HDMI (e)ARC and SPDIF - Fraunhofer-IIS/iec61937-13 GitHub Wiki

HDMI version 1.4b defines an Audio Return Channel (ARC) in Supplement 2 HEAC 4. The ARC is basically a single IEC 60958 link from the sink to the source device. If an HDMI device supports ARC, it shall support 2-channel 16-bit L-PCM audio at sample rates of 32 kHz, 44.1 kHz, and 48 kHz defined in IEC 60958-3. Support of other audio formats as defined in IEC 60958-3 or compressed audio as defined in IEC 61937 (all parts) is optional.

For the use of MPEG-H via ARC, it is strongly recommended that devices support an ARC channel with 192 kHz sampling frequency enabling HBR transmission with 4 times the basic transmission rate.

ARC uses the channel status bits for signaling of sampling frequency and other properties in the same way as IEC 60958. The audio format (type of used codec etc.) is signaled within the bitstream as defined in IEC 61937.

Since no EDID signaling is available in reverse audio direction, the CEC protocol can be used by the HDMI sink to enquire which audio formats are supported by the source for the reverse direction. This mechanism can also be used if a SPDIF link is used instead of ARC and CEC is available.

Support for this CEC feature is optional, but it is strongly recommended that MPEG-H capable devices using the ARC implement this feature. The sink sends a series of <request short audio descriptor> messages and the source replies with descriptors according to CEA 861. See chapter CEC 13.15.3 from supplement 1 of HDMI 1.4b for more details.

HDMI version 2.1a defines an enhanced Audio Return Channel (eARC) in section 9.5 of HDMI 2.1a. eARC shall support transmission of 16-bit 2 channel L-PCM audio at one or more of the following sample rates: 32 kHz, 44.1 kHz, or 48 kHz. eARC may optionally support up to 32 L-PCM audio channels and compressed audio at bitrates up to 24.6 Mbps.

eARC specifies a dedicated data communications channel for configuration and capabilities exchange that is independent of EDID or CEC. For signaling of compressed audio formats on this data communications channel CTA-861 short audio descriptors are used like in EDID.