Handbook for Sound Engineers

970 Chapter 25

25.21.5 EQ High-Frequency Response Anomalies

Odd phenomena occur when filters are attempted too
close to half the sample rate, 24 kHz in a 48 kHz system
for discussion here: partly as a result of the inevitable
zero in response at half the sample rate with bandpass
filters, as we saw, and partly an effect of the prewarping
in the transform calculations used to create the filter
coefficients. The effective Q of a filter as used in a para-
metric such as this appears to increase (become sharper)
and become asymmetric (high side gets steeper) as its
curve approaches Fs/2. Although this effect can be
considered unimportant, occurring at the audible
extreme as it does, this behavior can be improved by the
expedient of applying a subsidiary correction to the
desired Q value prior to warping, or by the more funda-
mental approach of oversampling. This basically means
running the EQ (or at least the HF bits of the EQ) at
twice the sample rate; upsampling to 96 kHz and down-
sampling (to get back to 48 kHz) are quite straightfor-
ward. This has the effect of pushing the squiffy zone up
toward the new Fs/2 of 48 kHz, where it simply won’t
matter, keeping the normal audio-frequency range of
EQ linear and tame. Under some conditions with some
program material, upsampled EQ (even though subse-
quently brought back down again) can sound better.
One has to be very careful with the nature of the recon-
struction filters in the upsampling in order not to imbue
even worse funnies in EQ frequency response than one
is trying to fix.
Fig. 25-141A shows the squiffy effect on a 16 kHz Q
of 2 parametric EQ section; a similar Q of 2 filter at
200 Hz is shown for comparison. Correction (not over-
sampling in this case) results in the improved
lower-frequency slope of the 16 kHz filter; this is now
comparable to the skirts of the 200 Hz filter (Fig.
25-141B. Unfortunately, there’s not a whole lot one can
do about that zero at 24 kHz without oversampling, so

EQ close up to the band limit will always be a bit

suspect.

25.22 Digital Dynamics

There are many approaches to dynamics processing in

digital, but most fall under one of two categories:

mapping and literal. Briefly, Mapping involves creating

a plot, a table, or a map describing what the desired

output level for any particular input level needs to be;

an input sample comes along and based on its value, a

gain-control value is picked out of the look-up table

map that is to be applied to that particular value of input

Figure 25-140. Shelving EQ using single-order filters.

+ Output

X

+

X

Output

delay 1

A1

Low-pass

Input

LF level

(+ or

+

X

Output

delay 1

HF level

coefficient

(+ or )

High-pass achieved

by subtracting a

low-pass

A1

Low-pass

B0

X +

B0

X

X

Figure 25-141. High-frequency anomaly

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AUDIO PRECISION STD LEVEL (dBu) VS FREQ (Hz) 30 JUL 101 18:32:0

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A. Uncorrected HF EQ anomaly.

B. Corrected HF EQ anomaly.