FabFilter Pro-L 2 Help
Digital audio processing, and especially ultra-fast limiting or hard clipping in the digital domain, can introduce inter-sample peaks. In this case, the analog signal that a D/A converter would generate from the sample values becomes higher than the individual sample values. Depending on how good the D/A converter is, this can cause audible clipping. The same happens during operations like MP3 conversion.
As illustrated by this image, the analog signal (blue) has a peak (green) that is higher than the sample peaks in the digital signal. Peaks like these are called inter-sample peaks or true peaks. Today, many commonly used metering/loudness standards, like EBU R128 or ATSC A/85 impose a maximum allowed true peak level.
To guarantee that FabFilter Pro-L 2's output doesn't exceed a certain maximum true peak level, enable the True Peak Limiting option in the bottom bar.
True Peak Limiting works at two different stages:
When using True Peak Limiting, it's best to keep True Peak Metering enabled as well, using the TP button next to the display and meter scale settings. The output level meter will then show the actual true peak levels (with level reading in green to reflect this). See Metering.
Of course, FabFilter Pro-L 2's true peak metering system fully complies with the ITU-R BS.1770 and EBU R128 standards, and is suitable for MFiT (Mastered for iTunes).
Instead of letting the true peak limiting process deal with remaining true peaks in the output, it's even better to prevent high true peak overshoot in the first place. The best way to minimize inter-sample peaks and distortion, is to use oversampling. In addition, increasing the lookahead can also help quite a lot, but may affect loudness and transparency.
In most cases, choosing 4x oversampling in combination with a minimum lookahead time of 0.1 ms, keeps inter-sample peaks within a range of 0.1 dB.