Info on frame rates/refresh rates etc.

This is a topic that's bothered me for quite some time and I'm looking to see if anybody can solve any of the questions I have.

As far as I'm aware, 25fps is the only frame rate used on British television, and when interlaced at 50i, it gives the impression of more frames and giving the picture a smoother and more "live" look. For example, the Strictly results show is as smooth as peanut butter (the smooth variety) and yet things look ever so slightly filmic during the credits when they're interviewing the celebrities. So anybody would assume this was 24fps given it's not as smooth... but how can it be when PAL television broadcasts at 25fps?

Another instance is the likes of Gavin and Stacey. That looks a lot more filmic than say a studio sitcom like Mrs Brown's Boys, yet isn't as smooth as something like Bargain Hunt which, like Gavin and Stacey, is filmed on location but with a seemingly smoother frame rate. Is Gavin and Stacey at 24fps? Because the motion certainly doesn't look like a Hollywood film. Neither movies or shows like Gavin and Stacey are as smooth as the likes of Strictly, but they don't look like they have the same frame rate.

And just one more case for s**ts and giggles, The Inbetweeners has a slightly filmic look about it (as in it's not "live" smooth) but an outtake featured from it in It'll Be Alright on the Night was perfectly smooth. There must be some postproduction technique involved, but how does it still look different when it's broadcast at 25fps like everything else on British television?

I hope I've perplexed you enough to render you to help me in some form of another, anything of which would be greatly appreciated.

The filmic look is when things are processed as 25p.

Normal broadcasting is 50i - it's not an interlaced version of a 25fps picture, it's generated as 50i in the camera. By switching to 25p you halve the field rate, resulting in the less smooth filmic look (and is broadcast [on most platforms] as 50i, with two individual fields carrying information from the same moment in time).

So is the 25p processing something done with the camera or is that done in post?

Can be either.

The camera can be set to record in 25p or 50i. With 25p its very much like film, in that you are shown/recording 25 whole frames per second and playing them back at the same speed. With 50i you are recording a interlaced/half frame followed by the other half and so on, this gives a smoother look to movement, but a more jagged edge to fast moving objects. Most notable on ticker text.

Cheers guys. That does clear up quite a bit, but I'm still confused as to why there seem to be different levels of "filmic" looks. For example, the pre-recorded items in Strictly seem a tad smoother than Gavin and Stacey, yet neither of them have that live smoothness to them.

In theory you aren't allowed to film-effect 50i to 25p in HD, and should shoot 25p natively (almost every camera does it these days). This is because the film-effects that do a 50i to 25p conversion usually soften the picture a bit too much. It was in the BBC HD Tech Specs that film-effecting 50i was forbidden, not sure if the same is true in the DPP specs. Of course for a production effect you can make an exception. Sport do it all the time on their montages.

Another thing that can play a part is the shutter used on 25p.

If you want to simulate standard film shot at 25fps (as European TV film is), then you also need to simulate the 180 degree shutter that is also standard in film production. A 180 degree shutter only exposes the film for half of the frame duration (it only opens for 180 degrees of a 360 rotation), so that equates to a 1/50th second shutter speed. If you shoot 25p video and want it to look like film, in motion terms, then you need to shoot with 1/50th shutter.

However it is possible to shoot 25p with 1/25th second shutter. This will give you more motion blur, and can look quite smeary (I hate it). This will also make motion look a bit blurrier and this can make things appear a bit less jerky.

Shooting 25p at 1/50th sec shutter, of course, halves the exposure time compared to 1/25th sec shutter, so means you need more light, or a wider aperture, or more gain to get bright enough pictures compared to shooting 25p with 1/25th sec shutter.

Similarly if you are shooting interlaced 50i (which normally would also shoot each captured image with 1/50th second exposure - equivalent to 1/50th shutter) you can use a 1/100th second shutter (kind of simulating film shot at 50fps with a 180 degree shutter) to give the video a jerkier effect which kind of nods to film effect. (Same lighting/aperture/gain issues apply)

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I thought that would normally be shot at 1/100th. I made some graphics for an ad once, and found that simulated 1/50th motion blur was way too much. 1/100th was just right.

A 1/50th also happens to match nicely with UK mains 240/50hz power, so lights don't appear to flicker,

If you mean 1/50th as a shutter speed, does it matter much when it comes to lights flickering? As long as the framerate is 50fps or 25fps, you'll be in sync with the lights whatever your shutter speed is, won't you?

No - most normal 50i interlaced video is shot with 1/50th shutter. In the days of tubed cameras you scanned from the top to the bottom of frame in real time - so captured 1/50th of a second per field with no shutter at all. 1/100th second shutter interlaced 50i looks quite jerky. It can be a good effect though. It was used very effectively on a singe rock number at the Eurovision Song Contest a few years ago. (The Turkish entry to the 2010 concert that NRK produced in Oslo. Was very effective)

When the first generation of 'film effect' processing was introduced by simply discarding every other field and repeating the one you kept, you were effectively creating a 25p 1/50th second video signal - which is pretty similar to film (albeit with horrendous vertical resolution loss and aliasing)