Other unique dbx circuit designs such as true rms-level detection, feed forward gain reduction, and automatically variable attack and release times provide the dbx 163 with audibly superior performance. The dbx 164 (shown above) is a stereo- tracking version of the dbx 163, working with a pair of channels as opposed to the single (mono) channel of the dbx 163. In all other respects the dbx 163 and dbx 164 are identical.
The dbx 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 O dB 0.775 V rms.)
The dbx 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 feed forward circuitry and rms-level detector combine to make it possible for the attack time to ‘track’ t the rise 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
The operation of the dbx 163 is greatly simplified because the compression ratios. 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) .
No manual attack-time adjustments are required. The release rate not time is fixed by the detector characteristics at a smooth-sounding 120 dB/ second. These unique circuit characteristics preserve the natural quality of any given sound. The dbx 163 is most significant advance in preserving natural sound quality is provided ‘by a continuously variable compression ratio whi.ch increases gradually from toward infinity as the Input level rises above the compression threshold.
With a conventional hard-knee compression curve, one typically hears normal program dynamics until the input level rises above threshold; then the gain is suddenly reduced by a fixed ratio and, depending on the nature of the program, the listener senses a ‘holding back.’ With the dbx 163’s Over Easy characteristics, the ratio of gain reduction increases gradually as the Input rises farther above threshold. Thus there is no sudden change and no feeling of holding back, just a natural—sounding program that avoids undesirably high output levels .