Compression Limiting and Equalization

Now you've got the tracks in balance, with fades up and down in all the right places, it's time to address three common mix problems:

• Some of the tracks are at an inconsistent level, and it would take forever to sort out with gain fader automation. (This is often the case with vocal tracks, particularly when the singer doesn't have much studio experience.)

• Transient bursts of sound push the meters into the red, even though the rest of the track is quiet.

• Many of the instruments have a muddy, booming sound, and it's hard to hear them individually.

Sound engineers like to attack these problems using the triple whammy of compression, limiting, and equalization. These are traditional techniques used since the early days of rock 'n roll, when the aim was to make recordings sound more exciting when played on a jukebox. Fortunately for you, these methods translate well into the digital domain, and Free Software plug-ins for Ardour cover all these effects and many more. LADSPA has been the audio plug-in standard on GNU/Linux for several years, and the API of its successor, LV2 (LADSPA version two), is starting to gain acceptance. For the time being, you're likely to find that the original LADSPA versions of the plug-ins are packaged for your

GNU/Linux distro of choice. The following examples use the swh-plugins package (available from Ubuntu users can install swh-plugins using the Synaptic Package Manager.

Compression is the tool used to solve the problem of inconsistent level. Every time the signal passes a predetermined threshold, the compressor plug-in applies a preset gain reduction on the part of the audio above the threshold. The effect is to make the loudest parts a little quieter, while the quiet parts are left alone. This creates more headroom, which is why a compressor often includes a boost control— raising level after the compression stage, for gain with reduced risk of clipping, but at the cost of an increased noise floor.

In Ardour's mixer, right-click in the black box above the Mute and Solo buttons for the track you wish to compress. This is the prefader plug-ins box; there's a corresponding post-fader plug-ins box further down. Select New Plugin from the pop-up menu, and then select Plugin Manager from the submenu. A LADSPA plug-in selection dialog opens, if your plug-ins are correctly installed (see Figure 10-6). (You can use the same right-click menu to delete a plug-in when you no longer need it.) Plug-ins are listed alphabetically by default—you may find it helpful to click the Category button, which resorts the plug-ins by their function. Under the Compressors category is SC4, which is a stereo in, stereo out plugin. An alternative SC4 Mono compressor is available for tracks with a single channel. Select SC4 or SC4 Mono, click the Add button, and then click the Insert Plugins button to close the dialog box.

ardour: piugins

AvaiFable P.lugins

C* Compress - Mono compressor C* Compress - Mono compressor Dyson compressor Dyson compressor

Category v

Compressors Compressors Compressors Compressors Compressors Compressors Compressors Compressors Compressors Compressors

Tim Goetze Steve Harris

Steve Harris Steve Harris

Plugins to be connected

* Cancel Insert Plugin(s)

Figure 10-6. Ardour's Plugin Manager dialog, featuring a broad range of LADSPA effects. A bug in Ubuntu means that each plug-in is listed twice in this screenshot.

The name of the plug-in you selected, SC4, appears in the prefader plug-ins box of the mixer strip. If a plug-in's name is in brackets, the plug-in is bypassed in the mixer's signal chain until you choose to activate it. Double-click SC4 to bring up the plug-in's dialog, and click the Bypass button—this plug-in's name is shown in brackets in the mixer strip. Click the Bypass button again, and the brackets disappear—the plug-in is ready for use. The name of the plug-in author is displayed proudly in the brown top bar of the plug-in's dialog—just as there is a community of Free Software application developers, there is a community of Free Software audio plug-in creators.

Next, start session playback with the play button in Ardour's transport controls; the Amplitude meter in the SC4 dialog shows some activity. The Gain Reduction meter probably isn't doing anything yet, because the default settings don't do any compression. You may need to bring the threshold level down to about -20dB or lower; and if the track is mostly transient sounds, you probably need to reduce the attack time to 10 milliseconds or so. The Attack control determines how long the compressor waits before applying the effect; if attack is set too long, the plug-in will miss transients, such as snare or bass drum hits. Finally, set the compression ratio to around 4.0. The "Gain reduction" meter starts to move (see Figure 10-7).

Figure 10-7. Some typical settings for the SC4 compressor. The "Gain reduction" meter shows a -2.68dB cut on the Audio 1 track, but this value varies continuously according to the input amplitude.

If only a few loud transients are reducing headroom across the track, as in the second typical problem, then a limiter may be the best solution. The swh-plug-ins package includes a plug-in called Hard Limiter, which, as the name suggests, produces a harsh clipped sound if overused (see Figure 108). However, with more subtle settings, you can use it to catch the occasional transient without audible side effects. Increasing the "Residue level" control softens the effect by mixing in some of the untreated signal, but allowing too much of this residue into the track's output works to negate the limiting.

Figure 10-8. You can use the Hard Limiter plug-in to prevent transients from causing overs.

To fix the third problem, muddy and indistinct tracks, you can use a frequency equalizer, such as the LADSPA plug-in Multiband EQ (see Figure 10-9). This equalizer plug-in is a little like the graphic equalizer on some hi-fis, which is designed to compensate for acoustic deficiencies in the listening room. Alternatively, a parametric EQ plug-in allows you to specify any problematic frequency you choose and apply boost or cut as required. The bandwidth control on a parametric EQ lets you set a wide or narrow boost or cut across the frequency range, as desired. Knowing the exact frequencies to boost or cut is part of the mixer's art; but generally speaking, you're looking to avoid large overlaps in frequency bands between different tracks—and cuts usually work better than boosts.

Figure 10-9. This multiband EQ with a low shelf enables you to shape the frequency curve of your track

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