You certainly can capture DNxHD36 from a live HD-SDI stream with Avid hardware or AJA i/o Express. Or you could capture via a Cinedeck or Sound Devices type device directly to DNxHD 36.
Are you planning on inserting the LUT into the HD-SDI stream with something like an HDLinkPro? make sure you don’t have any frame delays. Also, what will the common source ID be for the image being captured?
Unless the filename is being inserted into the HD-SDI and decoded, you will only have timecode – but to which file will be lost. I know the RED camera inserts this into the HANC – I don’t know about Arri Alexa unless it was added with the latest firmware.
Michael
Michael Phillips