Correct translation, but on a blocked-word list to prevent spam.
Got the spdif-toslink working 
in armbian buster. It showed up as a device (Sink #2, alsa_output.platform-spdif-sound.stereo-fallback in pulseaudio) and I could play sounds to it.
The connection to my amp using toslink works also.
Parts: JST PHR-3 connector, short cable (shouldn't be more than 2-3cm), PLT133/T8 transmitter.
Thought some more about it. Build my own image - cool build process .
Compared config.gz and dtb of Armbian to the ones of LibreElec and found differences.
Made some changes to .config of the kernel package in LibreElec (funny, I had to change the package directory to get a rebuild of the package - changing the kernel configuration didn't do the trick) and changed some minor difference in the dtb.
I'm still reading about these things as I'm new to them.
Well, I'm stuck with this message on serial console when starting:
[ 2.128560] rockchip-spdif ff870000.spdif: Missing dma channel for stream: 0
[ 2.129312] rockchip-spdif ff870000.spdif: ASoC: pcm constructor failed: -22
[ 2.130055] asoc-simple-card spdif-sound: ASoC: can't create pcm ff870000.spdif-dit-hifi :-22
[ 2.130946] asoc-simple-card spdif-sound: ASoC: failed to instantiate card -22Don't know how to cure this. The system is otherwise working fine, but no spdif out.
Sometime giving the thought about an approach some time helps. Not this time. I'm stuck.
I'd bet that there is only something really small to change to get the spdif output for rockpro64 working since the difference to the dtb and kernel of armbian is so slight. But I don't see my approach to pin it.
Nobody else out there running rockpro64 and thinking about using a digital output 
?
Anybody who can point me in the right direction to look to find the difference?
That's actually pretty easy to achive:
RockPro64 devicetree has all i2s engines enabled by default: You'll just have to disable the unused (i2s0, iirc - i2s1 is used for the analog codec and i2s2 for hdmi) and enable spdif (and spdif-dit). Reason behind is, that the dma controller does not support all channels which are connected to it at the same time and the upstream pl330 driver does not support virtual channels (yet).
Also you should check if the second pin of the spdif header outputs the voltage (> 3V) which is needed drive the optical output (maybe there is another pinctrl required - check schematics for that) - or try first by connecting via coax by using spdif_tx and gnd only.
works without errors or glitches
Sadly it doesn't. The sound is nearly good, but it is stuttering every ~10seconds.
Any idea what else I could do?
I successfully tested the output in armbian and it just worked fine. So it shouldn't be a hardware issue.
