codec/interlacing confusion for custom software

[Fred Jodry]

Answer: when you get the good advice, pencil it down.
Your HDMI card output to the display is usually 59~60 cycles per second because of two reasons, 1. less than 60 flickers per second would be too poor flickering, 2. Hi-def TV was developed largely as a reminder of NTSC TV. An typical example of an HDMI doing far away from a 60 cyles rate would be a Gamer running a 240 frames per second “low blur low flicker” game.

Your recorded DVD can put out almost any vertical rate within reason because it`s recorded audio-video data on a disk. Obviously most examples are same or high-quality cousins of 60 fields, 30 frames TV. Still, one can occasionally find that one has got a DVD that contains whatever field, frame, and line rates that were oddly made and can be viewed on a computered display but not the average TV.

Any composite video outputted (if NTSC) DVD player is primarily used for 60 fields, 30 frames TV but many component outputted types also run on other standards like 60 fields, 60 frames. This can be better if things match but is instantly worse if they don`t match.

So it can be worse. Enters GenLock. More important, enters odd-even field marking. Otherwise your desired video frames can get re- constructed straddling two frames. Also, standards converters can blur frame refresh times, and so on. For the for the freshest answers, often ask a Broadcaster, and some others who know the technical terms and the best equipment. By the way, if you want to check for field, frame blur, see your video on a oscilloscope. When you get the good advice, pencil it down.

[Fred Jodry]

Alex, my advice yesterday was a bit garden variety but essentially all good. Here are some additional comments:

you, “I’ve learned a bit about genlock but I can’t find a camera within my budget that has a 60p out.”
me, Those types of cameras were made and sold quite a bit in the second half of the 1980`s and through the 1990`s because improvements in the regular NTSC TV quality towards SMPTE-C and beyond required 60 actual frames per second, not 30 frames per second scanning. Also sports productions liked to take even more than 60 frames per second and drop most frames to make strobed study- like moving images. These cameras required the light of the mid-day sun to do it but for a baseball game, who cares? My advice on getting those cameras is to know that broadcast stations have been throwing away those cameras right and left, so the goal is simply to be lucky enough to ask a station or retiring employee of one, if you may have it before it gets chucked. A respectable bath and conversation does more to get this stuff than does a full wallet because the tosser can consider this stuff to be distracting junk, it`s not hi-def!, the same as older equipment. These cameras have horizontal scan rates of 31,000 Hz (or so, math) or higher and feed their outputs through “component” (analog, studio type) monster cables. Your only three competitors in this, three stiff competitors, are the garbage can, the DVD makers, and the (still NTSC) cable stations. Note, some cameras found in Europe also work. Get looking.

I noticed a new post in Creative Cow`s front page starting today, just to mention:

“Hot Industry News, News: Matrox Ships World’s First HD-SDI Scan Converter with Genlock Under $1000”

codec/interlacing confusion for custom software
by Alex Delany on Feb 19, 2010 at 11:21:02 am

I am developing some custom software and am hoping someone can help me figure out why or when my image is getting interlaced. The goal of my software is to allow me to aim my camera at a video monitor and record EACH FRAME displayed on the monitor (tv monitor).

I’m going out from my camera (via HDMI) at 720/60P into my Blackmagic Intensity Pro card. I’ve deduced that I’m actually capturing at 59.94. I am new to this but have gathered that, my DVD player connected to my TV is broadcasting at 29.97 fps.

When I playback my captured video, every other frame is clean. (and every other frame blurs with it’s preceding frame). This is expected, because for every broadcast frame (at 29.97) we are capturing two frames (from capturing at 59.94), and nothing is synced.

But once we compress using MJPEG the captured video becomes interlaced, giving twice as many half-hieght frames (120 per second).

I’m really unclear about what I’m actually viewing when I open the video up in Quicktime or Window’s Media Player. I can’t tell what the player is doing in terms of de-interlacing. Ideally I can just save the progressive frames. If anyone can help me conceptualize this, I’d appreciate it.

Also if anyone has any suggestions on how to properly sync, I’d love to hear. I’ve learned a bit about genlock but I can’t find a camera within my budget that has a 60p out.

Thanks,
Alex