I see some argues with ringing so I'll explain how interpolation works.
Oversample = just adding more points to the file.
Interpolate = upsample = adding more points and applying a filter to avoid imaging
DAC manufacturers interpolate the PCM file up to 512 times or 1024 times. But they don't do it at single stage. First, they oversample the file up to about 4-16 times(and match every files to similar sampling rate) and apply good anti-imaging filter.
Red line: Original signal
Orange line: Imaged signal
Then, they apply simple interpolation filters with poor filters because there is enough room for filter to attenuate.
And this is the final result
But it would have been better with sharper filter
This is what so many audiophiles have fought of. DAC performance don't degrade so much with slow filter and there is no
audible ringing issue with sharp filter. Frequency range can be converted to maximum steepness a wave can have. The wave won't ring if sufficient steepness to stop ringing is allowed. This is just like driving a car. A heavy car needs to turn widely and slowly.
An impulse signal with slow filters don't have so much ringing just because it contains higher frequency(In this case, its partial image of original file). Ringings are not natural waveform because nature is not frequency-limited. But our ears are frequency-limited and always hear ringings. In other words, even if your DSD256 high-res album is reproduced perfectly, your ears are not high-res and hear ringings generated by your ear itself.
Then, what is NOS dacs on the market? If you look at the waveform of the NOS dac, it will look like this.
The waveform of NOS dac is the one you can see at ladder dac with bypassed(or insufficient) low-pass filter. Actually, the NOS dac with commercial delta-sigma dac chip is
not 'Non-Oversampled'. The 2nd stage interpolator and delta sigma modulator is impossible to bypass and no signal can be printed out without them. It is
emulated through old interpolation technique called
zero-order hold interpolation.
This is what the original PCM file is representing. It is made of several points indicating the level.
Now lets add some more points to it.
That's it. This will be put into 2nd stage interpolator and modulator.
In frequency domain, this can be achieved by simply oversampling the signal and applying a filter like this.
In short, if you apply a very very poor filter after oversampling, a nice stair-shaped wave is made.
Normal delta-sigma DAC
View attachment 29463
NOS emulating dac
Cirrus Logic patent
The result is massive amount of image that is very likely to cause distortion problem unlike slow filter. Although a strict brick-wall filter is not so necessary to real performance, using NOS filter will reduce its SINAD performance. But I'm not sure about audibility of distortion caused by NOS filter.