The two types are not compatible, but I was able to borrow a Type I unit for my tests to develop my plugin method. The only hardware dbx unit that I still have is a Type II unit, that was designed for consumer equipment with its limited frequency response and dynamic range. The tapes that I wanted to transfer were encoded with dbx Type I, which was designed for professional and semi professional equipment capable of fairly flat and extended frequency response and reasonable dynamic range. The differences are all in the equalization curves. They are similar, but not the same, and further, Type II also has a Disk variation for playing the rare dbx encoded LPs. Moreover, there are two different versions of dbx NR, Type I and Type II.

Also, the attack and release characteristics of the expander, as well as the method of level detection are important. There are also a few equalization and filter curves to deal with, some in the audio path and some in the control path (the signal fed to the level detector that controls the gain of the compressing or expanding element, the VCA). This is indeed correct, but it isn't the whole story. Some audio people old enough to have worked with analog tape can tell you that dbx noise reduction is a companding process, wherein the audio going to the recorder is compressed at a 2:1 ratio, and expanded at a 1:2 ratio upon playback. These had to be determined experimentally. The manuals make no mention of the attack and release times of the expander. The dbx company published block diagrams with important information needed to allow us to decode our audio. The place to start is the owner's manuals of two dbx products from the '70s (models 122 and 150) that explain the process in surprising detail. You just have to understand what dbx noise reduction is, and how it works, to properly decode it.

But, it CAN be done from a technical standpoint.

There would likely be licensing issues and the market would probably be too small to be worth the bother and expense of developing software and bringing it to market. The answer is, no one has marketed such a plugin. A quick Google search revealed that many people have had the same question that came to my mind, 'Is there a way to decode dbx NR in software, such as a plugin?' The discussions on many audio forums were interesting, even entertaining, but full of misconceptions and downright nonsense. I recently had to transfer some of my old tapes into Pro Tools, and realized I had used dbx noise reduction on some of them, mostly the 1/2-inch 8-track stuff from the '80s. A Method For Decoding dbx Noise Reduction Using Standard Plugins

Waves noise reduction plugin