Interfacing and Processing 277Paradoxically, the compression threshold (at least for bass frequencies) should be
increased if the gain reduction exceeds about 6 dB. Also, attack and release times require
careful setting to avoid pumping on strong low bass.
Limiting is a higher ratio, more brute force (many dB-to- l) gain reduction. Its raison
d’etre is to catch fast peaks, hence “peak limiting. ” Attack times that are useful for
protecting most loudspeaker drivers are in the order of 10 μ S. Faster rising peaks that “ get
through ” rarely cause damage to hardware, but may be reproduced effi ciently by metal-
diaphragmed drive units ( cf. paper cones) and perceived and found highly unpleasant by
the ear. Hence faster-acting peak limiters may enhance sound quality under many real
conditions of “ operator abuse. ”
8.6.5 Clipping (Overload) Considerations
Driving any power amplifi er with excessive input results in clipping because the output’s
excursion is fi nite. Amplifi ers offering higher power into a given load impedance provide
a higher voltage swing into that impedance so clipping for a given sound pressure level
is less likely to arise. However, linear increases in power give only underproportionate,
logarithmic increases in headroom (in dB) and cost linearly ascending amounts of money.
At some point, whatever more swing could be afforded would make no difference, and
a limit is set. Exceeding this is clipping. For short periods it can be benign but else it
is unpleasant and potentially damaging to hearing and positively damaging to hf and
bass drive units in particular. Moreover, considerable overdriving, into hard clip, as can
happen at any time by accident, even with domestic systems, can heavily saturate and
thus vaporize the BJT output stages of inadequately designed power amplifi ers.
8.6.6 Clip Prevention
Destructive and antisocial clipping may be prevented with comparatively simple circuits
performing like a dedicated, fast limiter. There are as many names as there are makers.
Some examples are shown in Table 8.6.
In these and related schemes, clip prevention does not occur until a dB or so of clip. Using
the 100-W analogy, the usual low % THD does not rise until the signal passes above about 50
to 70 W. If headroom is adequate, this point should hardly ever be reached with the majority
of recorded sound. With live sound, it may be reached quite often, but the fact that the deeply
unpleasant point only l dB higher isnot crashed through is of far more importance.