Hi. I got an 18i20 3rd gen. I tested the output with stereo headphones. I can hear sound from my Macbook Pro with the Scarlett as the output audio device in System Preferences > Sound.
I also got a great deal on Sony MDR-RF6500 surround headphones from a Craigslist seller. The headphones have a base station with an optical Toslink input plug. (https://www.sony.com/electronics/support/res/manuals/4267/42672637M.pdf)
I run MacOS Catalina on my Macbook Pro. I configured Audio Midi Setup for the Scarlett 18i20
USB with "20 ch 24-bit Integer 48.0 kHz" output using the Internal clock source. I configured 5.1 speakers in the default order. (1-Left, 2-Right, 3-Center, 4-Sub, 5-LSurr, 6-RSurr.)
I hit the test button for each channel in the Configure Speakers section of Audio Midi Setup. In Focusrite Control > Output Routing, I see the corresponding ADAT output levels activated when the test sound plays. I turned them all off "stereo" to use individually. ADAT 1.1 input is Playback 1, 1.2 is Playback 2, etc. I can hear 1 & 2 on the stereo monitor headphones plugged into the 18i20.
I tried playing the DTS audio sound check file with Kodi. (hd_dts_hd_master_audio_sound_check_5_1_lossless.m2ts). As each speaker in the sound check file plays its test sound, the corresponding level bar for each ADAT output in Focusrite Control lights up from ADAT Output 1.1 to ADAT Output 1.6, in the correct order. I also tried the stereo monitor headphones and I can hear 1 and 2 from the 18i20.
I have the MDR-RF6500 processor (base station) set to digital input, with effect and compression off. If I turn the processor off, I hear the warning beep sound from the headphones, so I know they receive RF signal from the processor when it is turned on. (There is no problem with signal transmission from the processor to the headphones.)
I plug the Toslink cable into the right-hand optical-out port of the 18i20. It lights up inside. The cable is not kinked or folded tightly. I plug the other end into the MDR-RF6500 processor digital input port. Bupkis.
Notably, none of the decode lights on the processor turn on: Dolby Digital, Dolby PLIIx, nor DTS.
Focusrite Control Settings: Sample rate 48 kHz, Clock Source Internal. Digital I/O Mode I tried both S/PDIF optical and Dual ADAT. ("Device restart required" means cycling the 18i20 power switch, right? Is there a nicer way to reboot the device?) I figure it should be Dual ADAT so I have left it there. "Retain 48V" I tried both enabled and disabled. (What does this mean?)
I tested the MDR-RF6500 from my Panasonic BD player optical digital out with a BluRay disc. The DTS indicator lights up on the processor and the headphones work great.
Am I barking up the wrong tree here? Can I use these headphones through the 18i20 digital output?
Do I need another device between the 18i20 and the headphones to take the raw PCM signal from the 18i20 and encode it with Dolby Digital or DTS? If so, what would work?
Or, can I somehow encode the 18i20 output correctly with Dolby Digital or DTS so it works?