Coaxial Output: Digital Audio Output Specs (S/PDIF, 75 Ohm)
A coaxial output – labeled ‘coax out’, ‘digital out coaxial’ or ‘S/PDIF out’ – is the one-way 75 ohm port found on TVs, soundbars, receivers and source components. Understanding exactly what the port outputs: the electrical level, the formats that fit, the hard limits – is what lets you wire a home theater without surprises and stop guessing at menu settings. This page documents the specification, compares coax out to its siblings, and explains the settings on both devices that make or break the connection.
Quick Answer
A coaxial output is an S/PDIF transmitter: 75 ohm line, 0.5 V peak-to-peak, bit-level JDS encoding, one audio stream. It carries uncompressed PCM up to 24-bit/192 kHz and compressed Dolby Digital or DTS 5.1 only at 48 kHz. It is one-way, carries no video, and cannot pass Dolby Digital Plus, TrueHD or DTS-HD – those require HDMI. Set the output to PCM or bitstream to match the receiving device’s decoder, and lock the sample rate to what the receiver accepts.
The S/PDIF Output Spec in Detail
The electrical side is fixed: 75 ohm characteristic impedance, 0.5 Vpp bit stream into a matched load, and a transport rate of 32 times the sample frequency – 1.41 Mbps at 44.1 kHz, scaling to 6.14 Mbps at 192 kHz. The format side has a ladder: stereo PCM fits at every supported rate up to 24/192; compressed 5.1 (Dolby Digital, DTS) fits only at 48 kHz; everything above that ceiling lives on HDMI. Copy-protection flags and channel status metadata travel in the frame headers, which is why some players expose a ‘copy control’ setting that the receiver honors.
Encoding: BJD and the 32x fs transport rate
Bit-level JDS (bi-phase mark) encoding means every bit produces a voltage transition, so the receiver recovers its clock from the data itself. No separate clock line, no handshake – the stream is self-timing, which is also why a marginal cable shows up as random clock errors rather than a simple no-signal.
Format ceiling: what fits in the bitstream
The 48 kHz compressed limit is the one people hit first: a Blu-ray’s 5.1 Dolby Digital track passes, but the 7.1 DD+ track on the same disc cannot, because DD+ is not part of the S/PDIF container. The receiver will fall back to stereo or a 5.1 downmix depending on its menu.
Coaxial Output vs Optical Output vs HDMI ARC/eARC
Three outputs cover most consumer audio. Coax and optical are both S/PDIF with identical ceilings; coax wins on cable robustness and cost, optical wins on ground isolation. HDMI ARC/eARC is a different class: it carries the HD codecs, 7.1 and beyond, plus the return path from the TV’s tuner – but it requires both devices to support it and a compatible cable. The practical rule: use HDMI when the whole chain supports it, coax or optical when one side is legacy. A device with all three outputs lets you match each input type in the room.
Receiver and Source Settings Around a Coaxial Output
The source menu typically offers three choices. Format: bitstream (forward the encoded 5.1, receiver decodes) or PCM (source decodes, sends stereo). Sample rate: auto, 48 kHz, 96 kHz or 192 kHz – auto negotiates, but a mismatched auto is the top cause of ‘no signal’ on picky receivers, so locking 48 kHz for surround is the safe default. Downmix: off, for 5.1 content. On the receiver side: select the coax input, enable the decoder, and set zone routing if the unit drives multiple rooms. Power-cycle the receiver after menu changes; many units re-detect formats only at boot.
PCM vs bitstream: which to pick
Bitstream when the receiving device has the decoder and you want the source’s original encoding – the standard choice for a Blu-ray to receiver chain. PCM when the destination is a DAC or simple amp, or when you need a rate above 48 kHz for stereo music. The cable and port are identical either way; only the menu state changes what arrives.
Limits and Workarounds
Known limit one: a 96/192 kHz music source feeding a receiver that only accepts 48 kHz on coax – the fix is resampling at the source (set the output rate to 48 kHz) or moving the path to HDMI. Known limit two: HD surround codecs – no workaround on coax; HDMI is the only path. Known limit three: run length – past about 15 m the bit stream degrades; the workaround is a shorter route or a quality 75 ohm build. Known limit four: one program per port – multi-zone needs a receiver pass-through or a separate output, not a Y-split of the bit stream.
Coaxial Output Capability Matrix
| Format | On coax out? | Max rate | Notes |
|---|---|---|---|
| Stereo PCM | Yes | 24-bit/192 kHz | Resample if receiver caps at 48k |
| Dolby Digital 5.1 | Yes | 48 kHz | Receiver decodes |
| DTS 5.1 | Yes | 48 kHz | Receiver decodes |
| Dolby Digital Plus / TrueHD | No | n/a | HDMI only |
| DTS-HD Master | No | n/a | HDMI only |
Real-World Applications
The coaxial output remains a load-bearing port in specific scenarios: a living-room TV passing its tuner audio to an AV receiver that predates eARC; a soundbar with a coax input receiving the TV’s digital out; a source matrix in a commercial space where every player exposes S/PDIF; a zone-2 output from a main receiver feeding a kitchen amp; a studio or listening room where a transport’s coax out drives a dedicated DAC for its monitors; and a test bench, where the coax output gives instruments a clean 75 ohm reference. In each case the port’s value is its longevity: the spec has not changed in decades, so today’s cable still feeds tomorrow’s receiver.
Related Guides
Frequently Asked Questions
What is the maximum resolution a coaxial output can carry?
Uncompressed PCM up to 24-bit/192 kHz. Compressed surround is capped at Dolby Digital/DTS 5.1 at 48 kHz – anything above that ceiling needs HDMI.
Can a coaxial output carry video?
No. The S/PDIF stream is audio-only. Video travels on HDMI, component or composite paths.
Why does my TV’s coax out only offer 48 kHz with surround?
Because the S/PDIF container carries compressed 5.1 at 48 kHz only. Stereo can run higher rates, but surround is locked to the 48 kHz frame rate by the standard.
Is a coaxial output better than an optical output?
Audio ceiling is identical. Coax is more robust mechanically and cheaper per meter; optical isolates grounds between devices. Choose by the install – long or noisy runs favor coax, ground-loop-prone racks favor optical.