Plugins have been drastically increasing in quality over the last 10 years. We are at the point now where we have some very innovative developers creating some truly remarkable sounding plugins. Not just digital emulations of classic analog gear but also new types of processors that wouldn’t be possible in the real physical world.
Unlike hardware, plugins require the use of complex algorithms and the sound of the plugin is dependent on the coding of the developer. The better coders will be able to achieve better-sounding plugins much like a better electrical engineer can design a better circuit for a compressor. Trained ears matched with talented developers allow software companies to turn out some very high-quality plugins.
So, what is oversampling or upsampling?
Oversampling is when a plugin converts the audio to a higher sample rate for processing. Processing at the higher sample rate usually removes some of the negative artifacts associated with processing digital audio; mainly aliasing. Aliasing happens when information outside of the frequency response range of the digital converters and sample rate you’re using are interpreted by the converter to be different frequencies.
Oversampling mitigates issues including aliasing and will usually yield smoother more pleasant-sounding results at the cost of using more CPU power. But all oversampling algorithms aren’t made equal and some are better than others. You may even find that you prefer the sound of a plugin with the oversampling turned off. It’s not necessarily guaranteed that oversampling will make the audio sound “better.” If you see a plugin or DAW that offers oversampling and you have the CPU power to spare, try it out and see if you prefer the way it sounds. If you are short on CPU power you’ll probably want to keep oversampling off, unless you decide to freeze the tracks.