Handbook for Sound Engineers

802 Chapter 23

value but can also be configured so that the Q varies
with gain.

23.6.3.2 Symmetric or Asymmetric Q

Straightforward designs produce constant Q filters that
have the same Q for any amount of boost or cut. If the
frequency response curves for the same amount of boost
as cut are mirror images of each other across the unity
gain axis, the response characteristic is called reciprocal
or symmetrical. This means that the bandwidth of
frequencies affected when boost is applied is greater
than that affected when cut is applied. Fig. 23-30 shows
that in the symmetrical response, the cutoff frequency in
attenuation mode Fc is less than that in boost mode Fb.

This is not always the most musically useful response.
It is more common in spectrum shaping to want to gently
apply boost to a broader region. Boosting a narrow
region tends to lead to instability. At the same time, it is
more useful to be able to notch out a fairly precise
frequency, without removing a large portion of the

surrounding spectrum. For this reason, equalizers tend be

designed so that the bandwidth increases with gain.

23.6.4 Programmable Equalizers

All types of equalizers can be programmable. In digital

equalizers, the filter coefficients are stored in memory

and may be recalled or modified at will. Unless a digital

equalizer implements only a fixed set of coefficients it

is inherently programmable.

In programmable analog equalizers, a digital control

system is used to physically manipulate the analog

filters. This can be either by controlling switches that

swap components in or out of the circuit, or by using

voltage-controlled gain to alter the filter’s response. In

the case of switched capacitor filters, the digital control

system can adjust the filters by manipulating the

switching frequencies to adjust the equivalent resistor

values and thus the filter characteristics.

23.6.5 Adaptive Equalizers

The adaptive equalizers have long been used in commu-

nications systems for multipath echo cancellation. They

are the ultimate equalizers for sound systems that must

adapt to acoustic conditions that may change at any

time. A common example of an adaptive equalizer in

sound reinforcement is a feedback suppressor. In this

application, the equalizer monitors the signal passing

through it for the characteristic exponential increase in

level of a frequency that is associated with feedback

buildup. When this increase is detected, a very narrow

and deep notch filter is placed at that frequency to

suppress the feedback. This can typically operate in a

fraction of a second such that you were unaware that the

event occurred.

References

1. J. Blauert, and P. Laws, “Group Delay Distortions in Electroacoustical Systems,” JASA, Vol. 63, no. 5, pp.
   1478–1483 (1978).


2. S. Linkwitz, and R. Riley, “Passive Crossover Networks for Non-coincident Devices,” JAES, Vol. 26, no.3, pp.
   149–150 (1978).

Figure 23-30. Symmetrical response with different band-
width in cut and boost.

Gain

Fc Fb Frequency