Last week I went deep to the Closed Captions wrapping into MXF (op1a, as02)
so the task to extend our MXF writer to write captions track and extend reader to read captions track.
by captions I mean CDP (Closed Caption Data Packet)
I had bunch of op1a/as02 files containing captions track inside, sure what I did firts is made dump to reverse structure.
Any CDP should have header which starts with MAGIC 0x9669 so to read you dont even have to understand structure (in case you dont need afd or so) its enough to go through data after Captions Essence Element key and find CDP header start, but its not enough if you want to write captions or you want todo everything right.
captions data mapping in MXF described in SMPTE 436m it costs $75 before buying I decided to google if something was discussed on forums. Most usefull comment was by link (https://trac.ffmpeg.org/ticket/726#comment:10)
it more or less describes header before cdp packet except 8 last bytes as result i had to buy SMPTE 436m, but unfortunatelly I was really surprised that 436m says nothing about that 8 bytes 🙁
I read standard like 4 or 5 times and finally I found answer in line “The Payload Byte Array is an MXF array including an array element count”
so those 8 bytes its part of MXF Array structure:
first 4 bytes – number of elements (in our case size of packet)
second 4 bytes – element size, as we have byte array its always = 1
After that, will be logical to ask why is that size different of “payload sample count” mentioned by that link?
and this is the second thing not mentioned in that comment, standard says that payload should be double word aligned and if its not it should padded by zeros
so lets say you need to map CDP with size 15 to MXF header strcuture will be:
first 2 bytes: 0x00 0x01 (we have just 1 packet so its 1)
second 2 bytes: 0x00 0x09 (as a rule captions in line 9 but you can set number you need)
fifth byte: 0x02 (if its interlaced or psf frame or 0x04 if its progressive)
sixth byte: 0x04 (8-bit luma)
7th and 8th bytes: 0x00 0x12 (why 18? because we add 3 bytes before cdp: did, sdid and cdp size)
9th – 12th bytes: 0x00 0x00 0x00 0x14 (20 because we pad 18 bytes packet to 20 to be double word aligned)
13th – 16th bytes: 0x00 0x00 0x00 0x01 (always 1)
17th byte: 0x61 (did could be 0x80 as well)
18th byte: 0x01 (sdid 1 if cea708 or 2 if 608, could be also dfferent if did = 0x80)
19th byte: 0x0F (size of your cdp)
20th – 34th bytes: your cdp
35th – 36th bytes: 0x00 0x00 (padding)