Can RCA Cables Carry Digital Audio (SPDIF)?

1. The Short Answer: Yes, But With Caveats

Yes, a standard RCA cable can carry an S/PDIF digital audio signal.
In fact, most consumer gear uses exactly that – an RCA-terminated 75-ohm coax cable.
But not every RCA cable works well.
The difference comes down to impedance, shielding, and length.

2. What Is S/PDIF Over Coax?

S/PDIF (Sony/Philips Digital Interface) has two physical forms: optical (Toslink) and coaxial.
The coaxial version uses a single RCA connector.
The signal is not analog audio. It’s a bi-phase mark code (BMC) at a fixed bit rate.
For CD-quality audio (44.1 kHz, 16-bit), the raw data rate hits 2.8224 Mbps.
For 96 kHz / 24-bit, that jumps to 6.144 Mbps.
Dolby Digital (640 kbps) or DTS (1.5 Mbps) fits easily, but the clock is still embedded.

So yes – the plug is RCA, and the wire is coax.
But the specification calls for 75-ohm characteristic impedance.

3. Impedance Mismatch: The Real Problem

A standard analog RCA cable (red/white) is typically uncontrolled impedance – often 50–150 ohms, but not consistent.
Most fall into the 50–70 ohm range.
Some cheap cables drop to 40 ohms at certain frequencies.

Here’s the issue:
At 2.8 Mbps, the signal’s third harmonic reaches ~8.5 MHz.
At 6 Mbps, harmonics go past 18 MHz.
Impedance mismatch causes reflections.
Reflections blur the data edges.
That raises the jitter (timing error).
Jitter above 5–10 ns can cause clicks, dropouts, or no lock at all.

3.1 Real-world test data

In a 2017 test by Audio Science Review, a 2m generic RCA cable dropped S/PDIF lock at 96 kHz sample rate.
The same length of Belden 1694A (true 75Ω) worked fine at 192 kHz.
At 44.1 kHz, both worked – but the generic cable added ~2.1 ns of RMS jitter vs 0.7 ns on the proper cable.
Not audible for most, but borderline for sensitive DACs.

4. Why Do People Say “RCA Cables Don’t Work for Digital”?

Because they tried a long, cheap analog cable.
At 5 meters, many analog RCA cables fail completely.
The signal eye diagram closes.
The receiver chip (e.g., CS8416 or AK4118) loses PLL lock.
Result: no sound, or random static bursts.

Manufacturers don’t help.
They sell “digital coaxial cables” that look identical but cost 3x more.
In truth, any good 75-ohm video cable (e.g., RG-59, RG-6) with RCA ends works perfectly.
Monoprice’s 1.5m digital coax cable sells for $4.
A premium analog RCA interconnect might cost $50 – and perform worse for digital.

5. When Does It Actually Work Fine?

Short runs – under 1.5 meters.
Low sample rates – 44.1 or 48 kHz.
Decent analog cable – not the flimsy thin type that comes with a DVD player.

Example:
A 0.9m RCA cable from a 1990s VCR (unknown impedance) passed 44.1 kHz S/PDIF without errors in one informal test.
At 96 kHz, same cable failed after 20 minutes (heat build-up in the receiver’s PLL).
So “works” depends on your tolerance for random dropouts.

6. Technical Limits at a Glance

• S/PDIF voltage: 0.5V to 0.6V peak-to-peak into 75Ω.
• Max cable loss (per spec): ~6 dB at 6 MHz.
• 75Ω coax loss (RG-59): ~4 dB/10m at 10 MHz.
• Analog RCA loss (typical): higher and uneven – often 6–10 dB/10m at 10 MHz.

Uneven loss across frequency distorts the signal shape.
That adds jitter.
Too much jitter (above ~20 ns peak) = wrong bits.

7. Summary: Use the Right Tool, But Don’t Overthink It

Bottom line:
An RCA cable can carry S/PDIF digital audio, especially for short runs at CD quality.
But don’t push it.
If you hear clicks, stutters, or silence, swap in a proper 75-ohm coaxial digital cable.
Better yet – use an old composite video cable (yellow RCA). That’s already 75Ω.
Free, and technically correct.

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