The HDMI and HDCP specs have evolved over time. Your CX will support HDMI 2.0 features and the C1 HDMI 2.1.
If using newer HDMI 2.1 devices in a setup, I always recommend “Ultra Certified” HDMI cables. These have been tested and the “Ultra” designation is for their support of 48Gbps bandwidth. Let the devices communicate their capabilities to each other rather than have inferior cables impose any limitations on them.
The Monoprice™ 8K Certified Braided Ultra High Speed HDMI® Cable is the next generation of HDMI cable. Certified by an HDMI authorized testing center, these cables support up to 8K@60Hz/4K@1
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While a PC can decode various signals and output them as multichannel PCM to the LG CX and C1 which will pass it onto an AVP or AVR, the Rokus do things differently. Setting the Rokus to PCM output will result in the downmixing of any Dolby multichannel signals to two channel PCM for output rather than a decode and output of multichannel PCM.
When a source device is set to decode or bitstream a signal for delivery to an AVP or AVR through a TV using eARC, the TV HDMI port to which the device is connected must be set to bitstream so that it is unaltered by the TV. Setting it to PCM here will also result in the down mix of any multichannel signals. Once the HDMI ports are set to bitstream, eARC must also be turned ON or only the limited bandwidth of ARC will be available and will only work with HDMI-CEC turned on in the LG TV(Simplink) and also in the AVP.
The sound output settings of the Roku Ultra are a bit convoluted. Made worse for audio output of any streaming device are streaming service limitations based on a selected service tier. Dolby Vision and Dolby Atmos(Dolby Digital Plus 7.1 with Atmos metadata) output aren’t supported on every service and some that do support it require a subscription to their highest and/or ad free tiers.