I don't understand how that is an issue exclusive to NOS DACs. Doesn't the LPF take care of the images regardless?
Nope, only when you were to use a brickwall that is just above the 1/2 fs.
When a DAC can do multiple sample rates you either have to shift that filter or pass it though another filter.
No one does that and IF they did the squarewave and needle reproduction would not look as nice any more and that is what attracts buyers thinking that they have superior 'attack'. The squarewaves/needles would all be showing (severe) post ringing.
In Hans's case there is a brickwall filter in place though just above 12kHz and it is called TH-02
that will at least get rid of the generated 'bit steps' as the driver simply cannot 'follow' those fast transients (ultrasonic sample frequency steps) but the (low level) generated harmonics of all signals below 5kHz will still be present (not really audible though as the distance is too big).
The higher the sample rate the higher the frequency of the steps.
At say 88kHz no headphones will be able to 'follow' those steps but some amplifiers may have issues with it.
When you use the DAC with higher (upsampled) sample rates you 'shift' part of the garbage to higher frequencies (its where DeltaSigma is based on) so the DAC will perform better when you upsample redbook to 176 or 352.
In an upsampling DAC that basically does the same but the algorithm is locked in its firmware.
Hans has a point where he mentions you can choose the upsample algorithm to be used with a NOS DAC and is clearly shown that Peter's algorithm does this better than the ones in the PC by looking at the numbers.
As a side note... Both the Metrum and XXHighEnd are designed by fellow countrymen (Dutch)
On sound reproduction I am on Xnors camp...
All sounds on a CD are brickwalled and transients faster than a 22kHz sinewave can reach are NOT recorded but when looking at the bits (impulse and squarewave) do not have this 'limit' and reproduce a relatively slowly rising 20kHz sinewave as a 1MHz squarewave with all its harmonics.
BUT that sharp edge has NEVER been present in the original signal and if it has it was lost in the encoding so now ALL bit level changes (those in sinewaves as well) always produce very high harmonics that are NOT present in the original signal.
Yes these DAC's produce very nice squarewaves and needle pulses BUT these signals are NEVER present in ANY audiosignal and don't even sound nice. But it LOOKS really good and 'fast' on scope images.
Sound consists of sinewaves and they are NOT accurately reproduced by any NOS DAC instead they look like this:
the picture is from Computer Audiophile and uploaded by PeterSt (the XXHE developper) Pick your poison... a perfect looking squarewave or needle pulse as a sales argument (as they are never present in music, and also not in 'attack' of any instrument nor are these attacks encoded as Xnor explained briefly) but terrible at sinewave reproduction and high levels of distortion / HF energy.... versus ... excellent sinewave reproduction (music consists of sinewaves) and less good looking squarewave/needle
pulses that do not exist in music and have harmonics HIGH above the audible band and WON't be reproduced by the transducers anyway. Well maybe some good speakers but do you really want high amounts of supersonic, UNRELATED to the audiosignal, frequencies 'energy' in your tweeter nor in your amp.
What is clearly seen in the provided plots (I have an SB1240 myself as well b.t.w.) is the added harmonics all across teh audible band reaching -70dB and NOT decaying for higher harmonics.
This is just 1 simple tone imagine what amount of harmonics you would be getting when you play music with a plethora of frequencies between 60Hz and 10kHz mainly... they will ALL be producing harmonics at THESE level DISTANCES so a signal at -20dB will also have harmonics but 20dB below the other ones that are louder.
This basically means that when you do a S/N ratio test you will get impressive numbers but as soon as you play music the noise level rises to about -70dB !
Fortunately it only rises when music is present so in quiet passages there is still a >70dB S/N ratio but the S/N ratio embedded in the recording (mic noise) will always be present and almost at the same level so might 'mask' that added noise.
Distortion levels < 70dB are inaudible anyway (acc. to blind tests not acc. to sighted tests).
So even though the numbers appear quite bad I highly doubt it will be audible.
What is audible is the HF roll-off (certainly when playing redbook) but a lot of people do not find that objectionable (I do).
These DACs can still sound good simply because our hearing isn't as good as we think it is. It certainly isn't because of rise times.
Take a good look at the 'attack' in any music piece at 'step level' in something like Audacity and see what 'rise times' are actually present in that signal.
Good luck in hunting for steps that are faster than 1/2 fs. You will find even fast transients in music take a few samples at least.
Just my thoughts...
I would be using that expensive DAC with upsampling only and aim for the highest possible bit rate / bit depth it allows.
XXHE seems to deliver a better upsampled 'image' than the default windhose drivers do...