Signal Processing Fundamentals

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_pressure%22%3Eatmospheric+pressure%3C%2Fa%3E%3C%2Fi%3E%29.+Sound+is+a+disturbance%3Ci%3E+%3C%2Fi%3Ein+the+surrounding+medium.+A+vibration+that+spreads+out+from+the+source%2C+creating+a+series+of+expanding+shells+of+high+pressure+and+low+pressure+...%3Ci%3E+high+pressure+...+low+pressure+...+high+pressure+...+low+pressure%3C%2Fi%3E.+Moving+ever+outward+these+cycles+of+alternating+pressure+zones+travel+until+finally+dissipating%2C+or+reflecting+off+surfaces+%28nightclub+walls%29%2C+or+passing+through+boundaries%2C+or+getting+absorbed+--+usually+a+combination+of+all+three.+Left+unobstructed%2C+sound+travels+outward%2C+but+not+forever.+The+air+%28or+other+medium%29+robs+some+of+the+sound%27s+power+as+it+passes.+%3Ci%3EThe+price+of+passage%3A+the+medium+absorbs+its+energy%3C%2Fi%3E.+This+power+loss+is+experienced+as+a+reduction+in+how+%3Ci%3Eloud+%3C%2Fi%3Eit+is+%28the+term+%3Ci%3Eloudness+%3C%2Fi%3Eis+used+to+describe+how+loud+it+is+from+moment+to+moment%29+as+the+signal+travels+away+from+its+source.+%3Ci%3EThe+loudness+of+the+signal+is+reduced+by+one-fourth+for+each+doubling+of+distance+from+the+source%3C%2Fi%3E.+This+means+that+it+is+6+%3Ca+href%3D%22par-d.html%23decibel%22%3EdB%3C%2Fa%3E+less+loud+as+you+double+your+distance+from+it.+%5BThis+is+known+as+the+%3Ci%3Einverse+square+law+%3C%2Fi%3Esince+the+decrease+is+inversely+proportional+to+the+square+of+the+distance+traveled%3B+for+example%2C+2+times+the+distance+equals+a+1%2F4+decrease+in+loudness%2C+and+so+on.%5D%3C%2Fp%3E+%0D%0A++++++++%3Cp%3EHow+do+we+%3Ci%3Ecreate%3C%2Fi%3E+sound%2C+and+how+do+we+%3Ci%3Ecapture%3C%2Fi%3E+sound%3F+We+do+this+using+opposite+sides+of+the+same+%3Ci%3Eelectromagnetic%3C%2Fi%3E+coin.+Electricity+and+magnetism+are+kinfolk%3A+If+you+pass+a+coil+of+wire+through+a+magnetic+field%2C+electricity+is+generated+within+the+coil.+%3Ci%3ETurn+the+coin+over+and+flip+it+again%3A+%3C%2Fi%3EIf+you+pass+electricity+through+a+coil+of+wire%2C+a+magnetic+field+is+generated.+Move+the+magnet%2C+get+a+voltage%3B+apply+a+voltage%2C+create+a+magnet+...+this+is+the+essence+of+all+%3Ci%3Eelectromechanical%3C%2Fi%3E+objects.%3C%2Fp%3E+%0D%0A++++++++%3Cp%3EMicrophones+and+loudspeakers+are+electromechanical+objects.+At+their+hearts+there+is+a+coil+of+wire+%28the+%3Ci%3Evoice+coil%3C%2Fi%3E%29+and+a+magnet+%28the+%3Ci%3Emagnet%3C%2Fi%3E%29.+Speaking+causes+sound+vibrations+to+travel+outward+from+your+mouth.+Speaking+into+a+%3Ci%3Emoving-coil%3C%2Fi%3E+%28aka+%3Ci%3Edynamic%3C%2Fi%3E%29+microphone+causes+the+voice+coil+to+move+within+a+magnetic+field.+This+causes+a+voltage+to+be+developed+and+a+current+to+flow+proportional+to+the+sound+--+%3Ci%3Esound+has+been+captured%3C%2Fi%3E.+At+the+other+end+of+the+chain%2C+a+voltage+is+applied+to+the+loudspeaker+voice+coil+causing+a+current+to+flow+which+produces+a+magnetic+field+that+makes+the+cone+move+proportional+to+the+audio+signal+applied+--+%3Ci%3Esound+has+been+created%3C%2Fi%3E.+The+microphone+translates+sound+into+an+electrical+signal%2C+and+the+loudspeaker+translates+an+electrical+signal+into+sound.+One+capturing%2C+the+other+creating.+Everything+in-between+is+just+details.+And+in+case+you%27re+wondering%3A+yes%3B+turned+around%2C+a+microphone+can+be+a+loudspeaker+%28%3Ci%3Ethat+makes+teeny+tiny+sounds%3C%2Fi%3E%29%2C+and+a+loudspeaker+can+be+a+microphone+%28%3Ci%3Eif+you+SHOUT+REALLY+LOUD%3C%2Fi%3E%29.%3C%2Fp%3E+%0D%0A++++++++%3Ch3%3E%3Cb%3E%3Ca+name%3D%22crossovers%22%3E%3C%2Fa%3ECrossovers%3A+Simple+Division%3C%2Fb%3E+%3C%2Fh3%3E+%0D%0A++++++++%3Cp%3E+Loudspeaker+%3Ci%3Ecrossovers+%3C%2Fi%3Eare+a+necessary+evil.+A+different+universe%2C+a+different+set+of+physics+and+maybe+we+could+have+what+we+want%3A+one+loudspeaker+that+does+it+all.+One+speaker+that+reproduces+all+audio+frequencies+equally+well%2C+with+no+distortion%2C+at+loudness+levels+adequate+for+whatever+venue+we+play.+Well%2C+we+live+here%2C+and+our+system+of+physics+does+not+allow+such+extravagance.+The+hard+truth+is%2C+no+one+loudspeaker+can+do+it+all.+We+need+at+least+two+--+more+if+we+can+afford+them.+%3Ci%3EWoofers+%3C%2Fi%3Eand%3Ci%3E+tweeters.%3C%2Fi%3E+A+big+woofer+for+the+lows+and+a+little+tweeter+for+the+highs.+This+is+known+as+a+%3Ci%3E2-way+system%3C%2Fi%3E.+%28Check+the+accompanying+diagrams+for+the+following+discussions.%29+But+with+two+speakers%2C+the+correct+frequencies+must+be+routed+%28or+%3Ci%3Ecrossed+over%3C%2Fi%3E%29+to+each+loudspeaker.%3C%2Fp%3E+%0D%0A++++++++%3Ch4%3E%3Cb%3EPassive%3C%2Fb%3E%3C%2Fh4%3E+%0D%0A++++++++%3Cp%3E+At+the+simplest+level+a+crossover+is+a+%3Ci%3Epassive+%3C%2Fi%3Enetwork.+A+passive+network+is+one+not+needing+a+power+supply+to+operate+--+if+it+has+a+line+cord%2C+or+runs+off+batteries%2C+then+it+is+not+a+passive+circuit.+The+simplest+passive+crossover+network+consists+of+only+two+components%3A+a+%3Ci%3Ecapacitor%3C%2Fi%3E+connecting+to+the+high+frequency+driver+and+an+%3Ci%3Einductor%3C%2Fi%3E+%28aka+a+%3Ci%3Ecoil%3C%2Fi%3E%29+connecting+to+the+low+frequency+driver.+A+%3Ci%3Ecapacitor%3C%2Fi%3E+is+an+electronic+component+that+passes+high+frequencies+%28the+%3Ci%3Epassband%3C%2Fi%3E%29+and+blocks+low+frequencies+%28the+%3Ci%3Estopband%3C%2Fi%3E%29%3B+an+%3Ci%3Einductor+%3C%2Fi%3Edoes+just+the+opposite%3A+it+passes+low+frequencies+and+blocks+high+frequencies.+But+as+the+frequency+changes%2C+neither+component+reacts+suddenly.+They+do+it+gradually%3B+they+slowly+start+to+pass+%28or+stop+passing%29+their+respective+frequencies.+The+%3Ci%3Erate+%3C%2Fi%3Eat+which+this+occurs+is+called+the+%3Ci%3Ecrossover+slope%3C%2Fi%3E.+It+is+measured+in+%3Ci%3EdB+per+octave%3C%2Fi%3E%2C+or+shortened+to+%3Ci%3EdB%2Foctave%3C%2Fi%3E.+The+slope+increases+or+decreases+so+many+dB%2Foctave.+At+the+simplest+level%2C+each+component+gives+you+a+6+dB%2Foctave+slope+%28a+physical+fact+of+our+universe%29.+Again%2C+at+the+simplest+level%2C+adding+more+components+increases+the+slope+in+6+dB+increments%2C+creating+slopes+of+12+dB%2Foct%2C+18+dB%2Foct%2C+24+dB%2Foct%2C+and+so+on.+The+number+of+components%2C+or+6+dB+slope+increments%2C+is+called+the+%3Ci%3Ecrossover+order%3C%2Fi%3E.+Therefore%2C+a+%3Ci%3E4th-order+%3C%2Fi%3Ecrossover+has+%28at+least%29+four+components%2C+and+produces+steep+slopes+of+24+dB%2Foctave.+The+steeper+the+better+for+most+drivers%2C+since+speakers+only+perform+well+for+a+certain+band+of+frequencies%3B+beyond+that+they+misbehave%2C+sometimes+badly.+Steep+slopes+prevent+these+frequencies+from+getting+to+the+driver.%3C%2Fp%3E+%0D%0A++++++++%3Cp%3EYou+can+combine+capacitors+and+inductors+to+create+a+third+path+that+eliminates+the+highest+highs+and+the+lowest+lows%2C+and+forms+a+%3Ci%3Emid-frequency+%3C%2Fi%3Ecrossover+section.+This+is+naturally+called+a+%3Ci%3E3-way+system%3C%2Fi%3E.+%28See+diagram%29+The+%26quot%3Bmid%26quot%3B+section+forms+a+%3Ci%3Ebandpass+%3C%2Fi%3Efilter%2C+since+it+only+passes+a+specific+frequency+band.+Note+from+the+diagram+that+the+high+frequency+passband+and+low+frequency+passband+terms+are+often+shortened+to+just+%3Ci%3Ehigh-pass+%3C%2Fi%3Eand+%3Ci%3Elow-pass%3C%2Fi%3E.+A+3-way+system+allows+optimizing+each+driver+for+a+narrower+band+of+frequencies%2C+producing+a+better+overall+sound.%3C%2Fp%3E+%0D%0A++++++++%3Cp%3ESo+why+not+just+use+passive+boxes%3F%3C%2Fp%3E+%0D%0A++++++++%0D%0A++++++++%3Ch4%3E%3Cb%3EProblems%3C%2Fb%3E%3C%2Fh4%3E+%0D%0A++++%3Cp%3EThe+single+biggest+problem+is+that+one+passive+cabinet+%28or+a+pair%29+won%27t+play+loud+enough+and+clean+enough+for+large+spaces.+If+the+sound+system+is+for+your+bedroom+or+garage%2C+passive+systems+would+work+just+fine+--+maybe+even+better.+But+it+isn%27t.+Once+you+try+to+fill+a+relatively+large+space+with+equally+loud+sound+you+start+to+understand+the+problems.+And+it+doesn%27t+take+stadiums%2C+just+normal+size+clubs.+It+is+really+difficult+to+produce+the+required+loudness+with+passive+boxes.+Life+would+be+a+lot+easier+if+you+could+just+jack+everyone+into+their+own+cans+amp+--+like+a+bunch+of+HC+4+or+HC+6+Headphone+Amps+scattered+throughout+the+audience.+Let+%3Ci%3Ethem%3C%2Fi%3E+do+the+work%3B+then+everyone+could+hear+equally+well%2C+and+choose+their+own+listening+level.+But+life+is+hard%2C+and+headphone+amps+must+be+restricted+to+practice+and+recording.%3C%2Fp%3E+%0D%0A++++%3Cp%3EMonitor+speakers+on+the+other+hand+most+likely+have+passive+crossovers.+Again%2C+it%27s+a+matter+of+distance+and+loudness.+Monitors+are+usually+close+and+not+overly+loud+--+too+loud+and+they+will+feed+back+into+your+microphone+or+be+heard+along+with+the+main+mix%3A+not+good.+Monitor+speakers+are+similar+to+hi-fi+speakers%2C+where+passive+designs+dominate+...+because+of+the+relatively+small+listening+areas.+It+is+quite+easy+to+fill+small+listening+rooms+with+pristine+sounds+even+at+ear-splitting+levels.+But+move+those+same+speakers+into+your+local+club+and+they+will+sound+thin%2C+dull+and+lifeless.+Not+only+will+they+not+play+loud+enough%2C+but+they+may+%3Ci%3Eneed%3C%2Fi%3E+the+sonic+benefits+of+sound+bouncing+off+close+walls+to+reinforce+and+fill+the+direct+sound.+In+large+venues%2C+these+walls+are+way+too+far+away+to+benefit+anyone.%3C%2Fp%3E+%0D%0A++++++++++%3Cdiv+align%3D%22center%22%3E+%0D%0A++++++++++%3Cp%3E%3Cimg+src%3D%22n134fig1.gif%22+alt%3D%222-way+crossover%22+width%3D%22375%22+height%3D%22200%22+align%3D%22MIDDLE%22%3E%3C%2Fp%3E+%0D%0A++++++++++%3Ch4%3E%3Cb%3EFigure+1.+Passive+2-Way+Crossover%3C%2Fb%3E%3C%2Fh4%3E+%0D%0A++++++++++%3Ch4%3E%3Cimg+src%3D%22n134fig2.gif%22+alt%3D%223-way+crossover%22+width%3D%22375%22+height%3D%22255%22+align%3D%22MIDDLE%22%3E%3C%2Fh4%3E+%0D%0A++++++++++%3Ch4%3E%3Cb%3EFigure+2.+Passive+3-Way+Crossover%3C%2Fb%3E%3C%2Fh4%3E+%0D%0A++++++++%3C%2Fdiv%3E+%0D%0A++++++++%3Cp%3ESo+why+not+use+a+%3Ci%3Ebunch%3C%2Fi%3E+of+passive+boxes%3F+You+can%2C+and+some+people+do.+However%2C+for+reasons+to+follow%2C+it+only+works+for+a+couple+of+cabinets.+Even+so%2C+you+won%27t+be+able+to+get+the+high+loudness+levels+if+the+room+is+large.+Passive+systems+can+only+be+optimized+so+much.%3C%2Fp%3E+%0D%0A++++++++%3Cp%3EOnce+you+start+needing+multiple+cabinets%2C+active+crossovers+become+necessary.+To+get+good+coverage+of+like-frequencies%2C+you+want+to+stack+like-drivers.+This+prevents+using+passive+boxes+since+each+one+contains+%28at+least%29+a+high-frequency+driver+and+a+low-frequency+driver.+It%27s+easiest+to+put+together+a+sound+system+when+each+cabinet+covers+only+one+frequency+range.+For+instance%2C+for+a+nice+sounding+3-way+system%2C+you+would+have+low-frequency+boxes+%28the+big+ones%29%2C+then+medium-sized+mid-frequency+boxes+and+finally+the+smaller+high-frequency+boxes.+These+would+be+stacked+or+hung%2C+or+both+--+in+some+sort+of+%3Ci%3Earray.+%3C%2Fi%3EA+loudspeaker+array+is+the+optimum+stacking+shape+for+each+set+of+cabinets+to+give+the+best+combined+coverage+and+overall+sound.+You%27ve+no+doubt+seen+many+different+array+shapes.+There+are+tall+towers%2C+high+walls%2C+and+all+sorts+of+polyhedrons+and+arcs.+The+only+efficient+way+to+do+this+is+with+active+crossovers.%3C%2Fp%3E+%0D%0A++++++++%3Cp%3ESome+smaller+systems+combine+active+and+passive+boxes.+Even+within+a+single+cabinet+it+is+common+to+find+an+active+crossover+used+to+separate+the+low-+and+mid-frequency+drivers%2C+while+a+built-in+passive+network+is+used+for+the+high-frequency+driver.+This+is+particularly+common+for+super+tweeters+operating+over+the+last+audio+octave.+At+the+other+end%2C+an+active+crossover+often+is+used+to+add+a+subwoofer+to+a+passive+2-way+system.+All+combinations+are+used%2C+but+each+time+a+passive+crossover+shows+up%2C+it+comes+with+problems.%3C%2Fp%3E+%0D%0A++++++++%3Cp%3EOne+of+these+is+power+loss.+Passive+networks+waste+valuable+power.+The+extra+power+needed+to+make+the+drivers+louder%2C+instead+boils+off+the+components+and+comes+out+of+the+box+as+heat+--+not+sound.+Therefore%2C+passive+units+make+you+buy+a+bigger+amp.%3C%2Fp%3E+%0D%0A++++++++%3Cp%3EA+couple+of+additional+passive+network+problems+has+to+do+with+their+%3Ci%3Eimpedance%3C%2Fi%3E.+Impedance+restricts+power+transfer%3B+it%27s+like+resistance%2C+only+frequency+sensitive.+In+order+for+the+passive+network+to+work+exactly+right%2C+the+%3Ci%3Esource%3C%2Fi%3E+%3Ci%3Eimpedance%3C%2Fi%3E+%28the+amplifier%27s+output+plus+the+wiring+impedance%29+must+be+as+close+to+zero+as+possible+and+not+frequency-dependent%2C+and+the+%3Ci%3Eload+impedance+%3C%2Fi%3E%28the+loudspeaker%27s+characteristics%29+must+be+fixed+and+not+frequency-dependent+%28sorry%2C+not%3Ci%3E+%3C%2Fi%3Ein+%3Ci%3Ethis%3C%2Fi%3E+universe%3B+only+on+%3Ci%3EStar+Trek%3C%2Fi%3E%29.+Since+these+things+are+not+possible%2C+the+passive+network+must+be+%28at+best%29%2C+a+simplified+and+compromised+solution+to+a+very+complex+problem.+Consequently%2C+the+crossover%27s+behavior+changes+with+frequency+--+%3Ci%3Enot+%3C%2Fi%3Esomething+you+want+for+a+good+sounding+system.%3C%2Fp%3E+%0D%0A++++++++%3Cp%3EOne+last+thing+to+make+matters+worse.+There+is+something+called+%3Ci%3Eback-emf+%3C%2Fi%3E%28back-%3Ci%3Eelectromotive+force%3C%2Fi%3E%3A+literally%2C+%3Ci%3Eback-voltage%3C%2Fi%3E%29+which+further+contributes+to+poor+sounding+speaker+systems.+This+is+the+phenomena+where%2C+after+the+signal+stops%2C+the+speaker+cone+continues+moving%2C+causing+the+voice+coil+to+move+through+the+magnetic+field+%28now+acting+like+a+microphone%29%2C+creating+a+%3Ci%3Enew%3C%2Fi%3E+voltage+that+tries+to+drive+the+cable+back+to+the+amplifier%27s+output%21+If+the+speaker+is+allowed+to+do+this%2C+the+cone+flops+around+like+a+dying+fish.+It+does+%3Ci%3Enot+%3C%2Fi%3Esound+good%21+The+only+way+to+stop+back-emf+is+to+make+the+loudspeaker+%26quot%3Bsee%26quot%3B+a+dead+short%2C+i.e.%2C+zero+ohms+looking+backward%2C+or+as+close+to+it+as+possible+--+something+that%27s+%3Ci%3Enot+gonna+happen%3C%2Fi%3E+with+a+passive+network+slung+between+it+and+the+power+amp.%3C%2Fp%3E+%0D%0A++++++++%3Cp%3EAll+this%2C+and+not+to+mention+that+inductors+saturate+at+high+signal+levels+causing+distortion+--+another+reason+you+can%27t+get+enough+loudness.+Or+the+additional+weight+and+bulk+caused+by+the+large+inductors+required+for+good+low+frequency+response.+Or+that+it+is+almost+impossible+to+get+high-quality+steep+slopes+passively%2C+so+the+response+suffers.+Or+that+inductors+are+way+too+good+at+picking+up+local+radio%2C+TV%2C+emergency%2C+and+cellular+broadcasts%2C+and+joyfully+mixing+them+into+your+audio.%3C%2Fp%3E+%0D%0A++++++++%3Cp%3ESuch+is+life+with+passive+speaker+systems.%3C%2Fp%3E+%0D%0A++++++++%3Cdiv+align%3D%22center%22%3E+%0D%0A++++++++++%3Cp%3E%3Cimg+src%3D%22n134fig3.gif%22+alt%3D%222-way+crossover%22+width%3D%22381%22+height%3D%22186%22+align%3D%22MIDDLE%22%3E%3C%2Fp%3E+%0D%0A++++++++++%3Ch4%3E%3Cb%3EFigure+3.+Active+2-Way+Crossover%3C%2Fb%3E%3C%2Fh4%3E+%0D%0A++++++++++%3Cp%3E%3Cimg+src%3D%22n134fig4.gif%22+alt%3D%223-way+crossover%22+width%3D%22375%22+height%3D%22221%22+align%3D%22MIDDLE%22%3E%3C%2Fp%3E+%0D%0A++++++++++%3Ch4%3E%3Cb%3EFigure+4.+Active+3-Way+Crossover%3C%2Fb%3E%3C%2Fh4%3E+%0D%0A++++++++%3C%2Fdiv%3E+%0D%0A++++++++%3Ch4%3E%3Cb%3EActive%3C%2Fb%3E%3C%2Fh4%3E+%0D%0A++++++++%3Cp%3E%3Ci%3EActive+%3C%2Fi%3Ecrossover+networks+require+a+power+supply+to+operate+and+usually+come+packaged+in+single-space%2C+rack-mount+units.+%28Although+of+late%2C+%3Ci%3Epowered+loudspeakers+%3C%2Fi%3Ewith+built-in+active+crossovers+and+power+amplifiers+are+becoming+increasingly+popular.%29+Looking+at+the+accompanying+diagram+shows+how+active+crossovers+differ+from+their+passive+cousins.+For+a+2-way+system+instead+of+one+power+amp%2C+you+now+have+two%2C+but+%3Ci%3Ethey+can+be+smaller+for+the+same+loudness+level%3C%2Fi%3E.+How+%3Ci%3Emuch+%3C%2Fi%3Esmaller+depends+on+the+%3Ci%3Esensitivity%3C%2Fi%3E+rating+of+the+drivers+%28more+on+this+later%29.+Likewise+a+3-way+system+requires+three+power+amps.+You+also+see+and+hear+the+terms+%3Ci%3Ebi-amped%3C%2Fi%3E%2C+and+%3Ci%3Etri-amped%3C%2Fi%3E+applied+to+2-+and+3-way+systems.%3C%2Fp%3E+%0D%0A++++++++%3Cp%3EActive+crossovers+cure+many+ills+of+the+passive+systems.+Since+the+crossover+filters+themselves+are+safely+tucked+away+inside+their+own+box%2C+away+from+the+driving+and+loading+impedance+problems+plaguing+passive+units%2C+they+can+be+made+to+operate+in+an+almost+mathematically+perfect+manner.+Extremely+steep%2C+smooth+and+well-behaved+crossover+slopes+are+easily+achieved+by+active+circuitry.%3C%2Fp%3E+%0D%0A++++++++%3Cp%3EThere+are+no+amplifier+power+loss+problems%2C+since+active+circuits+operate+from+their+own+low+voltage+power+supplies.+And+with+the+inefficiencies+of+the+passive+network+removed%2C+the+power+amps+more+easily+achieve+the+loudness+levels+required.%3C%2Fp%3E+%0D%0A++++++++%3Cp%3ELoudspeaker+jitters+and+tremors+caused+by+inadequately+damped+back-emf+all+but+disappear+once+the+passive+network+is+removed.+What+remains+is+the+amplifier%27s+inherent+output+impedance+and+that+of+the+connecting+wire.+Here%27s+where+the+term+%3Ci%3Edamping+factor+%3C%2Fi%3Ecomes+up.+%5BNote+that+the+word+is+damp-%3Ci%3Eing%3C%2Fi%3E%2C+not+damp-%3Ci%3Ening%3C%2Fi%3E+as+is+so+often+heard%3B+impress+your+friends.%5D+Damping+is+a+measure+of+a+system%27s+ability+to+control+the+motion+of+the+loudspeaker+cone+after+the+signal+disappears.+No+more+dying+fish.%3C%2Fp%3E+%0D%0A++++++++%3Ch4%3E%3Cb%3ESiegfried+%26amp%3B+Russ%3C%2Fb%3E%3C%2Fh4%3E+%0D%0A++++++++%3Cp%3E+Active+crossovers+go+by+many+names.+First%2C+they+are+either+2-way+or+3-way+%28or+even+4-way+and+5-way%29.+Then+there+is+the+slope+rate+and+order%3A+24+dB%2Foct+%284th-order%29%2C+or+18+dB%2Foct+%283rd-order%29%2C+and+so+on.+And+finally+there+is+a+name+for+the+%3Ci%3Ekind+%3C%2Fi%3Eof+design.+The+two+most+common+being+%3Ci%3ELinkwitz-Riley%3C%2Fi%3E+and+%3Ci%3EButterworth%3C%2Fi%3E%2C+named+after+Siegfried+Linkwitz+and+Russ+Riley+who+first+proposed+this+application%2C+and+Stephen+Butterworth+who+first+described+the+response+in+1930.+Up+until+the+mid+%6080s%2C+the+3rd-order+%2818+dB%2Foct%29+Butterworth+design+dominated%2C+but+still+had+some+problems.+Since+then%2C+the+development+%28pioneered+by+Rane+and+Sundholm%29+of+the+4th-order+%2824+dB%2Foct%29+Linkwitz-Riley+design+solved+these+problems%2C+and+today+is+the+norm.%3C%2Fp%3E+%0D%0A++++++++%3Cp%3EWhat+this+adds+up+to+is+%3Ci%3Eactive+crossovers+are+the+rule%3C%2Fi%3E.+Luckily%2C+the+hardest+thing+about+an+active+crossover+is+getting+the+money+to+buy+one.+After+that%2C+most+of+the+work+is+already+done+for+you.+At+the+most+basic+level+all+you+really+need+from+an+active+crossover+are+two+things%3A+to+let+you+set+the+correct+crossover+point%2C+and+to+let+you+balance+driver+levels.+That%27s+all.+The+first+is+done+by+consulting+the+loudspeaker+manufacturer%27s+data+sheet%2C+and+dialing+it+in+on+the+front+panel.+%28That%27s+assuming+a+complete+factory-made+2-way+loudspeaker+cabinent%2C+for+example.+If+the+box+is+homemade%2C+then+both+drivers+must+be+carefully+selected+so+they+have+the+%3Ci%3Esame+crossover+frequency%3C%2Fi%3E%2C+otherwise+a+severe+response+problem+can+result.%29+Balancing+levels+is+necessary+because+high+frequency+drivers+are+more+%3Ci%3Eefficient%3C%2Fi%3E+than+low+frequency+drivers.+This+means+that+if+you+put+the+same+amount+of+power+into+each+driver%2C+one+will+sound+louder+than+the+other.+The+one+that+is+the+most+efficient+plays+louder.+Several+methods+to+balance+drivers+are+always+outlined+in+any+good+owner%27s+manual.%3C%2Fp%3E+%0D%0A++++++++%3Ch3%3E%3Cb%3E%3Ca+name%3D%22equalizers%22%3E%3C%2Fa%3EEqualizers%3C%2Fb%3E%3C%2Fh3%3E+%0D%0A++++++++%3Cp%3E+You+may+have+heard+it+said+that+equalizers+are+nothing+more+than+glorified+tone+controls.+That%27s+pretty+accurate+and+helps+explain+their+usefulness+and+importance.+Simply+put%2C+equalizers+allow+you+to+change+the+tonal+balance+of+whatever+you+are+controlling.+You+can+increase+%28%3Ci%3Eboost%3C%2Fi%3E%29+or+decrease+%28%3Ci%3Ecut%3C%2Fi%3E%29+on+a+band-by-band+basis+just+the+desired+frequencies.+Equalizers+come+in+all+different+sizes+and+shapes%2C+varying+greatly+in+design+and+complexity.+Select+from+a+simple+single-channel+unit+with+10+controls+on+1-octave+frequency+spacing+%28a+%3Ci%3Emono+10-band+octave+equalizer%3C%2Fi%3E%29%2C+all+the+way+up+to+a+full-featured%2C+two-channel+box+with+31+controls+on+1%2F3-octave+frequency+spacing+%28a+%3Ci%3Estereo+1%2F3-oct+equalizer%3C%2Fi%3E%29.+There+are+%3Ci%3Egraphic%3C%2Fi%3E+models+with+slide+controls+%28%3Ci%3Esliders%3C%2Fi%3E%29+that+roughly+%26quot%3Bgraph%26quot%3B+the+equalizer%27s+frequency+response+by+the+shape+they+form%2C+and+there+are+%3Ci%3Eparametric+%3C%2Fi%3Emodels+where+you+choose+the+frequency%2C+amplitude%2C+and+bandwidth+desired+%28the+%3Ci%3Efilter+parameters%3C%2Fi%3E+-+see+diagram+below%29+for+each+band+provided.+Far+and+away%2C+the+simplest+and+most+popular+are+the+1%2F3-+and+2%2F3-octave+graphics.+They+offer+the+best+combination+of+control%2C+complexity+and+cost.%3C%2Fp%3E+%0D%0A++++++++%3Cp%3EIn+selecting+graphic+equalizers%2C+the+primary+features+to+consider+are+the+number+of+input%2Foutput+channels%2C+the+number+of+boost%2Fcut+bands%2C+the+center-frequency+spacing+of+each%2C+and+the+accuracy+of+the+output+vs.+the+front+panel+settings.+Up+until+the+recent+development+of+true+response+graphics%2C+the+front+panel+settings+only+approximated+the+equalizer%27s+actual+response.+Prior+to+true+response+graphics%2C+adjacent+band+interaction+caused+the+actual+output+response+to+deviate+from+the+front+panel+settings.+Described+as+either+constant-Q+or+variable-Q+%28see+diagrams%29%2C+the+individual+filter+bandwidth+behavior+determined+the+interaction.+In+the+early+%2780s%2C+Rane+developed+the+first+constant-Q+designs+to+preserve+the+same+shape+%28bandwidth%29+over+the+entire+boost%2Fcut+range.+In+contrast%2C+variable-Q+designs+have+varying+bandwidths+%28the+shape+changes%29+as+a+function+of+boost%2Fcut+amount.+Rane%27s+constant-Q+design+offered+a+big+improvement+in+output+response+vs.+front+panel+settings+and+became+the+most+popular+design+until+Rane+and+others+developed+the+first+true+response+graphic+equalizers.+Now+true+response+graphics+offer+the+best+response.%3C%2Fp%3E+%0D%0A++++++++%3Ch4%3E%3Cb%3EUsing+Equalizers%3C%2Fb%3E%3C%2Fh4%3E+%0D%0A++++++++%3Cp%3E+Equalizers+can+do+wonders+for+a+sound+system.+Let%27s+start+with+loudspeaker+performance.+An+unfortunate+truth+regarding+budget+loudspeakers+is+they+don%27t+sound+very+good.+Usually+this+is+due+to+an+uneven+frequency+response%2C+or+more+correctly+a+non-flat+%3Ci%3Epower+response.+%3C%2Fi%3EAn+ideal+cabinet+has+a+flat+power+response.+This+means+that+if+you+pick%2C+say%2C+1+kHz+as+a+reference+signal%2C+use+it+to+drive+the+speaker+with+exactly+one+watt%2C+measure+the+loudness%2C+and+sweep+the+generator+over+the+speaker%27s+entire+frequency+range%2C+%3Ci%3Eall+frequencies+will+measure+equally+loud.%3C%2Fi%3E+Sadly%2C+with+all+but+the+most+expensive+speaker+systems%2C+they+will+not.+Equalizers+can+help+these+frequency+deficiencies.+By+adding+a+little+here+and+taking+away+a+little+there%2C+pretty+soon+you+create+an+acceptable+power+response+-+and+a+whole+lot+better+sounding+system.+It%27s+surprising+how+just+a+little+equalization+can+change+a+poor+sounding+system+into+something+quite+decent.%3C%2Fp%3E+%0D%0A++++++++%3Cp%3EThe+best+way+to+deal+with+budget+speakers+--+although+it+costs+more+--+is+to+commit+%3Ci%3Eone+%3C%2Fi%3Eequalizer+channel+for+%3Ci%3Eeach%3C%2Fi%3E+cabinet.+This+becomes+a+marriage.+The+equalizer+is+set%2C+a+security+cover+is+bolted-on%2C+and+forever+more+they+are+inseparable.+%28Use+additional+equalizers+to+assist+with+the+room+problems.%29+And+now+for+the+hard+part%2C+but+the+most+important+part%3A+If+you+do+your+measurements+%3Ci%3Eoutside%3C%2Fi%3E+%28no+reflections+off+walls+or+ceiling%29+and+%3Ci%3Eup+in+the+air+%3C%2Fi%3E%28no+reflections+off+the+ground%29+you+can+get+a+very+accurate+picture+of+just+the+loudspeaker%27s+response%2C+free+from+room+effects.+This+gives+you+the+%3Ci%3Eroom-independent%3C%2Fi%3E+response.+This+is+really+important%2C+because+%3Ci%3Eno+matter+where+this+box+is+used%2C+it+has+these+problems%3C%2Fi%3E.+Of+course%2C+you+must+make+sure+the+cost+of+the+budget+speaker+plus+the+equalizer+adds+up+to+substantially+less+than+buying+a+really+flat+speaker+system+to+begin+with.+Luckily+%28or+should+this+be+%3Ci%3Esadly%3C%2Fi%3E%29+this+is+usually+the+case.+Again%2C+the+truth+is+that+%3Ci%3Emost+%3C%2Fi%3Ecabinets+are+not+flat.+It+is+only+the+very+expensive+loudspeakers+that+have+world-class+responses.+%28Hmmm+...+maybe+%3Ci%3Ethat%27s%3C%2Fi%3E+why+they+cost+so+much%21%29%3C%2Fp%3E+%0D%0A++++++++%3Cp%3EThe+next+thing+you+can+do+with+equalizers+is+to+improve+the+way+each+venue+sounds.+Every+room+sounds+different+--+fact+of+life+--+fact+of+physics.+Using+exactly+the+same+equipment%2C+playing+exactly+the+same+music+in+exactly+the+same+way%2C+different+rooms+sound+different+--+guaranteed.+Each+enclosed+space+treats+your+sound+differently.%3C%2Fp%3E+%0D%0A++++++++%3Cp%3EReflected+sound+causes+the+problems.+What+the+audience+hears+is+made+up+of+the+%3Ci%3Edirect+%3C%2Fi%3Esound+%28what+comes+straight+out+of+the+loudspeaker+directly+to+the+listener%29+and+%3Ci%3Ereflected+%3C%2Fi%3Esound+%28it+bounces+off+%3Ci%3Eeverything+%3C%2Fi%3Ebefore+getting+to+the+listener%29.+And+if+the+room+is+big+enough%2C+then+%3Ci%3Ereverberation+%3C%2Fi%3Ecomes+into+play%2C+which+is+all+the+reflected+sound+that+has+traveled+so+far%2C+and+for+such+a+%28relatively%29+long+time+that+it+arrives+and+%3Ci%3Ere-arrives+%3C%2Fi%3Eat+the+listener+delayed+enough+to+sound+like+a+second+and+third+source%2C+or+even+an+%3Ci%3Eecho%3C%2Fi%3E+if+the+room+is+%3Ci%3Ereally+%3C%2Fi%3Ebig.%3C%2Fp%3E+%0D%0A++++++++%3Cp%3EIt%27s+basically+a+geometry+problem.+Each+room+differs+in+its+dimensions%3B+not+only+in+its+basic+length-by-width+size%2C+but+in+its+ceiling+height%2C+the+distance+from+you+and+your+equipment+to+the+audience%2C+what%27s+hung+%28or+not+hung%29%2C+on+the+walls%2C+how+many+windows+and+doors+there+are%2C+and+where.+Every+detail+about+the+space+affects+your+sound.+And+regretfully%2C+there+is+very+little+you+can+do+about+any+of+it.+Most+of+the+factors+affecting+your+sound+you+cannot+change.+You+certainly+can%27t+change+the+dimensions%2C+or+alter+the+window+and+door+locations.+But+there+are+a+few+things+you+can+do%2C+and+equalization+is+one+of+them.+But+before+you+equalize+you+want+to+optimize+%3Ci%3Ehow+%3C%2Fi%3Eand+%3Ci%3Ewhere+%3C%2Fi%3Eyou+place+your+speakers.+This+is+probably+the+number+one+item+to+attend+to.+Keep+your+loudspeakers+out+of+corners+whenever+possible.+Remove+all+restrictions+between+your+speakers+and+your+audience%2C+including+banners%2C+stage+equipment%2C+and+performers.+What+you+want+is+for+most+of+the+sound+your+audience+hears+to+come+directly+from+the+speakers.+You+want+to+minimize+all+reflected+sound.+If+you+have+done+a+good+job+in+selecting+and+equalizing+your+loudspeakers%2C+then+you+already+know+your+direct+sound+is+good.+So+what%27s+left+is+to+minimize+the+reflected+sound.%3C%2Fp%3E+%0D%0A++++++++%3Cp%3ENext+use+equalization+to+help+with+some+of+the+room%27s+more+troublesome+features.+If+the+room+is+exceptionally+bright+you+can+beef+up+the+low+end+to+help+offset+it%2C+or+roll-off+some+of+the+highs.+Or+if+the+room+tends+to+be+boomy%2C+you+can+tone-down+the+low+end+to+reduce+the+resonance.+Another+way+EQ+is+quite+effective+is+in+controlling+troublesome+%3Ci%3Efeedback%3C%2Fi%3E+tones.+Feedback+is+that+terrible+squeal+or+scream+sound+systems+get+when+the+audio+from+the+loudspeaker+gets+picked-up+by+one+of+the+stage+microphones%2C+re-amplified+and+pumped+out+the+speaker%2C+only+to+be+picked-up+again+by+the+microphone%2C+and+re-amplified%2C+and+so+on.+Most+often%2C+this+happens+when+the+system+is+playing+loud.+Which+makes+sense%2C+because+for+softer+sounds%2C+the+signal+either+isn%27t+big+enough+to+make+it+to+the+microphone%2C+or+if+it+does%2C+it+is+too+small+to+build-up.+The+problem+is+one+of+an+out-of-control%2C+closed-loop%2C+positive-feedback+system+building+up+until+something+breaks%2C+or+the+audience+leaves.+Use+your+equalizer+to+cut+those+frequencies+that+want+to+howl%3B+you+not+only+stop+the+squeal%2C+but+you+allow+the+system+to+play+louder.+The+technical+phrase+for+this+is+%3Ci%3Emaximizing+system+gain+before+feedback%3C%2Fi%3E.%3C%2Fp%3E+%0D%0A++++++++%3Cp%3EIt%27s+important+to+understand+at+the+beginning+that+you+cannot+%3Ci%3Efix+%3C%2Fi%3Eroom+related+sound+problems+with+equalization%2C+but+you+can+%3Ci%3Emove+%3C%2Fi%3Ethe+trouble+spots+around.+You+can+%3Ci%3Erearrange+%3C%2Fi%3Ethings+sonically%2C+which+helps+%3Ci%3Etame%3C%2Fi%3E+excesses.+You+win+by+making+it+sound+better.+Equalization+helps.%3C%2Fp%3E+%0D%0A++++++++%3Cdiv+align%3D%22center%22%3E+%0D%0A++++++++++%3Cp%3E%3Cimg+src%3D%22n134fig5.gif%22+alt%3D%22bandpass+filter%22+width%3D%22365%22+height%3D%22254%22+align%3D%22MIDDLE%22%3E%3C%2Fp%3E+%0D%0A++++++++++%3Cp%3E%3Cb%3EFigure+5.+Bandpass+Filter+Parameters%3C%2Fb%3E%3C%2Fp%3E+%0D%0@++++++++++%3Ch6%3E%3Cimg+src%3D%22n134fig6.gif%22+alt%3D%22variable-q+filter%22+width%3D%22375%22+height%3D%22264%22+align%3D%22MIDDLE%22%3E%3C%2Fh6%3E+%0D%0A++++++++++%3Cp%3E%3Cb%3EFigure+6.+Variable-Q+Graphic%3C%2Fb%3E%3C%2Fp%3E+%0D%0A++++++++++%3Ch6%3E%3Cimg+src%3D%22n134fig7.gif%22+alt%3D%22constant-q+filter%22+width%3D%22384%22+height%3D%22279%22+align%3D%22MIDDLE%22%3E%3C%2Fh6%3E+%0D%0A++++++++++%3Cp%3E%3Cb%3EFigure+7.+Constant-Q+Graphic%3C%2Fb%3E%3C%2Fp%3E+%0D%0A++++++++%3C%2Fdiv%3E+%0D%0A++++++++%3Cp%3EEqualizers+are+useful+in+augmenting+your+instrument+or+voice.+With+practice+you+will+learn+to+use+your+equalizer+to+enhance+your+sound+for+your+best+personal+expression%3A+deepen+the+lows%2C+fill+the+middle%2C+or+exaggerate+the+highs+...+whatever+you+want.+Just+as+an+equalizer+can+improve+the+sound+of+a+poor+loudspeaker%2C+it+can+improve+the+sound+of+a+marginal+microphone%2C+or+enhance+any+musical+instrument.+Equalizers+give+you+that+something+extra%2C+that+%3Ci%3Eedge%3C%2Fi%3E.+%28We+all+know+where+%26quot%3Bradio+voices%26quot%3B%3Ci%3E+really+%3C%2Fi%3Ecome+from.%29%3C%2Fp%3E+%0D%0A++++++++%3Ch4%3E%3Cb%3ESeeing+Sound%3C%2Fb%3E%3C%2Fh4%3E+%0D%0A++++++++%3Cp%3E+To+make+loudspeaker+and+sound+system+measurements+easy%2C+you+need+a+real-time+analyzer+%28RTA%29.+An+RTA+allows+you+to+see+the+power+response%2C+not+only+for+the+loudspeaker%2C+but+even+more+importantly%2C+for+the+whole+system.+Stand-alone+RTAs+use+an+LED+or+LCD+matrix+to+display+the+response.+A+built-in+%3Ci%3Epink+noise%3C%2Fi%3E+generator+%28a+special+kind+of+shaped+noise+containing+all+audible+frequencies%2C+optimized+for+measuring+sound+systems%29+is+used+as+the+test+signal.+A+measuring+microphone+is+included+for+sampling+the+response.+The+display+is+arranged+to+show+amplitude+verses+frequency.+Depending+upon+cost%2C+the+number+of+frequency+columns+varies+from+10+on+1-octave+centers%2C+up+to+31+on+1%2F3-octave+centers+%28agreeing+with+graphic+equalizers%29.+Amplitude+range+and+precision+varies+with+price.+With+the+cost+of+laptop+computers+tumbling%2C+the+latest+form+of+RTA+involves+an+accessory+box+and+software+that+works+with+your+computer.+These+are+particularly+nice%2C+and+loaded+with+special+memory%2C+calculations+and+multipurpose+functions+like+also+being+an+elaborate+SPL+meter.+Highly+recommended+if+the+budget+allows.%3C%2Fp%3E+%0D%0A++++%3Ch3%3E%3Cb%3E%3Ca+name%3D%22dynamics%22%3E%3C%2Fa%3EDynamic+Controllers%3C%2Fb%3E%3C%2Fh3%3E+%0D%0A++++++++%3Cp%3E%3Ci%3EDynamic+controllers%3C%2Fi%3E+or+%3Ci%3Eprocessors%3C%2Fi%3E+represent+a+class+of+signal+processing+devices+used+to+alter+an+audio+signal+based+solely+upon+its+%3Ci%3Efrequency+content+%3C%2Fi%3Eand+%3Ci%3Eamplitude+level%3C%2Fi%3E%2C+thus+the+term+%26quot%3Bdynamic%26quot%3B+since+the+processing+is+completely+program+dependent.+The+two+most+common+dynamic+effects+are+%3Ci%3Ecompressors%3C%2Fi%3E+and+%3Ci%3Eexpanders%3C%2Fi%3E%2C+with+%3Ci%3Elimiters%3C%2Fi%3E+and+%3Ci%3Enoise+gates%3C%2Fi%3E+%28or+just+%26quot%3Bgates%26quot%3B%29+being+special+cases+of+these.%3C%2Fp%3E+%0D%0A++++++++%3Cp%3EThe+%3Ci%3Edynamic+range+%3C%2Fi%3Eof+an+audio+passage+is+the+ratio+of+the+loudest+%28undistorted%29+signal+to+the+quietest+%28just+audible%29+signal%2C+expressed+in+dB.+Usually+the+maximum+output+signal+is+restricted+by+the+size+of+the+power+supplies+%28you+cannot+swing+more+voltage+than+is+available%29%2C+while+the+minimum+output+signal+is+fixed+by+the+noise+floor+%28you+cannot+put+out+an+audible+signal+less+than+the+noise%29.+Professional-grade+analog+signal+processing+equipment+can+output+maximum+levels+of+%2B26+dBu%2C+with+the+best+noise+floors+being+down+around+-94+dBu.+This+gives+a+maximum+%3Ci%3Edynamic+range%3C%2Fi%3E+of+120+dB+%28equivalent+to+20-bit+digital+audio%29+--+pretty+impressive+number+--+but+very+difficult+to+work+with.+Thus+were+born+dynamic+processors.%3C%2Fp%3E+%0D%0A++++++++%3Ch4%3E%3Cb%3ECompressors%3C%2Fb%3E%3C%2Fh4%3E+%0D%0A++++++++%3Cp%3E%3Ci%3ECompressors%3C%2Fi%3E+are+signal+processing+units+used+to+reduce+%28%3Ci%3Ecompress%3C%2Fi%3E%29+the+dynamic+range%3Ci%3E+%3C%2Fi%3Eof+the+signal+passing+through+them.+The+modern+use+for+compressors+is+to+turn+down+just+the+loudest+signals+%3Ci%3Edynamically%3C%2Fi%3E.+For+instance%2C+an+input+dynamic+range+of+110+dB+might+pass+through+a+compressor+and+exit+with+a+new+dynamic+range+of+70+dB.+This+clever+bit+of+processing+is+normally+done+using+a+VCA+%28voltage+controlled+amplifier%29+whose+gain+is+determined+by+a+control+voltage+derived+from+the+input+signal.+Therefore%2C+whenever+the+input+signal+exceeds+the+threshold+point%2C+the+control+voltage+becomes+proportional+to+the+signal%27s+dynamic+content.+This+lets+the+music+peaks+turn+down+the+gain.+Before+compressors%2C+a+human+did+this+at+the+mixing+board+and+we+called+it+%3Ci%3Egain-riding%3C%2Fi%3E.+This+person+literally+turned+down+the+gain+anytime+it+got+too+loud+for+the+system+to+handle.%3C%2Fp%3E+%0D%0A++++++++%3Cp%3EYou+need+to+reduce+the+dynamic+range+because+extreme+ranges+of+dynamic+material+are+very+difficult+for+sound+systems+to+handle.+If+you+turn+it+up+as+loud+as+you+want+for+the+average+signals%2C+then+along+comes+these+huge+musical+peaks%2C+which+are+vital+to+the+%3Ci%3Epunch+%3C%2Fi%3Eand+%3Ci%3Edrama+%3C%2Fi%3Eof+the+music%2C+yet+are+way+too+large+for+the+power+amps+and+loudspeakers+to+handle.+Either+the+power+amps+clip%2C+or+the+loudspeakers+%3Ci%3Ebottom+out+%3C%2Fi%3E%28reach+their+travel+limits%29%2C+or+both+--+and+the+system+sounds+%3Ci%3Eterrible%3C%2Fi%3E.+Or+going+the+other+way%2C+if+you+set+the+system+gain+to+prevent+these+overload+occurrences%2C+then+when+things+get+nice+and+quiet%2C+and+the+vocals+drop+real+low%2C+nobody+can+hear+a+thing.+%3Ci%3EIt%27s+always+something.%3C%2Fi%3E+So+you+buy+a+compressor.%3C%2Fp%3E+%0D%0A++++++++%3Cp%3EUsing+it+is+quite+simple%3A+Set+a+%3Ci%3Ethreshold+%3C%2Fi%3Epoint%2C+above+which+everything+will+be+turned+down+a+certain+amount%2C+and+then+select+a+%3Ci%3Eratio+%3C%2Fi%3Edefining+just+how+much+a+%26quot%3Bcertain+amount%26quot%3B+is.+All+audio+below+the+threshold+point+is+unaffected+and+all+audio+above+this+point+is+compressed+by+the+ratio+amount.+The+earlier+example+of+reducing+110+dB+to+70+dB+requires+a+ratio+setting+of+1.6%3A1+%28110%2F70+%3D+1.6%29.+The+key+to+understanding+compressors+is+to+always+think+in+terms+of+%3Ci%3Eincreasing+level+changes+%3Cb%3Ein+dB%3C%2Fb%3E+above+the+threshold+point%3C%2Fi%3E.+A+compressor+makes+these+increases+smaller%3Ci%3E.+%3C%2Fi%3EFrom+our+example%2C+for+every+%3Ci%3E1.6+dB+increase%3C%2Fi%3E+above%3Ci%3E+%3C%2Fi%3Ethe+threshold+point+the+output+only+%3Ci%3Eincreases+1+dB%3C%2Fi%3E.+In+this+regard+%3Ci%3Ecompressors+make+loud+sounds+quieter%3C%2Fi%3E.+If+the+sound+gets+louder+by+1.6+dB+and+the+output+only+increases+by+1+dB%2C+then+the+loud+sound+has+been+made+quieter.%3C%2Fp%3E+%0D%0A++++++++%3Cp%3ESome+compressors+include+%3Ci%3Eattack+%3C%2Fi%3Eand+%3Ci%3Erelease+%3C%2Fi%3Econtrols.+The+%3Ci%3Eattack+time%3C%2Fi%3E+is+the+amount+of+time+that+passes+between+the+moment+the+input+signal+exceeds+the+threshold+and+the+moment+that+the+gain+is+actually+reduced.+The+%3Ci%3Erelease+time+%3C%2Fi%3Eis+just+the+opposite+--+the+amount+of+time+that+passes+between+the+moment+the+input+signal+drops+below+the+threshold+and+the+moment+that+the+gain+is+restored.+These+controls+are+very+difficult+to+set%2C+and+yet+once+set%2C+rarely+need+changing.+Because+of+this+difficulty%2C+and+the+terrible+sounding+consequences+of+wrong+settings%2C+Rane+correctly+presets+these+controls+to+cover+a+wide+variety+of+music+and+speech+--+one+less+thing+for+you+to+worry+about.%3C%2Fp%3E+%0D%0A++++++++%3Cp%3ESystem+overload+is+not+the+only+place+we+find+compressors.+Another+popular+use+is+in+the%3Ci%3E+making+%3C%2Fi%3Eof+sound.+For+example+when+used+in+conjunction+with+microphones+and+musical+instrument+pick-ups%2C+compressors+help+determine+the+final+%3Ci%3Etimbre+%3C%2Fi%3E%28tone%29+by+selectively+compressing+specific+frequencies+and+waveforms.+Common+examples+are+%26quot%3Bfattening%26quot%3B+drum+sounds%2C+increasing+guitar+sustain%2C+vocal+%26quot%3Bsmoothing%2C%26quot%3B+and+%26quot%3Bbringing+up%26quot%3B+specific+sounds+out+of+the+mix%2C+etc.+It+is+quite+amazing+what+a+little+compression+can+do.+Check+your+owner%27s+manual+for+more+tips.%3C%2Fp%3E+%0D%0A++++++++%3Cdiv+align%3D%22center%22%3E+%0D%0A++++++++++%3Cp%3E%3Cimg+src%3D%22n155fig25.gif%22+width%3D%22720%22+height%3D%22648%22+align%3D%22MIDDLE%22%3E%3C%2Fp%3E+%0D%0A++++++++++%3Ch4%3E%3Cb%3EFigure+8.+Gate%2FExpander%2FCompressor%2FLimiter+Action%3C%2Fb%3E%3C%2Fh4%3E+%0D%0A++++++++%3C%2Fdiv%3E+%0D%0A++++++++%3Ch4%3E%3Cb%3EExpanders%3C%2Fb%3E%3C%2Fh4%3E+%0D%0A++++++++%3Cp%3E%3Ci%3EExpanders%3C%2Fi%3E+are+signal+processing+units+used+to+increase+%28%3Ci%3Eexpand%3C%2Fi%3E%29+the+dynamic+range%3Ci%3E+%3C%2Fi%3Eof+the+signal+passing+through+it.+However%2C+modern+expanders+operate+only+%3Ci%3Ebelow+the+set+threshold+point%3C%2Fi%3E%2C+that+is%2C+they+operate+only+on+low-level+audio.+Operating+in+this+manner+they+make+the+%3Ci%3Equiet+parts+quieter%3C%2Fi%3E.+The+term+%3Ci%3Edownward+expander+%3C%2Fi%3Eor+%3Ci%3Edownward+expansion+%3C%2Fi%3Eevolved+to+describe+this+type+of+application.+The+most+common+use+is+noise+reduction.+For+example%2C+say%2C+an+expander%27s+threshold+level+is+set+to+be+just+below+the+quietest+vocal+level+being+recorded%2C+and+the+ratio+control+is+set+for+2%3A1.+What+happens+is+this%3A+when+the+vocals+stop%2C+the+signal+level+drops+below+the+set+point+down+to+the+noise+floor.+There+has+been+a+step+decrease+from+the+smallest+signal+level+down+to+the+noise+floor.+If+that+step+change+is%2C+say%2C+-10+dB%2C+then+the+expander%27s+output+attenuates+20+dB+%28i.e.%2C+due+to+the+2%3A1+ratio%2C+a+10+dB+decrease+becomes+a+20+dB+decrease%29%2C+thus+resulting+in+a+noise+reduction+improvement+of+10+dB.+It%27s+now+10+dB+quieter+than+it+would+have+been+without+the+expander.%3C%2Fp%3E+%0D%0A++++++++%3Ch4%3E%3Cb%3ELimiters%3C%2Fb%3E%3C%2Fh4%3E+%0D%0A++++++++%3Cp%3E%3Ci%3ELimiters+%3C%2Fi%3Eare+compressors+with+fixed%3Ci%3E+ratios+%3C%2Fi%3Eof+10%3A1+or+greater.+Here%2C+the+dynamic+action+prevents+the+audio+signal+from+becoming+any+bigger+than+the+%3Ci%3Ethreshold+setting%3C%2Fi%3E.+For+example%2C+say+the+threshold+is+set+for+%2B16+dBu+and+a+musical+peak+suddenly+comes+along+and+causes+the+input+to+jump+by+10+dB+to+%2B26+dB%2C+the+output+will+only+increase+by+1+dB+to+%2B17+dBu+--+basically+remaining+level.+Limiters+find+use+in+preventing+equipment+and+recording+media+overloads.+A+limiter+is+the+extreme+case+of+compression.%3C%2Fp%3E+%0D%0A++++++++%3Cp%3EYou+will+hear+the+term+%3Ci%3Epumping+%3C%2Fi%3Eused+in+conjunction+with+poorly+designed+or+improperly+set+limiters.+%3Ci%3EPumping%3C%2Fi%3E+describes+an+audible+problem+caused+by+actually+%3Ci%3Ehearing+the+gain+change%3C%2Fi%3E+--+it+makes+a+kind+of+%26quot%3Bpumping%26quot%3B+sound.+This+is+particularly+a+problem+with+limiters+that+operate+too+abruptly.+Rest+assured+that+Rane+limiters+are+designed+not+to+have+%3Ci%3Eany+%3C%2Fi%3Eaudible+side-effects.%3C%2Fp%3E+%0D%0A++++++++%3Ch4%3E%3Cb%3ENoise+Gates%3C%2Fb%3E%3C%2Fh4%3E+%0D%0A++++++++%3Cp%3E%3Ci%3ENoise+gates+%3C%2Fi%3E%28or+%3Ci%3Egates%3C%2Fi%3E%29+are+expanders+with+fixed+%26quot%3Binfinite%26quot%3B+downward+expansion+ratios.+They+are+used+extensively+for+controlling+unwanted+noise%2C+such+as+preventing+%26quot%3Bopen%26quot%3B+microphones+and+%26quot%3Bhot%26quot%3B+instrument+pick-ups+from+introducing+extraneous+sounds+into+your+system.+When+the+incoming+audio+signal+drops+below+the+%3Ci%3Ethreshold+%3C%2Fi%3Epoint%2C+the+gate+prevents+further+output+by+reducing+the+gain+to+%26quot%3Bzero.%26quot%3B+Typically%2C+this+means+attenuating+all+signals+by+about+80+dB.+Therefore+once+audio+drops+below+the+threshold%2C+the+output+level+basically+becomes+the+residual+noise+of+the+gate.+Common+terminology+refers+to+the+gate+%26quot%3Bopening%26quot%3B+and+%26quot%3Bclosing.%26quot%3B+A+gate+is+the+extreme+case+of+downward+expansion.%3C%2Fp%3E+%0D%0A++++++++%3Cp%3EJust+as+poorly+designed+limiters+can+cause+pumping%2C+poorly+designed+gates+can+cause+%3Ci%3Ebreathing%3C%2Fi%3E.+The+term+%3Ci%3Ebreathing+%3C%2Fi%3Eis+used+to+describe+an+audible+problem+caused+by+being+able+to+hear+the+noise+floor+of+a+product+rise+and+lower%2C+sounding+a+lot+like+the+unit+was+%26quot%3Bbreathing.%26quot%3B+It+takes+careful+design+to+get+all+the+dynamic+timing+exactly+right+so+breathing+does+not+occur.+Rane+works+very+hard+to+make+sure+all+of+its+dynamic+processors+have+no+audible+funny+business.%3C%2Fp%3E+%0D%0A++++++++%3Cp%3EAnother+popular+application+for+noise+gates+is+to+enhance+musical+instrument+sounds%2C+especially+percussion+instruments.+Correctly+setting+a+noise+gate%27s+%3Ci%3Eattack%3C%2Fi%3E+%28turn-on%29+and+%3Ci%3Erelease%3C%2Fi%3E+%28turn-off%29+adds+%26quot%3Bpunch%2C%26quot%3B+or+%26quot%3Btightens%26quot%3B+the+percussive+sound%2C+making+it+more+pronounced+--+this+is+how+Phil+Collins+gets+his+cool+snare+sound%2C+for+instance.%3C%2Fp%3E+%0D%0A++++++++%3Cp%3E%3Cimg+src%3D%22pdficon.gif%22+alt%3D%22PDF%22+width%3D%2232%22+height%3D%2232%22+align%3D%22BOTTOM%22%3E+%3Ca+href%3D%22pdf%2Franenotes%2FSignal_Processing_Fundamentals.pdf%22%3E%26quot%3BSignal+Processing+Fundamentals%26quot%3B%3C%2Fa%3E+This+note+in+PDF.%0D%0A++%3C%2Ftd%3E+%0D%0A++%3C%2Ftr%3E+%0D%0A%3C%2Ftable%3E+%0D%0A%3C%21--+InstanceEndEditable+--%3E+%0D%0A+++%3Cdiv+class%3D%22content%22%3E%3C%2Fdiv%3E+%0D%0A+%3C%2Fdiv%3E%3Cdiv+id%3D%22layoutbottom%22%3E%26nbsp%3B%3C%2Fdiv%3E+%0D%0A%3C%2Fdiv%3E+%0D%0A%3Cdiv+id%3D%22footerbox%22%3E%3C%21--+InstanceBeginEditable+name%3D%22Footer%22+--%3E+%0D%0A++%3Cdiv+id%3D%22footer%22%3E+%0D%0A++++%3Cp%3E%26copy%3B+1997+Rane+Corporation%3C%2Fp%3E+%0D%0A++%3C%2Fdiv%3E+%0D%0A%3C%21--+InstanceEndEditable+--%3E%3C%2Fdiv%3E+%0D%0A%3C%2Fbody%3E+%0D%0A%3C%21--+InstanceEnd+--%3E%3C%2Fhtml%3E+

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