Digital Filter Mode
Digital Filter Mode describes whether a DAC uses oversampling, non-oversampling (NOS), or lets you switch between them for incoming PCM audio.
Oversampling creates intermediate samples before conversion, making unwanted high-frequency copies of the signal easier for the analogue output filter to remove. Non-oversampling (NOS) omits this digital interpolation. A NOS-only DAC has no oversampling option; Switchable offers both modes.
Digital Filter Mode — DAC
NOS does not mean there are no filters anywhere: analogue output filtering can remain. A choice of filter shapes is not necessarily an oversampling/NOS switch. Labels such as Super Slow or filter-off need checking, too; they do not establish what processing the complete DAC performs.
Check the product manual for the actual behaviour and applicable formats and sample rates. A converter chip’s feature list does not tell you which features the product implements. The mode name is not a sound-quality rating.