User manual SONY OXFORD LIMITER PLUG-IN

[. . . ] Oxford Limiter Plug-in Manual For Digidesign ProTools 1. Introduction. 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'). [. . . ] So it is a good idea to keep the auto gain active 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. 4. 5. 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 in the 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 distortions. 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. 4. 5. 1. 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 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 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 Oxford limiter can be used freely to make subtle modification to this process in order to achieve the best possible results. 4. 5. 2. 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. [. . . ] 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 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. [. . . ]