This curve, plus dbx’s true RMS level detector coupled to a wide-range voltage controlled amplifier in a feed-forward circuit, makes it possible to achieve larger amounts of compression without adverse audible side effects.
The AMS level detector is separately accessible, allowing various signal conditioners – equalizers, filters, delay lines, etc. – to be inserted in either the level detector or signal path independently. This feature aids in the compression of certain difficult types of material, as well as in the creation of special effects. In addition to dbx’s automatically variable attack and release time circuitry (as on the Models 160, 161, 162, 163 and 164), a front panel switch and controls provide for manually adjustable attack and release rates over a very wide range.
The dbx 165A also features stereo coupling for two dbx 165A’s (at the touch of a front panel button) LED indicators showing whether the signal is at, above or below the threshold of compression, a 30 dB dynamic range meter (indicating input, output and gain change levels), and a rear panel zero VU calibration adjustment.
A hard-wired bypass switch, also located on the front panel of the dbx 165A, is convenient for checking the effect of the compression, and assures fail safe flow of audio through the unit. The maximum input level is +24 dB (12.3 V RMS) and maximum output level is +23 dBm. The output amplifiers have a source impedance of 47 ohms and will drive input loads of 600 ohms or greater impedance. Output gain is also adjustable over a very wide range (+ 20 dB), so the dbx 165A is compatible with virtually all professional sound and creative audio equipment.
Perhaps the most flexible and useful compressor/limiter ever offered, the dbx 165A is well suited to a wide range of applications including: tape recording, disc mastering, radio and TV production and broadcast, live concert sound reinforcement, and theatrical production.
Level Detector Access
Because auxiliary sound equipment can be used to process the level detector signal but not the main audio input signal (or vice·versa), the dbx 165A offers the user an opportunity to create many unusual effects. By connecting it to additional signal processors, such as a parametric equalizer, the dbx 165A can be converted to a de-esser, a vocal stresser or a level·sensitive filter.
Certain musical or vocal elements in a program can be suppressed without affecting others. In addition to these signal conditioning functions, many creative special effects are possible.
Over Easy Compression
Conventional compressor/limiters have a sharp knee at the threshold point. That is, when the input signal is below the threshold, the gain remains fixed (1: 1 ratio or no compression), but when the input signal goes above the threshold, the gain abruptly decreases according to the compression ratio for which the unit is adjusted. This abrupt change in gain in a conventional compressor/limiter is often audible and therefore undesirable to the user.
The dbx Over Easy approach utilizes a soft knee at the threshold of compression. The Over Easy compressor/limiter gradually increases its compression ratio from 1:1 towards the set compression ratio as the input signal rises through the threshold region. Thus an age old dream can be realized…dynamic range restriction without audible, abrupt gain changes. When properly operated, the dbx 165A is a highly effective compressor/limiter that you don’t hear working.
The PeakStop feature allows you to control the maximum peak levels at the output of the dbx 165A irrespective of any other control. PeakStop comes after the coopression and other circuitry, including the output gain, so it lets an absolute limit be put on the peak-to-peak excursions of the output. PeakStop works instantaneously; you are able to apply moderate amounts of dbx’s Over Easy compression and will still be protected from large transients, other short-term overloads, and over modulation.
Technically, PeakStop on the dbx 165A consists of a sophisticated voltage-controlled clipper that produces a minimum of audible distortion. It rounds the corners of a peak rather than cutting it off sharply, as the word “clipping” usually implies. By making a signal’s leading and trailing edges curved instead of sharp corners, it reduces the amount of higher odd-order, offensive-sounding harmonics that conventional clipping causes. The level at which PeakStop is activated is adjustable from – 2 to +24 dBm.
Note that small signal excursions above the set value of PeakStop are possible, to allow the rounding to take place; therefore, for any applications where you must not exceed a given ceiling, set the PeakStop control 1-2 dB below it to be sure.
The red PeakStop LED flashes whenever peaks attempt to exceed PeakStop level and get reduced in amplitude. To disable the PeakStop function altogether, simply set the control to +24 dBm (which is the maximum output level of the dbx 165A anyway).
In use, the PeakStop function can prevent an amplifier from being driven into hard clipping, where it can lose control over the speaker system. PeakStop is a smooth, well-controlled clipper whose behavior is sonically similar to the gentleness of Over Easy compression; its clipping is much preferable to a power amp’s. As mentioned, control of speaker over excursion, broadcast over modulation, and harsh electronics clipping are all applications of PeakStop. With it and Over Easy. you have the best of both worlds: virtually inaudible RMS compression and peak protection downstream at the end.