The Orban 418A Stereo Limiter/Compressor was designed to address a specific gap in the market for audio processing equipment, offering a unique automatic gain control device that combined programme-controlled dynamics with a specialized high-frequency limiter. The manufacturer identified a need for a unit that could operate subtly with all types of programme material without requiring critical manual adjustments for attack and release times. The device featured a high-frequency limiter with several user-selectable time constants, allowing the limiting action to be optimised for different recording media. It comprised two stereo-ganged channels with excellent tracking stability over time and temperature, eliminating the need for readjustment. The control configuration was intentionally kept simple and user-friendly to permit rapid and efficient operation in high-pressure, real-world production situations.
The Orban 418A functioned as a complete limiting system, consisting of a pair of ganged broadband compressor/limiters with exceptionally smooth and subtle characteristics, followed by a high-frequency limiter. This high-frequency limiter offered four different time constants, which were user-selectable via a front-panel switch. This variable time constant feature was unique in the industry at the time, permitting the characteristics of the high-frequency limiter to be tailored to the recording medium following the limiter, such as disc, optical film, cassette, or 7.5 ips tape. The system was derived from research conducted for the Orban Optimod-FM, adapting seven years of development work to applications involving pre-emphasis and clipping, such as tape recording, cassette duplication, and optical film recording, at a significantly lower cost than the broadcast transmitter-oriented Optimod-FM.
The operating controls of the Orban 418A were limited to four adjustments: Input Level, Output Level, Release Time, and Time Constant. The Time Constant control determined the degree of high-frequency control obtained. The Release Time control was not a simple release time control in the classic sense but rather scaled the release speed of the internal multiple-time-constant release time circuit faster or slower. This circuit incorporated four time constants to automatically tailor the release time to the programme material, relieving the engineer of the task of determining a single compromise release time for each piece of programme material. If the engineer wished to make the recording denser, increasing the apparent loudness for a given peak level, they could operate with a faster setting of the Release Time control and large amounts of gain reduction. For a more natural, uncompressed sound, the engineer would avoid excessive gain reduction and operate the Release Time in its slow position. Regardless of the setting, the limiter was remarkably immune to classic limiter problems such as pumping or heavy transients punching holes in the audio.
The Orban 418A was not suited for driving broadcast transmitters directly. While it could be placed in the program line provided substantial RF fields were not present, the loudness would be 6 to 8 dB lower than that provided by the Optimod-FM given equal peak modulation levels. The manufacturer strongly urged the use of the Optimod-FM for this specific application, as it combined the natural sound of the Orban 418A with highly effective control of fast-peak modulation. However, the Orban 418A was useful in providing AGC at the studio end of an STL (phone line or microwave) to prevent STL overload. It was particularly ideal for use with an Optimod-FM 8000A, since the compression function provided by the 8000A could be provided instead by the 418A. The 8000A could then be adjusted to act only as a safety limiter to preserve the open, transparent Optimod-FM sound.
In broadcast production, the Orban 418A addressed issues arising from the “sweetening” of discs during transfer to tape, which often involved a boost in the upper midrange. This boost could interact with the high-frequency pre-emphasis inherent in 7.5 ips (19 cm/sec) tape recording to produce distortion. Use of the Orban 418A with 50 or 37.5 microsecond high-frequency limiting could eliminate this problem with minimal audible effect upon the brightness of the tape. The device also augmented the quality and efficiency of production for station promos, spots, and other live-mike or recorded production. In recording studios, the Orban 418A served as a mixdown machine, saving time by gain riding and controlling high-frequency energy. Its naturalness also lent itself to single-channel limiting chores, particularly for traditionally difficult sources like chorus, french horn, or piano.
For record masters, where time was available to mix without the aid of a limiter, the Orban 418A could still be employed profitably. If used at or below the threshold of broadband limiting, no gain-riding would occur, but the high-frequency limiter would still operate as necessary. This permitted the use of brighter equalisation than would otherwise be practical. If vocal sibilance was controlled by use of an Orban 516EC Dynamic Sibilance Controller on the vocal track alone, the resulting mix could have much more snap or sparkle than would be possible by manual mixing alone. When preparing reference cassettes for artists and producers, the Orban 418A could control high-frequency energy so that no audible high-frequency distortion was produced, even from bright mixes on 15 or 30 ips reel-to-reel tape. The simplicity of the operating controls made this process extremely fast and economically practical.
In cassette duplication, the Orban 418A effectively addressed the two main problems: high-frequency distortion and inadequate signal-to-noise ratio. The 75 microsecond time constant was ideal for cassette processing, and 4 or 5 dB of fast peak limiting could typically be employed without significant audible side-effects to improve cassette signal-to-noise ratio. Dolby-B processing could be employed after the Orban 418A if desired. For optical film recording, the extreme pre-emphasis employed by optical sound recorders in the production of 35mm and 16mm tracks required careful control of high frequencies to avoid severe distortion due to galvanometer shutter clashing or less than perfect cross-modulation control in printing. While this control had traditionally been performed manually by careful equalisation in the mixing stage, use of the Orban 418A in the magnetic-to-optical transfer chain permitted equalisation for greater brightness without risk of serious distortion, as high-frequency content could be controlled by the Orban 418A’s high-frequency limiter utilising a 75 microsecond time constant.
The sound quality of the Orban 418A was determined almost entirely by its dynamics, specifically how the gain changed when the input of the limiter was excited by complex audio material. The analogue release-time processor in the Orban 418A was an extremely sophisticated and relatively complex circuit. It employed four time constants, as well as non-linear smoothing of the gain-control voltage. The result was an extremely subtle characteristic which was matched psychoacoustically to the ear with unprecedented accuracy. The non-linear smoothing reduced steady-state harmonic distortion to truly negligible amounts. The attack times of the broadband AGC and high-frequency limiter sections of the 418A were on the order of several milliseconds. Therefore, the output contained considerable overshoot. The attack times had been adjusted by means of extensive listening tests so that any overshoot produced could be hard-clipped without audible side effects. This made the 418A suitable for any medium which could clip naturally without damage or other unacceptable effects, such as tape and optical film.
In a disc mastering situation, the Orban 418A’s high-frequency limiter would protect the cutter from thermal overload. However, overcutting or excessive acceleration due to fast-rising transients could still occur. It was noted that limiters which offered extremely fast attack times, such as 10 microseconds, reduced gain for many milliseconds on microsecond-wide peaks. The resulting holes in the output were far more objectionable than clipping of overshoots could possibly be. The Orban 418A used slewrate limited peak detectors in both broadband AGC and high-frequency limiters to determine the amount of control. These peak detectors ignored very fast peaks, producing more and more control as the duration of the peak increased. This approach contrasted with RMS detection, which was claimed by some manufacturers to correspond to loudness. In reality, the RMS value of a sound had almost nothing to do with its loudness, which was a weighted function of the spectral distribution among the critical bands of frequency, with further weighting related to the duration of the sound. RMS detection ignored actual peak levels in favour of power, which could be a liability when the purpose of AGC was to accommodate a sound to the peak level that a recording channel could handle without audible distortion.
The Orban 418A was constructed with industrial-quality materials, ensuring high reliability and first-class engineering. It came with a one-year warranty and offered fast and reasonably costed service. The device was designed to save time and money for users and their customers while producing better product, which was considered financially advantageous. The simplicity of the controls meant simple, hassle-free operation in any application, yet the experienced engineer could use the Release Time and Input Attenuator controls to increase density and average levels as desired.
Features
- Programme-controlled attack and release times for subtle operation with all programme material
- High-frequency limiter with four user-selectable time constants (75, 50, 37.5, 25 microseconds)
- Two stereo-ganged channels with stable tracking over time and temperature
- Simple control configuration with only four adjustments: Input Level, Output Level, Release Time, and Time Constant
- Internal multiple-time-constant release time circuit with non-linear smoothing
- Slewrate limited peak detectors for broadband AGC and high-frequency limiting
- Immunity to classic limiter problems such as pumping and transient punch-through
- Suitable for recording media that can clip naturally, including tape and optical film
- Industrial-quality construction with high reliability and one-year warranty
- Derived from Orban Optimod-FM research for lower-cost signal control applications