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This safety margin is known as headroom, and may range from 10 to 25 dB. Lowering our standard operating level to leave ourselves some headroom helps prevent distortion, but at the same time it moves our average program level closer to the noise floor, thereby compromising signal-to-noise performance. It becomes apparent that to get the most out of an audio system, you have to keep your standard operating level as high as possible without risking distortion. CLIPPING Significant Distortion Occurs AaoveThis Level Headroom [ Normal Operating Level Useable Dynamic flange NOISE FLOOR (Minimum Storage Transmission Level) Tape Hiss. Hum. Transistor No«se The useful dynamic range of any piece of audio equipment is limited by noise and distortion. 10 ASHLY Gain Riding One solution to the noise vs. distortion trade-off is to keep your hand on the level control and manually adjust gain to suit the program. Indeed, there are times when this approach is entirely satisfactory. However, in most types of music there are instantaneous, short duration volume peaks, or transients, which would be difficult to anticipate and impossible to respond to with manual gain riding; you simply could not bring the level down fast enough. In many situations, this can present real problems. For example, in recording, an extra burst of enthusiasm from a lead singer might overload the capabilities of your recording tape, causing ragged distortion and necessitating another take. In sound reinforcement, a sudden burst of energy through the system can blow fuses or even damage loudspeakers. In addition to the problem of response time with manual gain riding, it also requires your constant attention, which takes you away from more important jobs. The need for a fast-acting, reliable, automatic gain control is answered by limiters and compressors. What Compressors and Limiters Do Limiting and Compression are closely related effects, their differences sometimes being very subtle. Since limiting is generally an easier concept to deal with than compression, the following sections will talk about limiting first, followed by a discussion of compression. The use of the front panel controls is discussed in terms of limiting also. Bear in mind that these controls will have the same function when the CL-SERIES unit is used as a compressor. Limiting In any musical program there are constant changes in loudness. It is the job of a limiter to detect when the volume has exceeded a predetermined level, and to then turn down the volume. When the incoming signal returns to its original safe level, the limiter should respond by restoring the gain to normal. Thus, when the level is within a specified “safe* range, the limiter has no effect. When an occasional peak occurs, the limiter responds. This situation is completely analogous to manual gain riding, except that it occurs faster and more consistently. ABHLY 11 To determine which peaks are acceptable and which are too loud, the limiter establishes a reference level known as the threshold, and then responds to those peaks which exceed this point. In the Ashly CL-SERIES, this threshold is fixed at OdBV. When the input level reaches OdBV, the limiter will begin to react. If the source feeding the limiter is increased in overall level, then more of the peaks will now be above OdBV; those peaks which were previously just below threshold level will now be at or above threshold, in addition to the volume peaks which had already been above threshold. Thus, varying the input level to the limiter is inversely proportional to changing the threshold level. On the Ashly CL-SERIES, this effect is easily accomplished by changing the input GAIN control. Increasing the GAIN control is equivalent to lowering the threshold point. A gain control range of plus or minus 30 dB is provided. What the limiter does once a peak above threshold is detected is determined by the RATIO, ATTACK TIME, and RELEASE TIME controls. Compression There is a good deal of disagreement and confusion in the audio industry over the definitions of limiting and compression, and the terms are frequently and incorrectly swapped back and forth. The fact is, compression and limiting are closely related effects, and the differences between them are largely a matter of degree. Indeed, it could be said that the difference between compression and limiting is, more often than not, a matter of what you intend the device to accomplish. Returning to our original definition of limiting, we stated that gain reduction occurred when input signals rose above a predetermined threshold level of 0 dBV, and that the effective threshold could be varied by changing the GAIN control. A very significant difference in dynamic range is achieved simply by changing the relationship between nominal signal level and threshold, as a result of varying the GAIN control. The most interesting effect to be noted, however, is seen by comparing the original Input signal...

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