Audio Engineering

Interfacing and Processing 259

8.4.3 The Pro Approach

Subsonic fi ltering may be regarded as an essential part of editing and sweetening in
recording. “ Subsonic ” frequencies ( “ sub ” here being rather loosely designated as any
“ out of context/too-low bass information ” ) are usually removed before amplifi cation
by HP fi lters (HPF) with fi xed, switchable, or sweepable roll-off frequencies, usually
available on each channel or group of a mixing console. Alternatively, HP fi ltering may
even be available “ up front ” as a switch on some microphones or on portable, location
tape machines.

Generally, such fi lters are at least –12 dB/octave and, more usefully, the steeper –18-dB/
octave ( Figure 8.6 ) or even –24-dB/octave. They may be occasionally appended to
professional power amplifi ers, as well as to preceding active crossovers, on the basis of
providing “ maximum ” (read: brute force) protection at all costs, in this guise they are
described as “ subsonic protection ” (SSP). Often this facility is superfl uous and repeated
needlessly, as the mixer and active crossover already do or can provide subsonic fi ltering.

8.4.4 Logistics

The mixer can provide SSP most fl exibly per channel, solely for those sources requiring
fi ltration. The active crossover may provide overall back-up subsonic protection, in case a
mic without HPF’ing on its channel is dropped.

When subsonic protection is fi tted to and relied upon in amplifi ers alone, there will be an
enforced and unnecessary repetition and diversifi cation of resources in any more than the
simplest, two-channel PA. If subsonic fi lter provision is made in an amplifi er, it should be

Line

Signal

Bypass

Bypass

SSP in

4 .7u 18 dB/

1K

10 K

10 K 10 K

Set

alignment

Select slope

12 dB/octave

470 n 470 n





Figure 8.6 : Typical high-pass (subsonic protection) fi lter circuitry.