Tascam 488 PORTASTUDIO Owner's Manual

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Contents

How the dbx WorksDEGAUSSING (DEMAGNETIZING)A little stray magnetism can become quite a bignuisance in tape recording. It only takes a smallamount (.2 Gauss) to cause trouble on the recordhead. Playing 10 cassettes will put about thatmuch charge on the heads. A little more thanthat (.7 Gauss) will start to erase high frequencysignals on previously recorded tapes. You cansee that it's worth taking the trouble to degaussregularly.DEGAUSSING IS ALWAYS DONE WITH THERECORDER TURNED OFF. If you try it with theelectronics on, the current pulses produced bythe degausser will look just like audio signals tothe heads. These pulses are around 10,000Gauss, and will seriously damage the electronicsand/ or meters. Turn off your 488, then turn onthe degausser at least 1 m (3 feet) away from therecorder.Be certain that your degausser has either aplastic cover or plastic tape covering the tip.Make sure that no metal ever touches the tapeheads as it will scar them and ruin them.Slowly move in to the tape path. Move thedegausser slowly back and forth, touching lightlyall metal parts in the tape path. Slowly move itaway again to at least 1 m (3 feet) from therecorder before turning if off.Be sure to concentrate while you are degaussing.Don't try to hold a conversation or think ofanything else but the job you are doing. If thedegausser is turned on or off by accident while itis near the heads, you may put a permanentmagnetic charge on them that no amount ofcareful degaussing will remove. You will have toget the heads replaced. Make sure you are wideawake for this job.A clean and properly demagnetized taperecorder will maintain its performance withoutany other attention for quite a while. It won'truin previously recorded material, nor willgetting it back to original specifications bedifficult.CAUTION: If the surface of the unit gets dirty,wipe the surface with a soft cloth or use a dilutedneutral cleaning fluid. Clean off thoroughly. Donot use thinner, benzine, or alcohol, as they maydamage the surface of the unit.The DBX is a wide-band compression-expansionsystem which provides a net noise reduction(broadband, not just hiss) of a little more than 30dB. In addition, the compression duringrecording permits a net gain in tape headroom ofabout 10 dB.A compression factor of 2:1 is used beforerecording; then, 1:2 expansion on reproduce.These compression and expansion factors arelinear in decibels and allow the system toproduce tape recordings with over a 90 dBdynamic range - an important feature, especiallywhen you're making live recordings. The DBXemploys RMS level sensors to eliminatecompressor-expander tracking errors due tophase shifts in the tape recorder, and providesexcellent transient tracking capabilities.To achieve a large reduction in audible tape hiss,without danger of overload or high-frequencyself-erasure on the tape, frequency pre-emphasisand de-emphasis are added to the signal andRMS level sensors.SUBSONICS AND INTERFERENCEThe DBX incorporates an effective bandpassfilter. This filter suppresses undesirable subsonicfrequencies to keep them from introducing errorsinto the encode or decode process. However, ifrumble from trains or trucks is picked up by yourmicrophone and fed to the DBX, modulation ofthe program material during low level passagesmay occur. This low-frequency component willnot itself be passed through the recorder and so,will not be present at reproduce for properdecoding. It this low-level decoding error isencountered, and subsonics are suspected, wesuggest the addition of a suitable high-pass filterin the Microphone Line.45