There’s a lot of subtle incorrectness in your paragraph…let me clarify.
This is the case with every use of these interfaces, nothing special about Roon here, correct?
You are correct that Roon does not change how USB or S/PDIF works. No, it is not the case that every use of these interfaces works the same way.
Remember, we make software and protocols, not hardware, so the relevant comparisons here are against other software and protocols.
Some streaming protocols, including AirPlay and Songcast, use the system clock on the media server to determine the rate that the stream is transmitted to the playback device.
RAAT exposes the device’s clock over the network so that Roon can slave to it. In the case of a USB device, it’s exposing the clock embedded within the USB device. In the case of a S/PDIF transmitter, it is exposing the clock in the transmitter. In the case of a Roon Ready that doesn’t use USB or S/PDIF internally, it’s exposing the DAC’s internal clock.
Whenever you have a clock generating content at one rate, and a second clock receiving it at a different rate, something must be done to reconcile the discrepancy. Two clocks will always keep time at a slightly different rate. In most cases, the rate changes meaningfully based on ambient temperature.
Resolving that discrepancy involves engineering tradeoffs. Expensive systems use PLL to “bend” the receiving clock to match the sender. Less expensive systems, including most consumer-grade stuff, perform lossy signal processing (either re-sampling or “stuffing” and “dropping”) to reconcile the differences.
So, when you compare RAAT (a network protocol) to AirPlay (another network protocol), both sitting on top of the USB device, the AirPlay configuration has two clocks and a lossy clock drift compensation mechanism somewhere in the system, and RAAT is using a single clock and doing bit-perfect playback.
That is, my understanding is that the USB receiver chip will “reclock” the signal
The word re-clocking implies that there are multiple clocks. There’s no necessary **re-**clocking with async USB devices. The USB device contains a clock, and requests data from the computer based on that clock’s rate.
whereas the SPDIF signal will be the driver of the DAC clock.
This is only true of the cheapest DACs. Almost everything used in a HiFi/Audiophile context is designed to perform some form of re-clocking or asynchronous resampling of S/PDIF input.