Sound Effects and Filtering

Sound Effects and
Filtering

The next step is to apply effects or filtering to modify the
sound quality of your recording, if necessary. One situation where filtering
might be useful is when capturing live audio from a microphone. Depending on
the quality of the microphone and the acoustics of the recording space, you may
need to perform frequency filtering or apply minor effects to give the
recording a fuller sound.

Volume
Adjustment

Despite the precuations you may have taken setting the correct
recording volume, sometimes an audio clip is just too quiet. If you've obtained
the clip from somewhere else, you may have no choice in the matter. It's easy
to use SOX to boost the volume level of the file. In fact, this can be done
simultaneously with any other operation SOX supports. The first step is to find
out how much the volume must be boosted:

$ sox mysong.wav -e stat

Maximum amplitude: 0.803

Minimum amplitude: 0.000

Mean��� amplitude: 0.006

Maximum delta:���� 0.690

Minimum delta:���� 0.000

Mean��� delta:���� 0.003

Volume adjustment: 1.245

The command runs some statistics on the samples in the file
and prints the results. For us, the important one is the Volume adjustment
suggestion. This indicates that we need to boost the volume about 24%. And we can
immediately do this:

$ sox mysong.wav -v 1.245 newsong.wav

newsong.wav is created, with
an optimal volume. This technique can also be used to soften the volume of an
audio clip. Keep in mind, though, that if the recording volume was too high
when the clip was recorded, lowering the volume of the file will not make the
crackling go away. It will just be softer, like the rest of the file.

If graphical applications are your thing, you can use audacity
to do exactly the same thing. The menu option is under Effect/Amplify. You also
have the option of boosting the volume of certain sections, instead of applying
the change to the entire file.

Frequency
Filtering: High-Pass, Low-Pass, Band-Pass "Graphic Equalizer"

Frequency filtering is most familiar in the sense of a graphic
equalizer. In the earlier section Types of Digital Audio, I discussed how
frequency encoding converts an audio clip into a set of volumes. Frequency
filtering is a mathematical operation that changes these volumes relative to
one another. style='color:#003399'>Figures 19-4 and style='color:#003399'>19-5 show a sample frequency encoding. The
spikes you see are the pure tones with the highest volumes. Notice how, in style='color:#003399'>Figure 19-5, the lower-pitched frequencies are
suppressed.

style='font-size:10.5pt;font-family:Arial'>Figure 19-4. A frequency spectrum
before filtering.

style='font-size:10.5pt;font-family:Arial'>Figure 19-5. The same frequency
spectrum as in style='color:#003399'>Figure 19-4 after applying a high-pass filter.

Although frequency filtering can be done very
precisely, the human ear generally recognizes three broad categories: treble,
midrange, and bass. Too much bass or too little treble leads to a
"muffled" sound, whereas too much treble or too little bass leads to
a "tinny" sound. The human voice generally falls in the lower end of
the midrange frequencies, so vocals can be enhanced by boosting the midrange. Karaoke
machines work by selectively filtering in the 2- to 4-KHz range while allowing
other frequencies through.

Again, SOX excels at performing equalization.
SOX provides low-pass, band-pass, and high-pass filters, which decrease the
amount of treble, midrange, and bass, respectively. The filter called class=docemphasis1>tape-deemph is also useful when recording directly
from a television or DVD audio signal. All of these filters can be invoked
during file format conversion. Consider the following commands:

$ sox infile.au tape-deemph outfile.wav

lang=EN-GB>$ sox outfile.wav highp 4000 outfile-hp.wav

The first command performs a format
conversion while applying the tape-deemph filter, and the second applies an
additional high-pass filter centered at 4KHz and saves the recording into a new
WAV file. SOX is restricted to one filter per invocation, but it supports pipes
for input and output, so the previous pair of commands could be reduced to the
following command line:

$ sox infile.au tape-deemph - | sox - highp 4000 outfile-hp.wav

A word of caution: Many audio players for
Linux, such as XMMS, have built-in graphic equalizers. If you are using one of
these players to preview the "before" and "after" of
frequency filtering, make sure that all the equalizer knobs are set to a
central position. Otherwise, the "after" file that sounds best on
your own system may not sound that good on someone else's system, defeating
altogether the purpose of doing this kind of processing.

Sound Effects: Chorus, Delay, Echo, Reverb, Stereo Offset

Sound effects are even more complex than
frequency filtering; they include such well-known effects (especially in
electronic music) as reverberation, echo, delay, flanging, and chorus. The SOX
utility can add all of these effects to any uncompressed sound file, using the
same syntax as earlier:

$ sox mysong.wav -e reverb 1.0 500 250 mysong

Did I mention that audacity can apply effects
to all or part of a file, as well? There's more than one way to do it in the
Linux world.