interlace help

[nymph4444]

I need to know if I understand how interlace works.

The way CRTs displayed images was interlace.

It would start at the top left and draw a Zig Zag Line left to right and go all the way down the screen.
But this would be just one field so when it got to the bottum of the screen it would go back up and draw the other fild in.

Now I know that these 2. feilds are the top feild and the bottum feild.

So am I right the first pass it is drawling the Bottum field and the second pass it is drawling the Top fied??

[Baldrick]

Video streaming? Moving you to the general forum section.

[edDV]

I need to know if I understand how interlace works.

The way CRTs displayed images was interlace.

It would start at the top left and draw a Zig Zag Line left to right and go all the way down the screen.
But this would be just one field so when it got to the bottum of the screen it would go back up and draw the other fild in.

Now I know that these 2. feilds are the top feild and the bottum feild.

So am I right the first pass it is drawling the Bottum field and the second pass it is drawling the Top fied??

Not exactly. There are odd line fields and even line fields alternating during a broadcast. There is no concept of top or bottom field first until video is digitized and captured to a file. Some digital formats use TFF, some (e.g. DV-AVI) use BFF.

[nymph4444]

I sould say that I see a lot of things on the net that say the first pass draws the od Lines first.
And then it draws the Even Lines.

So am I right the Od Line would be the Top feild?

[edDV]

I sould say that I see a lot of things on the net that say the first pass draws the od Lines first.
And then it draws the Even Lines.

So am I right the Od Line would be the Top feild?

Yes when scanning the CRT but when stored in a file or TS packet the first field stored might be the odd or even field. TFF and BFF only relate to the digital data structures.

[nymph4444]

Thank you this makes sance to me because I use Movie Studio 8 from Sony.

And I save my work as Un Compressed AVi and it saves as Interlaced.

And if I look in Option it has Lower Field first.

So this is what you told me it saves Lower field first when saving it. But when playing it plays Top feild firsat.

[edDV]

Thank you this makes sance to me because I use Movie Studio 8 from Sony.

And I save my work as Un Compressed AVi and it saves as Interlaced.

And if I look in Option it has Lower Field first.

So this is what you told me it saves Lower field first when saving it. But when playing it plays Top feild firsat.

How did you capture this file? What capture hardware?

What is your Vegas project setting? It defaults to DV BFF.

Without getting into data structure details, you can tell if the field order is wrong when playback shows jerky or blurred motion. Usually you want project field order to match your source. DV is always BFF, most other capture and digital broadcast is TFF.

[jagabo]

I sould say that I see a lot of things on the net that say the first pass draws the od Lines first.
And then it draws the Even Lines.

So am I right the Od Line would be the Top feild?

It depends on how you number the lines. Computer programmers (depending on what language the program in) start counting with zero. So they would say the top fields is the even field, the bottom field odd. Normal people start counting with one so they would say it's the other way around.

[nymph4444]

Well even if I just open a file in my movie studio if I go to edit it and then I am ready to save it I go to Render As.

Then pick AVi Uncompressed.
And there is an opyions tab and when I look in it it has Interlace mathed Lowe field first.

But I think I know what you are getting at whatever you do when you save it it will be reversed when played back.
I will look into it more thanks

[edDV]

whatever you do when you save it it will be reversed when played back.

You need to get your field order correct. If you get it wrong, there will be jerky motion on playback.

Ideally the project field order should match the primary source format although Vegas can handle mixed TFF/BFF/progressive sources. You need to make sure field order is correctly identified in clip properties.

What are your source formats? Use Mediainfo if you don't know.

What is your Vegas project format? Best to set this to match the source.

Why are you saving uncompressed? How will the export file be used?

Don't confuse digital field order with analog CRT field scanning. Field order is determined at the point of analog to digital conversion.

[nymph4444]

Well CRTs play Top field first then Bottum field next.

So my gess is if I want to play it back on a CRT screen I sould save it Bottum field first because it will reverse it when played back

[edDV]

Well CRTs play Top field first then Bottum field next.

So my gess is if I want to play it back on a CRT screen I sould save it Bottum field first because it will reverse it when played back

Again, it is a digital data issue. Field reversal means the wrong field plays to the even and/or odd scan lines. You need to get the field order flag correct based on how it was digitized. Make an interlace DVD and check motion on playback.

You don't want to answer my questions so all I can say is good luck.

[jagabo]

Well CRTs play Top field first then Bottum field next.

No. Analog video is simply an alternating series of top and bottom fields. When you watch interlaced video on an interlaced CRT you never seen an entire frame. You see one field at a time. When one field is being drawn the previous field has already faded away.

When captured as digital frames pairs of fields are woven together into frames. The capture device can start with a top field then add in the next field (a bottom field) to create a top field first frame, or it can start with a bottom field then add in the next (top) field to create bottom field first frames. This is why digital interlaced video has to have a field order flag. So the playback device can display each of the fields in the correct order.

[nymph4444]

Soory I get ahead of myself.

I foregot you asked me.

When I capture video my program saves it as Interlaced and it says Bottum feild first.

And my capture device is the comeing into my computer through Firewire.

So let me sk you told me that whatever field is saved first it will be reversed when played back.

This may be why my program captures video with Bottum field first becasue it will play the bottum field second.

[jagabo]

So let me sk you told me that whatever field is saved first it will be reversed when played back.

No, it's not reversed when played back. It's played back in the same order it was captured.

Interlaced analog video has no frames. It is simply a sequence of alternating top and bottom fields. There is no top-field-first or bottom-field-first, just alternating top and bottom fields. If you capture them as TFF frames they you play them back as TFF frames (ie you display the top field first, then the bottom field). If you capture them as BFF frames you play them back as BFF frames (ie you display the bottom field first, then the top field).

A sequence of fields as displayed on an interlaced TV:

1t 2b 3t 4b 5t 6b 7t 8b...

I arbitrarily started with a top field but it's just an alternating sequence of top and bottom fields.

Capture as top field first frames:

1t+2b 3t+4b 5t+6b 7t+8b...

When played back they are played back top field first:

1t 2b 3t 4b 5t 6b 7t 8b...

If you play TFF frames as BFF you get pair of fields in the wrong order:

2b 1t 4b 3t 6b 5t 8b 7t...

This results in very fast-jerky motion.


When captured as bottom field first frames:

2b+3t 4b+5t 6b+7t 8b+9t...

they are played back bottom field first:

2b 3t 4b 5t 6b 7t 8b 9t...

[edDV]

When I capture video my program saves it as Interlaced and it says Bottum feild first.

And my capture device is the comeing into my computer through Firewire.

What is the capture device?

Capture via Firewire does not necessarily mean the video is BFF. While DV format video is always BFF for both PAL and NTSC, MPeg transport streams from tuners are usually TFF.

If you are certain your capture device creates BFF format video, then you would set your edit project to BFF and encode an interlace DVD as BFF. If you are mixing in TFF interlace video from other sources, make sure the clip properties identify the videeo as TFF.

[jagabo]

If you need to convert TFF video to BFF (or vice versa) you can separate the fields, throw away the first field, then recombine the fields into frames. Or you can shift the frame up or down by one scanline.

[nymph4444]

OK I think I get it now.

When played on a CRT screen it sould start at TOP of screen and draw the TOP feilds first all the way down.
The go back and Draw the Bottum feilds all the way down.

And as it is drawling the Bottum feilds all the way down the screen at the same time the first feild that it did draw starts to fade away.

I get all of this now this next part is were I get mixed up.

My capture device is an ADVC100 Analog to Digitla converter box. And it outputs the signal as DV through a Firewire to my computer.
My computer program Movie Studio captures the video as AVi Uncompressed.

And if I look in video settings I see Feild Order and it says Bottum Feild first.

Now if I play it back on a CRT scren the Bottum feild will be drawn first like you told me how ever it is captured it will play back the same way.

What I don't get is how can it start playing back the video by sdrawing the Bottum feild first?

The Bottum feild is always the Second Frame being drawn right?

[jagabo]

OK I think I get it now.

When played on a CRT screen it sould start at TOP of screen and draw the TOP feilds first all the way down.
The go back and Draw the Bottum feilds all the way down.

And as it is drawling the Bottum feilds all the way down the screen at the same time the first feild that it did draw starts to fade away.

Yes. Except interlaced video doesn't start with a top field then finish with a bottom field. It is simply an alternating sequence of top and bottom fields. When you turn on the TV it might start displaying with a top field or it might start with a bottom field, whichever being transmitted at the time it starts displaying the picture. It would be more proper to say the TV draws one field, then returns back to the top of the screen and draws the next field.

My capture device is an ADVC100 Analog to Digitla converter box. And it outputs the signal as DV through a Firewire to my computer.
My computer program Movie Studio captures the video as AVi Uncompressed.

And if I look in video settings I see Feild Order and it says Bottum Feild first.

Now if I play it back on a CRT scren the Bottum feild will be drawn first like you told me how ever it is captured it will play back the same way.

Yes.

What I don't get is how can it start playing back the video by sdrawing the Bottum feild first?

The Bottum feild is always the Second Frame being drawn right?

Again, interlaced video is neither top field first nor bottom field first, simply an alternating series of top and bottom fields. You can start displaying with a top field or a bottom field -- it doesn't matter which one.

[edDV]

...
What I don't get is how can it start playing back the video by sdrawing the Bottum feild first?

The Bottum feild is always the Second Frame being drawn right?

You are still confusing digital file format structure with analog video. As Jagabo said, analog just alternates odd and even line fields. The way the TV (or capture card) knows the odd from even field is the position of the vertical sync pulse in relation to equalizing pulses as shown here.



http://www.bewinstar.co.cc/2010/11/experiment-iii.html

Note that this picture is for 625/50 PAL but NTSC (RS-170A) is similar.

Here is the four color field sequence for NTSC RS-170A. Composite NTSC has a four field color frame. Componet analog or digital only need two fields to complete a frame so can be edited at the frame.



http://www.ntsc-tv.com/ntsc-index-02.htm

A composite PAL colour frame consists of eight fields.

Digital video uses different techniques to identify field one from field two using data flags. Digital video file formats can be stored TFF or BFF. So, in addition to field one vs field two flags, an editor program needs to know which field is to be read first for file playback. Edit programs need to match various BFF and TFF video files as a layered composite.

[nymph4444]

Thanks this makes sance now whenever you turned on the CRT TV whatever feild is being drawn at that moment is what you will see first I get it now.

But you say there is no Top Feild and no Bottum Feild.
But there is because everything I read and look at about Interlaced Images tell it this way.

One Image is made up of 2. Images.

The CRT draws a zig zag line from left to right from top to bottum of the screen.
And these lines are spaced out so there is a little bit of room right after every line.

When it get to the bottum of the screen this is nnot one Frame I know this but it is one image.

Then it goes back and starts drawling the next set of Line zig zag fromm top to bottum.
But this time it draws these line in the spaces that were left open.

And as this set is drawn the first set fades away and the eye can't see it because it is to fast.

Is this how it is doen if not let me read more??

[jagabo]

I sould say that I see a lot of things on the net that say the first pass draws the od Lines first. And then it draws the Even Lines.

When you see statements like that they are arbitrarily starting with the top field. They could just as well have started with the bottom field. Ie, there is no start and there is no end. You can start with a chicken or you can start with an egg. Either way you'll have an alternating sequence of chickens and eggs.

[edDV]

...
My capture device is an ADVC100 Analog to Digitla converter box. And it outputs the signal as DV through a Firewire to my computer.
My computer program Movie Studio captures the video as AVi Uncompressed.

And if I look in video settings I see Feild Order and it says Bottum Feild first.

The ADVC-100 converts analog NTSC or PAL to DV format. Lets assume NTSC for now. First composite input is Y/C separated. S-Video is already Y/C. The ADVC will start decode on a fresh NTSC color frame. Next C is decoded to Pb, Pr color components. Then Y, Pb, Pr are A/D converted to digital Y, Cb, Cr sampled 4:1:1, 720x480 interlace in DV 5x compressed format. DV uses a BFF format structure.

Question: Why are you capturing "uncompressed" rather than "NTSC DV"? Uncompressed requires a decode conversion and makes the file 5x larger. Some computers may have problems with that bit rate. Normal capture would be DV format to a DV-AVI file. This is 1:1 what the ADVC-100 exports. Normally a DV capture will be edited as an NTSC DV project.

Now if I play it back on a CRT scren the Bottum feild will be drawn first like you told me how ever it is captured it will play back the same way.

What I don't get is how can it start playing back the video by sdrawing the Bottum feild first?

The Bottum feild is always the Second Frame being drawn right?

This is all handled by MPEG encoding. If your project is BFF, then export (render as) MPeg2 BFF and all will be fine with the DVD player. The DVD player will export depending on your player settings to composite/S-Video, or analog component, or HDMI out. In each case the field order is handled so long as you specified BFF. If you get it wrong, the motion gets wild jerky.

[edDV]

Thanks this makes sance now whenever you turned on the CRT TV whatever feild is being drawn at that moment is what you will see first I get it now.

But you say there is no Top Feild and no Bottum Feild.
But there is because everything I read and look at about Interlaced Images tell it this way.

One Image is made up of 2. Images.

The CRT draws a zig zag line from left to right from top to bottum of the screen.
And these lines are spaced out so there is a little bit of room right after every line.

When it get to the bottum of the screen this is nnot one Frame I know this but it is one image.

Then it goes back and starts drawling the next set of Line zig zag fromm top to bottum.
But this time it draws these line in the spaces that were left open.

And as this set is drawn the first set fades away and the eye can't see it because it is to fast.

Is this how it is doen if not let me read more??

This is how a CRT is scanned but has a distant connection with DV format lower field (bottom field) capture.

Bottom line: If you get the digital field order wrong, everything that happens from then on will have jerky motion (i.e. two frames forward, one frame back, two frames forward, one frame back ... repeat).

[nymph4444]

OK I get what you are telling me just one thing.

Now I am right that if you stoped an image onscreen and if you could get it that fast I sould say amd I right on this?

One frame is made up of two diferant images?

You would see the first image and second image blanded?
But when they move it goes so fast you just see it as moving images?

[edDV]

OK I get what you are telling me just one thing.

Now I am right that if you stoped an image onscreen and if you could get it that fast I sould say amd I right on this?

If you press pause on a normal VHS VCR, you will see one field (half vertical resolution).

One frame is made up of two diferant images?

A frame capture requires the equipment to deal with two field capture. A straight capture will show 2 fields with motion displacement (aka "lines" "line split"). More sophisticated equipment will apply various methods of deinterlace.

You would see the first image and second image blanded?
But when they move it goes so fast you just see it as moving images?

This depends on how the application applies deinterlace.

Now, I think the reason you are asking for help is related to wrong field order. Can you get more specific?

[jagabo]

Now I am right that if you stoped an image onscreen and if you could get it that fast I sould say amd I right on this?

One frame is made up of two diferant images?

You would see the first image and second image blanded?

No, you would see only one field. By the time a new field is being drawn the previous field has faded away. That's why you don't see comb artifacts on interlaced CRTs. And it's also why they flicker when there are sharp, high contrast, horizontal edges.

[nymph4444]

And I get what you just told me.
If you stop a Tape in a VCR you see just one Feild.

Now is a DVD Interlaced?

[jagabo]

The video on DVDs is in the form of digital frames. They can be stored as interlaced frames or as progressive frames with pulldown flags (a few bits per frame) that tell the player how to produce the 59.94 fields per second required for analog output. Most film sources are stored on DVD as progressive frames with pulldown flags.

[nymph4444]

Now I know Prograsive Scan DVD Players just came out not to long ago.

Have DVDs always been Prograsive?