Video Frame Rates – 60i vs 60p vs 30p vs 24p – what it means
As a general rule of thumb we could say the following:
The ‘p’ stands for ‘progressive’ and the ‘i’ stands for interlaced.
If a camera records in progressive mode it will capture full frame images so and so many times per second, meaning that e.g. a 30p recording mode would capture 30 full images per second.
Interlaced is different: the term interlaced goes back to the TV era when lines (instead of pixels) were used to create the TV image. Interlaced means in video terms that 2 (video) fields are used to build one full image frame. One field contains the odd-numbered lines of a screen and the other field contains the even-numbered lines.
So, we could describe a field as a frame with half the information needed for a full image (== a full video frame).
Where to use 24p, 30p, 60p, 60i etc?
24p (equals 24 full frames per second) should be used if you want a cinema like effect — most movies are shot at that frame rate. It is a historical thing because it is (or was) the ideal fit for conversions to international TV standards and therefore the frame rate movie directors went for.
Nowadays, with latest HD TV systems that requirement might go away. Avatar and The Hobbit were already filmed in higher frame rates, the latter in 48fps.
30p is the standard for home movies or let’s say digital video outside of movies and sports. 30 frames per second capture normal paced movements quite well and in a natural way.
48p, which for example GoPro cameras offer, is well suited for faster action, but not too fast, and allows you still to slow the frames down later in editing when you aim at a 30p frame rate for your video export. A real world situation for that frame rate would be white water rafting, kayaking, etc. when you might want to slow down the motion but not to the point where you record water drops flying in slo-mo.
60p is suited for fast-paced sports where movements with 30p just don’t look as smooth as they should. Again, it is already suited to slow the motion down in an editing software and the motion looks still fluent. Slowing down for 48p and 60p is meant as ‘moderate’.
Modern video camcorders in the upper consumer class can usually record native 60p.
Just to mention 2 more popular frame rates: 120p and 240p. Both are great for better slow motion. You can slow them down 4 respective 8 times and end up at 30p normal speed. If you look at your camera make sure that you still get those frame rates at a high resolution and not just with a ‘mini’ resolution which would mean that you either have to reduce the dimensions of your result video or you need to enlarge the footage which results in not so sharp footage.
I talk about 60i in a minute…
Before, let’s mention 300p and 400p: those high frame rates are for slow motion recording. In consumer devices you might not get a full 1080 HD resolution and instead smaller frames like 720×400 but those results are definitely usable for real nice slow motion effects. Action cams often offer 120p or 240p but might deliver 300p soon.
60i: because it is an ‘interlaced’ format it describes a recording mode where 60 fields per second are recorded. As mentioned earlier, 2 fields make up on full frame, and the term field means that either the odd- or even-numbered lines of an image frame are recorded. The 60i format was developed for the NTSC color TV system standard of 1953. For technical reasons that standard defined an effective 29.97 interlaced frames per second for NTSC Color TV.
Interlaced recording: since the processor can only record one field at a time it handles them sequentially: one field after the other. That leads to time difference (=exactly the amount of time it needs to take 1 field) between two fields that comprise one full image frame. If you record fast moving objects you run into the issue of composing a full frame from two fields (=half frames in an interlaced manner) which are slightly ‘off’ in time and motion. The result could be that motion artifacts (‘interlace artifacts’) appear, it is also known as a comb-effect.
Interline twitter is an another issue which is well known from TV programs where people are wearing shirts with vertical or horizontal fine lines. A ‘flickering’ effect and a low degree of sharpness can be observed.
A 60i video camera recording leads to effective 30 frames per second because 2 fields are required for one full image frame. Since 60i refers to NTSC (color) standard the effective frame rate is actually 29.97 (because the standard used a certain algorithm to get around some issues).
(Analog to the 60i field rate there is a 50i field rate for European TV standards PAL and SECAM. This is just mentioned in case you have a camcorder that offers 50i recording).
Brief excursion into the Interlaced past and the digital future of broadcasting:
Interlaced formats became the choice of analog broadcast in order to reduce bandwidth. Nowadays, broadcast systems switches from analog to digital. For PAL that means DVB, Digital Video Broadcast standard, adopted by Europe, Australia, NZ, and for NTSC that new digital standards are ATSC (Advanced Television System Committee) adopted by the United States, Canada, Mexico, and others. However, there are more standards, as usual.
60i vs. 60p: as long as you are producing for computer (Internet) video and not for older TV systems (PAL/SECAM/NTSC) with interlaced format it is better to go with a camcorder that records 60 progressive frames per second.
60p recording means that real 60 full frames per second are recorded and it is no surprise that 60 frames per second capture more details of a motion than 30 frames per second. Therefore 60p is a good format for action sports.
Video Frame Rate Conversions
When a 60i raw format is de-interlaced to a 30 frame per second video it depends on the quality of the de-interlacing software (algorithm) how great your video looks, how sharp it is. For most cases you will probably see no difference to a video that was shot in 30p native format, however, since theoretically a 30p must be better than a de-interlaced 60i format I would always go for a 30p recording mode (provided that I do not intend to produce for NTSC TV formats or any output that works in an interlaced mode).
The less conversions you need the better it is.
If you need to convert …then it depends on several things what the best way to go is and even different de-interlacing software makes a difference.
Any conversion from a higher to lower frame rate has to throw away a certain number of frames (e.g. 30p to 24p) and artifacts may become visible in the video, … again, depending on the quality of the conversion software.