Two questions though - Why the need for a lowpass filter in the first place, and why the need for a second lowpass filter?
The primary one is the reconstruction filter - essential for recreating the original waveform.
Julf, could you explain how this works?
...on conversion back to analogue a reconstruction filter removes the spuriae above this same bandwidth to leave just the reconstructed input signal.
The 'spuriae' presumably being jitter induced?
Oversampled; bunch of high frequencies undesirable but necessary (or at least v. important for accuracy)
Here is an illustration:
<snip> The reconstruction filter interpolates between the points to reconstruct the original waveform (gray). Forget the "staircase" pictures, they are very misleading (and actually wrong).
‘Holy Wars’ are all well and good but all I care about is whether it sounds good or not. Well actually, that’s not quite true - I’m interested in why it sounds good too, and Martin, Julf, your comments are much appreciated and understood. I have been busy reading up on Sallen-Key, Bessel and Butterworth filters (Art of Electronics et al) and trying to relate what I’m reading to the actual layout and specific applications therein....(yet we are still left with holy wars about the shape, nature and flavour of how the very last part 'should' be done, from the ignorant 'NOS 16/44.1' heresy to 'apodising' which conflates effects of the digital oversampling filter with manufacturing desire to get good measurements in magazines & use the cheapest-possible analogue output stage, but claim a 'win' in perceptual BS terms. It's late, and I'm too tired to wade-in there...)
‘Holy Wars’ are all well and good but all I care about is whether it sounds good or not. Well actually, that’s not quite true - I’m interested in why it sounds good too, and Martin, Julf, your comments are much appreciated and understood. I have been busy reading up on Sallen-Key, Bessel and Butterworth filters (Art of Electronics et al) and trying to relate what I’m reading to the actual layout and specific applications therein.
Why am I bothering with an age old CD player? Because it’s what I’ve got and I love it.
I also get the impression that op-amps are somewhat frowned upon in audio circles and if that is the case, then why would a company like Naim use them?