The core tools of the digital audio mastering engineer aren't all that different from those used in the mix—compression, limiting, and equalization again. The main difference is that mastering plug-ins are generally less invasive, because they're not meant to have an obvious effect. You're only supposed to notice the increase in average level and how much better the mix translates to different playback systems, not any change in tonal color or increase in distortion. It's possible to master a session using Ardour's LADSPA plug-in support, but the Ardour mixer isn't geared to the task. Instead, you can use JAMin (GNU/Linux), the name of which stands for the JACK Audio Mastering Interface. JAMin is available from http://jamin.sourceforge.net or in most good GNU/Linux distributions. Ubuntu users can install the jamin package with the Add/Remove Applications tool. Because it's a stand-alone program, you can use JAMin with any JACK application, not just Ardour.
JAMin provides a custom interface for a collection of the most frequently required stereo mastering plug-ins and provides global preset save and recall facilities for them. It includes a 1023 band equalizer with parametric controls and a neat hand-drawn frequency curve feature. There's also a more traditional 30-band equalizer with faders and a built-in spectrum analyzer. Next in the signal chain is a three-band mastering compressor, which means you can adjust bass, middle, and treble frequency bands independently. On the output stage, a look-ahead brick-wall limiter catches any transient peaks that exceed OdBFS. The look-ahead feature means the effect of limiting isn't as harsh and obvious as a simple hard limiter that square-waves every peak.
You must have JACK running before you attempt to start JAMin, or it won't work. Next, open your previous Ardour session. You need to use JAMin's bypass button to make subjective comparisons between processed and unprocessed sound, which means JAMin has to sit in the signal chain between Ardour's master bus outputs and your soundcard's playback ports (see Figure 10-11).
Because JAMin connects to your soundcard by default on startup, the easiest way to accomplish this is to use Ardour's mixer to disconnect the master bus outputs from the soundcard and then connect them to JAMin's inputs. You can use JAMin to manage JACK connections by using JAMin's own Ports menu, but make sure you disconnect any previous direct connection from Ardour's master outputs to your soundcard first.
Don't be too alarmed if the DSP load figure reported by JACK shoots up. JAMin is CPU hungry—it has to be, to perform complex audio processing tasks in real time. Just like Ardour, JAMin works best on a system with a real-time kernel, but you shouldn't need one just to check out how the program works.
Back in Ardour, to the right of the clock on the top bar is a small drop-down menu that says Internal by default; switch this to Jack. (You may recall setting this menu option when working with Hydrogen in the last chapter.) You can now remote-control Ardour's transport using the play, rewind, and pause buttons at upper left in the JAMin window. You can also press the space bar on your keyboard to toggle play or pause, just as in Ardour. This saves a lot of clicking back and forth between windows during the mastering process.
Now you're ready to begin mastering. First, click the left-pointing arrow in the JAMin transport controls to rewind to the beginning of your Ardour session. Then, click the triangle play button, and the JAMin clock to the immediate right starts counting upward. To the right again, the transport indicator says Rolling. If all is well, the green input and output meters at the extreme left and right of the JAMin window, respectively, flicker, and you hear your Ardour session playing back through your soundcard (see Figure 10-12).
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