Digital Video - How many frames ?

[ScottV]

I was playing with my Panasonic DS-99 this morning and something really surprised me.

I paused the playback and then used the jog button to advance one frame at a time. What I noticed was that there were 50 discrete frames per second !!

This isn't what I was expecting. Can somebody enlighten me please as I was expecting 25 (PAL country).

Does this mean that when I capture at 25 frames per sec I'm losing half the information? If I transferred directly using firewire how many frames per sec would I have then.

[Brian Ellis]

Scott

PAL (and other standards) are interlaced. You have 25 full frames per sec, each comprising of 2 half frames, one with the odd-numbered lines and the other with the even-numbered ones. This reduces flicker to the eye, while keeping the required bandwidth within reasonable bounds. All a magic illusion, or at least an optical one.



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Brian (the terrible)

[ScottV]

But I still don't understand why I see 50 perfectly formed still images per second, each one slightly different from the last.

Surely it should pair the odd and even ones and just display a single image for each pair.

[ScottV]

Could it be that when the camera's in pause mode its only displaying half the number of lines? Just a thought, I really don't know what I'm talking about.

[Grigory]

Hi,
Typically, DV camcorder displays only single field of a frame. This is done to escape picture flickering on TV in paused mode. The field is interpolated over entire frame, of course. The vertical resolution is reduced by a factor of two. This is clearly visible on my D8 camcorder on test pattern in pause.

I have noticed this and decided to not use paused playback for still captures.

If the camcorder can display one field, it probably can display the other, going later. So, you have 50 images per second, but shown as full frames, one after one.

With DV raptor, I can set camcorder to display odd or even, or both fields on the output.
Camcorder itself cannot show both fields in a frame in pause.

To get full frame, capture frames from avi, but not capture single frames.

Grigory

[ScottV]

Thanks very much Grigory. I guessed it must be doing something like that. I'd just never noticed it before. I was previewing the frames on the LCD display so I wouldn't have noticed any reduction in quality.