In the Roon online resource titled DSP Engine: Sample Rate Conversion and in reading outside of this forum there is often mention of any given DAC having a “Native Sampling Rate”. I take that to mean a sampling rate that, when fed to the DAC, is simply converted directly to analog with no additional “internal up/down sampling”. In other words, a rate at which the DAC is doing the least work and altering the signal the least.
First, is my understanding roughly correct? And, if so, what would that native rate be for the Lumin T2?
Yes, roughly. Many DACs have two internal up-sampling filters and only one can be bypassed. Lumin states that the T2 uses the ES9028PRO chip, which has an 8x FIR up-sampling filter and an 8x IIR one. The latter can’t be bypassed, not even with DSD, and it seems that the chip has to be explicitly configured to bypass the former. It’s unclear to me if the T2 does that at all, considering that it maxes out at 384kHz PCM, which is supported by the chip in non-bypassed mode. Your best bet would be to use 384kHz PCM or DSD, but you’ll still be at the mercy of the IIR. I don’t think it’s worth it.
For simplicity of discussion from a setting and SQ point of view, without going into DAC chip internals, please A/B compare DSP Engine off vs Roon upsampling to DSD512, or PCM352/384 and see what sounds best to you, for 44.1kHz tracks. I believe an approach like this is required instead of deriving theory-based SQ optimization based on assumption of who does what in better ways. Even if something is mathematically better does not necessarily mean you always get better SQ from it because SQ is subjective.
For users who did this test with our products, I have the impression that more users tend to prefer keeping sample rate as is without upsampling.
Please leave Lumin internal re-sampling to Off when using Roon.
When doing DSD upsampling or PCM upsampling tests for 44.1kHz tracks, check for bass (drum, percussion instruments), treble, imaging in complex music, etc. Your choice of linear phase filter (perhaps precise) vs minimum phase (perhaps smooth) also affects percussion instruments.