Sony OXFORD LIMITER PLUG-IN User Manual
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’).
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.
2. System requirements.
2.1. Pro Tools | TDM HD and HD Accel Systems
• Pro Tools 5.3 (PT 6.0 for OSX)
• Approved Pro Tools CPU and hardware configuration
• Approved Pro Tools Mac operating system (depending on Mac) or Windows XP
• 1024x768 minimum display
• RAM 384Mb minimum (512Mb recommended)
• iLok USB key required
2.2. Pro Tools | MIX Systems
• Pro Tools 5.1.1
• Approved Pro Tools CPU and hardware configuration
• Approved Pro Tools Mac operating system (depending on Mac) or Windows XP
• 1024x768 minimum display
• RAM 384Mb minimum (512Mb recommended)
• iLok USB key required
2.3. Pro Tools LE
• Pro Tools 5.3 (PT 6.0 for OSX, or 6.1 for WinXP)
• Approved Pro Tools CPU and hardware configuration
• Approved Pro Tools Mac operating system (depending on Mac) or Windows XP
• 1024x768 minimum display
• RAM 384Mb minimum (512Mb recommended)
• iLok USB key required
2.4. Instantiations per DSP Chip.
Pro
Tools
* Due to processing load restrictions, the Reconstruction meter and Auto compensation options are not available
DSP TYPE
Mix 1 1 - - - -
HD 1 1 1* 1* - -
Accel 4 2 2 1 1* 1*
48kHz 96kHz 192kHz
MonoStereoMonoStereoMonoStere
o
3. Control Functions.
The limiter is comprised of four functions; pre-process dynamic control, enhancement, output metering and dithering
The GUI is arranged according to these in order.
3.1. Input section.
Input gain fader (1).
Controls the input gain to the limiter processing from –18dB to +18dB. The threshold for the onset of dynamic gain
control action in the pre-process section is fixed at 0dBr, therefore a possible 18dB of gain reduction is available from a
full level input signal.
Input meter (2).
Displays the effective input level from –20dBr to +18dBr. Gain settings producing effective programme levels above
0dB are subject to dynamic gain control; the meter scale above 0dB displays the range of level where action occurs.
The red overload indication operates on input signal levels prior to the limiter processing regardless of gain setting in
order to provide input overload warning at all times.
Numerical gain display (3).
Displays effective gain at all times and allows settings to be typed in manually. All numerical displays allow manual
type in setting.
3.2. Pre-process section.
Attack control (4).
Provides setting of attack timing from 0.05mS to 1mS (ref 10dB gain change).
Release control (5).
Provides adjustment of release timing from 0.05mS to 1.038 seconds (ref 10dB gain change).
Soft Knee control (6).
Adjusts the soft limiting threshold from 0dB (hard limiting) to 10dB (maximum soft limiting).
Gain reduction meter (7).
Permanently displays the total peak gain reduction due to pre-process dynamic gain control action.
Auto Gain switch (8).
Switches in an additional long-term gain process, providing automatic gain scaling compensation to accommodate wide
input programme level ranges.
Safe mode switch (9).
Switches plug-in into safe mode, which employs the enhance section to control peak levels at all times.
3.3. Output section.
Enhance control (10).
Controls the level of the dynamic enhancement from 0 to 125%. In normal mode full sample value limiting occurs only
with settings at or above 100%. In Safe mode sample value limiting occurs permanently and the enhance slider controls
the degree of dynamic loudness boost.
Output Fader (11).
Adjusts output level from -20dBr to 0dBr.
16 bit dither selector (12).
Selects 16bit output word length dithering.
Dither type selector (13).
Selects output dithering type, which includes conventional TPDF and 4 types of noise shaping options.
Depth selector (14).
Selects the effective degree of action for any selected noise shaping option from 0% to 100%.
Output meter (15).
Displays final output levels for the plug-in.
Peak Hold selector (16).
Selects meter display peak hold function, for long term monitoring of peak levels.
Recon Meter selector (17) *.
Switches meter to display actual reconstructed programme signal levels, i.e. the signal that will occur in the analogue
domain after decoding with a digital to analogue converter. When uncorrected, the overload region is displayed in red.
Auto Comp selector (18) *.
Enables extra processing that allows automatic compensation of potential reconstructed programme over loads. For
monitoring purposes, the meter overload region remains visible during auto compensation, but it changes colour to
indicate that these signals are no longer present at the output and therefore do not represent actual programme error.
Please note that the red peak indicator responds to normal peak sample values at all times. This provides indication of
possible numerical clipping for signals that may not provoke a full-scale value readings in reconstruction mode **.
* Due to processing load restrictions, the Reconstruction meter and Auto compensation options are not available for
4FS (192KHz) Accel and 2FS (96KHz) HD versions of the TDM plug-ins.
** The output peak indication within the Limiter plug-in always displays legal peak sample values for the intended
24bit or 16bit target formats. In the case of LE versions for both PC and Macintosh the red overload indicator
within the mixer may not light at all when the plug-in is in use. This is because the Limiter generates an output
signal that precisely matches fixed point target media (i.e. CD, DVD), which differs from the overload indication
point within the LE host application. It is therefore NOT an error of the plug-in and the internal plug-in overload
indication should be trusted at all times in order to avoid unwanted overloads.
4. Operation.
The limiter plug-in is comprised of four cascaded processes in the order below.
- Pre-processing gain control section,
- Programme enhancement and overshoot control section,
- Reconstruction metering and compensation section.
- Dithering and noise shaping section.
The primary purpose of the plug-in is to control peak levels and increase the volume, density and presence of musical
programme without the 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 provide a means of allowing
the sound of transient and dynamic information within your programme to exist in the output from 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 is provided to allow 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.
4.1. 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’s own right.
For clarity of operation the plug-in gain structure is arranged with the threshold permanently set to 0dB ref input
normal level. Dynamic gain reduction is achieved by increasing the input gain sufficiently for the internal signal to
surpass the 0dBr reference level. A total of 18dB 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 peaks are encountered)
coupled with a logarithmic side chain employing multiple interdependent timing functions. Timing controls are
provided to modify its action depending on programme type and production style. In order to reduce excessive short-
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