
Features
  • Unique Enhance section - provides sample value limiting and enhances perceived loudness
  • Ability to add harmonic distortion, where artistic style requires
  • Variable soft knee control
  • Attack and Release controls
  • Reconstruction Meter shows 'actual' signal rather than sampled signal
  • Auto compensate feature - corrects recon errors without losing overall programme loudness
  • 16 and 24 bit dithering with 5 selectable types
  • User has control over variable depth noise shaping
  • 16 presets - some quite extreme !
  • Platforms Supported
    Pro Tools HD (TDM), Pro Tools LE (RTAS), M-Powered (RTAS), PowerCore, AU, VST

    General
    The limiter plug-in comprises of four cascaded processes in the order below.
  • Peak programme limiting section.
  • Programme enhancement section.
  • Reconstruction metering and compensation section.
  • Dithering and noise shaping section.
  • Pre-process Section
    The Pre-process section is a musical programme levelling function. Its primary purpose is to control programme 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 levelling section in it ís own right.
    Enhancement Section
    The purpose of the enhancement process is to provide sample value limiting and overall programme 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 programme density at the expense of increasing potential distortion artefacts.

    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 programme 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 programme dynamics can occur even for a minimum setting of 0%.

    The enhance process improves the perceived loudness and presence of the programme 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 programme 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 favourable improvement in the sonic qualities of the peak reduction process by allowing peak programme 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, we provide 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.

    System Requirements
    Pro Tools
  • Pro Tools 6.0 or above
  • Approved Pro Tools CPU, OS and hardware configuration. More details: www.digidesign.com
  • Mac OSX 10.3 or later (Leopard supported see Digidesign for details)
  • Windows XP / Vista32
  • RAM 1GB minimum
  • iLok key with latest drivers

     

    Audio Units
  • Audio Units compatible application (Logic, Digital Performer etc.)
  • Mac OSX 10.4 or later (including Leopard)
  • RAM 1GB minimum
  • iLok key with latest drivers

     

    VST
  • VST compatible application (Cubase, Nuendo, Acid etc.)
  • Mac OSX 10.4 or later (including Leopard)
  • Windows XP / Vista32
  • RAM 1GB minimum
  • iLok key with latest drivers

     

    PowerCore
  • TC PowerCore hardware
  • PowerCore version 3 drivers
  • AU or VST host application
  • Mac OSX 10.4 or later (including Leopard)
  • Windows XP / Vista32
  • RAM 1GB minimum
  • iLok key with latest drivers
  •  

    roduce signals that are considerably in excess of maximum modulation.

    Since the vast majority of metering within workstation environments responds to sample value only, the above example would show a level of around -3dB below clipping. However any further increase in the level of the signal would result in a potentially illegal output level from the system converter. As this error would not be reported on metering within the workstation, in this particular case a possible 3dB overload can result if the signal is increased to a maximum reading on the workstation meters. This phenomenon is sometimes termed 'inter-sample peaking'.

    Although the above example is somewhat extreme and specific, there is plenty of potential for this to occur within the mixing environment. Combining a number of processed contributing tracks and limiting the result to the maximum possible modulation level in order to satisfy current industry trends, using only peak value metering, provides a recipe for such hidden errors.

    Since the very purpose of a limiting application is most often to increase average modulation levels, a reconstruction meter with both manual and automatic correction processing has been included in the Sonnox Oxford Limiter plug-in, in order to provide the user with a method to avoid or repair such errors.

    Back to Top

    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, as illustrated below using the previous example.
    Two methods are provided to correct for this.

    Since the output level fader precedes the metering, errors may be corrected manually by simply reducing the output level setting by the same amount as the maximum error level reported on the meter.
    Auto Comp
    Under normal circumstances errors are interspersed throughout the programme often restricted to certain specific events. The Auto Comp function is provided to address the situation where it may be undesirable to reduce the level of the whole programme to avoid transient errors. When Auto Comp is selected the level of the output is automatically controlled to repair reconstruction errors by the minimum amount required and only for the duration of the error. In this way the loudness of parts of the programme unaffected by the errors remains as high as possible.

    A combination of Auto Comp and manual output level reduction can be used to strike a compromise, if the action of the error correction becomes intrusive in the presence of very large and intermittent error conditions.


    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. Such discussion may lead to possible misconceptions, which could prevent the user from achieving maximum performance from systems in use. Therefore the dithering options provided in the Sonnox Oxford Limiter warrant some prior explanation.
    Conventional Dither
    In both 24bit and 16bit output word length selections, high pass TPDF (triangular probability density function) dithering is applied to the output of the plug-in. Since any signal related error caused by finite word length limitation is turned into constant random noise with no relation to the signal itself, such dithering provides complete removal of harmonic distortion due to precision limits, which are an inescapable result of any numerical signal representation. Dithering also suppresses any possibility that the programme will suffer loss of harmonic signal resolution due to word length restriction. The following plots illustrate this in action.

    The next plot shows the exact same signal and truncation to 16 bits but with the HP TPDF dither applied.
    It can be seen that all harmonic errors have been removed. Also since the FFT analysis method provides an enhanced view of the signal below the noise floor, it can also be seen that there is effectively no low level floor below which a signal will fail to pass. To illustrate this fact the following plot shows a 1KHz signal at -120dBr passing through a dithered 16bit system. This corresponds to a signal 24dB below the level of the least significant bit, the effective channel SNR is added in blue for illustration purposes.
    This shows that dither turns a quantised numerical signal conduit into the equivalent of a naturally continuous (un-quantised) system, which exhibits a finite signal to noise ratio with no practical limit to harmonic signal resolution. In other words the inescapable presence of quantisation in numerical systems does not forcibly lead to 'discontinuity' or 'resolution loss' in the signal. Misunderstandings of this fact underpin many of the most damaging misconceptions surrounding digital audio systems. It can also be deduced from the above plots that any undithered digital representation of an audio signal is effectively illegal.


    Noise Shaping dither
    If for some reason Signal to Noise Ratio (SNR) figures of 93dB at 16bits (or 143dB at 24bits) prove insufficient, noise shaping can provide an apparent increase in SNR, but there are some potentially hidden costs. 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 they are at the limit of our hearing within ambient noise conditions. The following plot illustrates this process.
    The red line shows the original 16bit dithered output with the -120dBr signal passing. The blue line shows the effect of noise shaping (type 1 at 100%) on the same signal transfer.

    It can be seen that the noise has been substantially reduced in the regions up to around 8KHz where we are most sensitive, at the expense of extra noise energy in the higher ranges above 10KHz at which we are less sensitive. Such processing can have a dramatic effect on the perceived intrusion of low-level noise.

    However as is always the case, one cannot get something for nothing and it can be seen from the above plot that the total noise power across the whole range must remain constant to satisfy the dithering requirement. This means the noise level necessarily increases in some ranges of the spectrum. A level increase anywhere in the spectrum must be accommodated by an increase in total peak noise level. The following plot illustrates this in action with the previous test conditions.

    The first plot shows a sample value plot of the conventional TPDF dithered signal. The second shows the increase in values caused by the application of the type1 noise shaping at 100%. Focussing the dither energy into a more restricted range than would naturally occur causes the level to increase.

    From the effective level values highlighted in the plots it can be seen that the application of noise shaping has increased the effective noise level from around -93dBr to -80dBr, an increase of roughly 13dB.

    From this it can be understood that the design of suitable noise shaping frequency curve is a trade off between the perceived loudness of the noise under certain conditions and the increase in overall level of the dither signal, much of this trade off relies on what we can hear (psycho-acoustics). Significant research has been carried out over the years into various approaches to this issue and several accepted curves are in use around the industry.

    The Sonnox Oxford Limiter includes 4 noise-shaping curves. Types 1 and 3 are fifth order and types 2 and 4 are third order designs representing a varied set of trade-offs to suit most programme types, as illustrated below.

    Whilst it is understood that the selection of noise shaping type is largely a matter of user preference, generally speaking types 1 and 2 produce the most dramatic reduction in overall noise loudness, with type 1 being the most effective of all. Types 3 and 4 provide gentler responses, which under some circumstances may produce less intrusive sounding spectrums, at the expense of higher audible residual noise. Type 3 also provides greater noise attenuation in the range between 10KHz and 16KHz at the expense of higher noise levels in the mid ranges.


    Noise shaping Depth control
    From the previous section it can be seen that noise shaping can potentially cause unwanted effects in equipment and processes down line, particularly if the programme is to be further modified, such as in mastering situations. Some unwanted effects may include:

    Marked increase in noise levels if the file is not transferred intact bit for bit, (i.e. if further processing is done).
    Premature meter readings in silence.
    Premature peak level over loads (as increased dither levels add to peak signal value).
    Unwanted low-level behaviour in dynamics processing.
    Disturbance caused to data reduction encoding processes such as MP3, WMA etc.
    Increased audibility (unmasking) of various errors that may occur in play-out systems.

     

    Generally speaking high levels of noise shaping renders a signal that is more fragile. Almost any change to the produced audio file after noise shaping could potentially result in unwanted effects.

    For these reasons the depth control is provided to put you in charge of the degree to which noise shaping is applied. When any of the noise shape curves are selected, the depth control varies the degree of noise shaping from 0% to 100%. At 0% the dither is conventional HP TPDF dither (as if noise shaping were not selected), at 100% full noise shaping is applied. All control positions within the range produce legal proportions of dither.

    The action of the depth control is illustrated below with type 1 selected.

    There is no technical (or philosophical) advantage to noise shaping above and beyond that which can be actually heard directly. Therefore the decision to use noise shaping (and to what extent) is basically determined by what might actually be heard in practice. If conventional TPDF dither provides sufficient audible dynamic range such that noise never intrudes within the programme, it is safest to avoid noise-shaping altogether.

    Because of the potential fragility of a noise shaped signal, it is better to ensure that it is carried out only at the final stages of mastering, immediately prior to release. If your mix is not already a final master, it is technically preferable to send a TPDF dithered 24bit file to mastering rather than a 16bit noise shaped file.

    Another important factor is that the effectiveness of psycho-acoustic noise shaping relies heavily on our sensitivity to noise spectra at the threshold of hearing. Therefore if noise shaped dither actually gets to be heard directly it will often sound quite strange and intrusive and may detract from the listener's experience of the programme. Therefore the most effective and safest approach is to apply noise shaping at the minimum amount necessary to render the noise inaudible within the conditions the programme is destined to be auditioned.




    Browse for more products in the same category as this item:

    Pro Audio Software
    Allpro Electronics

     About Us
     Become an Affiliate
     Privacy Policy
     Send Us Feedback
     Bookmark Us
    Company Info | Advertising| Product Index | Category Index | Help | Terms of Use
    Copyright © 2004 Allpro Electronics. All Rights Reserved.
    Built with Volusion
    google-site-verification: google70c7f7d6747c81ab.html