Interlaced Video

My understanding is interlacing is also a way to squeeze more out the available bandwidth. It's true that very early CRTs could only scan a limited number of lines per second (as opposed to displaying them) so for interlaced video they'd refresh every even line, then every odd line, which also helped reduce flicker and increase the smoothness of motion. At 50Hz the limit was originally a little over 200 scan lines per 1/50th second, so the UK system began as 405 line system and later increased to 625 line as technology improved. I don't know when the UK switched but I'm pretty sure the very early episodes of Doctor Who were 405 line. I'd imagine the motivation for interlacing today would still be bandwidth/compression related, but 1080i has a higher temporal resolution than 720p at the same frame rate, so I assume it'd be more likely to be used for sporting events where there's lots of motion, or maybe broadcasters think 1080i sounds more impressive than 720p, although I'd prefer the latter most of the time. In Australia the switch from mpeg2 to mpeg4 has been slow because many early HD TVs and set top boxes can't decoded mpeg4. I watch almost no free-to air TV, but I think some broadcasters are using mpeg4 for some non-primary channels these days, while the rest is still overly-compressed mpeg2 crap. The latest "Freeview certified" devices only have to support 576p, 720p and 1080i, although mpeg4 is mandatory. We might have 4k TVs but I can't see free-to air broadcasts being 4k for a long time. I don't know what the situation is in the UK.

Thank you hello_hello Ok from a broadcast / bandwidth pov it has some merits, but on a brand new Blu-Ray filmed in 2016, broadcast and released in 2017, surely it would be filmed and mastered in 1080p. Then again doctor who didn't move from standard definition to HD until. 2010. Interlacing may use less bandwidth, but imo the loss of quality is not worth the compromise.

Movies are actually transmitted as progressive (at least this is what my TV is telling me), so no problem here.

ok poisondeathray has hit the nail on the head. the uk pal standard of 25 fps is not blu-ray compliant at 1080p resolution, so as to be encoded as interlaced, or in the case of the blu-ray i'm referring to which is encoded as MBAFF. so in all likely hood it is filmed as progressive but encoded as interlaced / MBAFF to be compliant with the blu-ray spec. I Guess when they created the blu-ray spec they didn't take the PAL 25fps standard into consideration :/ Kolak also makes a very valid point about the cost of upgrading / replacing outdated equipment.

Problem is that some older broadcast equipment doesn't even support PsF as this was introduce bit later than original interlaced specs. This is probably how movies are transmitted- as PsF when they start at transmission room. At consumer end (on TVs decoders) this is treated as progressive and it's indistinguishable from "real" progressive.

How does the decoder know it's fake interlaced though, as opposed to the real thing, and how would the player/TV know not to de-interlace it but treat is as progressive? The same questions in relation to chroma subsampling. The way I understand it the chroma for each field is subsampled individually for interlaced video, given each field is a different moment in time, but how would the decoder know the "fake interlaced" video has progressive chroma subsampling rather than interlaced chroma subsampling? I'm thinking it should matter, but maybe not?? I assume for PsF, how it's done is part of the specification (although I can't find definitive information on how PsF's chroma-subsampling works) but "fake interlaced" seems like it'd be a different story.

x264 "fake interlaced" is PAFF. In the bitstream there are flags to show the decoder which frames are progressive and which are made of two fields. This is allowed. And then x264 simply doesn't mark any frame as interlaced.

Which should translate to perfect progressive decode. PsF is related to SDI and handling it is defined by strict SDI spec. PsF doesn't really exist outside SDI world. PsF SDI signal by many video cards is translated to progressive HDMI signal (sometimes to interlaced). It's all legacy.

Sorry, I just checked and the UK uses PAL, not NTSC. I could have sworn that I once saw a standards map that showed the UK as NTSC. I stand by everything else that I said.