Input Gain, Output Volume, and Clipping

The next step is to set the properties of one of these audio devices, namely, the mixer. Input gain and output volume control are important considerations when dealing with digital audio files. If the output volume is set too high, audio files that contain perfectly good samples can overdrive the soundcard, causing a type of distortion called clipping. Clipping is often described as a "crunchy" or "crackling" sound that usually occurs during the loudest parts of an audio file. To avoid clipping, you can turn down the output volume just far enough to remove any audible crackling.

Likewise, if the output volume is set too low, you may have to turn up the volume significantly on whatever system is receiving your soundcard's output (your audio receiver, a stereo, PC speakers , etc.). This can introduce additional noise during playback, which is not very desirable from a fidelity point of view. Improper volume adjustment is one of the major causes of problems and complaints with digital audio.

There are a variety of ways to adjust the output, or PCM, volume on Linux systems; players like XMMS or FreeAmp have a volume control that adjusts the PCM volume. Under KDE, the kmix program is accessible from the Control Center. The PCM volume is the second slider from the left, with the waveform icon above it ( Figure 19-1 ). On my system, I need to turn the PCM volume down to 58% to avoid clipping during audio playback.

Figure 19-1. KDE volume control utility.

Analogous problems can occur while recording. Recorded audio files can be clipped, or too quiet, depending on the input gain. Luckily, quiet files can be boosted in volume (see the later section, Sound Effects and Filtering), although this may introduce some noise. On the other hand, clipped files cannot easily be repaired. The solution is to ensure that the input gain is near the top of the optimum range before you begin recording.

The optimal input gain depends on the properties of your soundcard, so you'll need to determine the appropriate value by trial and error. Use a recording source that is optimally loud; the line out of a CD player is usually a good bet (refer to the later section, Writing Your Own Audio CDs). Note that the CD player's volume control does not affect the volume of the line-out signal, only the volume of the speakers. You'll need to play a CD and try recording short clips using various values for the input gain. Make sure that the clips you record are the loud parts of the CD, or you'll overestimate the required gain. Finally, pick the highest gain that does not exhibit clipping when you play it back from the computer. On my system, I have to turn the input gain all the way down to 25% to avoid clipping.