My friend has an evaluation board for the SOM and, although HDMI works without the magic cable, it also has some small issues with the image, there are some minor interferences sometimes (a few pixels blinking, an occasional horizontal line).
I meant to add that the "magic 5m cable" solves these small issues on the SOM's evaluation board.
Yeah, at least as of today, there's only one such SOM available so we can't test another one. I'll suggest it to him, to get a another SOM.
Maybe make a second 'inverse' adapter and stick it between some other HDMI things you have on hand.
What do you mean by the inverse adapter?
To clarify what we have:
[carrier board with SOM plugged in | female mini HDMI connector] -> [male mini HDMI connector | small adapter with ferrites and trace len fix | female HDMI connector] -> (HDMI cable / cable set) -> [Monitor]
This all works better with lower resolutions (some cable combinations other than the magic one show lower resolution formats, but with lots of image quality problems and frame loss, while at full HD it basically doesn't show anything).
By "voltage conversion" I meant voltage level conversion, level shifter, there are other names. It's not an active repeater, it's just ESD protection for the differential pair lines (the chip hangs on the lines, it's not in series) and digital (I2C....) level conversion (SOM -> 1V2, HDMI -> 5V) and 5V HDMI power line current limitation, I don't have the chip reference handy.
I understand the reflection theory. I don't have the knowledge to evaluate the PCB layout on that light. As far as I can see, the basic rules were followed, both lines of the same pair are close, more space between pairs, uninterrupted GND plane underneath the pair, all differential lines the same length (within 3mm) (both inside a pair and between pairs)... Most non-ideal situations is that the signals change layer through vias some 3 or 4 times and there's a spot near the (mini) HDMI connector where the GND plane was interrupted because of the layer changing vias, but it's still very close; I'll see if I can get/post shots of that. Maybe the layout should be re-designed to have more direct (less vias) path from SOM to HDMI connector...
Many thanks for the help.