Help explaining 60i (30p sensor output)

[x2x3x2]

Can some kind soul explain what some cameras do when the specs says it produces 60i video but has 30p sensor output?
Is there any benefit to this besides compatibility on HDTV playback?
Does it degrade the video by interlacing a perfectly clean progressive input?

Thanks!

[jagabo]

An interlaced camera takes 60 pictures a second and sends half of each picture to the TV (every other scan line of the picture, called a field). So you see 60 different half-pictures every second. In 30p mode the camera takes 30 different pictures every second and sends the two halves sequentially (ie, two fields). Yes, it is done for compatibility with TVs that only accept 1920x1080i. Depending on how the camera stores the video you may get slightly less sharp colors.

Interlaced video is smoother because you see 60 different motion increments. 30p is easier to deal with on computers because computers are designed for progressive display.

[x2x3x2]

cool, i kinda understand the difference between interlaced and progressive video.
my question is what does these camera which have specs stated as 60i video (30p sensor output) do? these are usually on DSLR's with video recording capability.

true 60i footage would give smoother motion as each set of scan lines show different motion.
but if the sensor output is 30p, and then converted to 60i, does this mean that every 2 interlaced frames are just duplicated of the original progressive frame? thus the motion fluidity wouldn't gain any, would this process also then make the video seem choppY?

[Cornucopia]

These are also known as 30 PsF (Progressive, Sequential-Frame). They ARE Choppy-er (than 60i or true 60p). They're probably stored as 30p just like any other 30p, but are SENT down the cable as 60i. Those two 1/2-frames are "sampled" at the same time period, but they're sent with a slight delay on the 2nd 1/2frame. If you need to return them to progressive, it's easy.

Another example of fancy marketing. If your final output needs aren't mainly web-based, this might not be the best VIDEO tool to use.

Scott

edit: I checked - it's actually "Progressive, Segmented Frame", but otherwise just like I said. Similar to 2:2 pulldown telecine, but without the vertical pre-filtering.

[x2x3x2]

Thanks for that explanation Corn.

If i open such footage in Premiere Pro, should they be opened/interpreted as 60i or 30p?

[Cornucopia]

If it were me, I'd be opening/interpreting them as 30p.

Scott

[jagabo]

There are no frames in interlaced analog video, only fields. You see one field at a time. A field is every other scanline of the face of the CRT. The signal alternates between even fields (all the even numbered scanlines, 0, 2, 4, 6...) and odd fields (all the odd numbered scanlines), every 1/60 second.

When those fields are digitized pairs of fields are woven together to create a frame. At playback the fields are sent one at a time to the TV, restoring the original 60 field per second analog signal. In a fully interlaced video each field is a unique point in time, every 1/60 of a second. In video from a 30p camera the two fields come from the same point in time. When displayed you still see one field at a time but every pair of fields come from the same point in time. So motions aren't as smooth.

If i open such footage in Premiere Pro, should they be opened/interpreted as 60i or 30p?

It depends on how the video is stored. If they are stored as interlaced YV12 frames (eg, HDV MPEG 2) you should open them as 60i (more properly called 30i but that's a losing battle). Otherwise the chroma channels will be screwed up. A DSLR probably stores them as 30p.

[aedipuss]

HDV 30p is stored and output as true progressive. full frame.