09-25-15: Robertsong
Okay, that makes complete sense now. Thanks!
So even with a NOS DAC I assume there is still benefit of using hi-res files? Why would somebody want to up-sample redbook in their software (foobar, JRMC, Amarra, etc.)? I haven't tried this in a few years but I recall that I preferred the sound w/o the upsampling (in JRMC).
Hmmm.
when you input 16/44.1 into the NOS DAC, the output contains not only the converted analog equivalent but also all the clock energy at high frequencies above 22.05KHz to 60Khz, 100KHz & beyond albeit in decreasing amplitude as the frequencies keep getting higher. By using LPF from a tube buffer or a sinc filter some of this noise is reduced but not eliminated. There could be sufficient energy in the higher freq for the power amp to amplify it & create distortion thru the speakers.
You can eliminate this by upsampling redbook by, say, 4X to 176.4KHz using software (like the examples you gave). Now the noise will contained in frequencies 88.2KHz (Fs/2) & above rather than 22.05KHz & above. By upsampling you just moved the problem up in frequency to a place where most power amps are likely to have very little power gain hence the probability of distortion is much, much reduced. Additionally you can employ an analog LPF with a -3dB cut-off at a much higher frequency, say, 44.1KHz (or even higher since the software upsampling has ensured there is no noise in the 0-88.2KHz region) such that you do not roll-off any of the high freq response of the DAC.
Since the digital signal is upsampled in software & given your NOS DAC can accept 176.4KHz you can feed this signal directly into the NOS DAC without any hardware digital filtering.
You reap all the benefits of the NOS DAC & you dont have to worry about high freq noise.
That's why one would want to upsample redbook in software. Hope this makes sense.....
