I have now tested on a newer receiver.

Then I use the display port the receiver say it get a Dolby D signal.
Then I use the dock and Displaylink the receiver says “Multi in”.( So the 5.1 sound works on the newer receiver)

I thought Displaylink just pass-through the sound, but it seems it process and changes it in some way. The source is Dolby D.

I have tried to google it and I think it makes it to PCM multichannel. I don’t think my old receiver can handle PCM multichannel so that should explain my problem. And if I understand it right, if I set the source(computer/Displaylink) to Bitstream it will pass the signal unprocessed to the receiver.

Have I understood it right and is it possible to set Displaylink to Bitstream so it just pass-through the signal?

I've checked with the engineers and yes, your understanding is correct. It is PCM only, not compressed bitstream.

Wim