Variable Ratio Limiter-Linear Integration Detector scheme makes response variable depending on waveform complexity, fundamentally different from any other limiter/compressor.The Gain Brain II was a departure from traditional variable ratio limiters in the way Paul approached the problem of dynamic gain control. A “traditional” variable ratio limiter uses a feedback controller in which the audio level is detected after the gain control element. The voltage corresponding to the level is then used to set one variable in an analog computer, compared to the threshold setting, and the computer generates a correction voltage for the gain control element. Feedback limiters have the virtue of being able to compensate for non-linearity in the gain control element, but they aren't very fast, and with short release times, they can be quite audible. That is sometimes just what the user wants, but not usually. Usually, one uses a variable ratio limiter to control transient levels or to emulate the operation of a compressor. This implies the device should be as transparent as possible.
Paul's departure was the development and application of a feed-forward approach to control the variable gain element. The approach depended upon the accuracy of both a logarithmically responsive detector circuit, and of the dB/V gain port response of the new log/antilog VCA circuit. These two functions were so closely complementary that a properly designed feed forward limiter could perform 40 or more decibels of gain reduction with only a fraction of a decibel error. In addition, the use of antilogarithmically controlled, high compliance current sinks in the attack and release circuitry of the analog computer resulted in a startling difference in preserving the natural decay characteristics of the processed sound. One could also alter the release rate in such a way as to artificially shorten the decay time of a sound, but still impart a more natural-sounding exponential tail-out of the sound.
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