Understanding 1080-60i (1920×1080, 29.970 fps)

[Yuan Soong]

Thank you much John!

[John Rofrano]

You’re welcome. Glad I could help.

~jr

http://www.johnrofrano.com
http://www.vasst.com

[Gary Abraham]

Thanks Ken for asking the question I’ve wanted to ask and to John for the great explanation!
Regards,
Gary Abraham

[Jay Beckman]

I have been working on converting my huge family VHS collection for a decade now. Every time I believe I have interlacing figured out, I come across another issue, resulting in me needing to start all over which is beginning to drive me insane. I have a DVD/VHS combo upscaling unit. Due to HDCP, It does not function directly with any modern display, instead I get a blue screen. So I was forced to use a splitter to strip the HDCP.

(Combo output HDMI to splitter HDMI input, then splitters HDMI output A to display, and splitters HDMI output B to hd recorder.)

I capture the footage with 1280x720p @ 59.94fps. For some reason, all of the footage I convert has one dropped frame every second, the missing frame is replaced with a copy of the previous. I know for a fact that the hd recorder is not the cause, because I can notice some of the frame drops while watching my plasma display, which is connected directly from one of the splitters two HDMI outputs. Sadly I am unable to test the combo without the splitter and have no other equipment to test the splitter on, so I do not know which, if any, is at fault. For all I know, this could be a natural occurrence from the nature of VHS, or any particular VHS. Six months ago I thought that most or all pre digital interlaced broadcasts and VHS recordings were 30i, which was 30 frames per second, with each frame containing an upper field and a lower field. I know there is a 1/60th difference in each fields time. I thought both fields that made a single frame were part of a single image, until someone told me otherwise. He said that the fields were separate images.

At that point I began to wonder if the upper field appears, then the lower field appears, then the upper field is replaced with the next upper field, then the lower field is replaced with the next lower field, and so on. But if that is the case, then what happens at the first frame? Is the first frame missing one of the fields? Or is the first frame given a temporary first field that only lasts half as long as the other fields? Or is a special, unique frame created for the first frame? Is this why 60 is actually 59.94 and 30 is actually 29.97, to make pasting multiple recordings together easier, so the fields continue to flow in the same sequence uninterrupted? If I have a 59.94 fps recording that is exactly one second in duration, will it have 60 unique frames, or 59 unique frames, or 59 unique frames with a 60th frame being a duplicate?

Another issue I am faced with, is the fact that, even though I always capture VHS at 59.94, some of the footage shows movement every other frame, while other footage shows movement every frame. Could this be a timing issue with one of the devices, or did some pre digital broadcasts send out progressive signals instead of interlaced?

Why is it that some interlaced footage that I have worked with, such as 2006 era mini-DV footage, has very noticeable artifacts such as combing during horizontal movement, while late 1980 era VHS recordings that my grandfather made, has no noticeable artifacts whatsoever?

Someone please help me.