Apple TV and frame rates

Tom Wolsky wrote:

Movies are still delivered in 24 for the most part. Is the Apple TV box using some kind of pull down to get to 29.97 or 25 in PAL frame rate countries?

Specifically about movies from the iTunes Store as opposed to other sources: movies are 24 fps in the cinema. On UK broadcast TV they are run at 25 fps (so 50Hz with interlacing or a double - progressive - scan) to avoid frame rate conversion problems, resulting in a 4% speed increase and a pitch rise of 2/3 of a semitone. (The 60Hz vs 50Hz scan rate arose for historical reasons, being matched to the mains frequency so that any hum getting onto the signal, which would cause a horizontal area of different brightness - all too common on early TV sets - would stay stationary rather than rolling and thus becoming much more obvious.)

American TV converts 24 fps to 60Hz by using what is called 2/3 pulldown (or these days probably something a bit more sophisticated) which, at basic, scans alternate film frames for 2 field and 3 fields, which converts to exactly 60Hz with the disadvantage of slightly jerky motion since alternate frames are held for half as long again. You do at least get the correct pitch (though the slight rise on UK TV is barely noticeable unless you have perfect pitch).

iTunes Movies on the Apple TV are all in 60Hz, and thus display the same issue. It's most noticeable on camera pans, which can look worryingly jerky (I suspect most Americans are so used to it that they don't notice). Although UK TV is 50Hz, any TV with an HDMI connector will also run at 60Hz when fed with that scan rate. (PAL/NTSC doesn't come into it since this is digital and they apply only to the method for squeezing colour into the analogue system.)

As far as I can tell output frame rate is fixed at either 50 or 60 Hz to suit the output device, and the ATV handles the conversion of the source media as required. See https://forums.developer.apple.com/thread/16971 for example.

tt2

I've never been able to get the ATV4 to output anything other than 60Hz when showing iTunes movies - the actual files, transfers from film, are evidently in 60Hz (the same is true of the Amazon Fire TV). I suspect that were you able to connect it to a TV which could only handle 50 Hz (which seems unlikely) you would get 24fps converted to 60Hz then converted to 50Hz which would be nasty.

American video would be 60Hz but many series, such as Star Trek, are filmed in 24 fps - though often then transferred to 60Hz video for editing and transmission (older ones were often distributed world wide on film as being much easier to handle in the days when frame conversion was very scruffy). Some recent reissues have gone back to the the original film.

There was a famous row about 'Dallas' many years back - it was very popular on BBC television and originally was distributed on 35mm film which looked very good in colour. Then suddenly the producers took to editing and distributing it on NTSC video with a very noticeable drop in quality - the public normally aren't too aware of quality, but the sudden drop in it was too much and there was a huge outcry.

24 fps at 60 Hz requires a 3:3 pulldown, 24 fps at 60 Hz require a 2:2:2:2:2:2:2:2:2:2:2:3 pulldown

24 fps to 60Hz requires 2:3 pulldown (odd frames 2 fields, even frames 3 fields, so 24 frames in 60 fields)

24 fps to 50 Hz would require every twelfth frame to be 3 fields instead of 2, which looks pretty horrible; which is why it's hardly ever done and films are run at 25fps.

60Hz to 50Hz would require dropping every sixth field, though nowadays much more sophisticated methods are available (the difference between the early and later showings of Star Trek The Next Generation was a very noticeable improvement).

For me the issue here is needing the Apple TV, and some of it's apps, to support changing the output frame rate based on the content playing.

As has been noted in this thread, depending on where the content originates from it can be 24p, 25p, 30p, 50i or 60i (so far...) and at present the ATV has a fixed output depending on what you select in settings. Auto simply negotiates 1080p/60 as it thinks that's the best as it's the highest frame rate.

60Hz is rubbish for any (mainly European) 25p/50i content including apps like BBC iPlayer, but is best for 24p, and largely US sourced material. Many TV's can now correctly display 24p from a 60Hz playback, but what's best for 60Hz is rubbish for 50Hz and vice versa.

In the US this isn't a problem generally as many US TV's don't support 50Hz at all, so I imagine either the BBC for example generate 60Hz friendly content for the US shows they export, or people in the US are used to lots of judder for European content.

However in Europe all HD TV's support 50Hz and 60Hz modes, and using the right one makes a huge difference to content quality.

So really, ATV needs a mode that apps can use that allows them to change the output frequency based on the content, and perhaps a setup wizard where users can select all the modes their TV supports, so that you can disable 50Hz for example in the US where support is lacking.

That would make TV apps on ATV far more friendly for content viewing. OK, there would be a bit of screen flicker when the TV changes modes, but I'd much prefer that to constant judder and then having to manually mess around in setup depending on content type which you're not always aware of.

I've never been able to get the ATV4 to output anything other than 60Hz when showing iTunes movies - the actual files, transfers from film, are evidently in 60Hz

The frequency at which the Apple TV outputs the files has nothing to do with the files themselves, the files are what they are, (24 fps for most movies) irrespective of what frequency you choose to use to send the output from the Apple TV to the TV.

Think of it like the dpi example I gave you for the iPhone camera. the iPhone camera takes a picture at 4,000 x 3,000 pixels (or whatever it is), dpi means nothing to the iPhone, it isn't until you put them on a Mac (72 dpi) or a PC (96dpi) or put it through a printer (300dpi - for example) that dpi means anything. Same with the video, Hz doesn't mean anything until the TV gets involved.

I suspect that were you able to connect it to a TV which could only handle 50 Hz

TV's generally work at either frequency, mine do. You can set which frequency the Apple TV outputs at in it's settings, the TV detects what is being sent and adapts to it accordingly.

Indeed I often switch between which frequency the Apple TV outputs at because 30 fps and 24 fps content looks best at 60 Hz and 25 fps content looks better at 50 Hz.

Here's some iTunes content at various frame rates.

"24 fps at 60 Hz requires a 3:3 pulldown, 24 fps at 60 Hz require a 2:2:2:2:2:2:2:2:2:2:2:3 pulldown"

24 fps to 60Hz requires 2:3 pulldown (odd frames 2 fields, even frames 3 fields, so 24 frames in 60 fields)

24 fps to 50 Hz would require every twelfth frame to be 3 fields instead of 2, which looks pretty horrible; which is why it's hardly ever done and films are run at 25fps.

That was a typo, it should have read 2:3. For 24 at 50 Hz it is 3 for 2 every twelfth frame which is 2:2:2:2:2:2:2:2:2:2:2:3

which looks pretty horrible; which is why it's hardly ever done and films are run at 25fps.

24 fps at 50 Hz does look horrible on the Apple TV which is why I suspect it is done on the Apple TV and not just speeded up

60Hz to 50Hz would require dropping every sixth field

This doesn't mean anything, you don't convert 50 Hz to 60 Hz you convert (say) 24 fps to be displayed at 50 Hz or you convert it to be displayed at 60 Hz. The source file is neither 50 or 60 Hz it's 24 fps, whether you display it at 50 or 60 Hz dictates which conversion method is used.

Quoting is getting to be impossible, I've used blue for quotes and blue italic for quoted quotes.

The Apple TV will play all frame rates up to 60 fps (I believe it will play obscure ones too, it did with earlier models but I haven't checked this recently)

Yes there has to be some kind pulldown otherwise I believe there would be a noticeable change in pitch, it's also noticeable that 24 fps content is much smoother at 60 Hz than it is at 50 Hz which suggest to me that the output is being converted

iTunes Movies on the Apple TV are all in 60Hz, and thus display the same issue. It's most noticeable on camera pans, which can look worryingly jerky

I'm not sure that's quite right, iTunes movies aren't in 60 Hz, that's a bit like saying all photos from an iPhone are at 300 dpi. They are at 24 fps and it depends on the output settings for the Apple TV whether you see this at 50 or 60 Hz.

I'm also not certain but I do think I've come across some iTunes movies that aren't at 24 fps (although I do believe virtually all are), however I am certain that TV shows come in 25 and 30 fps and largely depend on where they are made.

As for jerkiness when watching 24 fps content, it's significantly less obvious if your output is set to 60 Hz, which I believe is for the reason previously stated. 24 fps at 60 Hz requires a 3:3 pulldown, 24 fps at 60 Hz require a 2:2:2:2:2:2:2:2:2:2:2:3 pulldown. Obviously another method would be to speed the playback up by letting it play frame for frame as you stated, but I suspect this isn't what the Apple TV does since there is no change in pitch and playback is far from smooth.

There are numerous resolutions at two frequencies at which it will output.

1080p HD - 60Hz

1080p HD - 50Hz •

1280 x 1024 - 60Hz

1280 x 1024 - 50Hz

720p HD - 60Hz

720p HD - 50Hz

1024 x 768 - 60Hz

1024 x 768 - 50Hz

800 x 600 - 60Hz

800 x 600 - 50Hz

576p - 60Hz

576p - 50Hz

480p - 60Hz

480p - 50Hz

640 x 480 - 60Hz

640 x 480 - 50Hz

But it will play any resolution up to 1080 and any frame rate up to 60 (or at least that was the case with earlier models except the framerate was limited to 30 fps)

Good idea, well stated. But telling us won't get the job done. You should send it to Apple at https://www.apple.com/feedback/appletv.html.

Those are output sizes Tom, they are to suit different TV's and projectors rather than the source content.

As for content sizes I'm not aware of any restrictions except for maximums, although it wouldn't surprise me if it ruled out some daft ones.

So for example an old movie that I took on my Sony Pxxx before the iPhone came out that was shot at 320 x 240 and 15 fps can be viewed on my old projector which works at 800 x 600. In such a case if my projector did both 50 and 60 Hz, I'd set my Apple TV to output at 800 x 600 at 60 Hz, because 15 divides into 60 without any spillage.

I'm being perhaps a bit picky, but PAL specifically refers to colour - 'Phase Alternation by Line'. France used SECAM for SD television, but still at 50Hz. But I understand what you mean - basically America is 60Hz mains and I think everywhere else is 50Hz (hopefully no-one is still using DC) except Japan where half the country is 50Hz and the other half 60Hz ⚠ and a few small countries which are on 60Hz; and the TV rate follows the mains. Probably South America is NTSC but I wouldn't know about the smaller countries, though NTSC and 60Hz tend to go together.

Well, I'll have another look - I only ever watch movies - and everything I see is coming over in 60Hz with consequent motion artefacts. I seem to remember trying 50Hz to no effect (but as you say 24 to 50 would be worse). The annoying thing is that my TV is capable of handling 24fps and does exactly that with HD Blu-Rays.

Roger Wilmut1 wrote:

Well, I'll have another look - I only ever watch movies - and everything I see is coming over in 60Hz with consequent motion artefacts. I seem to remember trying 50Hz to no effect (but as you say 24 to 50 would be worse). The annoying thing is that my TV is capable of handling 24fps and does exactly that with HD Blu-Rays.

Yes Apple need to introduce an output option for TV's that support 24 fps