It’s the analog signal that carries the digital signal (voltage fluctuations) over the copper Ethernet cable.
Yes the data is transmitted accurately. However, we’re streaming music, not moving text documents over network. So in our systems the Ethernet cable is a digital cable. Just like any other cable - quality of the conductor, dielectric, shielding, connectors, etc. Everything matters for the quality of the signal received and processed by the component that’s fed by that cable.
Your system will reproduce music just fine with a cheap cable as long as it works. You don’t need to spend a ton in Ethernet cables.
If you have a revealing system you are very familiar with, you will hear differences between Ethernet cables. I hear it in my system.
How can different CAT5/6 cables affect sound.
While is is beyond doubt that analog cables affect sound quality and SPDIF, TOSlink and AES/EBU can effect SQ, depending on the buffering and clocking of the DAC, I am at a loss to find an explanation for how different CAT5 cables can affect the sound.
The signals over cat5 are transmitted using the TCP protocol. This protocol is error correcting, each packet contains a header with a checksum. If the receiver gets the same checksum then it acknowledges the packet. If no acknowledgement is received in the timeout interval the sender resends the packet. Packets may be received out of order and the receiver must correctly sequence the packets.
Thus, unless the cable is hopeless (in which case nothing works) the receiver has an exact copy of the data sent from the sender, AND there is NO timing information associated with TCP. The receiver must then be dependent on its internal clock for timing.
That is different with SPDIF, clocking data is included in the stream, that is why sources (e.g. high end Aurenders) have very accurate and low jitter OCXO clocks and can sound better then USB connections into DACs with less precise clocks.
Am I missing something as many people hear differences with different patch cords?