Question about line scanning in Interlaced Camera

[Movie-Maker]

An interlaced display will alternate between the odd and even lines on the screen. I believe the beam is actually 'shifted' everytime to scan the other set of lines.

But what happens in the video camera ? Lets assume a digital camera(DV). Does it capture even and odd lines seperately too ? In that case what would be equivalent of 'shifting the beam' in a camera ?

Or does it always capture the lines at the same position, except it does it twice as fast to create two fields ?

[Cornucopia]

(Assuming interlaced here:...)
The imagers in cameras (these days, not talking about tube imagers) have CCD or CMOS etc chips that either:

1. Read all individual pixels at time T, store/buffer them, and spit out the odd lines at time T' and wait 1/2 a frame and spit out the even lines at time T'+ 1/2 frame

-or-

2. Read the odd line pixels at time T, store/buffer them, and spit them out at time T', while reading the even lined pixels at time T+ 1/2frame, store those and spit them out at time T'+ 1/2frame.

-or-

something very similar to one of those methods. #1 above is usually called SF (segmented frame).

The lines are always at the same position (whether for display or capture), and the rate is double the frame rate--that's why it's interlaced fields and not frames. The only system that is double rate whole frames is 720p60 video (which is great for sports/motion).

Clear as mud, huh!?

Scott

[Movie-Maker]

Got it. Although only #2 looks more in sync with the way interlaced displays work. i.e. display odd lines from the first field and then display even lines from the second field. The sequence of fields and lines in both the capture and the display are same.

But that doesn’t seem to be the case with #1 though. Wouldn’t the second field’s lines be captured at an earlier point in time as compared to when they would actually be displayed ? It’s like capturing in a progressive fashion and artificially interlacing them. Maybe the difference is too insignificant to notice ? Or is it ?

[Safesurfer]

Have a read here

http://lurkertech.com/lg/fields/

[jagabo]

Whether the camera actually captures the entire frame every 1/60 of a second and only saves one field, or only one of the fields is captured every 1/60 of a second, is immaterial. The end result is that one field is saved every 1/60 of a second and the fields do not overlap spacially or temporally.

When this is displayed on an interlaced display they are displayed the same way they were captured. One field at a time, non-overlapping (although the electron beam in most CRTs is wider than a scanline so the fields actually do overlap in that sense).