You bought an Atmos soundbar, you press play on a disc, and the bar lights up as plain surround. Nothing is broken. Somewhere between the player and the bar, the sound had to squeeze through one HDMI socket on your TV, and that socket has a width. This piece is about that width, and about the one cable run that decides it.

The pipe behind the TV

ARC (Audio Return Channel) lets your TV send sound back down an HDMI cable to a soundbar or receiver. It arrived with HDMI 1.4 and carries about 1 Mbps. eARC (enhanced ARC) came with the HDMI 2.1 specification and carries roughly 37 Mbps, enough for up to 32 channels of uncompressed audio at up to 24-bit and 192 kHz. An optical cable is narrower still, about 0.4 Mbps. The bars below are drawn to true scale, which is why ARC looks so thin.

Follow the sound

1. Where does the sound start?

2. How does the TV reach the soundbar?

What fits through ARC

ARC carries stereo PCM and compressed surround: Dolby Digital, Dolby Digital Plus and DTS. It does not carry the lossless formats found on discs, Dolby TrueHD and DTS-HD Master Audio, or the Atmos and DTS:X tracks built on them. eARC adds all of those, plus uncompressed multichannel PCM.

That explains a puzzle many people hit. Netflix, Prime Video and Disney+ encode Atmos as Dolby Digital Plus, a small, lossy stream, so Atmos from an app on the TV rides over plain ARC. A 4K disc stores Atmos as TrueHD, which is far bigger, so over ARC it will likely arrive as ordinary 5.1.

Five things to check, in order

  1. Use the right sockets. The port must be the one marked ARC or eARC on both the TV and the soundbar. Many TVs have only one eARC port, even with four HDMI sockets.
  2. Switch it on. Enable ARC or eARC in the TV's audio or system settings, and turn on HDMI-CEC (often called HDMI Control) if ARC needs it. If the TV speakers keep playing, mute them.
  3. Know your TV's passthrough. Some TVs downmix multichannel sound to stereo before sending it on. If a disc only ever gives you stereo, this is the first suspect.
  4. Skip the TV for discs. If the soundbar or receiver has its own HDMI input, plug the disc player into it and send only the picture to the TV. ARC is then never involved.
  5. Mind the cable. Sources differ here: some say an existing High Speed cable is fine for eARC, others recommend High Speed with Ethernet or Ultra High Speed. Try the cable you have first, then a short certified one.

No sound, or sound that is late

If the handshake fails, switch inputs and back, reseat both ends of the cable, try a shorter cable, then power everything off, unplug it and start again. For lip-sync, correction is automatic and mandatory in eARC devices but optional in ARC, which is one more reason to have eARC at both ends. If the picture and sound still drift, many TVs and bars let you nudge the audio delay by a few milliseconds.

Where to look next

Before you buy anything, read the spec sheet for the words eARC and HDMI in. A bar with a single HDMI socket must take everything through the TV; a bar with an input lets a disc player skip the pipe entirely. Availability and features vary by country and model, so check the exact model you are looking at.

Sources