Quick Answer: Coax S/PDIF carries 192 kHz PCM to about 10 m on 75 ohm RG6, and 3 to 5 m on thin RCA cable. TOSLINK holds 96 kHz PCM over 5 to 10 m of 1 mm plastic fiber, with glass fiber reaching 30 m. The coax limit is reflection and jitter, not level loss. The optical limit is attenuation and pulse widening in the fiber. Both are rules of thumb from field practice, not published standards. The limits also depend on the transmitter, not only on the cable length. In practice, treat both figures as ceilings.
Digital audio cable ratings are rarely about bandwidth. A 192 kHz stereo S/PDIF stream runs at 12.288 Mbit/s, which any 75 ohm cable passes over 100 m. What fails first is timing: reflections at the connectors and a receiver that recovers its clock from the data edges. Once the eye closes, the link does not degrade gracefully, it clicks. The numbers below follow from that failure mode.
Coax S/PDIF: 10 m Is the Practical Ceiling
A coax S/PDIF link is a 75 ohm unbalanced line driven at 0.5 V peak to peak. A proper S/PDIF cable is 75 ohm coaxial and loses almost nothing at 12 MHz, roughly 0.05 dB per meter. What matters is return loss at the RCA plug, where the geometry rarely holds 75 ohm for more than a few millimeters.
What limits coax
Two reflections and a jitter budget. A mismatch at each end produces echoes that land on the following bit, and the receiver’s clock recovery translates that into jitter. Longer cable does not fix it, because the echo arrives further from the edge and smears across more of the bit. This is why a 15 m run of generic RCA cable can fail while a 100 m run of proper RG6 passes.
TOSLINK: 5 to 10 m of Plastic Fiber
TOSLINK uses a 1 mm PMMA core, a 660 nm LED and a photodiode. Plastic fiber attenuates about 0.2 dB per meter and spreads the pulse, so a 10 m cable already looks like a low pass filter to the receiver.
What limits optical
Attenuation plus modal dispersion. The LED launches light into many modes, and the slow ones arrive late and widen every pulse. Consumer transmitters are built to a 96 kHz PCM budget, which is why 192 kHz over TOSLINK often works on a 1 m cable and drops out on a 5 m one. Glass fiber cores solve the attenuation, not the transmitter’s rise time.
Length Limits at a Glance
The figures below assume a receiver that meets its own specification. A marginal transmitter shortens every number.
| Link | PCM rate | Practical length | Limiting factor |
|---|
| Coax, thin RCA | 192 kHz | 3 to 5 m | Return loss at plug |
| Coax, RG6 | 192 kHz | 10 m | Jitter from echoes |
| TOSLINK, plastic | 96 kHz | 5 to 10 m | Attenuation, dispersion |
| TOSLINK, glass | 96 kHz | up to 30 m | Transmitter rise time |
| AES3, 110 ohm | 192 kHz | 100 m | Balanced drive level |
Extending Beyond the Limit
Do not add a longer cable and hope. Convert instead. A 75 ohm balun pair sends S/PDIF over Cat5e to 100 m because the twisted pair is driven balanced.
The cheap workaround is to keep the digital run short. Put a DAC or an active speaker at the source end, or an HDMI audio extractor near the amplifier. Before any of that, check the lead: half the field failures are a 50 ohm or unscreened cable on a 75 ohm link, which is where the digital coax guide starts.
Choosing Cable for a Fixed Install
For runs under 5 m, any well-made 75 ohm RCA cable with a molded plug will do; for anything permanent, prefer a solid center with foil plus braid. Beyond 5 m, use RG6 quad shield with compression RCA or BNC ends and keep it away from mains runs. The coaxial vs optical audio comparison explains why coax wins on long runs and loses on ground isolation.
Practical Scenarios
- TV to soundbar, 1 m: optical is fine and breaks the ground loop. Plastic TOSLINK carries 96 kHz at this length without trouble.
- Preamp to active speakers, 8 m: use RG6 coax at 192 kHz, or step down to 96 kHz if the source allows it. Do not use a 10 m plastic optical cable for 192 kHz.
- Two rooms, 20 m apart: coax balun over Cat5e, or AES3. A passive RCA extension will fail intermittently, usually at the loudest passage.
- Recording interface to monitor controller: AES3 on 110 ohm cable, not S/PDIF on an RCA lead, because the balanced link tolerates studio routing and ground differences.
FAQ
Can I use a long RCA cable for digital audio?
Up to about 5 m with a decent 75 ohm lead. Beyond that the reflections at the plugs dominate and the receiver starts muting on loud passages. If you need 10 m, use RG6 with compression ends rather than a thin audio RCA cable.
Why does optical drop out at 192 kHz?
Consumer TOSLINK transmitters are specified around 96 kHz PCM. At 192 kHz the pulse width halves, and the plastic fiber plus the LED rise time can no longer deliver a clean eye. The receiver mutes rather than playing distortion, which is why the symptom is silence.
Does a longer digital cable reduce sound quality?
It does not add noise or soften the top end. It increases the chance of a bit error, and a S/PDIF receiver has no error correction, so the result is a click, a mute or a lost lock. Below the length limit, cable length has no audible effect.
