"If you upsample a nonDSD file to DSD512/1024, iDSD first oversamples to 705.4/768kHz. None of the filters mentioned above applies in those 2 modes. You can turn the knob, and may appears that you are switching digital filters, but they are not doing anything." I don't think this is true. At lest [USER=361812]@iFi audio[/USER] has confirmed that any of the PCM filters can be selected and works if PCM files are used. The DSD upsampling just takes place after the selected filter is applied.