One Finite Planet

One Finite Planet

Revolution?: HDMI ARC, eARC and S/PDIF

Date Published:

The move from S/PDIF to HDMI ARC failed in bringing even a small revolution, but the introduction of eARC in HDMI 2.1 may now actually allow a major revolution.


  • S/PDIF: History
  • Audio Return and Directional Audio/Video Cables
  • Pre ARC (before HDMI 1.4, 2009 )
  • HDMI 1.4 with ARC (Audio Return Channel)
  • Sonos Beam – ARC at work!
  • TV: A HiFi capable Digital Pre-Amp?
  • eARC – HDMI 2.1 (2019)
  • The Audio Revolution

S/PDIF: History

The traditional way to send digital audio within HiFi, has been the S/PDIF coaxial or optical connection.  The connection originated from the desire to send digital information from a CD player to an amplifier, in order to use the D/A conversion within the audio amplifier.  The system was designed to send stereo signals of CD resolution, but within that design has also been able to send compressed surround sound.

Audio Return and Directional Audio/Video Cables

An important concept to have clear before discussing Audio Return Channel, is that audio and video cables always (or almost always) have a direction.  One end of the connection will act as an output, and the signal is sent to the other end which will act as an input.  Even though the connectors on each end of the cable are the same type of connector, there is a different role for each end of the cable.

So an RCA connector from a CD deck to an amplifier connects the output of the CD deck, to an input on the amplifier.  With a tape recorder, there are output connections for when a tape is in playback, and input connections for when a tape is being recorded, and the same pin type is used for each end.  With RCA cables, two sets of red and white leads, one set from the outputs of the tape player to the a set of two inputs on the amplifier, and an identical cable confusingly carrying signals in the opposite direction.  To simply things, there was a ‘DIN’ plug with 5 wires. A ground level, plus and left and right signal for one direction, and another left and right signal for the opposite direction.  Of course, if you tried to use this cable to connect to tape recorders to copy a tape…. the two inputs were connected and the two outputs were connected, so it would not work.

The point is, that every wire in an audio/video cable has a direction: either input or output.  Normally, all HDMI ports on a TV are inputs, and all HDMI ports on DVD players are outputs.  In fact, even DVD recorders have normally have all HDMI ports as outputs, and a different system for video inputs.

So if all HDMI ports on a TV are inputs, how does the TV output anything?  An Audio/Video amplifier, or a centre speaker, need sound output from a TV when TV programs are being watched.   The sound and video are inputs to TV during a DVD, but the sound is an output when watching live TV, so while video is always ‘in’, audio can be in or out.

Pre ARC (before HDMI 1.4, 2009 )

Without ARC, the answer is to have a separate audio connection running back from the TV to a speaker or Audio using SPDIF.  Of course, SPDIF was designed for sound that does not have control, so even with a centre speaker, the speaker will need its own volume control.

HDMI 1.4 with ARC (Audio Return Channel)

Recall that ‘DIN’ plug for tape deck connection?  Both in and out in the one cable.  HDMI 1.4 decided to use the same concepth with ARC.  ARC adds the audio reverse direction to the HDMI cable, so now and HDMI cable can be thought of a Audio and Video going in one direction, plus audio returning in the other direction. However while the audio and video sent to the TV have no control information (volume or tone for Audio, brightness or contrast for video), the return audio that has been added to the specification  does have added control information.

This added control information is extremely important. This cable, for the first time,  provides control information with a digital audio signal.  This gives the return direction of digital audio additional uses that the ‘normal’ direction of HDMI audio does not have.

The problem with ARC, is the specification was too flexible, which has allowed different interpretations, which leads to interoperability problems. The problems have led to a common suggestion being “avoid ARC, it is too flaky”.

The Sonos Beam – ARC at work!

Sonos has become a major brand. In wireless multiroom, the Sonos is the clear industry leader.  Which means Sonos introducing a sound bar with HDMI ARC as the only input, creates pressure on the major TV brands to get their ARC actually working properly.

When working properly with ARC, the Sonos Beam gives a preview of the revolution that will be fully enabled by eARC in HDMI 2.1

The Sonos sound bar being designed ARC is more significant than it sounds.  The significance is that by using an digital source which has control information already present, the Sonos beam does not have to act as a digital preamp.  The Sonos Beam can concentrate on converting digital to analog, amplification and being a speaker.

That difference may appear subtle, but in reality it is a revolution.

A pre-amp selects which input is to used and controls volume etc.  Most soundbars try and take over the function of being a digital pre-amp, which is why they, duplicate the multiple HDMI inputs that are on the TV.  Many will even have host an app store, because most digital sources have moved to being through the web, so to select these inputs you need to have app store. Then, even though the TV has a volume control, so does the soundbar, and when moving the soundbar you need to switch app stores.

None of this is needed when relying on ARC.  If ARC actually works properly, the TV can be digital pre-amp, leaving a single app store, and a single set of inputs. Nothing else changes when you turn on the soundbar other than the sound quality.

What Changes When Relying on ARC?

Using S/PDIF, the alternative to ARC, means no control information.  With no control information, the signal must enter a ‘pre-amp’ or ‘control-amplifier’ stage, even though the TV already has, in the digital domain, perfectly adequate control amplifier capability.

Samsung TV connections (2018)

Note a smart TV has a volume control, an app store as well as physical HDMI and usually other inputs. A sound bar repeats (see below) repeats most of these connections.

Yamaha YSP-5600 sound bar connections

The Sound bar does add analog audio connections, plus an HDMI output for connecting the TV, but otherwise the sound bar generally repeats the connections that were on the TV. This means to use the sound bar as designed, you must move all the connections from the TV to the sound bar, then use the sound bar to select inputs.  Unless the input is from an App such as Netflix, or Spotify….. oh wait there is a Spotify app on the sound bar also, but not a Netflix app.  Confused? So the sound bar has its own remote, connections and apps, so when not relying on ARC, you get two remotes, two sets of input connections and two sets of apps.  But if you rely on ARC? One remote, no duplicated connections or Apps spread between two different app stores.

Connections when relying on ARC

TV: A HiFi capable Digital Pre-Amp?

Consider a digital preamp that accepts digital data in, and sends digital data with control information out.  This means a digital pre-amp that does not actually amplify, as digital signals are not amplified. The actual amplification must happen after a signal is converted to analog, in other words after a DAC.  Dealing with analog signals and retaining hi fidelity is the domain of high end HiFi, but while the signal stays digital, it is purely about computing.  TVs can have high powered CPUs, and can do digital. This means provided the Analog processing is handled externally to the TV, a TV can perform the digital part of processing the signal.  That means retrieving the digital information from the net with an app, or simply routing the input stream from a DVD or other device through to the output.  For all cases, simply adding an additional information source of playback settings such as desired volume, or other information the external HiFi ‘digital power amp’ will use in the real HiFi task, turning bits into sound. What a TV as a digital pre-amp should not do, is try to be an analog pre-amp as well and process analog inputs.

eARC – HDMI 2.1 (Nov 2017)

The original ARC introduced back in 2009 was a typical first try. Things were not perfect. Firstly, the new specification had so many optional elements that implementations vary too significantly for a real standard. Secondly, the bandwidth for the digital signal, although an improvement on S/PDIF, was not sufficient to handle multichannel or newer object based advanced sound encoding. Thirdly, the control channel information was best provide by the ethernet data, but ethernet data was optional in the standard.

The HDMI 2.1 specification mandates eARC.  Nothing required for eARC is still optional.  The bandwidth available copes with all current codecs, and there is room for the future as well.  Ethernet is no longer optional allowing more robust control information, and evolution of control specifications.

The Audio Revolution

In simple terms, once HDMI 2.1 is the normal, and thus eARC becomes common in homes, you could design a ‘Sonos Beam’ alternative that works as simply as the Beam, but with the clarity and perfection of the best HiFi systems, both for audio, and for Audio/Video.   That is, using the inputs and of the TV, the remote of the TV, and app store and source selection of the TV, but with multiple speakers and amplification optimised for HiFi and/or Home Cinema.  Sound  delivered with the sound qualify of the highest end home theatre or HiFi system.  HiFi and high end home theatre as simple as the Sonos Beam.

What about sources that do not connect to TV?  There are questions left unanswered by this post.  The answers do exist, they are just too long for this one post, and I will add these answers in other posts.

Table of Contents


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