All times UT unless otherwise noted
IOTA-VTI - Determining the proper field order for LiMovie analysis of occultation videos?
by Derek Breit
John Wright noticed an issue displaying fields and thought it had to do with the copying from DVD to a format a computer can read. He wrote this up and it is posted on Brad Timerson's North America Asteroid Page. The direct link is http://www.asteroidoccultation.com/observations/NA/DVD_issues_and_WWVB_time_synchronization.pdf
I was aware of this subject via offline emails, and at one point John said "I don't think it is the IOTA-VTI". I had noticed this behavior as well, but the only common device from our respective setup's was the time inserter. This page covers the issue and some tests I did.
Here is an 18 megabyte file that shows the issue, which is the order in which the fields are displayed. I setup a camera with the IOTA-VTI and videotaped the 1PPS LED. It is extracted from the middle of a larger video where I was disconnecting and reconnecting power...
You will note that on the first power up the decimal seconds are displayed on the left, but on the next power cycle, they are on the right. Viewing a frame in LiMovie or Virtual Dub, sometimes the left field time is larger than the right. Other times, the larger field time is on the right.
So the question is.. How does the field time position affect analysis in LiMovie when examining a video to the field level?
First up is what I call "Forward" with LiMovie measuring "Odd Field" first (which comes out correctly). Note- The step to GPS time is irrelevant to the matter at hand.
Now we have "Forward Even First" which comes out wrong
Next, we have "Reverse (where the larger field end time is on the left) and LiMovie examines Odd Field first"
This comes out correctly.
Finally, we have "Reverse, Even First" which comes out wrong
It makes no difference how the IOTA-VTI displays the fields. If you previously tested the Field Order of your KIWI OSD and set LiMovie to display the fields in the proper order as I previously documented, then the IOTA-VTI / LiMovie combination will accurately evaluate your videos regardless of whether the field order is displayed properly or reversed.
For my setup, including how I import the video into the computer, setting LiMovie's "Field Order" switch to "odd first" is the proper position. Each observer should determine what their setup does via this simple one time test.
Note the Field Count at the right has the higher numbered frame in the top half of the window, regardless of where the decimal seconds are located.
The IOTA-VTI makes no distinction as to whether a given field is the even or odd field of a frame. It takes the first field it receives and places the decimal seconds in the left location. Every field after that is written to the alternate location.
In "Forward Fields" above, this happened to come out "correctly"; in "Reverse Fields" it happened to come out "incorrectly".
To better understand how this happens, remember the IOTA-VTI makes no distinction as to an odd or even field, marking the first field time to the left; and that the NTSC / EIA Standard states a "frame consists of an Even and Odd field".As an example consider four sequential fields with a sample time.
1 = 0.1 = Even = Left / 2 = 0.2 = Odd = Right / 3 = 0.3 = Even = Left / 4 = 0.4 = Odd = Right
With this example, the playback device (such as a monitor, camcorder, VCR, Computer, etc) will assemble field 1 and field 2 and the timestamps will appear at the left and right, respectfully. But if the first field the IOTA-VTI encounters happened to be odd..
1 = 0.1 = Odd = Left / 2 = 0.2 = Even = Right / 3 = 0.3 = Odd = Left / 4 = 0.4 = Even = Right
then the playback device would assemble the frames from field 2 and field 3, and the left decimal seconds would be higher than the right. Your device may vary as to how in the order it assembles fields into interlaced video, but the process will be the same. The end result will always be linear - the fields will be sequential and individually timestamped and LiMovie will analyze them without issue if you have determined the proper field order outlined in Part 1 above.
LiMovie reads the IOTA-VTI timestamps accurately because, as outlined in PART TWO, the video is always sequential and individually timestamped.
Note the time display in the video image has the left decimal seconds larger than those on the right.. "Reversed".
Also note under the light curve where it says..
Frame No 23.0 / Frame Center 18h29m54.0262s, Frame End= 54.0429s
PART FOUR (optional)
I like to post "proof" of what I do to my website and I like things as "perfect" as I can get them. While this page has shown it is completely unnecessary, my setup allows for a rather simple way to always acquire "forward" video where the larger timestamp is always to the right.
In the 18MB video in PART ONE, the first half of the video shows the decimal seconds all on the left, the second half shows them on the right.
If, prior to recording your event of interest, you make a short recording, you can then play it back on the recording device. I use a Canon ZR300 DV Camcorder. If you play back the short clip and press pause, the ZR300 will display the decimal seconds in either the left or right location. If the decimal seconds are in the righthand location, then the video will be "correct" and always have the larger numbers on the right.
If, however, the numbers are on the left, you can press the RESET on the IOTA-VTI and record another short clip. Repeat as necessary until the numbers, when paused, are on the right. It is a simple procedure as it is literally a coin toss to which way the fields align to the interlaced frame, so it wont take more than a two or three attempts.
Various devices may do things differently, but once you determine how your device works, it is easy enough to repeat so you always get the result you desire.
This site was last updated 08/10/16
© Copyright, Derek C Breit. All rights reserved.