Coaxial digital to Toslink adapters, both directions

A soundbar has an optical input and nothing else. The TV box in the cabinet has a coax digital output and no optical port. Both run S/PDIF, both are digital, and neither will talk to the other through a plug adapter, because one end is copper and the other is light. The small powered box that sits between them is the only part in the chain doing real work, and knowing what it does makes the shopping much easier. It is also the part most often bought by mistake, because several products look identical and do different jobs.

What happens inside the converter box

On the coax side the box sees the familiar 75 ohm single-ended S/PDIF signal. A receiver chip locks to the bit clock, recovers the frames, and hands them to a transmitter. On the optical side a light-emitting diode is modulated to produce the same stream as pulses of red light, which a Toslink receiver at the far end turns back into logic levels. Nothing is decoded and nothing is resampled. The audio data passes through untouched; only the physical layer changes. That is why a good converter is audibly invisible, and why a bad one shows up as dropouts rather than as a change in tone. It also explains the price spread: you are paying for a clean clock, a stable optical driver, and a power supply that does not inject noise into the coax side.

Re-clocking is not decoding

Re-clocking means the converter regenerates a clean clock for the outgoing signal instead of forwarding a jittery one. It does not touch sample values, and it does not convert to analog. Some units use a fixed local oscillator while others recover timing from the incoming stream. Either way the format, the channel count, and the sample rate stay the same from input to output.

The optical side of the box

The box also drives a plastic fiber Toslink transmitter, and the light level has to be set so the receiving side can lock onto the clock. Too little light is the most common failure mode, and it looks like the port simply will not grab a signal. The small glass or plastic connector is fragile, so handle it by the body and never by the tip when you seat it into the port.

Coax in, optical out

This is the common direction, because many TVs and media players still offer a coax digital output while soundbars and AV receivers increasingly favor optical. Connect a 75 ohm coax cable from the source to the adapter input, then a Toslink cable from the adapter to the soundbar. Keep the adapter near the soundbar when the optical cable is short, since optical cable is less tolerant of tight bends than copper. Power the adapter from a USB port on the TV if one is available, so it switches on with the set. If the coax run from the source is long, keep it under the usual five to ten meter range and route it away from mains cabling, because the adapter cannot repair a signal that already arrives damaged.

Optical in, coax out

The reverse direction exists for the opposite problem: an older receiver or a car processor with a coax input, fed by a source that only has Toslink. The signal path mirrors the first case, with the light receiver feeding the coax driver. Not every adapter is bidirectional, and some that claim to be need a switch flipped or a second input selected. Check the listing for the direction you actually need rather than assuming both work out of the box, because buying a one-way unit for the wrong direction is the most common mistake in this category. A unit that only converts coax to optical will stay silent when you feed it light, and no amount of cable swapping will change that.

Direction is set by the ports, not the label

Look at the silkscreen on the box. Inputs and outputs are usually marked, but inexpensive units sometimes label both ports simply as coax and optical, leaving you to guess. A safe rule is that the port fed by your source is an input and the port fed into your sink is an output. If the markings are ambiguous, buy from a listing that shows a wiring diagram.

Coax output level and termination

When the box gives coax out, the output is a 75 ohm S/PDIF line on an RCA style plug at a fixed level. Make sure the receiving coax input is genuinely a digital S/PDIF port and not an analog RCA input, or you will hear noise. A short 75 ohm coax cable then completes the run, and the length budget is the same as any other coax digital connection.

Power, formats, and the rate ceiling

The adapter needs power, usually five volts over USB or a small barrel jack, and it needs that power continuously while the system runs. Unplugging it with the TV is fine, but a unit that loses power mid-stream normally produces a loud click. On formats, the ceiling is S/PDIF itself: two-channel PCM to about 96 kHz and 24 bit, or a compressed multichannel bitstream such as Dolby Digital or DTS. Higher-rate PCM and the lossless formats carried by HDMI will not pass. If the source outputs a format the converter cannot carry, the usual result is silence rather than degraded sound, which makes the fault easy to diagnose once you know to look for it. Set the source to a fixed output format and leave it alone.

When this box is the right answer

Use a converter when you have exactly one digital port of each type and no way to change it: a TV with coax out feeding a soundbar with optical in, or a streamer with optical out feeding a DAC with coax in. Do not use one to fix a level problem, to add channels, or to connect a USB source, because it does none of those. Do not expect it to improve the sound either. Its job is to move a bitstream from one physical layer to another without losing anything on the way. If you need analog output, buy a converter with a DAC inside instead, since this class of box has no analog stage at all. Read the port list twice before ordering.

Direction, ports, and power for each case

Real-World Application Scenarios

Three cases come up repeatedly. A living room TV with a coax digital output feeds a soundbar that only accepts optical, so the adapter sits behind the bar on a USB supply. A desktop DAC has a coax input while the streamer offers only optical, which is the reverse direction of the same box. In a small venue, a media player with optical out feeds a rack processor that has a spare coax input, and the adapter lets the operator keep the existing cabling instead of replacing the processor. Each of these installs has one thing in common: the digital format never changes, only the shape of the signal that carries it.

Related Guides on This Site

Our digital coax to Toslink page lists the common adapter types and their ports. If the far end needs analog, the Toslink to RCA converter note explains that different job. For the underlying comparison, the S/PDIF versus optical article covers distance and immunity, and the S/PDIF coax overview covers formats.

Frequently Asked Questions (FAQ)