Image Processing Reference
In-Depth Information
Figure 37-6 Small-speaker EQ setting in iTunes.
If you know that the audio is going to be played on a handset with a small loudspeaker,
then boosting the bass frequencies may help a little at the receiving end. Figure 37-6 shows
the appropriate equalizer settings for small speakers in the iTunes interface.
Encoding audio for use on 3GPP devices benefits from the application of a little com-
pression. This is analog compression, the sort that fattens the bass end of the frequency
If you know that the target device has a limited-frequency response, you may be able
to reduce the work done by the audio codec if you band-limit the audio before it goes
through the codec.
Analog Compression
Compression in the context of analog audio also describes a process where the waveform
is altered in a nonlinear way in order to prevent clipping and to add a pleasing richness to
the sound. The compressor dynamically alters its behavior according to the loudness of
the passage being processed. It is often combined with a noise gate and limiter so that they
all work together.
The concept of a compressor revolves around a mapping of the input level to the out-
put level. As the input level increases, the output level has progressively more attenuation,
so the relationship is nonlinear. This has the effect of flattening the waveform at the max-
imum excursion and squashing the waveform more at its extremes. Figure 37-7 shows
how a compressor gently flattens the waveform.
The effect is subtle but noticeable, and somewhat pleasing when you hear the audio.
A similar effect is attained when you turn on the loudness circuit in your hi-fi.
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