Frame Rate Conversion Simplified
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Frame Rate Conversion Simplified

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Point 360 IVC DIGITAL FILM CENTERFrame Rate Conversion SimplifiedJim James, Chief 
Engineer, IVCWhy So Many Frame Rates?Motion pictures, whether film or television, are 
nothing more than a series of rapidly changing still images, or frames. The faster they 
change the smoother the motion. A long time ago it was determined that 24 frames per second 
was acceptable for motion, but left a noticeable flicker due to the screen going dark each 
time a frame was pulled into place. By putting a two bladed rotating shutter in the 
projector to create a flicker rate of 48 frames per second the illusion of a steady moving 
picture was created. When the NTSC television standard was created a frame rate of 30 
frames per second was selected because it was half the electrical line rate of 60Hz. To 
avoid flicker it was necessary to create an effective frame rate of 60 frames per second, 
but the technology of the time could not transmit that much information that fast. The 
solution was to first scan only the odd lines, then the even ones, dividing the picture in 
half as two interlaced fields. The result of this is a full resolution image if an object 
is standing still, but two slightly offset versions of the object if it is in motion. When 
color was added it was necessary to adjust the frame rate slightly to prevent the color 
information from interfering with the audio carrier in a television broadcast signal. Thus 
the frame rate for NTSC became 29.97.In Europe the electrical line rate is 50Hz, so the PAL 
standard set the frame rate at 25 frames per second, and like America used interlacing to 
gain a flicker rate of 50 fields per second. Fortunately PAL did not have a problem between 
color and audio, so PAL still runs at 50 fields per second.An example of a soccer ball in 
motionWhen a committee was formed to set a standard for high definition television 
they decided not to set one standard, but allow many. The 29.97 interlaced frame rate was 
preserved for compatibility with existing systems, and even frame rates (30 & 60) were 
added as well. In addition, progressive rates, where the entire frame is sent together, 
were added. Thus 29.97 progressive, 30 progressive, 59.94 progressive, and 60 frame 
progressive were added to the mix. Later the obvious step of adding a high definition 
standard that matched film was taken and 24 and 23.98 progressive were added. (23.98 has 
the same offset from 24 as 29.97 has from 30 and was necessary to be compatible with NTSC.) 
In PA
Converting Between Frame RatesSome conversions are easy. Converting between 24 and 25 just 
requires speeding up or slowing down the source by around 4% (Audio may require separate 
pitch and data-rate conversion). Converting between fractionally adjacent speeds (59.97 to 
60) is even easier. Converting a slower frame rate to a faster one requires duplicating 
frames or fields. People have been transferring film to video for a long time. For NTSC the 
film is slowed down to 23.98, and then every other film frame is held for one extra field. 
(see diagram) This is called 3:2 pulldown. Some of the original frame 
scaffoldingnow begin on the second field of the video frame. This looks fine while 
being played, and can easily be removed for conversion to 25 frame PAL or 24 fps for DVD 
compression. However, if this material is edited without attention to keeping a steady 
3:2:3:2:3:2 cadence, a clean frame rate conversion becomes nearly impossible.Recently some 
computer systems, and tape formats, have chosen to convert 24 to 30 by simply duplicating 
every fourth frame. This makes editing simpler, and works well for computer formats that 
can not handle interlaced images, but it results in a noticeable stutter six times every 
second. Also professional systems that are expecting a 3:2 cadence will not know how to 
remove the extra frames.Where conversions become difficult is when an interlaced format is 
converted to a progressive one, or when large frame rate reductions are needed. When an 
interlaced format is converted to a similar progressive one (i.e. 30i to 30p) either every 
other field is discarded, resulting in an image with only half the vertical resolution, or 
the fields are blended, creating a distorted image (all moving objects will have serrated 
edges). The only way around this is to interpolate, or create an entirely new image out of 
the two fields. How well this works depends entirely on the quality of the hardware or 
software performing the

frame sets, frame scaffolding

frame sets, frame scaffolding

frame sets, frame scaffolding

rely new sequence using the information 
in the old ones to estimate where things would have been had it been shot at the new frame 
rate. This is a very complicated process that takes a lot of computer power and time. 
Depending on the quality of the software used the results can vary greatly.How To Survive 
In A Multiple Frame Rate WorldIt is not uncommon for a project to be distributed in 
multiple fra

frame sets, frame scaffolding

frame sets, frame scaffolding

rican television runs at either 29.97i or 59.94p. Europe and many 
others are 50i, while film is still shown at 24 frames per second. For a project to be 
easily and inexpensively made available for all of these markets it is important to take 
the frame rate into account from the beginning. For a universal format nothing beats 
23.98psf. With no quality loss and little expense it can convert to any format. However 
many smaller format HD cameras that claim to record 24p don’t. These cameras capture images 
at 24 frame 
sets per second, but the tape records at 60i. If they use a standard conversion format, 
such as 3:2 pulldown, this is not a problem, as many systems can easily remove the extra 
fields. But if a non-standard conversion is used it may be difficult, or even impossible to 
create a satisfactory 24 frame master. Also, if using a 24/60i camera be sure to convert to 
24 frames per second (actually 23.98) before editing, or you may end up with an unusable 
master.

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