FabFilter Pro-L 2 Help
At the righthand side of the bottom bar in the interface, FabFilter Pro-L 2 has a Bypass button and an Output Level button that opens a panel with a large Output Level knob and various input and output options.
The Output Level knob plays an important role: it sets the desired maximum output level for the limiter. It seems natural to set it to 0.0 dB: you want the output to be as loud as possible, right? Not so fast.
Due to its ultra-fast behavior, the limiting process can generate inter-sample peaks, also known as true peaks: while none of the outgoing sample values are higher than 0 dBFS, the analog wave form that will be constructed out of the samples by the D/A converter can actually exceed this by several dB. Actually, any conversion that reinterprets the wave form, especially AAC/MP3 conversion, can expose inter-sample peaks. This will lead to unpredictable clipping and therefore possibly audible distortion.
"How bad is clipping caused by true peak?"
This is very hard to tell: it depends on many things, such as the quality of the D/A converter or AAC/MP3 conversion process and the character of the music. Many professionally mastered albums contain inter-sample clipping and this doesn't have to be a problem. The main effect is that the music may suffer from slight distortion when played by low-quality D/A converters, or when converted to AAC/MP3 format.
"So what do you recommend?"
We recommend to turn on True Peak Metering to visualize the inter-sample peaks so you are at least aware of them. If the true peaks exceed your desired maximum output ceiling, you can either dial back the Output Level knob to accomodate for the true peak overshoot, or simply enable True Peak Limiting so that true peaks will never exceed the current Output Level (then showing as "dBTP" instead of the normal "dB").
The final Output Level ceiling you choose, depends on the metering standard you need to comply with and/or the medium you are mastering for. Theoretically, you could leave it at 0.0 dBTP with True Peak Limiting enabled, but commonly used metering standards like EBU R128 or ATSC A/85 require a specific maximum dB True Peak level (-1.0 dBTP and -2.0 dBTP respectively), just like popular streaming services as Spotify and Youtube (-1.0 dBTP). When mastering for CD or iTunes, you're safe with -0.1 dBTP. Note that metering standards and requirements tend to change once in a while, so it's always wise to stay updated about the current developments.
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. There are two ways to accomplish that: turning on oversampling, and slightly increasing the Lookahead setting in the Advanced panel. In most cases, choosing 4x oversampling in combination with a minimum lookahead time of 0.1 ms, keeps the true peaks within a range of 0.1 dB.
Right under the Output Level knob, there are four small option buttons:
You can bypass the entire plug-in with the Global Bypass toggle button to the left of the output level button. While most hosts already provide the ability to bypass plug-ins, our internal global bypass feature is guaranteed to correctly compensate the latency of the plug-in and it also applies soft bypassing to avoid clicks. While the plug-in is bypassed, the display dims and a red light glows at the top of the bypass button itself.
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