Do I need an expensive digital cable?

@audio-b-dog  With a USB connection into your Moon 280D, the data stream is asynchronous (untimed) until it reaches the DAC’s internal clock. With Toslink/coax/AES EBU/I2S, the source device (e.g. your CD player) holds the master clock, and the data transmitted to the DAC is synchronous (timed). Clock quality is most certainly a factor in the overall result, and another variable to consider as to why one type of connection to a DAC can bring a different result than another. 

The proper characteristic impedance for both coax cable and connectors is 75 ohms. Audio RCA connectors are typically 20-50 ohms, so a better coax cable will use connectors of the correct impedance. Proper impedance minimizes unwanted reflections. Where one is flexible about cable length, 1.5m (~5ft)  coax cable length has been demonstrated as theoretically "ideal" for reflection mitigation. Splitting hairs possibly, but worth a mention. 

@richardbrand you do have to remember though that streamers buffer the incoming data stream in memory. TCP is a clock-less protocol: by design it does not have nor need a physical clock, unlike USB etc. which do.

@devinplombier not entirely true. coax is a copper cable. It is susceptible to EMI and RFI. If subjected to either or both, the electrical noise may impact timing and interfere with data transfer. In addition the noise will make its way into the DAC which again will negatively impact sound quality.
Connections doing digital data transfer not impacted by emi and rfi are optical and fiber optic.

@audphile1 

I do apolcogize on the bit depth. It is a term chatgbt recognizes. I'll paste its answer below. I don't think that invalidates my discussion of what a bit is. We talk about "words" a 16 bit word length or 24 bit word length. Thos bits are still on/off switches, which is how digital works. I think we were talking past each other. On the other issue, chatgbt has assured me more than once that the asynchronous DAC in the Moon 280D will supercede the transport's clock. It was chatgbt who told me to try an RCA interconnect. And the interconnect works. I'll compare it to a cheap digital 75 ohm cable. Here is chatgbt's answer on "depth." i think depth talks about the effect of word length, but not what a word length is. That's where we were talking past each other.

What Bit Depth Actually Is

Bit depth determines how precisely the amplitude (loudness level) of a sound wave is measured at each sample in a digital recording.

Think of digital audio as:

• Sample rate = how often you take a picture of the waveform (time resolution)

• Bit depth = how finely you measure how tall that waveform is (amplitude resolution)

Bit depth controls the number of possible volume steps between silence and full scale.

Simple Analogy

Imagine a staircase.

• 8-bit audio = very few steps. Big jumps between levels.

• 16-bit audio = many more steps. Smaller jumps.

• 24-bit audio = an enormous number of tiny steps. Almost continuous.

The more steps you have, the smoother quiet details and dynamic transitions sound.

The Math (Important but Simple)

Each additional bit doubles the number of amplitude values.

• 16-bit = 65,536 possible levels

• 24-bit = 16,777,216 possible levels

That’s not a small difference. That’s massive.

On the other issue, chatgbt has assured me more than once that the asynchronous DAC in the Moon 280D will supercede the transport’s clock.

@audio-b-dog I did some reading on this unit, which claims to be asynchronous, although it doesn't go into further detail. It would be unusual if it is. The question to ask Simaudio directly (not ChatGPT) is if the unit is asynchronous across all digital inputs.