FabFilter Pro-L online help

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Accurate metering is extremely important in a limiter plug-in. To give you a perfect view of what's happening to your audio, FabFilter Pro-L offers very accurate output and gain change meters, including a textual representation of maximum peak levels, as well as a large real-time level display, showing levels and limiting over time. Using the Meter Scale button, you can choose a scale that fits your need. You can choose between three normal scales and three K-Metering scales.

Normal metering scales

FabFilter Pro-L has three normal metering scales. All three general scales have linear precision in the upper part of the metering, offering the best precision where limiting mostly happens:

  • 16 dB: Showing the top 5 dB of input, output and gain reduction meters in the linear upper part of the metering, this scale offers a precise view of limiting in the top ranges.
  • 32 dB: With a bit less, but still enough detail, combined with a fairly large overall range, this scale offers the best of both worlds; a good insight in the applied limiting and a proper impression of overal levels.
  • 48 dB: Covering a wide range of 48 dB, using this scales gives you the best a general overview input and output levels.

Like the K-System meters described below, the normal output meter shows the RMS level and the peak level at the same time, but with a longer RMS integration time of 2000 ms. Above the meters, the maximum peak output level and gain reduction is displayed. Click on the level text to reset it.

K-System metering scales

The K-System, introduced by mastering engineer Bob Katz in 1999, is a protocol for setting mix and monitor calibrations in a studio environment. It is an attempt to standardize leveling practices throughout the audio industry. It uses three separate standards known as K-20, K-14, and K-12. With each step (from K-20 to K-12), the available dynamic range decreases as the average level increases. The top label of the meter scale indicates the maximum head-room (either 20dB, 14dB or 12dB), and just as with normal metering this matches the the full-scale digital. Your monitor gain should be calibrated carefully, so that the level at the 0 dB label of the meter matches 83 dBC.

The K-System meters show both peak and RMS level at the same time. The top red zone of the meters is the loud or fortissimo zone. In music recording, the RMS level should only reach the red zone in the loudest passages, climaxes or occasional peak moments. If you find yourself using the red zone all the time, you might want to check whether your monitor gain is properly calibrated.

  • K-12: This scale is intended to be used exclusively for broadcast material, be it radio or television. With this system, -12dBFS = 0VU = 85dB SPL. The limited headroom of 12dB explains its exclusiveness to heavily compressed broadcast material.
  • K-14: This should be the standard for the majority of commercial recordings created for home listening. Pop music and home theatre mixes are examples of material that would fall under K-14, where -14dBFS = 0VU = 85dB SPL. The available headroom is 14dB. The K-14 scale is probably the most widely used of the three standards.
  • K-20: Offering the widest available dynamic range of the three systems, this scale should be used primarily for large theatrical mixes, dynamic music mixes, and Classical style mixes. Any material with a wide dynamic range should be reserved for the K-20 standard. In this system, -20dBFS = 0VU = 85db SPL. As you might guess, 20dB of headroom is available.

To read more about the K-System and how to use it properly (including monitor gain calibration), read Bob Katz' article: An Integrated Approach to Metering, Monitoring, and Levelling Practices .

The real-time level display

The large real-time level display shows input level (grey), output level (light blue), gain reduction (red) and RMS level (white line) at the same time. It gives you a very good insight in the amount of limiting going on, and the overal peak and average levels.

In case you don't want to be distracted by the display, you can simply disable it using the Show Display button, right under the meter scale select button, left of the level meters. The Show Meters button next to it disables the level meters so you can turn off all visual feedback.

ISP (Inter-Sample Peak detection)

Digital audio processing, and especially ultra-fast limiting or hard clipping in the digital domain, can introduce harmonic frequencies that can't be expressed properly with the sample rate you're using. Still, a D/A converter needs to interpret that signal and translate it to an analog wave form. At some points, especially at sharp transients as a result or limiting/clipping, the resulting wave form that is constructed out of the samples, can have peaks that are higher than the peaks in your original digital signal: inter-sample peaks. The quality of your D/A converter will determine how these peaks are handled, and how they affect the sound.

Of course, it's always best to minimize inter-sample peaks, and at the same time ensure that any peaks in your audio, both normal or inter-sample, stay within the 0 dBFS range. Then you can be sure that D/A conversion (and also other operations like MP3 conversions), do not introduce unwanted distortion.

Using the ISP button, you can enable inter-sample peak detection (also known as, and equal to True Peak metering from the EBU R128 / ITU-R 1770 standards). Pro-L's output level meter will then show the inter-sample peaks in the resulting audio, so you can correct the output level for them. The level readout above the output meter will show exactly how much more headroom is needed. After adjusting the output level, click on the level text to reset it.

Normally, using Pro-L's oversampling (preferably 4x) in combination with a minimum lookahead time of 0.1 ms (which is still very fast), brings down inter-sample peaks to a range of only about 0.1 dB. However, the ISP option will show you exactly what's going on, so you don't have to guess.

To learn more about metering and inter-sample peaks, check out the Advanced Metering Tutorial by Dan Worrall .


  • When skipping through presets, the current metering settings are not changed, but they are saved in sessions. In addition, any change you make is automatically saved as the default setting, used when opening a new instance of Pro-L.
  • You can also switch to the Compact interface layout to hide the real-time display and see larger output meters. See Full Screen mode and resizing.
  • Note that Pro-L's ISP metering fully complies to the True Peak metering as used by the EBU R128 / ITU-R 1770 standards.

Next: Advanced settings

See Also
Recommended workflow

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