Coaxial S/PDIF Sony/Philips Digital Interface over Coaxial Cable (IEC 60958)

What coaxial S/PDIF is

S/PDIF, the Sony/Philips Digital Interface, is standardized as IEC 60958 type II (IEC 958 before 1998). It is built on the professional AES3 interconnect but swaps the 110 ohm balanced twisted pair and 3-pin XLR for an unbalanced 75 ohm coaxial cable and an RCA or BNC connector. The RCA jack is typically color-coded orange to separate it from composite video and analog audio. Optical TOSLINK carries the identical bitstream as light; Wikipedia states that signals over consumer TOSLINK are identical in content to those over coaxial connectors.

The same coax can carry a compressed bitstream. IEC 61937 defines how Dolby AC-3, E-AC-3, DTS, and other compressed multichannel formats ride inside the S/PDIF frame, which is how a DVD player's coax output delivers 5.1 to a receiver.

How coaxial S/PDIF works

The format has no defined bit rate. Data is sent with biphase mark code, which has one or two transitions per bit, so the receiver can pull the sample clock out of the signal itself. For IEC 61937 data the symbol rate is usually 64 times the sample rate, which gives 2.82 Mbit/s at 44.1 kHz, 3.07 Mbit/s at 48 kHz, 6.14 Mbit/s at 96 kHz, and 12.29 Mbit/s at 192 kHz (arithmetic from that 64x figure). Each audio block holds 192 frames, and each frame has two subframes carrying 20 or 24 bits of audio. IEC 60958-3 channel status can signal 192 kHz, so one link can carry two-channel 24-bit/192 kHz linear PCM when both ends support it.

Wikipedia's AES3 and S/PDIF comparison table lists a consumer S/PDIF output of 0.5 to 0.6 V peak-to-peak into 75 ohm, a minimum input level of 0.2 V, and a maximum distance of 10 m. The ratio of 0.5 V to 0.2 V is 2.5, or about 8 dB of margin before a receiver loses the signal. That margin is generous for a short cable.

Coaxial vs optical

Content is the same, so the choice is physical. Coax has no fiber to bend. The Audio Stuff glossary says coax commonly handles 24-bit/192 kHz while many optical ports top out at 24-bit/96 kHz. Optical's advantage is galvanic isolation: Wikipedia notes it can help address ground loops, which a copper shield cannot.

On timing, the evidence is mixed. Archimago's 2018 jitter tests found that TosLink tends to have higher jitter than coaxial S/PDIF, but that it depended on the DAC: a TEAC UD-501 improved with coax at 24 bits yet worsened at 16 bits, and an Oppo and an RME showed no significant difference. He concluded that clock fluctuations are primarily a function of the DAC and that the jitter was low and inaudible. Wikipedia's S/PDIF article puts the mechanism plainly: the receiver cannot control the data rate, so clock recovery may produce jitter, and a DAC without a stable clock reference adds noise to the analog output.

Distance is a wash for living-room use. Wikipedia's table gives S/PDIF a 10 m maximum and the TOSLINK range as 5 to 10 m. The professional unbalanced AES3 variant on BNC runs 1.0 to 1.2 V peak-to-peak and is listed for 100 m, ten times the consumer figure.

The cable: why 75 ohm matters here

Digital audio is a square-ish wave at megahertz rates, so the cable should be a true 75 ohm coax. Blue Jeans Cable specifies 75 ohm coaxial construction for its digital audio cables and offers Belden 1694A (solid core), Belden 1505F (stranded, high flex), and Belden 1695A (Teflon dielectric, plenum rated), with the 1694A, 1505F, and 1505A described as sweep-tested to 3 GHz. Belden's 1505F datasheet lists 75 +/- 3 ohm impedance, 17 pF/ft of capacitance, and two 94% tinned-copper braids.

Loss is a non-issue at audio distances. The 1505F is rated at 0.89 dB per 100 ft at 10 MHz, so a 6 ft jumper loses about 0.05 dB, far inside the 8 dB margin above. Reflections from a mismatched cable are the real risk. The Audio Stuff glossary says an ordinary RCA lead often works over a short run, but over longer runs its reflections can corrupt the stream. The same 75 ohm construction is what many sellers label a subwoofer cable (see subwoofer cable vs RCA cable).

Where it lives

Receivers are where coaxial digital inputs show up. The Denon AVR-X3900H lists two optical and two coaxial digital inputs, in addition to six HDMI inputs. Wikipedia describes the typical source as a sound card sending S/PDIF over coaxial cable with RCA connectors, and the 48 kHz format as the one used on DVDs and 44.1 kHz as the CD format. On the TV side, LG's 2025 C5 pairs a single optical output with eARC; see optical vs HDMI ARC.

Common confusions

Coax does not carry more formats than optical. The format ceiling is the S/PDIF bitstream, so TrueHD, DTS-HD Master Audio, and multichannel PCM need HDMI on both connection types. If a receiver shows a Dolby Digital or DTS badge off a coax input, that is the compressed-core path, not lossless. See bitstream vs PCM and LPCM.

Coax also does not break ground loops. An unbalanced coax shield ties the two chassis together, so hum on a coax digital link is a reason to try optical. The mechanics are in ground loop hum.