PFL: The Sound Tech's Secret Weapon Part 1 of 2
A high-performance sports car can be exciting to drive even if you don't know anything about auto mechanics. But if you are having performance issues or you want to tune the motor to perfection, you're going to have to look under the hood. With all the great features that the latest generation of mixers have to offer it is easy to get distracted by bells and whistles and overlook how useful the simple tools can be in helping you to achieve a great sound. One of those important tools is PFL (which is an abbreviation for 'pre-fader listen'). If the Digital Signal Processor on your mixer is like the sports car's turbo boost, then the PFL would be like the headlights. Sometimes you need to be able to focus your ears on individual sounds in order to make sure you're going in the right direction with your mix. But to get there you need to learn the techniques which make PFL the Sound Tech's Secret Weapon.
What is PFL?
Pre-Fader Listen is a system that allows you to monitor individual channels, sub-mix groups, and sends in isolation or alongside a group of other channels. Most mid-to-large size mixers have a PFL system. The PFL system lets you create your own separate headphone mix without affecting the main mix. This gives you direct access to listen-in on almost any part of your mixer's signal path.
Each channel on your mixer has a little PFL switch and LED 'active' indicator somewhere close to the channel fader. These allow you to assign that channel to your headphone mix (PFL). As the name implies, the PFL is not affected by your channel fader, so you can listen to signals that aren't even in the main mix. Depending on your individual mixer, many models assign the PFL post-EQ so you can monitor your EQ adjustments in the phones too. This means you can compare the relative signal level and sound quality of all your inputs, optimize your input gain, EQ, and more. If your mixer has sub-mix groups with their own faders these will usually have a PFL switch also. Full-featured mixers like Carvin Audio's Concert Series 12-32 Channel Mixers provide PFL listening.
The Sound Tech's Stethoscope
You might have seen someone use the old mechanic's trick of using a piece of tubing like a stethoscope to search for the source of an unusual noise inside an engine. With PFL you can listen very closely to what's on a channel without the rest of the mix competing with what you need to hear. One of the most useful techniques is to check each individual channel in the headphones to make sure everything sounds right. Is the signal clean? Or is there noise present that shouldn't be there? Can you hear 60Hz hum or other instruments bleeding into the signal? Is there any feedback or ringing going on? What is the EQ actually doing to the sound? Is there any distortion that shouldn't be there? Is this the drum mic that is picking up that annoying squeaking sound you need to isolate?
Once you're familiar with your PFL system you will be able to employ selective-listening to dial in each part of your mix just like a high-priced foreign mechanic fine-tunes that sports car. Get some high quality closed back headphones and become an audio detective and find out how much better your mixes start to sound.
But wait! There's more!
Just when you were almost convinced that this PFL thing was really cool, you discover that there is another very useful feature that makes it an even more powerful audio engineering tool. In Part 2 of this series and we'll talk PFL and the Meter Bridge. I once discovered the guitar player had a rattlesnake tail inside his acoustic guitar! What is the most unusual discovery you ever made as an audio detective?
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