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Here are some manufacturers that you should check out:
The CBS Audimax processor (developed at CBS Laboratories in the early 1960s) was a gated wideband compressor that successfully eliminated the noise-breathing problem of earlier compressors. The Volumax was a clipper preceded by a limiter with a moderate attack time. The moderate attack time prevented the unit from punching holes in the program, while the clipper controlled the peaks that the preceding limiter did not catch. To eliminate "pumping" and "breathing", a number of companies introduced multiband processor designs. In the early 1970s, Dorrough Electronics introduced the "Discriminate Audio Processor" ("DAP"). The DAP divided the audio spectrum into three bands with gentle crossover slopes and compressed each band independently. Additionally, many engineers adjusted the three bands for different gains, using the device as a dynamic program equalizer as well. Also, a number of radio stations and studios designed their own multiband processors, such as the 4-band processor described here that was used at WITR for a while. However, the REAL breakthrough in audio processing for FM transmission occurred in the mid-1970s, when Robert Orban developed a produce that combined compressor, limiter, high-frequency limiter, clipper, 15kHz low-pass filters, and stereo multiplex encoder in a single unit, the "Optimod FM". This greatly reduced the possibility of misadjustment of the processing chain, as the unit was designed to produce a composite signal that did not exceed modulation requirements, as opposed to prior designs that limited audio, but when combined with a standalone stereo generator, could still produce a composite signal that overmodulated the transmitter. The "Optimod FM" started showing up in radio stations throughout the country, and it was easy to recognize a station that had the "Optimod sound", full volume without distortion AND without degradation of stereo separation. In the years since, a proper use of multiband audio processing, combined with a heightened awareness of the effects of the baseband signal on the stereo composite has resulted in a stable marketplace of good designs from Orban, Omnia, CRL, and many others. Further, the shift from analog processing to digital processing (which can include digital delay to permit the processor to "look ahead" at the incoming audio, and make smoother gain adjustments) has changed audio processing from a "black art" to a well-understood science.
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