Digital
Interface clocking
How each digital connection handles the clock — the only place interfaces truly differ.
Simple
Digital connections all carry the same bits, but they differ in who keeps time. With asynchronous USB the DAC uses its own clock, so the source and cable can't disturb timing at all. Coax and AES send the clock mixed into the data, and the DAC has to recover it. Optical adds electrical isolation (no hum loops) but tops out at lower sample rates. None of this changes a working link's sound — it's about engineering margins.
The theory
S/PDIF (coax/optical) and AES3 are self-clocking: the receiver extracts the sample clock from the biphase-mark-coded stream, so timing quality depends on the receiver's clock-recovery (a PLL, or better, a FIFO buffer re-timed by a local oscillator — "reclocking"). Asynchronous USB inverts the relationship: the DAC's clock pulls data from the host at its own pace, making source jitter structurally irrelevant. I2S carries clock and data on separate lines — transparent, but only standardized for very short runs. TOSLINK's optocouplers give inherent galvanic isolation but limit reliable bandwidth to ~96 kHz on many parts.
For experts
The engine reports the topology of the connection in use and ranks alternatives on architecture, not folklore: async USB (source-clock-immune) ≥ reclocking S/PDIF inputs (recovered clock never reaches the converter) > PLL-cleaned S/PDIF > plain clock recovery, with a bonus for galvanic isolation where ground loops are plausible and a penalty where the connector's bandwidth caps the source's format. What it will not do is translate any of this into a verdict downgrade: J-test artifacts on competent modern receivers measure 110+ dB down, below every published audibility threshold. The ranking exists because when you have three ways to plug the same two boxes together, one of them usually has more engineering margin — not because the others sound different.