Digital audio coaxial cable impedance explained

Your streamer drops out for half a second every few minutes, and the spare cable you found measures fine on a multimeter. It reads a fraction of an ohm end to end, so the copper is continuous and the plugs are not loose. What the meter cannot show is characteristic impedance, the property that decides whether a digital pulse arrives with clean edges or bounces off the far end of the line. For S/PDIF over coax that number is 75 ohm, and a cable that is not built for it can pass a continuity test while still corrupting the data clock.

Characteristic impedance is not DC resistance

Put a multimeter across the center pin and the shell of a coax cable and you read a fraction of an ohm. That is DC resistance: the opposition a steady current meets in the copper. Characteristic impedance is a different quantity. It is the ratio of voltage to current in a wave traveling along the line, and it is fixed by conductor diameter, the spacing to the shield, and the dielectric material. A finished 75 ohm coax typically measures a few tenths of an ohm over a short run while still presenting 75 ohm to a fast pulse. Confusing the two numbers is the most common reason people conclude that any cable with continuity is good enough for a digital link.

What a resistance reading really proves

A low DC resistance reading confirms only that the copper is continuous and the plugs are properly crimped or soldered. It says nothing about the geometry that sets impedance. A cable with a broken shield braid can still pass a continuity test on the center conductor, and a cable with one stray strand bridging the shield to the pin can read correctly at rest and fail once signal is applied. Treat continuity as a pass or fail check on assembly quality, never as a statement about impedance.

Where the 75 ohm number comes from

The 75 ohm value is a compromise inherited from broadcast video and radio frequency practice: it keeps loss low for a given dielectric size and matches equipment that already used it. S/PDIF adopted the same 75 ohm coax and an RCA-style plug so consumer gear could reuse existing video cables and connectors. That history explains why a digital audio coaxial cable looks like a video cable and why its impedance figure matters far more than its gauge.

Why a mismatch corrupts the data clock

S/PDIF is a square-wave signal that carries a clock and the audio data together, with edges in the low nanosecond range. At those speeds the cable behaves as a transmission line, not as a simple wire. When the impedance changes at a connector, a splice, or anywhere the cable was never 75 ohm, part of the energy reflects back toward the source and part continues. The receiver sees a smeared edge instead of a clean transition, and the timing window it needs to decide whether a bit is a one or a zero shrinks. Short, quiet runs usually absorb the damage. Long runs, high sample rates, and noisy grounds push the same link into audible dropouts. The cable is not filtering the sound; it is delivering a stream the receiver cannot trust.

How the failure actually presents

Failures are rarely subtle distortion. A marginal link either locks or it does not: the receiver mutes, clicks, or drops the stream for a fraction of a second and then recovers. Some processors show a brief error message on the display. If the same source and cable work at 48 kHz but not at 96 kHz, signal integrity is the first thing to suspect, not the source firmware.

Reflection grows with every joint

Each additional connector, barrel adapter, or wall plate adds another impedance discontinuity. One good joint costs little. Three cheap joints in series on a ten meter run can cost more margin than the cable itself, which is why a chain of short extensions often behaves worse than a single longer cable of the correct type. Every adapter you add is also a mechanical load on a small plug, and a plug that sags under its own weight will eventually lose contact on the shield.

50 ohm coax, speaker wire, and other substitutes

A 50 ohm lab coax is a good cable aimed at a different job. Used for S/PDIF it creates a deliberate mismatch at both ends, which costs margin you may not notice on a one meter run and will notice on an eight meter run. Speaker wire is further off: it is unshielded, usually two parallel conductors, and its impedance is nowhere near 75 ohm. It can appear to work because the receiver applies error correction and because very short runs tolerate a great deal of abuse. The risk is intermittent, hard to reproduce, and easy to blame on the converter or the DAC when it finally shows up. If you only have a two meter run and the system is stable, the substitute may hold indefinitely; if the run is long or the rack is noisy, replace it.

Reading a coax spec sheet in practice

A useful spec lists impedance in ohm, capacitance per unit length, shield coverage, and conductor size. Impedance should read 75 ohm with a tolerance, often plus or minus 3 ohm. Capacitance in the low 60 pF per meter range is typical for a 75 ohm polyethylene dielectric; a far higher figure suggests a cable built for analog line level rather than a digital line. Shield coverage above 90 percent is a reasonable target for a run that passes near power cables. Conductor size, usually quoted in AWG, matters less than people expect at these lengths: 22 to 24 AWG is normal for a flexible consumer coax, while a stiffer 18 AWG build mainly buys mechanical durability. Treat vendor numbers as typical rather than guaranteed unless the datasheet names a test method.

Length changes how much mismatch you can afford

Every joint and every impedance error adds reflection, and reflections accumulate over distance. That is why a cable which is marginal at one meter can be fine at three meters and unusable at ten. Practical coax S/PDIF runs are typically quoted in the 5 to 10 meter range, and the ceiling drops as the sample rate rises. If you need a longer run, the answer is not a thicker cable with the wrong impedance; it is a correctly rated 75 ohm line, better connectors, or a repeater that regenerates the signal before it degrades. Keep the joint count low and keep the run away from mains cabling, and a run that already works at three meters will usually keep working when you add a fourth.

Impedance by cable type and what it means for S/PDIF

Real-World Application Scenarios

Three situations come up most often. In a project studio, a 50 ohm patch cable borrowed from the test bench introduced a faint tick every few minutes until it was swapped for a 75 ohm line. In a living room with a ten meter run to a projector shelf, a thin RCA cable produced dropouts at 96 kHz that disappeared once the run was shortened and re-terminated. In a hotel lobby system, a subwoofer cable was pressed into service for a coax digital link and held for months before failing in summer heat. Each case traced back to the cable, not the source.

Related Guides on This Site

For the electrical detail behind the number, read our guide to the 75 ohm coaxial digital audio cable. If you are still choosing a product family, the coaxial digital audio cable guide covers construction and shielding. The S/PDIF coax overview explains formats and rate ceilings, and our note on whether all coaxial cables are digital clears up the labeling confusion.

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