Um. NOS stands for "no oversampling (filter)". The filter is the
digital filter which performs an oversampling which is necessary to prevent stair-stepped response at for example 44kHz, and to prevent aliasing in the analogue signal.
If you don't understand this at all I would suggest reading up on the principle
https://www.audiosciencereview.com/...ng-differ-from-over-sampling.3440/#post-83318 there's many other information sources online.
In any case the D90 does not have a "NOS" mode. You can only make the digital filter steeper or slower (or set it to linear phase or minimum phase, which is yet another distinction here. Linear phase has worse latency but keeps phase in tact). To make matters worse people generally can't hear a difference in linear/minimum phase, nor in digital filters. They are a really subtle thing and a lot of devices just have the manufacturer set a default filter and that is it. Exposing the digital filter to the end user only confuses people who do not really know what this even affects/is in the first place.
If you use a low sample rate like 44kHz and process it "NOS"/without proper filtering, the end result looks like so:
Thankfully your ears will probably not hear much of a difference besides some loss in treble. Our ears are very forgiving even when crucial principles like these are not respected. But you can understand now I assume, why the love for "NOS mode" in the audiophile community is something silly. At low sample rates it distorts the signal severaly, at a high sample rate like 88/192kHz and up NOS mode will not harm the signal any more, but it won't make it magically sound "better", either
...
