adt-audio
ToolMod
®
Pro-Audio Module System
TM215b Stereo Mastering Peak-Limiter
5
sical problem with limiting that is discussed below, the LONGEST PUMP FREE RELEASE
TIME is the best possible setting.
➔
Start with a release time of about 1.5 sec and
go down until the pumping disappears.
The second issue is the physical effect of low frequency distortion with short release
time settings. Since the control voltage that causes the regulation has to be a dc vol-
tage that is actually the rectified audio signal, there are always some remains of the
ac audio signal (ripple) on the dc control voltage. Since the main part of the filtering
takes place in the attack/release circuitry, the ripple depends on the settings of at-
tack and release, where the release time setting is a lot more important than the at-
tack time setting. In addition, low frequency signals cause more ripple than mid and
high frequency signals. The ripple of a low frequency audio signal causes an additional
regulation that modulates the audio signal. Since the waveform of the ripple is - peak
rectifier circuits assumed - a saw tooth with twice the audio frequency, the regulation
from the ripple causes distortion of the audio signal.
Unfortunately, there is no safe range between audible pumping and the occurrence of
LF distortion - it simply depends on the gain reduction. The higher the gain reduction
is, the worst it is to find a decent compromise.
Let‘s sum this up. If the release time is too long, you get audible pumping, if it is
too short, you get low frequency distortion.
➔
Set the release time to the longest
value without pumping and check for low frequency distortion.
If you get low frequency distortion set the Dynamic Release to a value that reduces
the distortion.
Dynamic release increases the static release time depending on the den-
sity of the signal. High level bass signals increase the density while short peaks in the
high and mid range don‘t change the density. With dynamics release you get a longer
release time with low frequency signals while the release time remains low with short
peaks. Since the longer release time is only present for the time of the higher density
(the high low frequency levels), it is automatically reduced after the high density si-
gnal. This is not a perfect remedy for all lf distortion problems, but it helps.
