On cheap SDI converters and Decklink SDI capture cards or: How I learned about the incompatible 3G SDI channel mappings

To test the Blackmagicdesign Decklink 4k Pro card I want to use to test my open source SDI implementation photonSDI against, I connected a cheap HDMI to 3G SDI converter to a HDMI port on the computer, configured the HDMI output to 1080p60 mode (60 full frames per second with 1920x1080 pixels each) and the SDI output to the input of the Deccklink card. To transfer the 1080p60 video stream, the link has to run at 3G SDI line rate, but since both converter and capture card support 3G SDI this should work out of the box. At least that’s what I thought. Turns out that it didn’t and the capture card detected the SDI format as 1080p30, only managed to capture a frame every 5 to 6 seconds and even those frames were garbled.