The dbx Model 163 and 164 compressor/limiter feature dbx's Over Easy compression. Over Easy refers to a "soft-knee" compression characteristic that provides virtually inaudible gain control at ratios up to infinite compression. Other unique dbx circuit designs such as true-rms-level detection, feedforward gain reduction, and automatically variable attack and release times provide the 163 with audibly superior performance. The Model 164 is a stereo-tracking version of the 163, working with a pair of channels as opposed to the single (mono) channel of the 163. In all other respects the 163 and 164 are identical, and throughout this manual we make reference only to the 163, for the sake of convenience. The 164 accomplishes its stereo tracking by feeding the signal in each channel to a special stereo rms detector, which computes the true-rms sum of the two channel levels. This signal is then fed forward to activate the gain-change circuitry for both channels, ensuring an accurate and constant image. Anyone who has ever tried to fine-tune a studio-type compressor/limiter will appreciate the Model 163's ease of operation. A single front-panel COMPRESSION knob simultaneously adjusts the threshold of compression and the overall system gain, thereby maintaining essentially constant output level. Just slide the knob to get any desired amount of compression. This ability to "squeeze" the dynamic range of a program-with a single control is the reason the Model 163 has become known as "THE ONE-KNOB SQUEEZER." (While the 163 is factory-set for nominal -10 dB line levels, the rear-panel controls enable the user to select any convenient nominal level from -20 to +10 dB, with 0 dB = 0.775 V rms.) The 163 features proprietary dbx true-rms detection. Most compressor/limiters use some form of peak detection, with fast-response characteristics that can have a disturbing effect on program material. True-rms-level detection closely simulates the response of the human ear. Even at 15 to 20 dB of compression, the gain-changing action of the Model 163 is highly listenable and natural-sounding. Because of its feedforward topology, the dbx 163 compressor/limiter can achieve effectively infinite compression with complete stability and low distortion. The dbx approach is quite unlike gain reduction in a conventional compressor/limiter. Traditional gain reduction is accomplished by sensing signal level at the device's output and applying a correction signal via a feedback loop. At progressively higher compression ratios, the feedback loop's gain reduction increases, distortion increases, and eventually instability or oscillation occurs. To avoid this problem, many conventional units restrict the maximum amount of feedback, thus restricting the maximum compression to some lower ratio, such as 10:1 or 20:1. Free of the instability of excessive loop gain, the dbx 163 therefore can provide effectively infinite compression. In addition to increasing both the stability and the available range of compression, dbx’s feedforward circuitry and nns-level detector combine to make it possible for the attack time to "track" the rire of the signal envelope. In conventional compressor/limiters, the attack time depends on feedback-loop gain, which means it must be continually readjusted for best results at different compression ratios. The operation of the 163 is greatly simplified because the attack time is controlled solely by the rms detector and varies automatically with the amount of level change in the program material (the shape of the envelope).