TOSLINK vs coaxial: how S/PDIF digital audio connections work

What S/PDIF carries, on wire or light

S/PDIF, the Sony/Philips Digital Interface, is the consumer version of a digital audio link defined in IEC 60958. It sends one stream: two channels of PCM at up to 24 bits, or compressed surround such as Dolby Digital and DTS, along with status bits that describe the stream, its sample rate and copy flags among them. The clock travels inside the data. Each bit cell starts with a transition and a 1 adds another in the middle, so the receiver can recover the timing from the signal itself. At 44.1 kHz the link runs at about 2.8 million bits a second.

That stream goes out in two physical forms. Coaxial S/PDIF is an electrical signal of about half a volt on a 75-ohm cable, usually with RCA plugs and sometimes BNC. TOSLINK, a name that comes from Toshiba, turns it into pulses of red light from an LED, sent down a plastic or glass fiber between square plugs. If both links deliver it without errors, the DAC receives identical numbers either way. The professional sibling, AES3 or AES/EBU, carries nearly the same data as a balanced signal on 110-ohm cable with XLR plugs.

What each link can carry

Coaxial inputs and outputs commonly handle 24-bit/192 kHz. Optical is where limits appear. A 192 kHz stream runs more than four times as fast as a 44.1 kHz one, and the LEDs and receivers in many optical ports weren't built for that speed, so plenty stop at 96 kHz. Others go all the way: the SMSL PS100 takes 24-bit/192 kHz over optical as well as coaxial. Check the spec sheets at both ends before relying on optical above 96 kHz.

Neither form carries lossless surround. Dolby Digital and DTS fit because they're compressed, while uncompressed multichannel PCM and the lossless formats on Blu-ray need HDMI. DSD travels only packed inside PCM, which the DSD explainer covers. Several DACs reviewed here also take HDMI ARC from a TV, which carries the same kind of stream over the HDMI cable, and its enhanced version, eARC, has room for lossless surround.

Optical links and ground loops

A fiber carries light and no current, so an optical link leaves the two devices electrically separate. A coaxial cable's shield usually joins their grounds, and between a TV or a computer and a hi-fi system that connection can close a ground loop and bring hum or buzz with it. Switching that one link to optical removes the path, which makes it one of the simplest fixes for a hum that appears only when a digital source is connected.

The same property keeps a source's electrical noise out of the DAC. The SMSL PS200 review suggests its optical input for a noisy source such as a computer's motherboard, since light can't carry electrical noise across. Some coaxial inputs are isolated with a small transformer, but you can't tell that from the socket.

Timing, and whether optical sounds different

Both links embed the clock in the data, so the DAC has to recover the timing from the incoming signal, and the jitter explainer covers how that timing can reach the converter. The optical parts switch more slowly than a coaxial line's electrical edges, and a slower edge is easier for noise to shift in time, so an optical input can arrive with more timing variation than a good coaxial one.

Measurements show that difference at the output of some DACs. Whether it is audible through a well-designed DAC is disputed, and how much reaches the output depends on the design: a DAC that buffers the incoming data and replays it on its own clock leaves little of the source's timing behind. The Henry Audio DA 256 works that way on both inputs, and its review still preferred USB by a small margin, hearing coax and optical as a touch mellower.

Cables, plugs and run lengths

A coaxial digital link wants a real 75-ohm cable. An ordinary analog RCA lead often works over a short run, but its impedance isn't controlled, and over longer runs the reflections that causes can corrupt the stream. With the right cable, coaxial copes with long runs well.

Plastic fiber, the usual kind, loses light over distance and at sharp bends, so it suits the short runs of a TV cabinet or a desk, and glass fiber goes further. New optical cables often come with small caps over the tips, which have to come off before the plug will click home. Some laptops and portable players hide a mini-TOSLINK socket inside the 3.5 mm headphone jack, which takes an adapter or a cable with the small plug.

Data errors are easy to spot: a marginal connection gives dropouts, clicks or silence as the receiver loses lock. A cable that delivers the stream without them has passed the data intact, and what's left is timing, where the DAC's input decides how much the link contributes.

Choosing a connection for each source

• From a TV, use optical, or HDMI ARC where both ends have it. TV sound is usually 48 kHz, well within optical's range, and the fiber keeps the TV's ground out of your hi-fi. The Taga Harmony TAV-500B v.2 active speakers take this route, with an optical input their review lists for CD or DVD players, consoles and TVs.
• From a CD transport, either works, since a CD's 44.1 kHz sits within both. Coaxial reaches further, and optical breaks a ground loop.
• From a computer, USB is usually the better choice: an asynchronous USB input lets the DAC's clock set the timing and takes higher sample rates. Optical is the alternative when the computer brings noise with it, as the PS200 review suggests.
• For 176.4 or 192 kHz, use coaxial or USB unless both optical ports are rated for it.

When you compare converters in the DAC guide, count their optical and coaxial inputs against the sources you plan to connect.