Quick answer: There is no such thing as a passive RCA to optical cable, because RCA carries an electrical S/PDIF or analogue signal and TOSLINK carries light. The conversion needs an active box with a receiver, a clock recovery stage and an LED or laser transmitter. The direction matters: coaxial digital RCA to optical is a simple format converter, analogue RCA to optical needs an analogue-to-digital converter, and optical to analogue RCA needs the reverse. Buy the box that matches your direction, not a cable with two different plugs.
Search results are full of products described as RCA to optical cables, and most of them contain a small converter hidden in a plug-sized housing. That is fine when the direction and the signal type match, and useless when they do not. Knowing which of the three conversions you need prevents buying the wrong one twice.
Two physical layers that never meet
Electrical versus optical
Coaxial S/PDIF is a 75 ohm electrical interface carrying a biphase-mark-encoded digital stream at about 0.5 V. TOSLINK is a plastic optical fibre carrying the same S/PDIF frame as pulses of red light at 650 nm. The data format is identical; the medium is not, so a converter only has to receive, retime and retransmit.
What the box contains
A coaxial to optical converter has a 75 ohm input, a comparator that recovers the bit clock, and an LED driver. An optical to coaxial converter has a photodiode, a comparator and a line driver. Neither is expensive, which is why these boxes are cheap; what varies is the supported sample rate, the quality of the clock recovery and whether the output is truly 75 ohm.
Which conversion do you actually need
| Source | Destination | Device needed | Formats kept |
|---|
| Coax digital RCA | TOSLINK | Coax to optical converter | PCM and compressed 5.1 |
| TOSLINK | Coax digital RCA | Optical to coax converter | PCM and compressed 5.1 |
| Analogue RCA, red and white | TOSLINK | ADC, analogue to digital | Stereo PCM only |
| HDMI ARC | TOSLINK | HDMI audio extractor | Up to 5.1 compressed |
| TOSLINK | Analogue RCA | DAC, digital to analogue | Stereo, decoded |
What to check before buying
Sample rate and format support
Cheap converters handle 48 kHz and sometimes 96 kHz, and many refuse 192 kHz. If the source is a CD transport at 44.1 kHz, anything works; if it is a hi-res streamer at 192 kHz, check the specification or the box will mute. Compressed 5.1 passes through because the converter does not decode it, but a DAC that also decodes will only output stereo.
Power and earth loops
Most converters come with a small switching supply, and that supply can inject noise into the audio chain. Optical isolation is one of the advantages of TOSLINK, so a converter that reintroduces a mains earth at the receiving end partly defeats the purpose. If hum appears after adding a converter, try a different outlet or a linear supply before blaming the cable.
Cables on each side
On the electrical side use a 75 ohm cable rated for digital audio, not a thin analogue RCA lead: the S/PDIF signal at 6 MHz suffers from reflections on a mismatched cable, and the symptom is intermittent dropouts rather than obvious distortion. A RCA patch cable made from coaxial stock is fine; a twisted-pair audio lead is not ideal.
On the optical side, keep the run under 5 m with plastic fibre, and avoid sharp bends. The TOSLINK audio cables guide covers cable grades, and the TOSLINK vs coaxial digital comparison explains why the two formats carry the same data with different practical limits.
Jitter and clock recovery
S/PDIF carries the clock inside the data stream, so the receiving device reconstructs it from the transitions. A cheap converter with a slow comparator adds jitter, and a DAC with a poor phase-locked loop turns that into timing error on some material. The effect is small compared with loudspeaker and room problems, but it explains why two converters that both pass 96 kHz can sound different. A converter that re-clocks with its own oscillator, rather than passing the recovered clock straight through, is the safer choice for a critical chain.
Finally, check the connector on the TOSLINK side. A square plug that is not pushed fully home passes light and produces intermittent dropouts that look like a converter fault. Listen for the click, and keep a spare cable: optical cables fail by cracking at the bend near the plug, which is invisible until the link stops.
Practical scenarios
- Soundbar with optical input only, TV with coax digital output: a coax to optical converter behind the TV solves it, powered from the TV’s USB port if it has one.
- CD player with coaxial output into an optical-only input: the same converter works, and 44.1 kHz is within every model’s range.
- Turntable into an optical soundbar: this needs an ADC, not a format converter, because the phono signal is analogue; add a phono preamp first.
- Long run to a second room: optical is immune to electrical noise over 10 m, so convert to optical at the source and back at the far end.
FAQ
Can I buy a cable with RCA on one end and optical on the other?
Only as a converter in cable form. A passive cable cannot turn electricity into light, so any product sold that way contains active electronics, usually in a small housing at one end. Check that it converts in the direction you need.
Will an RCA to optical converter carry 5.1 surround?
It will pass an already-encoded Dolby Digital or DTS bitstream unchanged, because it does not decode. It cannot create surround from a stereo analogue input. If the source outputs PCM stereo, the converter passes stereo.
Why does the converter work at 48 kHz but not 96 kHz?
The receiver chip is limited to the lower rate, or the optical transmitter cannot switch fast enough. Most inexpensive boxes specify 96 kHz maximum and some only 48 kHz. Set the source to a supported rate or replace the converter.
