Weird offset image when panning - see screenshot

[Alex Wren]

Hi Forum,

I have XM1 (PAL), when panning slowly I get a weird 'interlaced?' offset issue.

Anybody any ideas why this is happening - when I switch the camera to 'frame' mode it is better, however the manual recommends not to use this for everything.

Thanks in advance

Alex

[Alex Wren]

See attached for 'frame' mode version.

Alex

[John Miller]

Such is the nature of the interlaced beast.

Although PAL frame rate is 25 frames per second, each frame is really made up of two interlaced fields. The first field contains all the odd lines that make up the frame, the second contains all the even lines (or the other way around - depends - but the principle is the same). There are 50 fields per second. The first field is effectively a half-resolution image, as is the second but the second is "taken" 1/50 of a second later. So, if you are panning or an object is moving, you will see the effect of interlacing the two halves because they were captured by the camera 1/50 of a second apart.

Frame mode doesn't show it because the whole frame - line by line - is captured in one go, every 1/25 of a second.

As easy demonstration is to simply wiggle your finger in front of the lens and record in both modes. The effect of interlacing will be very apparent. You could reduce the effect somewhat by using a shutter speed that is slower than the field rate but to eliminate it entirely, use frame mode.

(For NTSC, the values 29.97 and 59.94 apply instead of 25 and 50).

[Alex Wren]

Thanks John,

Maybe I am a bit simple but this seems quite daft!

Do I assume that frame mode should be used whenever there is some movement (i.e. all the time as this is video...) If that is the case how comes the manual seems to think you might only want to use this occasionaly.

Are there any downsides to using frame mode?

Thank you for your quick reply by the way!

Alex

[Jarrod Whaley]

Have a look at the same footage on a TV. I bet you won't see the artifacts. They tend to show up on a computer monitor, because monitors are usually progressive scan. SD TV's are usually not.

For what it's worth, I always use frame mode. There's supposedly a slight drop in resolution, but I can't really see the difference.

[Alex Wren]

OK - good point.

I was starting to get depressed and everything....

Alex

[John Miller]

Originally Posted by Alex Wren

Thanks John,

Maybe I am a bit simple but this seems quite daft!

Do I assume that frame mode should be used whenever there is some movement (i.e. all the time as this is video...) If that is the case how comes the manual seems to think you might only want to use this occasionaly.

Are there any downsides to using frame mode?

Thank you for your quick reply by the way!

Alex

Until recently (the last handful of years), all video was recorded interlaced (i.e., field-based). Nearly every program on TV is this way. If you look at a still frame from almost any TV broadcast, it will show the same "problem" you have seen. Historically, it came about because the electronics needed to drive TVs etc at the speed required for true frame-based imaging were too expensive and required too much bandwidth to transmit. To make devices affordable and limit the bandwidth, an electronics engineer at RCA came up with the rather clever trick of interlacing. This was in the 1920s!!! There's an excellent description of interlacing on Wikipedia:

http://en.wikipedia.org/wiki/Interlace

It is a perennial problem for anyone wanting to capture still images from video. Whether you choose to record frame mode or not also depends on how you will view the recording. For example, if you are going to only ever watch them after transferring to DVD and using a computer, go with frame mode since computer displays are nearly always non-interlaced these days.

[Alex Wren]

Thanks again.

So this means I have a little problem.

Some footage I recently shot was not in frame mode - however I need to use this for a short clip on a web site (non interlaced).

Any idea how I can 'convert it' or at least reduce the effect?

Alex

[John Miller]

Originally Posted by Alex Wren

Thanks again.

So this means I have a little problem.

Some footage I recently shot was not in frame mode - however I need to use this for a short clip on a web site (non interlaced).

Any idea how I can 'convert it' or at least reduce the effect?

Alex

Depends on the size of the clip for the web site - i.e., if you want to keep the same image size (720 x 576) or reduce it to, say, 360 x 288. If the latter, you won't need to worry since the software you use to convert the image to half the resolution will either just ignore every other line or blend the two lines into one. If you plan to maintain the original image size, a lot of editing software has various deinterlace filters - some better than other.

Best thing is to experiment and decide what looks best to you.