1.What is the audio return function?

ARC is called Audio Return Channel, as the name implies, and the purpose of ARC is to make the entire digital home appliance easier to install on the wire. Its function is to transmit the sound signal, specifically on the HDMI interface that supports ARC, the TV can send back the audio signal through the ARC to the amplifier or other audio devices with HDMI ARC function through the HDMI cable, so as to save the separate digital output cable.

HDMI ARC Connection-BONESTEC

Audio Return Function (ARC) eliminates the need for additional composite audio cables or optical cables when an audio/video (A/V) receiver or surround sound system is connected to the TV. ARC enables sending from the TV to the A/V receiver to support watching and listening to movies and other content from the source device.

a) ARC audio return channel

ARC (Audio Return Channel) audio return function was introduced in HDMI version 1.4, similar in function to the optical digital audio port (optical audio), with a bandwidth of approximately 1Mb, and can return the audio formats.
Dolby Digital 5.1, DTS 5.1, Dolby Digital Plus 5.1, LPCM 2.0
In other words, ARC can only transmit basic lossy (compressed) 5.1 channel audio tracks and lossless LPCM stereo audio tracks.
Does that mean that ARC can’t support Dolby Atmos?
No, Dolby Atmos is not an audio encoding, it is a wrapper format for the metadata describing the orientation of the sound information inside a TrueHD or Dolby Digital Plus track, which in layman’s terms is an object-based description file that describes when the sound is present at that location.
So when watching Dolby Atmos tracks encapsulated with Dolby Digital Plus 5.1 core, the ARC port will still support Dolby Atmos.

b) eARC Enhanced Audio Return Channel

eARC is the enhanced ARC, a standard feature of the HDMI 2.1 interface. The bandwidth has become 37Mb, and finally lossless audio tracks can be transferred to.
Dolby TrueHD, AC-4, Dolby Atmos, DTS HDMA, DTS:X, LPCM 7.1
In other words, the eARC channel supports all audio formats on the market, whether lossless or 32-channel.
There is one more propaganda trap about eARC
Since the eARC data channel uses 14 and 19 of the HDMI interface, independent of the four high-speed data channels of HDMI 2.1, this feature can be downlinked to the HDMI 2.0 interface.

HDMI 2.1 Source pin-BONESTEC

We generally refer to the HDMI interface with a transmission bandwidth >= 40Gb as the HDMI2.1 interface, but many manufacturers have devices with a bandwidth of only 18Gb, which is actually the HDMI2.0 interface, but because it is equipped with the eARC function it is advertised as the HDMI2.1 interface.

2.Does the optical HDMI cable support ARC/eARC audio return?

Optical HDMI cables are supported for ARC/eARC audio return. Fiber HDMI cable is directional, the interface at both ends of the cable, a Source port, a Display port, that since the signal can only flow from the source to the display side, how is the audio return achieved?
In fact, optical HDMI is not the entire HDMI cable is optical fiber, but only four high-speed data channels using optical fiber, the rest of the low-speed channels or copper wire, four fiber optic cable is connected to the two ends of the photoelectric conversion chip, while the copper wire is directly connected to the jack, optical HDMI cable, in fact, only four high-speed data channels have directionality, while ARC and eARC using copper wire, and and there is no directionality, so it supports audio return.
So why do some optical HDMI claim not to support ARC and eARC?
In fact, it is still a matter of distance.
General fiber optic cable, 5M start, the longest can reach 300M. fiber optic anti-interference ability, low loss, even 200M signal is still very strong; but copper cable more than 15M if the general material, shielding is not well done, then the audio digital signal attenuation serious, may be to the amplifier and echo wall distortion is very serious, basically can not be used. Of course, BONESTEC optical HDMI cable eARC function to do better.