USB vs S/PDIF for DACs: does the digital input matter?

A customer used to come into my Fitzroy shop every few months with a new theory about his digital front end, and the one that stuck with me was: "Priya, I switched from USB to optical and the soundstage opened right up." I didn't have the heart to tell him his DAC's optical input was capped at 96kHz and he'd been playing 192kHz files through USB the whole time before that. He'd actually changed two things at once and credited the wrong one. That conversation is basically the entire USB-versus-S/PDIF argument in miniature: real engineering differences exist, but most of what people hear is something else entirely.

I run valves and vinyl at home, so I'll admit digital isn't my first love. But I've got a streamer feeding a DAC in the system too, because like it or not that's half of modern listening, and the question of which cable carries the bits comes up constantly from readers. So let's actually sort out what's true, and what's just a story we tell ourselves after we've fiddled with a setting menu for twenty minutes.

What USB and S/PDIF are actually doing

S/PDIF, the family that includes coaxial (RCA or BNC) and TOSLINK optical, was standardised back in the 1980s as a consumer version of the professional AES/EBU interface documented by the Audio Engineering Society. It's a one-way street: the transport sends a continuous stream of data with an embedded clock, and the DAC has to extract that clock on the fly and lock to it. That extraction process is where jitter, timing error in the digital signal, can sneak in. A cheap coax cable, a dodgy connector, even cable length can all nudge the numbers slightly. This is a real, measurable phenomenon, not audiophile folklore. Optical TOSLINK has the added quirk of a light-to-electrical conversion at both ends, which historically made it a bit noisier than coax, though it does offer full electrical isolation, which matters more in certain home cinema setups with ground loop problems.

USB audio works differently. In the async mode that essentially every decent DAC has used since the 2010s, the DAC's own internal clock sets the pace and the computer or streamer buffers and drip-feeds the data on demand. The USB Implementers Forum covers how this negotiation happens in its audio device class specifications. Because the DAC's clock is already free of recovery timing issues, async USB sidesteps the main vulnerability that plagues S/PDIF. This is why most modern DAC designers treat USB as the reference-quality input and optical as the one with real bandwidth ceilings.

It's worth sitting with why async matters so much, because a lot of customers assume USB is better simply because it's newer. In the older "adaptive" USB mode, still found on some ten-plus-year-old interfaces, the DAC actually took its timing cue from the host computer's clock, the same basic weakness as S/PDIF. Async flipped that relationship. The DAC says "send me data when I'm ready for it", the host buffers accordingly, and the DAC's own crystal oscillator, often a genuinely good one on anything above entry level, does the timing work. That's the actual engineering win, not the connector shape.

The bandwidth ceiling nobody mentions

Here's the bit that actually trips people up in practice, and it's not subtle once you know it: standard TOSLINK optical is limited to 24-bit/96kHz, a constraint rooted in the format's original Toslink specification from Toshiba in the early 1980s, detailed on its Wikipedia entry. Coaxial S/PDIF can usually stretch to 24-bit/192kHz depending on implementation, though it's not guaranteed across every DAC. USB has no such inherent ceiling; it's limited only by what the DAC chip and USB receiver support, which these days is often well into DSD and 32-bit/768kHz territory on higher-end gear.

So if you're streaming a Qobuz hi-res album at 24/192 through an optical connection, your DAC is either going to refuse the signal outright or silently downsample it, and you won't necessarily get an error message telling you so. That's almost certainly what happened to my customer. He wasn't hearing a magical optical improvement; he'd simply started getting his actual purchased resolution through USB after unknowingly being capped for months. I've written before about how your streaming app's displayed sample rate rarely matches what your DAC is actually receiving, and this is exactly the kind of silent mismatch that causes it.

Worked example, since this is easier with actual numbers in front of you. Say you've got a 24-bit/176.4kHz file, a DSD64 remaster, or anything from a high-resolution reissue house. Run that through optical into a DAC with a 96kHz ceiling and one of two things happens: either the DAC mutes and shows an error, which at least tells you something's wrong, or worse, some streamers will quietly resample the file down to something the optical link can carry, feed it through without complaint, and you'll never know unless you go digging in a settings menu. I've seen both behaviours on gear that otherwise measures beautifully. The only way to know for certain is to check the sample rate indicator on the DAC's own front panel, not the app, while a hi-res track is actually playing.

Where the real-world difference actually lives

In a quiet room, with a well-engineered modern DAC and a half-decent cable either way, I don't reckon most listeners will reliably pick USB from coax in a level-matched blind comparison, provided both are carrying the same resolution file. The engineering has matured. Chip-level jitter rejection in current DAC designs, the kind covered in our piece on DAC chips versus DAC engineering, is good enough now that the theoretical jitter advantage of async USB rarely translates into an audible one on a well-designed product.

Where it does matter is at the edges. Cheap, no-name DACs with poorly implemented S/PDIF receivers can genuinely sound softer or less focused through coax than USB, because their clock recovery circuitry is the bit they skimped on. Laptops and PCs with noisy USB power rails can, in rare cases, introduce their own interference into a USB-fed DAC, which is part of why isolated or battery-powered interfaces exist. And optical remains the one to avoid if you're chasing hi-res resolution, full stop, unless you only ever play CD-quality material anyway.

There's a middle category worth naming too: the twenty-year-old CD player with a coaxial digital out, sitting in plenty of Melbourne lounge rooms still, bolted to a new DAC bought on a whim. That pairing is almost always fine. CD is 16-bit/44.1kHz, comfortably within coax's headroom, and the transport's job, spinning a disc at a constant rate and reading error-corrected data, is a solved problem from decades ago. I wouldn't tell anyone with that setup to go buy a USB-equipped transport just for the sake of it. The money's better spent on a cartridge, honestly, if we're talking priorities.

Home cinema changes the calculus

This whole conversation shifts once you're talking HDMI-based home cinema gear rather than a dedicated stereo DAC. Your AV receiver or processor is handling object-based surround formats over HDMI, and that's a different beast entirely from a two-channel S/PDIF or USB conversation. If you're trying to get a separate stereo DAC into a home cinema chain, you'll run into the limitations I covered in HDMI eARC and a separate DAC: your home cinema's digital compromise, and honestly that's a messier problem than anything in this article. Multichannel bitstreams, lip-sync handshakes and HDCP all add complications that a simple stereo DAC input was never designed to deal with, so don't assume the USB-versus-coax logic above maps cleanly onto a theatre setup.

What I'd actually tell you to do

If your DAC has a USB input and you're using a computer or network streamer as the source, use it. Not because coax is broken, but because USB removes resolution ceilings and generally represents the more modern, better-supported engineering path on current DAC designs. The RME ADI-2 DAC FS (check price), for instance, treats USB as its primary high-resolution path for good reason, and gear like the Chord Electronics Mojo 2 (check price) was designed around USB's async advantages from the outset.

If you're feeding a DAC from a CD transport or an older streamer that only outputs coax, don't panic and don't go buying an exotic digital cable to fix it. A competent coaxial connection on a well-built DAC is genuinely fine for the vast majority of listening, and the money is better spent elsewhere in the chain. I'd rather someone spend $150 on a better phono stage or a proper equipment rack than on an "audiophile" digital cable promising to fix jitter that their DAC's own clock has probably already dealt with.

A practical checklist, for anyone setting a system up fresh. First, confirm what resolution your actual music library sits at; there's no point agonising over 192kHz transmission if your whole collection is ripped CDs. Second, check your DAC's input sample rate display while something plays, not the source app's claim. Third, if you've got both USB and coax available and you're chasing the last few percent, try both with a level-matched comparison over a week of normal listening rather than a rushed A/B on day one; ear fatigue and expectation bias do more damage to these comparisons than any cable ever could. Fourth, only consider optical if isolation from electrical noise is the actual problem you're solving, not resolution.

Avoid optical if you can, purely on the resolution ceiling, unless electrical isolation from a noisy source is your specific problem to solve, in which case it's still a legitimate tool. That's a genuinely useful exception, not a cop-out: some AV setups with ground loop hum benefit enormously from the electrical break that TOSLINK provides, even at the cost of bandwidth. I've recommended it myself to a customer on Smith Street running a turntable's phono preamp and a DAC off the same noisy power board, where the hum disappeared the moment we swapped coax for an optical link between two components, and nobody missed the extra resolution on what was, admittedly, a mostly CD-quality collection anyway.

The bit that actually matters more

Here's my mildly contrarian take, and it's one I'll defend over a coffee at Vinyl Diggers any day: the input type on your DAC is nowhere near as important as what's feeding it. A beautifully implemented USB connection from a poorly configured streamer, playing a lossy file through an app that's secretly resampling everything, will sound worse than a humble coaxial link from a transport doing bit-perfect output. We've covered how bit-perfect playback depends on software settings well upstream of the cable, and that's where I'd put my attention first. The connector is the last ten centimetres of a signal chain that's usually ten metres long. Worry about the cable last, not first.

And if you take nothing else from this, take the troubleshooting order. When something sounds off in a digital chain, check the file resolution first, check the software's output settings second, check the DAC's actual received sample rate third, and only then start thinking about the physical connection. Most of the "my new cable transformed everything" stories I hear across the counter collapse the moment you walk back through those first three steps, exactly the way my Fitzroy customer's did.

If you want a sanity check before you spend anything, it's worth reading our rundown of the best DACs and network streamers currently worth owning, because the input flexibility on offer varies more between models than most spec sheets let on.

— Priya Anand, Vinyl & Valves, Sound Technology