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Edit: While the reason for 2 vertical lines (interlacing) is correct, it seems that the 4 horizontal lines is from a compromise between NTSC and 525-line systems' frequency (explained in length here: https://news.ycombinator.com/item?id=28203942, thank you keithwinstein), standardised as Rec. 601. One of the proposals is 3:2:x, but it was both worse analogue-speaking and harder digitally-speaking. 3:1:x was used for HDVS, which was the uncompressed storage format of Sony. It was essentially inherited for the MUSE, D-MAC and unreleased prototype American Analog HD systems (note that ATSC was essentially a reboot, trying to outcompete MUSE and DVB). Okay, the real reason, as far as the bundles of paper I have* is accurate, is that digital chroma subsampling was first invented for MUSE, a Japanese analogue HD video standard (with pre-broadcast digital components). They chose four for horizontal because it's relatively easy to manipulate using their digital systems at the time and two for vertical so that it's easy to handle interlacing stuff. Unfortunately, I'm not Sony or NHK so I can't say for certain why not eight or any other powers of two. Also, Americans (aka the SMPTE) set the 1,080 lines (the Japanese standard is 1,025), the 16:9 compromise (between the European and Japanese 15:9 and cinema 21:9) and the "limited RGB" dilemma that is experienced in digital video systems (that's literally from the days of NTSC signalling!). Both the Japanese NHK/Sony MUSE system and the British IBA (adopted as European) D-MAC system uses the full-range 8-bit system that is used for JPEG (pre-broadcast to analogue, of course). Analogous to this, the reason why CD audio is 44,100 Hz is because that's the commonality between NTSC (System M, 525-line 480-visible 60-Hz) and 625-line (576-visible 50-Hz) systems. Digital audio was literally stored on U-matic systems at the time, and it was initially only 14-bit PCM rather than the 16-bit PCM of CDs. * or rather, my employer's mini-library. |