Better Squashing through Multiband Dynamics Processing

by Craig Anderton

A multiband dynamics processor isn't a processor shared by several different bands, but a device that combines elements of both EQ and dynamics control. If used properly, multiband dynamics (also called "multiband compression" because that's the most popular application) can give more transparent and effective dynamics control than traditional, single-band compressors.

Plug-in mania has given a big push to multiband compression, because hardware units required a costly collection of knobs and switches, while software versions are not subject to those limitations (Fig. 1).

Fig. 1: Two multiband dynamics processors: Waves' C4 Multiband Parametric Processor (right), and Cakewalk LP-64 Linear Phase Multiband (left).

Formerly used almost exclusively for mastering, multiband dynamics processing is common and inexpensive enough to be used for all types of dynamics control. They're particularly useful for instruments with wide frequency and dynamic ranges, such as piano and drums.

DYNAMICS CONTROL BASICS

A compressor is a special-purpose amplifier. This amplifier turns down its gain whenever the input signal exceeds a certain user-definable threshold, thus lowering the level of peaks. As the peaks are lower and no longer use up the maximum available headroom, it's possible to turn up the level of the overall compressed signal, thus making it sound louder.

A limiter clamps the output so that within reason, it won’t exceed a particular threshold, regardless of the input level.

An expander does the inverse of a compressor: as a signal falls below a threshold, the amplifier turns down the signal so it loses level at a faster rate than normal.

MULTIBAND COMPRESSION BASICS

A multiband compressor splits an incoming signal into several bands (typically 3 to 5), like a graphic equalizer or spectrum analyzer. A compressor follows each band, so that each compressor affects only a specific band of frequencies. The compressors can also usually serve as limiters or expanders.

The rationale behind band-splitting is simple. With a traditional compressor, a strong signal that exceeds the threshold brings down the gain, but this affects all frequencies. So, if a strong kick drum hits, it will bring down the level of the cymbals and other high-frequency sounds as well as the kick.

With a multiband compressor, you would dial in the frequency range of one compressor to that of the kick, another to lower midrange, another to upper midrange, and another to treble. Thus, when the kick hits, it's compressed but this doesn't affect the other bands, which are compressed according to their own needs.

TWEAKING MULTIBAND COMPRESSOR PARAMETERS

A lot of musicians have a hard time getting a grip on how to use single-band compressors. Often, they set the compression so high that there are "pumping" and "breathing" artifacts, or set the attack so fast that all percussive transients are lost. Tweaking a multiband compressor is far more complex, because now several bands have to be set just right.

Here are some general guidelines on setting up multiband compression.

1. Listen to the sound before you touch any knobs, and analyze what needs to be done. If you just want a hotter, louder sound, that's the simplest application: just split the signal into bands that divide up the spectrum, and set the compressor parameters similarly. In this case, the multiband compressor acts like a standard compressor, but gives a more transparent sound because of the multiple bands.

Using a multiband compressor to fix response problems is more complex. For example, suppose you're mastering a tune, and the bass end sounds kind of muddy because the kick and low toms ring too long, the high end is shrill, and the vocals in the midrange sound buried in the mix. You don't want to compress the low end and make the mud louder, nor do you want to emphasize the high end. But you do want to compress the midrange to bring the vocals more to the front. Once you've assessed what you need to do, it's time to...

2. Set the frequency ranges. Most software multiband compressors let you edit the frequency range, from narrow to broad, covered by each compressor. Generally, you can also solo individual bands to hear what they contribute to the overall sound. Set the compressors for a 1:1 ratio and high threshold so they don't affect the signal, then solo a band and listen. In our example above with the muddy low end, you might omit compression entirely and add some expansion to reduce ringing. Tweak the band's range so that it covers the muddy bass area but nothing else.

As most multiband compressors have level controls for each band, I find it handy to first treat the device like a graphic EQ. If turning up a band improves the sound, that may indicate that a little compression is in order. If turning a band down helps, then I generally don't compress it, or use subtle expansion to emphasize peaks more while de-emphasizing lower-level signals.

3. Start editing the compression settings. This is the trickiest part, because anything you do in one band affects how you perceive the other bands. For example, if you compress the midrange, the treble and bass will appear weaker. You don't want to get into a situation where you bring up the midrange, which makes you then want to bring up the bass and treble, and then you have to compensate for that.

Avoid going over 1.5:1 or 2:1 compression ratio at first, and keep the threshold relatively high, such as -3 to -9 dB. This will tame the highest peaks, without affecting too much else of the signal. Listen after each change, and give your ears a chance to get acclimated to the sound before making additional changes. If your multiband compressor lets you save presets, save them periodically as temp 1, temp 2, temp 3, etc. That way you can go back to a previous, less radical setting if you start to lose your way.

In general, it seems best to work on any "problem bands" first. Get them sounding right, then tweak the other bands to work well with them. For example, suppose you compress the upper midrange to better articulate voices, or bring out melodic keyboard lines. Once that's set, adjust the bass to support (but not overwhelm) the midrange, then tweak the treble to suit.

4. The final step: it's an equalizer. After the dynamics are under control, my final tweak is usually adjusting each band's output level for the best overall balance. In fact, one of the nice things about a multiband compressor is that some bands can compress while others expand, and still others just do nothing—if set for zero compression, they act like bands in a graphic EQ.

MORE THAN PROGRAM MATERIAL

In addition to mastering, multiband dynamics can also dress up individual instruments. Bass is one of my favorites; compress the high end to bring up finger pops and slaps, compress the very low end for a smooth, sustained sound, but leave the lower midrange alone.

Piano is also a great candidate for multiband compression. Leave the low end alone except for very light limiting, so that a good hit in the bass range has strong, prominent dynamics—don't squash the low end too much, or the notes will lose drama. But compressing the upper midrange a bit can help melody lines cut through a mix better, and boosting—but not compressing—the very highest frequencies (around 8kHz and above) adds "air."

Drums work well with multiband compression, because each drum tends to have its own slot of the frequency spectrum. A multiband compressor can almost remix a drum part by compressing or boosting certain drums, and using expansion to reduce "ringing" if you need to tighten up a drum sound.

So once again, let's hear it for plug-ins...the world of software has turned a formerly esoteric piece of hardware into something cost-effective enough for just about any studio. To me, serious progress.  -HC-

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