Digital Connections: USB, S/PDIF, AES and I²S

The back of a modern DAC offers a row of sockets that all carry the same thing — a stream of digital audio samples — and yet they are not interchangeable. They differ in the maximum resolution they will pass, in whether they electrically isolate the two devices, and in the single most consequential difference of all: which end of the link owns the clock that determines when each sample is converted. This guide takes them one at a time and ends with a practical order of preference.

S/PDIF Coaxial

Coaxial S/PDIF sends a self-clocking serial stream down a single 75 Ω cable terminated in an RCA connector. Because the clock is embedded in the data, the receiving DAC has to extract it — usually with a phase-locked loop — and any timing irregularity in the transmitted stream or the cable can appear as jitter at the point of conversion.

In practice, coaxial is a robust and capable connection. It comfortably handles 24-bit/192 kHz and often more, it is electrically simple, and its bandwidth is generous. Two details matter: the cable should be genuine 75 Ω video-grade coax rather than a repurposed analogue interconnect, because impedance mismatches cause reflections; and because the connection is electrically continuous, it carries ground between the two devices and can therefore participate in a ground loop.

S/PDIF Optical (TOSLINK)

Optical S/PDIF carries the same data format over light, through a plastic or glass fibre. Its defining advantage follows immediately: because nothing conductive crosses the link, it provides complete galvanic isolation. No ground connection, no ground loop, no electrical noise conducted from a computer or television into the DAC. When you have a hum problem between two mains-powered devices, optical often solves it outright.

The trade-offs are bandwidth and timing. The optical transmitter and receiver add conversion steps, and cheap plastic fibre limits bandwidth — most TOSLINK implementations top out at 24-bit/96 kHz, with some reaching 192 kHz over short, good-quality runs. Optical also has a reputation for higher jitter, largely from the rise and fall times of the optical receiver, though a DAC with good reclocking makes this a non-issue.

AES/EBU

AES/EBU (formally AES3) is the professional cousin of S/PDIF: the same fundamental data format, but sent balanced over a 110 Ω twisted pair with XLR connectors, at a higher signal level. The balanced transmission gives it excellent common-mode noise rejection and allows much longer cable runs — tens of metres rather than a few — which is why it dominates in studios.

For domestic use, AES/EBU is a solid choice where both devices offer it. It handles high sample rates comfortably, the locking XLR connector is mechanically superior to RCA, and the balanced line is more robust in electrically noisy environments. It does not, however, solve the ground-loop problem: like coaxial, it is an electrical connection with a ground path. And like coaxial, the DAC still has to recover the clock from the stream.

USB Audio, and Why Asynchronous Changed Everything

USB is the most capable of the common connections and the one that most repays understanding. Early USB audio used adaptive mode, in which the DAC slaved itself to the computer's timing — an arrangement that deserved its poor reputation, because a general-purpose computer is a poor master clock. Modern implementations use asynchronous mode, and the relationship is reversed: the DAC's own clock governs conversion, and it tells the computer when to send more data.

That inversion is the key point. In asynchronous USB, the timing of the source stops mattering, because the DAC is no longer trying to follow it — buffering absorbs the arrival jitter and the DAC's local clock determines when samples are converted. This is why asynchronous USB is generally the best-performing input on a DAC that has one, and why it supports the highest formats: 32-bit/768 kHz PCM and DSD512 are within reach, far beyond what S/PDIF carries.

USB's weakness is electrical. It is a computer bus carrying power and ground alongside data, and a computer is a noisy environment. Good DACs address this with galvanic isolation on the USB input; where they do not, the noise can find its way into the analogue stages. If you are choosing a DAC for USB use, isolation on that input is worth looking for specifically.

HDMI and I²S

HDMI carries audio alongside video and is essential for multichannel formats and for anything routed through a television or AV receiver. For two-channel hi-fi, it is usually a convenience rather than a preference: the audio clock is derived from a link designed principally for video, and the results depend heavily on implementation. Where a source only offers HDMI, it works perfectly well; where alternatives exist, they are usually simpler.

I²S is a different proposition. Inside almost every digital source and DAC, audio moves between chips on an I²S bus, which carries data, bit clock and word clock on separate lines. Because the clock never has to be embedded in and recovered from the data, the jitter problem that S/PDIF creates simply does not arise. Some manufacturers expose this bus externally, usually on an HDMI-style connector, so a matching transport and DAC can be linked I²S to I²S.

The catch is that there is no standard for the pinout. Different manufacturers assign the signals differently, so an I²S connection between two brands may not work — and in some cases can be electrically unhappy. Treat external I²S as an excellent option within a manufacturer's own ecosystem and check compatibility carefully otherwise.

Choosing an Input in Practice

Once the properties are clear, a sensible default order emerges — with the caveat that a well-implemented lesser input beats a poorly-implemented better one every time.

• Computer or phone as source: asynchronous USB, preferably into a DAC with galvanic isolation. Optical if you have ground noise you cannot otherwise fix.
• Network streamer as source: whichever of USB, coaxial or AES/EBU the pair supports best; the streamer has already removed the computer from the equation.
• CD transport: coaxial or AES/EBU; I²S if the transport and DAC are from the same manufacturer and designed for it.
• Television or console: optical, which isolates as well as connects. HDMI if you need multichannel or lip-sync handling.
• Long cable run: AES/EBU for its balanced transmission, or optical if isolation matters more than bandwidth.

A last practical note on cables: for coaxial use true 75 Ω cable; for AES/EBU use 110 Ω cable rather than a microphone lead, which is a different impedance; for USB, a decent standard cable of a sensible length is all that is required. Digital cable choice is a matter of meeting the specification, not of exceeding it.

Quick Reference: Digital Connections Compared

Connection	Typical Maximum	Clock Owner	Ground Isolated?
S/PDIF coaxial (RCA, 75 Ω)	24-bit/192 kHz and above	DAC recovers from stream	No
S/PDIF optical (TOSLINK)	24-bit/96 kHz, sometimes 192 kHz	DAC recovers from stream	Yes — fully
AES/EBU (XLR, 110 Ω)	24-bit/192 kHz and above	DAC recovers from stream	No (but balanced)
USB (asynchronous)	32-bit/768 kHz, DSD512	DAC's own clock	Only if the DAC isolates it
HDMI	High, including multichannel	Recovered from the link	No
I²S (external)	Very high	Separate clock lines	No; pinout is not standardised

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