HDMI (e)ARC audio quality and implementation.

[Grand Tezsto]

I am planning to purchase a headphone/Dac setup to listen to movies via headphones. I naturally want the best audio quality I can achieve.
My understanding thus far is that out of the three options being optical/coaxial and arc, arc provides the best SQ. Correct so far?
Now, while some Dacs do provide an Arc input my question is why don't manufacturers provide an eArc input?
Is it because there needs to be some further processing in the Dac for the digital signal to be decoded and therefore some sought of license is now
mandated from Dolby/THX for this to happen?
What is known: Arc only allows compressed 5.1/7.1 etc to pass while eArc allows due to it's increased bandwidth uncompressed like formats to pass.
While the surround multi-channels enter compressed or uncompressed are all these channels converted to plain stereo for headphone use?

 

[MCH]

From what I understand, that isn't much to be honest, ARC is a close cousin of SPDIF and it is very easy to convert one to the other, being the advantage of ARC, well hdmi, the additional control possibilities.
eARC on the other hand is a different beast that requires dedicated hardware. If there are additional licenses to be paid even when not decoding anything, I don't know. There are very cheap eARC > HDMI audio converters available, but without decoding.
In your case toslink or arc for TV volume control if necessary, could be good enough imo.

 

[Grand Tezsto]

I find your comment about ARC being a close cousin of SPDIF interesting. I might email SMSL and ask them. As far as I know, they are the only DAC manufacturers that supply on specific models with such an input.

 

[staticV3]

My understanding thus far is that out of the three options being optical/coaxial and arc, arc provides the best SQ. Correct so far?

No. Identical SQ with IMO optical having the edge due to electrical isolation.
(But ARC having the edge in UX due to CEC commands)

While the surround multi-channels enter compressed or uncompressed are all these channels converted to plain stereo for headphone use?

Depends on your DAC/processor.

Many don't support multi-channel at all (e.g. SMSL), in which case the TV will fall back to sending regular stereo.

Others may support multichannel decoding and headphone downmixing (of various quality), in which case they'll accept encoded MCH audio from the TV.

 

[Grand Tezsto]

After having read my Sony TVs manual it states:

HDMI ARC
Two channel linear PCM: 48 kHz 16 bits, Dolby Digital, Dolby Digital Plus, DTS

DIGITAL AUDIO OUT (OPTICAL)
Digital optical jack (Two channel linear PCM:
48 kHz 16 bits, Dolby Digital, DTS)

From Internet:
In the context of HDMI ARC, the maximum achievable bitrate for an uncompressed stereo signal is likely around 768 kbps
For a stereo signal, a commonly used bitrate for optical transmission is 1,536 kbps (96 kHz, 24-bit PCM)

So, from the above it looks very much that Optical for an uncompressed stereo signal is superior.