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Old Mar 19, 2009, 8:28 PM   #1
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Video Frame Rates (24p, 25p, 30p, 60i)

[align=left]The i's and the p's[/align] [align=left]Simply stated, the "i" or "p" after the capture rate indicates whether or not the video is interlaced or progressive. 60i for example indicates that the video will be 60 "fields" per second where each field is only half of a frame. The word "interlaced" means that each field consists of (typically) the odd rows or even rows in the picture. Since each update (very 1/60th of a second) only updates half the frame, you never actually see an entire frame. What you are actually seeing is half a picture mixed with the other half taken 1/60th of a second later. Of course, this happens so fast that our eyes perceive the data as if it were full frames. In contrast, 30p video also displays one entire frame every 1/30th of a second so an entire frame is displayed at once.[/align][align=left]Here's the catch and why 30p is generally regarded as better than 60i as far as resolution. Because 30p video displays a full frame at a time, an entire frame or snapshot of the scene is displayed for 1/30th of a second and then the next frame is displayed for 1/30 of a second and so on. If you could freeze time at any point in the video and look at your screen, you'd see a proper (full and unbroken) photo of the scene. With 60i, only half of the entire picture is updated every 1/60 of a second which means at any given time, only half the frame is displayed and the other half is 1/60th of a second "old". This can cause artifacts and can tend to blur detail of moving objects because mixing half of one frame with half of a different frame is obviously not optimal. Such is the case with any frame rate that is followed by the letter "i" to indicate interlacing. On a positive note, 60i can give the impression of less flicker when viewing fast moving objects such as sports in which case the 60i may look smoother even if slightly lower resolution.[/align]
[align=left]60i[/align] [align=left] As mentioned earlier in this article, 60i video basically amounts to 60 fields per second because you are not capturing entire frames. Simply put, 60i amounts to capturing 60 half frames per second. At first glance, it may sound like 60i and 30p are equivalent but they are not. 30p video generally results in better resolution and sharpness than 60i video while 60i video excels in fast action video such as sports. Since 60i video updates half the frame each cycle, resolution is reduced due to the fact that an entire frame is never captured at once. Since at least part of the image is updating every 1/60th of a second, however, flicker/strobing is reduced and 60i can look smoother to the eye even though overall, the data update rate is the same as 30p. Simply put, 60i can be useful if you plan to video a sporting event or if you are taking video where you must do a lot of panning from side to side. 30p is generally better for most slow to average speed pans and motion due to the ability of 30p to capture more detailed information and sharpness.[/align][align=left]

by Barry Green


While most of the camcorders on the market have traditionally used CCD sensors, many new camcorders are coming on the market are now using CMOS sensors. While CMOS and CCD both have unique properties that make each one suitable for one type of job or another, there are unique characteristics to each that need to be understood in order for a user to know whether a CMOS or CCD camera is more appropriate for the type of shooting they'll be doing.


The CCD artifact of Smear is well known, and for the most part shooters know how to deal with it and minimize or avoid it. But the new artifacts/characteristics that come from the new CMOS technology are likely to provide some nasty surprises for unprepared shooters. And camera manufacturers are likely going to continue to offer more and more CMOS camcorders. Which means we're likely to see more and more rolling shutter issues crop up, as the technology becomes more widespread. Shooters, educate yourselves as to what the issues are, how they affect the footage, and prepare yourselves for whatever steps you need to take to make sure that there are no unwelcome surprises in your footage.


[line] http://www.ptgrey.com/support/kb/ind...?a=4&q=115

Key differences between rolling shutter(CMOS sensors)and frame (global) shutter(CCD sensors). [line] SUMMARY:
This article describes the differences between the rolling shutter used in CMOS sensors and the frame (a.k.a. global) shutter used in CCD sensors.


Some types of CCD image sensors, such as the interline CCDs used by most PGR Imaging Products, provide an electronic shutter mechanism known as a frame (or global) shutter. In imagers that use a frame shutter, the entire imager is reset before integration to remove any residual signal in the sensor wells. The wells (pixels) then accumulate charge for some period of time, with the light collection starting and ending at exactly the same time for all pixels. At the end of the integration period (time during which light is collected), all charges are simultaneously transferred to light shielded areas of the sensor. The light shield prevents further accumulation of charge during the readout process. The charges are then shifted out of the light shielded areas of the sensor and read out.

This means that with a frame shutter sensor, the scene will be "frozen" in time, provided that the integration time is short enough i.e. there is no motion blur. An example of an image taken using a frame shutter is below.

The rolling shutter in a CMOS image sensor works differently, in that the photodiodes (pixels) do not collect light at the same time. All pixels in one row of the imager collect light during exactly the same period of time, but the time light collection starts and ends is slightly different for each row. The top row of the imager is the first one to start collecting the light and is the first one to finish collecting. The start and end of the light collection for each following row is slightly delayed. The total light collection time for each row is exactly the same, and the delay between rows is constant.

The time delay between a row being reset and a row being read is the integration time. By varying the amount of time between when the reset sweeps past a row and when the readout of the row takes place, the integration time can be controlled. Since the integration process moves through the image over some length of time, some motion blur may become apparent. An example of an image taken using a rolling shutter is below.
Rolling Shutter [align=center][/align][align=center][/align] [align=center]
[/align] [align=center]Frame (Global) Shutter[/align]

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Old Mar 19, 2009, 9:04 PM   #2
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The other really big issue with frame rate, and the reason that the Canon HF100 is without peer (in consumer camcorders) at 24P, is "image smear." As objects move, the human eye expects that movement to appear as edge-smear, and if the shutter isn't open for the complete frame time, usually 1/24 sec, the motion does not look natural.

It is amazing to see output of an HF100 in 'movie mode' at 1/12 shutter speed. Rather than being 'jerky,' it is blurred, but not at all objectionable.

The advantage of using slow shutter speeds is, of course, that the camcorder will give a good image in almost total darkness Additionally, since YouTube and Vimeo down-sample the frame rate, sending video to them already 'pre-smeared' makes that video look a lot less jerky than footage shot at 60p

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