frame per second - Q's on creating content

[caine]

hello,

i've been making loads of my own clips lately from scratch, not just my usual circuit bent glitch stuff, but animating stills and 3d objects, but i'm always confused as to the frames per second rates i should be using.

i see alot of people mastering to 120bpm, which is 20fps i think? since i use arkaos and its bpm function from an audio signal, does it really matter what rate i use since it will loop it to the beat ratio i specify for that clip?

i've tried rendering the same clip at both 20fps and standard (pal) 25fps rates, and havent really noticed much difference..... any advice?

i could just send Pointless Dav another email since i see their content packs are 20fps as well, but i feel guilty for pestering him so much of late with random questions.... sorry dav! ;)

[many2]

You are acutally mixing different concepts here.

FPS : how many frames (temporal samples) in a second. When creating a clip you should choose a framerate that is the same as the one supported by your display. North-american NTSC is 29.97 fps and European PAL is 25 fps. Film is 24 fps. This information is NOT giving you any clues about a clip's lenght nor about any event in its content.

BPM : how many beat per minute. Here we are talking about events. In one minute you will have X number of events evenly spread in time.

How can you relate those two things together ? You simply cannot. If you want to design clips for a particular BPM then you should question yourself about the duration of the action in it. Let's say you have a looping clip of a bouncing ball. If you want it to match a rythm of 120 BPM then you will make the loop half a second long (60 seconds / 120 beat = 0.5 sec per beat), of 2 seconds long if you want it looping on the fourth beat (60 seconds / 120 beats X 4 = 2 sec per beat). This looping clip can then be sampled 30 times per second (NTSC), 25 times per second (PAL), or any other frame rate, without changing the duration of the action itself.

To summarize : what is of interest to you is the duration of your clip's action, not its framerate.

[caine]

How can you relate those two things together ? You simply cannot. If you want to design clips for a particular BPM then you should question yourself about the duration of the action in it. Let's say you have a looping clip of a bouncing ball. If you want it to match a rythm of 120 BPM then you will make the loop half a second long (60 seconds / 120 beat = 0.5 sec per beat), of 2 seconds long if you want it looping on the fourth beat (60 seconds / 120 beats X 4 = 2 sec per beat). This looping clip can then be sampled 30 times per second (NTSC), 25 times per second (PAL), or any other frame rate, without changing the duration of the action itself.

To summarize : what is of interest to you is the duration of your clip's action, not its framerate.

i understand that as thats how i've been workjing(at 25fps), but i guess what i'm trying to ask, having thought about it..... is, what benefit is there in creating loops with shorter frame rates, or to a particular bpm, when a program such as arkaos, will speed the clip up/down to loop in time to the bpm?

although think i'll just stick to what i know best.... works so far.

[jimmyogenic]

the difference really is size of the file after its been rendered...

ie a file thats encoded at 15fps will be smaller than one thats 25fps, also if there's less data ie 15fps you will loose frames, so some fast moving clips may not look as good after being rendered..

think thats it anyway!

[sleepytom]

Oh well so much for me not posting for a bit...

The subject of BPM is perhaps one of the most misunderstood concepts in VJing. Lets first examine the problem and indeed why it is a problem before we can look at ways to work around it.

Video is composed of a sequence of frames - in PAL video we have 25 frames per second (NTSC has 29.97 - we will round this up to 30 as is common practice for simplicity's sake).Video comes in whole frames only - you cannot have half a frame of video.

Music has a tempo this is expressed in Beats Per Minuet - music also groups beats into Bars (in most modern dance music there are 4 Beats in a Bar know in music terms as 4/4 music - notice that this is still true for drum and bass etc)

VJs often want their video to go well with the music - they think it would be nice to have some loops that were at 123BPM and were one bar long (for example). now we can see the problem, video has only got 25 frames per second and a loop needs to be composed of whole frames yet you want to make a loop that is one bar at 123BPM. To be able to make a PAL video loop consisting of whole frames we need to divide the length of the bar by 25 and get a whole number.

to work this out we first need to know how long 1 Bar lasts for:-

1 beat @123BMP is

60/123=0.48780487804878048780487804878049 seconds long

so 1 BAR @123BPM is

0.48780487804878048780487804878049 *4=1.951219512195121951219512195122 seconds long

to be able to make a PAL video loop consisting of whole frames we need to divide the length of the bar by 25 and get a whole number. As we can see 1.951219512195121951219512195122 / 25 = 0.078048780487804878048780487804878 which is not a whole number!

(you can do the maths yourself for NTSC)



So what can we do to have a loop at 123BPM? There are a number of different options depending on which software you use to playback your loops.

Firstly the best option is to make you loops at a convenient whole frame BPM eg 125BPM (for PAL clips) By timestreching your motion to fit the new BPM. You can then change their speed in you VJ software when you play them back - this is of course dependent upon your VJ software supporting precise speed change, the best software for this is VJamm 3 which can very precisely change the speed of clips. If you use resolume for example then your limited to changing the speed in massive increments (25FPS>24FPS! useless for our purpose)

Secondly you could render your clips at the "wrong" BPM eg 125BMP and make sure that the motion you wish to loop lasts only for the length of a bar at 123BPM - you will have to re-trigger this clip every bar to keep the motion in sync which can get to be quite hard work!

The last option is to render you clip at a different frame rate so that you make a whole bar last a whole number of frames. for 123BPM you could make a clip at 24.6FPS which would be a perfect loop at 123BPM. - I do not like this option as it leads to having a lot of non standard video clips - if you ever want to make a DVD say then you will be in trouble because a DVD has to be 25 FPS so you will have a nightmare incorporating all your clips at weird frame rates.

Personally I make all my visual clips at 100BPM these days - this is nice round number that fits with the PAL standard of 25FPS - it also means that my clips are generally "oversampled" ie i play them speeded up as most music is faster than 100BPM - so if i want to slow them down and have them loop every 2 bars say they do not become too jittery.

I should add a note here to point out that most VJ software is inconsistent at looping and causes a slight delay when going from the end of the clip to the beginning - this combined with the lack of precise speed control means that making accurate BPM locked clips is more trouble than its worth for a lot of people. I will recommend VJamm here as being the only software that i have used that is fully aware of these issues and features the required perfect looping and precise speed control as well as more advanced functions to be able to automatically stretch your clips to the right length for any given BPM.I will also recommend Sony Vegas as being the only editing software that has the ability to set timeline markers using BPM or musical Bars and Beats - this can be extremely useful when trying to make clips at a certain BPM

[many2]

If you can throttle the playback speed then the frame rate can be seen as a slow-motion flexibility meter : the higher the frame rate, the easier it will be to slow-down the clip without stuttering. If you encode you clip at 60 fps and play it back at 50% speed you still get 30 fps on screen, which is equal to the full-frame rate of your display device (in the NTSC world). At 25% speed, you get 15 fps, which is a bare minimum but still acceptable (if your application supports real-time frame blending in slow-motion you can even go to slower frame rate than that). If you had encoded the same clip at 30 fps then you would get unnaceptable frame rate when playing at 25% speed : only 8 fps.

[caine]

cool, cheers guys!

i've been a dj and producer of music for about 10 years, so i understand the BPM side of things, but i read an article somewhere about how certain frame rates work best with speeding up/down in time with the music bpm, so thats probably what confused me. i'll just forget all that and work off your examples....

thanks for being er.... gentle with me. :up:

[videoswitchboard]

Hi Bod,

here's an overdue answer:

we do not always produce our loops at 20fps- in fact we think that the fps of base loops is mostly irrelevant for live work (as graphics cards/ projectors are capable of varied frame rates anyway- it's totally different if you are planning to use your loops for an edit though).

what's important to us is the composition process to achieve glitchless and "rhytmical" loops. [here comes the trade secret] We use the drum machine/ step sequencer analogy when building our content (flash/afterFX/C4d) and make sure that the lenght of our loops are always in base 2 (8,16,32,64,128 frames etc..) and place the keyframes of the visual elements on the strong beats of the musical genre (visualise where you put your drum beats on your TR808 for example).

these loops are then exported at the "right" fps to match the tempo we're aiming for (use the reverse equation from Tom's- welcome back by the way)

The 20fps came about when doing the Radar visuals (average tempo 140bpm)- and we found that having your tempo a bit higher than the music worked well for the above mentioned reasons (possibility to slow down without jitter+ includes the lag time if you're planning to trigger a new loop on every beat).
Back then we were working at 640x480 with 1 layer+effect in arkaos on a 500Mz powerbook.
We are now producing most our stuff at 25fps.


bottom line: some VJs can't live with jittery visuals. We think that it can be an interesting stylistic effect (stroboscopic vision? what if people's visual perception was to evolve to distinguish faster frame rate than the standard 25-30fps- think eastern microtonal music scales).
"standardised" video output has got it's own "grain" which I believe makes people switch to "TV zombie" mode. fucking around with the frame rate is a very efficient (yet can be subtle) way to tell viewers "it's not TV you're looking at".(this is only a personal opinion)

dav.

[Hambone]

I render everything at 24fps to coincide with 120bpm for consistency, then use Ableton Live to scale the Arkaos playback speed so that everything stays beatmatched and in sync. Also, keyframing is easier at 120bpm, as it's a nice round 0.5sec/beat. I don't use 25fps, as it's a lot easier to divide 24 in fourths.

For example, if I've got a dancy loop, and the track is 130bpm, I let Live scale the playback speed to (130bpm/120bpm)*24fps, or 26fps. Even at 80bpm, the scaled playback speed of 16fps is still quick enough to be smooth (just...)

This technique lets me use my loops irrespective of tempo, and they always stay beatmatched and in sync.

[caine]

nice one, cheers for the tips guys!

mow its time to go and re-render alot of clips.....