Channel dynamics – Part 2 – Limiters, Expanders, and Gates

February 13, 2009 at 6:33 am Leave a comment

OK, peeps – let’s finish up dynamics (at least as much as I’m covering for now). If my last post on compression made sense, then extrapolating limiters, expanders, and gates should be pretty easy. Let’s start with limiters.

A limiter is nothing more than a compressor with an infinite ratio. Any signal level greater than threshold will be reduced to threshold; in effect, limiters govern the maximum amount of signal that can pass through them. Their main purpose is protection of your ears or equipment. If you’ve ever looked at our old Hear Back mixers, you’d notice they have a limiter. So do many iPod-like music players, and they’re often hiding along the output stages of large PA systems. You set the threshold to the maximum level your ears or the equipment can tolerate and that’s the level to which the limiter will limit. And unlike compressors, which may repeatedly compress a signal throughout a song, you rarely want to be engaging a limiter. If you are, you’re probably running something too hot.

Now let’s talk about expanders. These great devices expand dynamic range, but not by making a signal louder, as you might at first expect. They actually expand dynamic range by reducing the amount of signal (by a ratio) once it drops below threshold.

Expander Input/Output Characteristics

Expander Input/Output Characteristics

Make sense? If you can imagine a signal getting progressively quieter (i.e., reading right-to-left in the figure), an expander doesn’t do anything to the signal until it falls below threshold, at which point the expander gets busy and reduces the signal even more rapidly (per the ratio). As with compression, you can often set an attack and release time. The attack time refers to the time it takes the expander to stop expansion once the signal rises above threshold; the release time determines the duration until a falling signal is further decreased by the set ratio. At first, it may seem a little counter-intuitive, but think about it for a few minutes and hopefully it’ll make sense.

Just as a compressor with an infinite (or very high) ratio is called a limiter, an expander with a very high ratio has new name: a noise gate. Gates have numerous applications in live sound. For example, we’ve all run into the problem of audible noise from an instrument during moments of silence. This is usually easily remedied by inserting a gate and increasing threshold just until the gate kicks in when the performer isn’t playing (and, for miked instruments, when the rest of the room is relatively quiet, as well). I start with a fast attack, so the gate lets go as soon as the performer begins playing, and a relatively slow release, so it doesn’t chop off any intentional sustain.

Another gate application I’ve read about (but haven’t actually done, since we’ve had fake drums) is in shaping the kick drum sound. Think of a good kick sound – it has a fast attack followed by a fast decay. A common problem is a slow decay – if you’ve heard such a kick, you’d probably describe it as “muddy,” “flabby,” or “lacking punch.” It’s that long decay tail you’re hearing, and that’s where a gate can help. A fast attack is required to allow the gate to let go quickly when the beater strikes the head, but the important parameter is the release, which lets you shape the decay tail (i.e., a fast release chops off more of the decay tail than a slow release). I’ll try to dig up a picture that illustrates this over the next few days.

One final note about our dbx 166, which contains both compression and expansion in one unit – make sure you’re using only the feature (i.e., compression or expansion) that you want on an instrument. I’ve occasionally seen a gate inserted on a channel to reduce noise, but the compressor functions were also (and I suspect, inadvertently) in effect. It’s not inappropriate, per se, to use both compression and expansion simultaneously, but make sure it’s intentional.

Rob

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Entry filed under: Audio.

Exposure, Focus & White Balance – Part Deux Basic Cues

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