Unexpected Field order variation

[Peter van der Burgh]

Dear all,
I’ve questioned the field order judgement from TMpgEnc in a previous post (http://forum.vcdhelp.com/viewtopic.php?t=105583) and got nice responses from BJ_M and LanceSteel.

I did some more testing and followed the instructions from the suggested link (mainly about using AviSynth with the ‘Fields’ script to determine field order).

I’M TOTALLY CONFUSED NOW

In short (for more details, click above link):
I’m capturing a home video (702x576) with VirtualDub/HuffYUV (UYVY - Predict Left).
Windows98 SE, so multi segment capture.
Canon Hi8 camcorder connected to the S-Video connector of my Asus V7700 DeLuxe video card and SoundBlaster Live sound card.

I suggested that:
A) when the hardware and software settings don’t not change, the capture sessions (capture1.00.avi, capture2.00.avi, etc.) should always have the same field order. This was more or less confirmed.
B) all the individual segments from a capture session (capture1.00.avi, capture1.01.avi, etc.) should have the same field order. This was also more or less confirmed.

What was confirmed is that TMpgEnc does not (always) judge the field order correct.

The results from the AviSynth/Fields script check:
A) The capture sessions (I have 3 separate sessions – same settings are used for all 3 during capture) DON’T HAVE all the same field order (2 have Top, 1 has Bottom field first)
B) From a single capture session NOT ALL segments have the same field order!

TMpgEnc did indeed, on 2 occasions, an incorrect field order judgement, but these segments had very little motion (at least at the beginning of the file).

It’s getting even ‘better’:
Because of an error on my tape (result: a scene that has some real distortion), I used VurtualDub (direct video/audio stream) to create two separate AVI files: 1 from frame X to the frame just before the error and one from the frame just after the error to frame Y.
(X was not the start and Y not the end of the AVI segment, for testing I took 6 and 4 seconds only).
GUESS WHAT: the 2 individual ‘new’ AVI’s have different field order (one Top & one Bottom field first).

This all is suggesting that events (even cutting in an AVI) are FIELD and not FRAME based. I can’t believe this, but it’s the outcome of my test.
Or does VirtualDub 'screw up' the field order when starting to capture; starting another segment file and/or saving a selection from an AVI file using the direct stream setting?

Can someone confirm that what I see is either ‘normal’ or not or what I could possibly do wrong.
Please help, I’m totally lost because I can’t believe this all, especially not the results after ‘cutting’.

Peter

[Spicuzza]

Your capture card drivers determine the field order at the time of capture.

If sometimes you get Field A and other times you get Field B, you have a hardware and or driver problem.
Nothing to do with software editing or encoding....the problem exists before that.

From Ulead's Tutorial site:

Video capture
Capture card drivers often offer the option of capturing one field per frame (sometimes you can choose the first or second), or capturing both fields. In addition, when capturing both fields there is a question of what order the fields are packed into the frames of the file on disk. Most drivers pack them in the order they are scanned, field-A first, but some pack them field-B first. Your capture card's documentation should tell you which field order it uses, but if you can't find it try order A and do a field-dependent trial edit. If the result plays badly, try order B. DV Type-1 is always order A.

[Peter van der Burgh]

Spicuzza, thanks for the reply.

I will sertainly follow up on your suggestions, but:

If I only had the problem during/as a result of capturing you are 100% right, it is most probably the hardware or drivers, but what about cutting two fragments from an single AVI file and ending up with two different field order files?
This can't be driver/hardware related, can it?

Peter

[Spicuzza]

but what about cutting two fragments from an single AVI file and ending up with two different field order files?

I don't have an answer for that one. Sounds odd. I chop up uncompressed AVI all the time and have never encountered that problem.

What tells you the field orders are different?

[Lamont Cranston]

Hi Peter,

Have you determined the field order swapping is real? For example, the clip you cut in segments have you encoded both segmets with a mpeg 2 encoder using the same field order for both, viewed the results and verified that one was jumpy and the other was not? If indeed the field order is being swapped during capture it sounds like the capture card has problems. However, I have one souce that captures a field first while all the other sources I have capture b field first. Everything works fine except the field order is reversed for that one source. Some codecs can be setup to flip the field order and the wdm->vfw wrapper will filp the field order but in all cases the field order is not changed randomly.

In the case where I have one source where the field order is flipped I suspect there is an anomaly in the video sync pulses that causes the capture card the flip the field order but that is only a guess and I have not had a chance to instrument it to find out why.

I would suggest you try another video source and see if the if the problem persists.

I am using AVI_IO to capture NTSC 720 x 480 using MainConcept's DV codec (DV type 2) with wdm->vfw wrapper from an ATI AIW 128 Pro.

[Peter van der Burgh]

Both, Spicuzza and Lamont thanks for the reply.

Spicuzza, it sounds like you don’t believe me. Can’t blame you, I don’t (want to) believe it myself.
I used VirtualDub with AviSynth and the ‘fields’ script to determine the field order (http://www.lukesvideo.com/ select ‘Interlacing’ in the left column and down to ‘Determine the Field Dominance’ in the document).
It will output the two fields as two separate frames. You will have a picture with half the height and the normal width. When played you will notice a small up-down motion, the difference in placement between the top and bottom field.
If you however also see a left-right motion with moving objects then the field order is incorrect.
AviSynth can reverse the field order, so it will not have the jerky horizontal movement.

Lamont, the field swapping is as real as I can get it (not that I want it). I think the above explains how I determined it and believe me I checked and double-checked the results. (I will re-re-check this evening). I also have the impression that the above method is a good and clear one.
No, I did not encode to MPEG2 – why would I?
With the above method I determined the field order and I’m sure that if I encode using the wrong field order the result will be a more or less (depending on the amount of horizontal motion) jerky video.
I did join the two AVI’s together and confirmed, using the above method, that one half was jerky and the other one was not.

Are you sure that all, but one, of your sources is b-field first? How do you determine this?

Since I wanted to know the ‘default’ field order my capture card is using, I never touched the ‘flip field order button’. I don’t think I can set this on capture card/driver level. I seem to remember I can set this in VirtualDub, but I never touched that one as well.

I might try another source one time, but I still want to capture the home video’s using my camcorder, so that wouldn’t help a lot. At the same time it does not explain why the problem pops-up when cutting an AVI. There must be something else going on.

More suggestions are welcome

Peter

[Spicuzza]

Peter, you're right, I doubt what you "think" is happening is actually happening.

Lukes's Video States:
"In general, there is no consistent field dominance. Each capture card and video source can lead to a different field dominance. One way to determine the correct field dominance is to play a segment of your video on a TV. If the motion is jerky, try changing the field dominance so the other field is displayed first (dominant)."

Lamont Cranston gave you the proper answer:
"......the clip you cut in segments have you encoded both segmets with a mpeg 2 encoder using the same field order for both, viewed the results and verified that one was jumpy and the other was not?"

Until you encode both ways and watch it on your TV I don't think you will be able to determine the field orders by looking at interlaced video on a computer monitor.

[dphirschler]

"Because of an error on my tape (result: a scene that has some real distortion), "

I was waiting for you to say that. That was my suspiscion all along. Bad sections sometimes cause dropped frames. Sometimes, they cause dropped fields, which means your fields will be out of order.

I've had it happen numerous times. It's difficult, but you have to try to start capturing that segment (if you want EVERYTHING) right after the bad section. In other words, try to start the capture on the first good frame.

Also, to find your field order, try this:

In VirtualDub, use the deinterlace filter, unfold fields side by side. then find a scene change in your video. The field that changes first (top is left, bottom is right) will give you your field order.


Darryl

[Lamont Cranston]

Lamont, the field swapping is as real as I can get it (not that I want it). I think the above explains how I determined it and believe me I checked and double-checked the results. (I will re-re-check this evening). I also have the impression that the above method is a good and clear one.
No, I did not encode to MPEG2 – why would I?
With the above method I determined the field order and I’m sure that if I encode using the wrong field order the result will be a more or less (depending on the amount of horizontal motion) jerky video.
I did join the two AVI’s together and confirmed, using the above method, that one half was jerky and the other one was not.

Are you sure that all, but one, of your sources is b-field first? How do you determine this?

Since I wanted to know the ‘default’ field order my capture card is using, I never touched the ‘flip field order button’. I don’t think I can set this on capture card/driver level. I seem to remember I can set this in VirtualDub, but I never touched that one as well.

I might try another source one time, but I still want to capture the home video’s using my camcorder, so that wouldn’t help a lot. At the same time it does not explain why the problem pops-up when cutting an AVI. There must be something else going on.

More suggestions are welcome

Peter

Peter,
I suggested encoding mpeg 2 because I don't trust most of these field order test methods like TMPGEnc. By encoding a mpeg 2 clip and making a DVD or an SVCD then playing the result on a interlaced tv it is immedialtely obvious by the jerky motion when the field order is swapped. Yes, it is time comsuming but I only have to do this test once and there is no guesswork.

The way I arrived at B field first was encoding wth the Ligos LSX35, TMPGEnc and finding that B field first produced a non-jerky mpeg 2 (DVD). I repeated the test with Ulead's Ligos Encoder in MSP6.5 and Ulead's Mpeg 2Now encoder in DVDWS and produced a non-jerky mpeg 2 (DVD) with A field first. No, that is not a conflict. Ulead calls A field first what all the rest of the world calls B field first. Search the forum for verification if you are in doubt.

One explanation for the field reversal popping up in the middle of the cut is you happen to pick the point to cut where your capture card flipped the fields.

I recommend you try another video source just to help isolate your problem. The symptoms you describe are bizarre and I suspect your video signal.

[Peter van der Burgh]

Dear all,

I’m happy to announce that we can close this one.
You were all more or less on the right track.

Lamont, I know what you mean by ‘encoding to MPEG and watch it on TV’. It's (almost) impossible to determine the (correctness of the) field order by looking at an interlaced video on a computer sceen, but with the AviSynth method I look at a non-interlaced field-after-field video. You and Spicuzza should really try to let AviSynth frame serve into VirtualDub using the ‘fields’ script. You will view your AVI at a double fps rate, each frame containing one field of the original. It is easy to see the incorrect field order, even on parts with hardly any motion.

DPHirschler, I will certainly try your ‘VirtualDub – Deinterlace’ suggestion.

Conclusion:
A) With the same hardware/software settings, capturing ALWAYS starts at the same field. (In my case the Top field), but don’t start capturing without a decent video signal present (e.g. at the very beginning of a tape).
B) All segments of a capture have the same field order, unless ‘something’ caused the field order to change. This is not (necessarily) happening between two segments, but somewhere else. If the field order of the START of segment 2 is not the same as that of the START of segment 1, the END of segment 1 has also a different field order. In 2 occasions I could not see why the field order had changed, but on all other occasions it was a bad video signal from the camcorder.
C) And of course, if you cut and save parts of an AVI the resulting AVI’s have all the same field order as the original. Indeed DP, I did cut just before and after a bad section and in that bad section the field order changed. It was there in the original AVI as well.

I presumed that capture cards would somehow (timing control) preserve the field order during capture even when they, during a short period, loose track of signal, but I was wrong.

Thanks again for all your suggestions, it really helped.
Peter