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| The Mehikon - broadcast TV color eraser |
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| vk6zgo:
--- Quote from: NiHaoMike on August 21, 2020, 02:38:03 am ---The part I like least about NTSC nowadays is the oddball 29.97 FPS frame rate, specifically because it's sometimes used in modern video standards instead of being rounded up to 30 FPS. The tolerances on analog TVs are loose enough that they won't even notice a slight increase in frame rate. --- End quote --- The old BW system was exactly 30Hz-----when NTSC colour came along, the horizontal rate was changed to avoid interference problems with the colour subcarrier frequency, so the vertical field (& frame) rate had to be changed as well, to maintain the normal raster. Analog TV is interesting----if you look at a BW (luma) video signal with a Spectrum Analyser, you will notice that the signal actually consists of multiple harmonics of the 15kHz horizontal rate, each with sidebands at multiples of field rate. There are gaps in the spectrum between these. For colour TV, the colour information is quadrature modulated upon the colour subcarrier (3.58MHz approx in NTSC). This signal also has a spectrum with large gaps between the sidebands. By judicious choice of the subcarrier frequency, these sidebands fit in the gaps in the luma signal spectrum. What might be called "frequency interlace"! This is why the strange frequencies are used as horizontal, vertical & colour subcarrier rates. |
| NiHaoMike:
--- Quote from: vk6zgo on August 21, 2020, 03:30:03 am ---The old BW system was exactly 30Hz-----when NTSC colour came along, the horizontal rate was changed to avoid interference problems with the colour subcarrier frequency, so the vertical field (& frame) rate had to be changed as well, to maintain the normal raster. Analog TV is interesting----if you look at a BW (luma) video signal with a Spectrum Analyser, you will notice that the signal actually consists of multiple harmonics of the 15kHz horizontal rate, each with sidebands at multiples of field rate. There are gaps in the spectrum between these. For colour TV, the colour information is quadrature modulated upon the colour subcarrier (3.58MHz approx in NTSC). This signal also has a spectrum with large gaps between the sidebands. By judicious choice of the subcarrier frequency, these sidebands fit in the gaps in the luma signal spectrum. What might be called "frequency interlace"! This is why the strange frequencies are used as horizontal, vertical & colour subcarrier rates. --- End quote --- If you speed up all those frequencies in proportion so that the frame rate is exactly 30 FPS, wouldn't they still have the same relation to each other? |
| vk6zgo:
--- Quote from: tooki on August 20, 2020, 07:16:43 pm --- --- Quote from: Halcyon on August 19, 2020, 11:25:25 pm --- --- Quote from: janoc on August 19, 2020, 10:20:54 pm ---FYI, both SECAM & PAL were designed explicitly to fix the problems of the "American method" (aka NTSC). --- End quote --- There is a reason why NTSC was commonly referred to as "Never Twice the Same Colour". I always did appreciate the extra resolution of PAL. --- End quote --- Whereas to me, as a highly flicker-sensitive individual, the trivially lower resolution of NTSC was more than a fair price to not have to deal with the flicker of a standard 50Hz PAL TV. :P --- End quote --- Was that with normal TV programming or did you have problems using a PAL TV as a monitor, as many of us did with the Commodore 64, etc? On the resolution side of things, before I went to the UK in 1971, I assumed I would see a huge difference in resolution between the BW 625 line system used in Australia, & the Brit 405 line system. As most of the Brit TV shows I'd seen were "kines" so were pretty dire, I had a definitely jaundiced view. The first "guest house" I stayed at had a "dual standard" BW TV for us "guests". Watching both 625 & 405 line Stations, I was surprised that the difference was quite slight. OK, the line structure, which was apparent with both, was a bit coarser, so vertical definition suffered slightly, but the horizontal resolution was pretty much the same. As the horizontal rate is slower, with 405, a given resolution requires lower frequency response, so, the narrower passband doesn't matter that much. One thing which was immediately noticeable was that with positive modulation, occasional very bright pictures would cause loss of vertical syncs. On the BBC, these were rare, but on the commercial channels, they did occur in the form of ads. |
| vk6zgo:
--- Quote from: NiHaoMike on August 21, 2020, 03:47:06 am --- --- Quote from: vk6zgo on August 21, 2020, 03:30:03 am ---The old BW system was exactly 30Hz-----when NTSC colour came along, the horizontal rate was changed to avoid interference problems with the colour subcarrier frequency, so the vertical field (& frame) rate had to be changed as well, to maintain the normal raster. Analog TV is interesting----if you look at a BW (luma) video signal with a Spectrum Analyser, you will notice that the signal actually consists of multiple harmonics of the 15kHz horizontal rate, each with sidebands at multiples of field rate. There are gaps in the spectrum between these. For colour TV, the colour information is quadrature modulated upon the colour subcarrier (3.58MHz approx in NTSC). This signal also has a spectrum with large gaps between the sidebands. By judicious choice of the subcarrier frequency, these sidebands fit in the gaps in the luma signal spectrum. What might be called "frequency interlace"! This is why the strange frequencies are used as horizontal, vertical & colour subcarrier rates. --- End quote --- If you speed up all those frequencies in proportion so that the frame rate is exactly 30 FPS, wouldn't they still have the same relation to each other? --- End quote --- No, the spectrum gaps don't quite work out. Bear in mind, this was designed as a whole system to be used in thousands of units, so they could afford to be "fussy". We could probably tweak it to exactly 30Hz & not notice, but the FCC would have! It's ages since I read the complete specs for 625 line PAL, but I'm pretty certain they kept 50Hz & adjusted the other frequencies. I do remember that the "slack" way of looking at a field rate signal with a 'scope was to just trigger from the 50Hz Mains. The display did drift, but it was so slow as to be effectively imperceptible. . 30Hz/30fps frame rate put the designers of telecine chains in a world of hurt, as there was no easy relationship between that & the 24 fps used for film. They devised ingenious mechanical systems which presented the same frame several times to get around this. Very, very, clever, & very complex to explain, especially for myself, as I never had to work with them. 50Hz systems just "cheated", ran their projectors at 25 fps, & said "to hell with the 4% difference!" |
| basinstreetdesign:
--- Quote from: NiHaoMike on August 21, 2020, 03:47:06 am ---If you speed up all those frequencies in proportion so that the frame rate is exactly 30 FPS, wouldn't they still have the same relation to each other? --- End quote --- Yes it would but you would have to move the sound carrier up by the same amount to avoid the original problem: If there were any non-linearities in the video system then an annoying stationary dot pattern would be caused by the sound carrier at 4.5 MHz. In that case, all NTSC receivers would have to be returned as well, and that wasn't about to happen. All other components of the signal were slowed down to maintain their respective relationships, of course, and this lowered the frame rate from 30 Hz to 29.97002997… Hz, or approximately 0.1%. The exact ratio of new frequency to old is 1000/1001. This seemed innocuous enough for the system designers since their first priority was to make sure that the system stayed compatible with all of the installed receivers in America; and a shift of about 0.1% was sufficiently small that those TV sets could easily track the change in scanning rate. However, when this was done the distinction between video time and real time came into sharp focus. With the widespread usage of time code 1 hours worth of video, or 108,000 frames of videotape would last too long! It now became 60 x 60 x 29.97… = 107,892 frames, or 108 frames too short! It was imperative for the producers (and sponsors) of an hour’s worth of programming to see that the length of a show was exactly one hour. This discrepancy was fixed in the application of the code by losing or “dropping” 108 frame counts spread over the course of the hour. So, during the progress of the frame count through the hour, two frames (the counts only NOT the actual frames of video), numbers 00 and 01 were skipped or “dropped” at the start of each minute with the exception of minutes which were a multiple of ten. Sounds complicated? It gets worse. Now every manufacturer of equipment that generated, read, computed, or otherwise used “drop frame” time code had to take this stuttering time reference into account when dealing with programming length. Every producer, director, editor who wishes to create accurately timed commercials or programs had to be aware of this disjoint “real-time” reference. As it happens, the above discrepancy of time is not exactly 108 frames per hour. I rounded the frame rate to 29.97 f/s but it is actually a repeating decimal, 29.97002997002997… This means that a fraction of a frame is left over each hour. In fact a real hour is 107,892.1079… frames long. If the time code generator were to run for an entire day it would count 2,589,410.589... frames. It would NOT be an even number of frames due to the repeating decimal. The “drop frame” code would reach the terminal count of 24:00:00:00 with an error of [2,589,410.589 - (30 x 60 x 60 x 24) - (108 x 24)] or 2.5894 frames or 86.4 milliseconds. Each and every day a time code generator would be this amount out of time every day. It drove many people nuts to have to deal with this crap until the software in the editing systems became sophisticated enough to remove much of the burden from the users. It also gave the Europeans much cause for amusement to look at the mess the TV engineers across the pond had created for themselves. |
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