Internal or External Clock

[Ermos (Erno)]

This is how a dCS external clock works, as explained by @James

[…] it is worth noting that the use of a Master Clock in a dCS system does not replace the internal clock inside of the DAC. It simply acts as a stable reference for the DAC to lock itself to, and allows for DAC and source to be properly synchronised without issues such as intersymbol interference causing jitter within the audio data. The DAC’s internal clock still dictates when samples are converted, it simply adjusts its frequency over time to match that of the Master Clock. This means the DAC still benefits from having a high-quality clock close to the DAC circuitry. The clock directly controlling the audio is still part of a tightly controlled environment, while also being in sync with the rest of the system.

From:

dCS Ring DAC - A Technical Explanation

Part 4 – Clock Synchronisation There is a problem posed when multiple digital audio devices, each with their own internal clocks, need to work together. Take the example of feeding a CD transport into a DAC. The DAC has a buffer – a section of temporary memory which stores the audio samples it receives from the CD transport. The transport’s clock dictates when a sample is sent out to the DAC, and the DAC’s clock dictates when the sample is used and converted to an analogue voltage. In an ideal…

So, in a dCS system, there is synergy between the internal and external clock.