U. Glaeser

FIGURE 39.69
gain of the filter. It is possible to sweep these parameters in real time, so it is important to ensure filter
stability under time-varying conditions. Stability criteria are outside the scope of this discussion, but
it is not difficult to drive IIR filters to instability when varying coefficients in the presence of an input
signal.
Three-dimensional positional audio and environmental simulation also demand the use of filters;
however, low-pass resonators are not the ideal choice. The head-related transfer function (HRTF) describes
the filtering performed by the shape of the human head, ear lobes, and ear canal [8]. In addition to
loudness and inter-aural time delay, the brain uses cues provided by this filtering to determine the position
of sound emitting objects. The most common implementation of an HRTF uses a FIR structure such as
the Gossett–Smith interpolator filters; however, both the impulse response and the usage of HRTF filters
are much different from that of Gossett–Smith interpolator filters.
Obstruction and occlusion are other filtering effects that occur when sound sources move in relation
to other objects. For example, obstruction is the effect caused by an obstacle between the listener and a
sound source in the same room. This applies a low-pass filtering effect on the direct sound, but not on
the reverberation. In contrast, occlusion is the effect caused by a sound source located outside the same
room as the listener. This applies a low-pass filtering effect on both the direct sound and the reverberation.
Low-pass IIR filters are appropriate for these applications, although resonance is neither needed nor
desired. The system can simulate these effects with the low-pass resonators used for music synthesis, but
the typical −12
dB per octave attenuation slope is generally too steep. A gentle −3
or −6
dB per octave
slope is more appropriate.
The audio system of the PC also performs the role of a typical component stereo system, playing
prerecorded music from CD or other sources. Thus, the software distributed with soundcards often includes
equalization, such as tone controls and graphic equalizers. Simple tone controls, such as bass and treble,
often use shelving filters as shown in Fig. 39.70. Graphic equalizers can use a bank of either band-pass
filters or parametric equalizers into a summation matrix.
It is the designer’s choice whether to implement the filter types required by the digital audio system
of the PC in hardware or software. The system often includes a programmable digital signal processor
(DSP), enabling a software implementation. As the required filter count increases, it often becomes more
© 2002 by CRC Press LLC
B0
Two-pole IIR filter block diagram.
Z -1
Z -1
A1
A2