Originally Posted by Mudge
The upsampler has no idea what it's upsampling which is part of the reason for the low pass filter. Upsampling is basically educated mathematical guess work where a few conditions are known. Nyquist theorem suggests that the highest frequencies present in the original signal are half the sample rate. When you calculate an intermediate sample you will almost inevitably introduce high frequency spuriae into the data. Therefore you can be pretty sure that everything in your new 88.2/96KHz data stream above 22KHz is noise, so you filter it out with a low pass filter.
The catch-22 is that with a limited amount of DSP power (the operations occur in real time) you can either use an algorithm that produces less noise and a less severe noise filter or a noisier algorithm and a severe noise filter. Or you can hope and pray that the losses from your tradeoff between the two are compensated by the gains from running your Delta-Sigma DAC at a higher rate.
Apologies if I've over-simplified upsampling, I'm aware that the reality is even worse than I've made out.
I understand basically how upsampling works which is why I asked why you said:

Originally Posted by Mudge
The G68 rolls off the mid-highs and emphasize the bass. This is to be expected, since most upsamplers don't have a flat frequency response.

By "rolls off the mid-highs", "emphasize the bass" and "don't have a flat frequency response" do you mean that it filters out frequencies above 22.05kHz? If not what do you mean by those phrases? Filtering out content above 22.05kHz occurs whether you upsample or not. I am also curious how you were able to determine that the upsampling algorithm in the G68 falls in the group of "most upsamplers" which don't have a flat frequency response. Which upsamplers fall within the group that do have a flat frequency response?

561, DSP5500s, DSP5000C, analog sides and rears