Introduction

The Oxford Limiter plug-in has been developed from decades of professional audio experience to provide a very high degree of quality and capability 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 offers exemplary performance, whether one is seeking general transparent level control, programme loudness maximisation or heavily applied artistic audio 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 included, 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 is included, with selectable and variable depth noise shaping, which ensures first class mastering output quality in either 24 bit or 16 bit modes.

Overview

The Limiter plug-in is comprised of four processes, cascaded in the following order:

The primary purpose of the plug-in is to control peak levels and increase the volume, density and presence of musical programme without an excessive loss of transient and dynamic information that normally results from conventional peak limiting applications.

In order to achieve this, the application employs gain scaling and compression in the Pre-Process section, and peak overshoot control in the enhancement section. These processes are used in conjunction to enable the sound of transient and dynamic information within your programme material to reach the output of the plug-in, despite very tight control of maximum peak sample values.

The normal ENHANCE control setting for this action to fully occur is 100% and above in Normal Mode, (or any enhance setting with SAFE MODE selected). A variable control allows the enhance process to be adjusted or removed completely (i.e. at 0% setting), in which case the Pre-Process section may be used as a conventional programme levelling application if desired.

A SAFE MODE is provided which uses the enhance processing to permanently control peak levels. In this mode the ENHANCE control varies the perceived loudness boost of the programme by modifying the processing law.

Pre-Process Section

The Pre-Process section provides a musical programme levelling function. Its primary purpose is to control programme level over a wide range, in order to create optimum conditions for the following enhancement stage. When the ENHANCE control is disabled in Normal Mode (at 0% with SAFE MODE disabled), the Pre-Process section can be used as a conventional levelling section in its own right.

The processing threshold is set to 0 dBFS by default. Dynamic gain reduction can achieved by increasing the INPUT GAIN sufficiently for the internal signal to surpass the 0 dBFS reference level. A total of 18 dB gain boost is available for this purpose, and the orange section of the INPUT meter illustrates the level range within which gain reduction occurs when the plug-in is active. The final output modulation level is set by the OUTPUT LEVEL control, which can be adjusted to compensate for dynamic conditions produced by the programme and Limiter settings, or produce a lower level mastered output if required.

Programme gain is accurately controlled by look-ahead detection (to allow action before any peaks are encountered) coupled with a logarithmic side-chain employing multiple interdependent timing functions. Timing controls are provided to modify its action depending on the programme type and production style. In order to reduce excessive short-term gain modulation, a selectable AUTO GAIN function is included, which compensates for wider input level variations by imposing a longer-term time constant that underlies the peak timing. A variable progressive SOFT KNEE function allows varying degrees of soft limiting (for lower settings), right up to large-scale gain management active over the final 10 dB of programme dynamic range.

Threshold

The THRESHOLD parameter adjusts the processing threshold of the Pre-Process and Auto Comp processing, and the ceiling of the Enhance/Safe Mode processing.

Sometimes it may be necessary to limit programme to less than 0 dBFS. In this case, simply set the THRESHOLD parameter below the default 0 dBFS setting.

One scenario in which this may be necessary is when the programme must be limited to less than 0 dBTP (dB 'True Peak') in order to comply with programme broadcast or delivery requirements. See True Peak Reconstruction Metering for details.

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 ENHANCE 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 allows 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 fader 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 material 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, while 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.

Also, since the plug-in has internal headroom, transient levels greater than notional maximum modulation can pass from the Limiter section into the enhancement stage. This means that percussive overshoots, that would normally be lost in a conventional limiter (or would give rise to overloads), may be included within the sonic results of the plug-in, producing both richer dynamic sonic detail and a useful reduction in the perceived artefacts of the limiting process, all without giving rise to any sample value overload.

This property enables slower attack times to be used in the Pre-Process section, without creating output overloads, which would otherwise result in a need to reduce output levels.

The enhancement section can be used effectively on its own to produce programme enrichment and peak value limiting, by using minimal gain reduction and slow timing settings in the Pre-Process section. Or it can be used to enhance highly processed content from the Pre-Process section to achieve even greater perceived loudness.

Because the enhancement process adds harmonic distortion during dynamics within the programme, under some conditions side effects may occur depending on the content of the programme material. Generally speaking side effects should be minimal when in Safe Mode and for boost settings up to 100% when mastering in the presence of most commonly occurring complex and dense composite material. However, some extra care may be needed in the case of single solo instrument tracks where there may be a predominance of sustained lower and middle frequency content. Settings above 100% are most useful when the programme type is intended to be very loud, or where extra distortion may actually prove advantageous within the style of the production.

Attack Timing

The addition of an ATTACK timing control is a significant departure from conventional limiter applications, and requires some explanation for the best results.

Because the level detection within the plug-in looks ahead of the gain control, peaks in the programme material are acted upon in advance of the gain reduction process. Therefore, at the fastest setting of the attack control, programme peaks are controlled within a very small margin (less than +0.25 dB, with respect to continuous sine input conditions).

The ATTACK fader allows the attack time to be increased 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 are not clipped.

Peak overshoots resulting from a combination of the programme material and the action of the Pre-Process stage are then passed to the enhancement section where their sonic signatures can be added to the final programme sound. Providing SAFE MODE is selected, or the ENHANCE fader is set to 100% or more in Normal Mode, no output sample value overloads will occur from the plug-in, regardless of ATTACK or RELEASE time settings.

A combination of slower attack times and the enhancement process is therefore a very powerful way to include transients in the output programme that would normally be removed by conventional limiting processes. It can create a sonic quality and impression of dynamic range that belies the degree to which the programme is actually being limited.

As an example of the difference this can make, if slower attack settings are used without the enhancer, and the output gain is reduced to accommodate the overshoots (avoiding overloads), using the same attack settings with the enhancer can create up to (and beyond) a 3 dB increase in average level, being able to legally increase the output gain setting and around another 2 dB of perceived loudness due to the enhance action itself. This can result in a perceived loudness increase of 5 dB to 6 dB!

In general, very fast attack times will more readily remove extremely fine detail and short- term events, but will produce greater harmonic disturbance.

Slower attack times will progressively allow finer detail to escape the harsh sound of fast limiting, and longer term events will tend to assume a more rounded peak profile. Such settings are usually kinder to the musical programme.

The only way slower attack settings can be used without a potential need for a significant reduction of the output level control is when the ENHANCE control is set at, or above, 100% or with SAFE MODE selected. These methods will ‘soft clip’ transients, i.e. compress their level before the onset of clipping, providing a warmer, valve-like saturated sound. Alternatively, enable 16 bit or 24 bit dither, which will ‘hard clip’ over-unity transients, ie. squaring the top/bottom of waveforms and resulting in a more distorted sound, often found in transistorised circuits.

Using Pre-Process without the Enhance Section

You should be aware that, since the Pre-Process section is a programme gain controller rather than a simple sample clipper, programme peaks can cause a small increase in maximum output sample value, even at the fastest attack time settings. If SAFE MODE isn’t selected, or the ENHANCE fader is not set at, or above, 100% in Normal Mode, these peaks will appear at the output of the plug-in. Increasing the attack times will further increase peak overshoots, so if tight level control is required without the enhancer, it is best to leave the attack at minimum setting.

Since the plug-in has internal level headroom, the output level control can be safely used to compensate for any artistically intended overshoot without fear of causing internal signal clipping.

Release Timing

The RELEASE control has a very wide range to accommodate the maximum possible extent of programme and production style. The ability to set very fast release times specifically allows for the modelling of short-term peaks over restricted gain reduction ranges, up to a maximum of around 4 dB. Such settings will result in high levels of distortion for larger gain reduction ranges, and are therefore unsuitable for overall level control situations.

The AUTO GAIN function can be used very effectively to compensate for large level changes, while still allowing fast peak modelling for shorter peak events, as the ATTACK and RELEASE controls are permanently functional. So it is a good idea to keep AUTO GAIN selected under most circumstances.

Generally speaking, faster release times produce the greatest perceived loudness, since gain recovery happens quickly after peak events have passed, and average programme levels are affected only during the shortest possible periods. However, since the gain recovery can begin to occur between the waveform peaks of lower frequencies in the programme, there is a trade off to be made between the speed of release and the generation of distortion. Such distortion may be desirable under many conditions, particularly in loud popular music productions, where some low frequency harmonics may add warmth and presence to the programme. Adjusting release timing over a wide range provides a method to ‘tune’ these effects to suit the production style.

Longer release times are far more forgiving of gain changes, and allow greater overall compression, but will result in a quieter sounding output programme.

If the AUTO GAIN function is not selected moderate release time settings (above around 0.2 seconds) may produce audible gain ‘pumping’ due to longer and more noticeable recovery periods. If such settings are needed the aim becomes one of ‘fitting’ the release timing to the natural rhythm of events in the programme. Under these conditions better results may be achieved by increasing the input gain somewhat thereby compressing further and applying the Soft Knee function in order to compress gently over an increased portion of the dynamic range. In this way the transition in and out of compression will become gentler and less obvious.

Programme Limiting Procedures

There are many approaches to limiting within current productions trends, but most approaches fall into two categories: loudness maximisation and general gain control. A very wide range of control is provided by the Oxford plug-in to make both these situations possible with ease.

The key to successful limiting is to understand that we are much more sensitive to the rate of change of gain than we are to absolute level. Therefore successful limiting has a tendency to fall into an appropriate mixture of two simultaneous but conceptually separate actions:

Fast control over small level ranges

– because they are too quick for us to notice and too small to produce damaging harmonic distortion.

Slow control over larger level ranges

– because the gain changes are slow enough to escape obvious notice and the rate of change of level is slow enough to avoid intrusive modulation effects and distortion.

Loudness Maximisation

The aim of this procedure is to achieve an overall average increase in the level of the programme by reducing the size of short-term peaks, and applying extra gain to move the programme up into the extra range freed up by the removal of the peaks.

Signal before limiting

Signal before limiting

Signal after limiting

Signal after limiting

To achieve this, it is customary to select relatively fast attack and release times whilst judiciously increasing the input levels so that only the offending programme peaks are subject to reduction by the Limiter, and the average modulation level is increased.

The Sonnox Oxford Limiter can produce significantly superior results in loudness maximisation because it can fully limit the signal even when using slower attack times. This leads to much lower distortion and less removal of dynamic programme information. The timing controls on the Pre-Process section of the Limiter can be used freely to make subtle modification to this process, in order to achieve the best possible results.

General Gain Management

The aim of this procedure is usually to preserve the short-term dynamics of the programme as far as possible, whilst ensuring that no levels surpass maximum peak modulation. This most often entails responding to the peak level of the programme as quickly as possible, and re- scaling the gain in the longer term, such that musical dynamics are only minimally affected in the short term. The Limiter can excel in general gain management because of its wide range of control in the Pre-Process section, and the ability of the Enhance stage to control the level of short term peaks, which means that musically kinder attack and release times can be used without risk of transient overloads. Moderate attack times and slower release times usually perform best for this function.

The AUTO GAIN function is particularly useful in this case, as it provides a method to achieve long-term gain control whilst allowing a degree of fast gain riding over a reduced range. Moderately fast recovery times can be arranged to control short-term events with the auto gain function managing long-term level changes. Higher SOFT KNEE settings can also considerably improve perceived quality, as this acts in the same way as a variable ratio compressor that starts at lower levels. This allows the Limiter to preview signals at moderate levels, and reduce the rate of change of gain in the loudest peak regions.

True Peak Reconstruction Metering

An important fact, which is often overlooked, is that in any discrete time sampling system, it is possible to create sample values that may not be decoded and reconstructed correctly. Whilst it is true that, if left unchanged, a signal properly converted into the digital domain by a perfect ADC will always produce sample values that can be legally decoded in a perfect DAC, further processing of those samples can result in a decoded signal that is illegally high, and therefore may not be faithfully reproduced, even if no sample value limiting occurs.

The following is an illustration of one example of this situation.