Hi
This comes from AMR's Chief Designer:
Oversampling is not automatically ZOH, in FACT, ZOH would be pointless in an oversampling filter, as it, in effect does not " oversample" nor "filter" at all, might as well use Non-Oversampling.
However Up-conversion to 352.8KHz (for 44.1/882/176.4 KHz Sample rate recordngs) or 384KHz (for 48/96/192KHz Sample rate recordings) with ZOH can be used to in effect place the iDSD into what amounts to non-oversampling mode. The downside is that ZOH attenuates the HF response of the resulting signal, for example CD-Standard PCM audio will face 3.2dB attenuation at 20KHz starting to rolloff as low as 5KHz - this can make the music sound too dull.
Linear interpolation is equally pointless, it does provide some mild filtering but again does not really help, but it produces even more HF roll-off than ZOH.
Modern Digital filters use different structures (tapped delay lines with multiple weights to mix together the different delayed outputs - this is somewhat reminiscent to the old style of analogue tapped delay line chroma filters in old Colour TV's). This rejects ultrasonic images depending on filter slope.
The "standard filter" on the iDSD in effect performs "sinc" type interpolation, which has the best "on paper" performance, with completely flat frequency and phase response up to half the sample rate and then a very steep rolloff. It also results in a symmetrical impulse response with filter ringing BEFORE and AFTER the main impulse.
However many listeners tend to associate this type of filter with "digital sound", for reasons that so far lack any reliable proof, though many theories to the "why" exist.
The iDSD offers an alternative filter that is generally evaluated as more "analoge" or "musical" than "best sinc". This alternative minimum phase filter on the iDSD is a different animal to anything discussed so far. This type of filter was pioneered by Wadia as "Digimaster" and Pioneer as "Legato Link" and has since been picked up by a number of manufacturers.
These filters do not attempt perfect response flatness and allow some HF attenuation at half the sample rate (but much less than ZOH, so the "dull sound" effect does not happen). And they are less steep, so some ultrasonic images are allowed through (though much less than ZOH) but the impulse response lacks pre-ringing.
We find these filters a good compromise between filtering ultrasonic noise and time domain fidelity vs. amplitude response flatness. The type of filter embodied by the iDSD's minimum phase filter is generally not available in resamplers such as those based on SOX (most common in playback software).
Additionally the digital filter in the iDSD is adaptive, in terms of sample rate. The higher the source sample rate, the less the oversampling ratio. So for example CD standard PCM is oversampled eight Times, but 192KHz audio is oversampled only two times, in effect the oversampling filter always performs integer ratio oversampling to 352.8KHz or 384KHz with either sinc or minimum phase filtering/interpolation (selectable).
We recommend to leave the digital processing to the built-in digital filter in Minimum Phase mode, though of course our customers are free to choose whatever way they wish, however we would strongly recommend to try the internal minimum phase filter FIRST, before trying other options.