udness, presence... just three f the many terms used t describe the effects f c mpressing and limiting n an audi signal.The terms c
mpressi n and limiting have been in the audi v cabulary f r years, yet there is s me c nfusi n ver their definiti ns. The c nfusi n arises fr m the fact that b th the c mpress r and the limiter are devices that restrict the dynamic range f a signal, and the difference between them is ne f degree, with the limiter having the m st effect. T simply define each:C
mpress r: An amplifier, wh se gain decreases as its input level is increased.Limiter: A c
mpress r, wh se utput level remains c nstant, regardless f its input level.B
th definiti ns are valid nly after the signal being pr cessed reaches a certain level. Theref re, ne m re definiti n needs t be thr wn ut f r c nsiderati n:Thresh
ld: The level at r ab ve which the c mpress r r limiter begins functi ning.In a situati
n where input and utput are idealized f r a c mbinati n c mpress r/limiter, as the input level increases fr m -10dB t 0dB, the utput level, likewise, increases fr m -10dB t 0dB. Here the device is functi ning as a simple unity gain amplifier, with n effect n the signal level.C
mpressi n nce the signal level exceeds the c mpressi n thresh ld f 0dB, the utput level will f ll w the c mpressi n curve, assuming a c mpressi n rati f 2:1, as the input increases 10dB; the utput will yield nly 5db m re gain. Limiting
In a limiting situati
n, with a limiting thresh ld f +20dB, nce the input level reaches +20dB, there is n further increase in utput level. Hence, the device is perating as a limiter,. In actual practice, c mpressi n rati s f greater than 10:1 are c nsidered as limiting. nce the limiting thresh ld f +20dB has been reached, the utput level remains at +10dB, despite further increases in input level. Theref re, it sh uld be underst d that the limiter thresh ld d es n t necessarily indicate the maximum all wable utput level f the device. Rather, it indicates the input level at which the limiter begins w rking. Variable Thresh
lds It sh
uld be n ted that the same c mpressi n rati f 2:1 menti ned earlier will have different effects n the verall dynamic range depending n the p int at which c mpressi n begins (thresh ld). Als , the p siti ning f the c mpressi n thresh ld will influence the p int at which limiting must begin, if a certain maximum utput level is n t t be exceeded. Variable C
mpressi n Rati sM
st state- f-the-art c mpress rs ffer audi engineers a variety f c mpressi n rati s fr m which t ch se. Assuming that an audi signal must be kept bel w +10dB, using higher c mpressi n rati settings will all w f r a greater dynamics range f the signal being pr cessed. Pumping and Breathing
It is relatively easy t
determine the c mpressi n thresh ld and rati needed t prevent a wide dynamic range signal fr m exceeding a specified utput level. H wever, it sh uld be realized that - especially at high rati s - the acti n f the c mpress r might bec me audibly btrusive. T understand why, remember that the c mpress r is a variable gain device. The higher the c mpressi n rati the greater the change in gain. A c nstant high level signal, say +10dB will cause m re gain reducti n. When the high level is rem ved, the am unt f gain reducti n decreases as the c mpress r returns t unity gain. If the gain reducti n fluctuates rapidly, it may be quite audible as the backgr und n ise g es up and d wn in time with the c mpress r acti n, i.e. attack /release times, causing a breathing like s und. This can be used s metimes as an effect pr ducing s me killer results.Example: The use
f extremely sh rt attack times and l nger release times may create a backward-like s und, especially n percussive instruments. The fast attack immediately dr ps the signal, and then as the signal naturally decays, the release time setting brings up the gain, w rking against the n rmal decay. This effect is particularly n ticeable n a drum set, and particularly n cymbals. Pr
gram Limiting ften c mpressi n may be applied t the verall pr gram rather than t an individual instrument. Kn wn as pr gram limiting, this practice will prevent cumulative levels f the vari us instruments fr m getting t high r falling t l w. This type f gain c ntr l must be appr ached with care, since the adverse effects f c mpressi n are heard n the entire pr gram.Pr
gram limiting is ften used t raise the apparent l udness f a rec rd. Since the ear averages the s und level ver a peri d f time, a l w level pr gram with ccasi nal high level peaks will n t seem as l ud as an average level pr gram with n high level peaks. (C nfused yet?). In the quest f r l uder s unding rec rdings and br adcasts this type f l udness b sting is ften verd ne, much t the detriment f the finished pr duct. Meaning, what y u hear c ming ver the radi r televisi n is ften much different than the riginal pr ducers, engineers and mastering pe ple had in mind. Stere
Pr gram LimitingWhen a stere
br adcast pr gram is limited, the gain regulating secti ns f the left and right track c mpress rs must be electr nically interl cked, s that the c mpressi n in ne track causes an equal am unt f c mpressi n in the ther track. This keeps the verall left-t -right stere pr gram in balance.C
nsider a stere pr gram in which the right track ccasi nally needs s me c mpressi n. During c mpressi n, a center placed s l w uld apparently drift t the left whenever the gain f the right track is affected by the c mpress r. T prevent this center channel drift, the stere interl ck functi n reduces the gain in b th channels simultane usly whenever ne exceeds the thresh ld f c mpressi n. This keeps the center placed inf rmati n fr m m ving fr m side t side with each acti n f ne r the ther c mpress r channel.
