The Mehikon - broadcast TV color eraser - Page 1
Forum section: electronics / general

author: Bud
author_rank: Super Contributor
timestamp: August 19, 2020, 07:37:44 pm
content: A curious piece of history of broadcast television in Israel that I never heard of before (source: Wikipedia):Quote

author: tom66
author_rank: Super Contributor
timestamp: August 19, 2020, 09:22:36 pm
content: This reminds me of the reason that SECAM was popular behind the Iron Curtain: because most of Europe used PAL, it made it difficult to receive (in colour) European signals without illegal imported sets. Of course, black and white video was still receivable, as SECAM and PAL did not differ there.It also gives rise to the alternate acronym for SECAM: System Entirely Contrary to the American Method.

author: janoc
author_rank: Super Contributor
timestamp: August 19, 2020, 10:20:54 pm
content: Quote from: tom66 on August 19, 2020, 09:22:36 pmThat is nice story but very unlikely to be true.The originally French (and European standard) SECAM was used in the Eastern bloc because it was used in Soviet Union since 1967. For example, Czechoslovakia didn't start with regular color broadcasts until 1973-75. So it was logical that they picked the same technical standard as what was used by the Soviets already (all the labor division within COMECOM).Why Soviets picked SECAM is likely due to the fact that it was an European standard, it wasn't German (all the WWII stuff) and, crucially, PAL didn't even exist when the decisions were made yet.And given the geography, people in Moscow (and most of the Soviet Union, really) really didn't need to worry about receiving any illegal foreign signals in offensive color.FYI, both SECAM & PAL were designed explicitly tofixthe problems of the "American method" (aka NTSC).

author: Benta
author_rank: Super Contributor
timestamp: August 19, 2020, 10:53:37 pm
content: Quote from: tom66 on August 19, 2020, 09:22:36 pmLike janoc says, nice story for the pub, but that's about it.All the systems have B/W compatibility, only colour coding is the issue. SECAM was probably chosen by the USSR as it seemed the most mature system (development started in the 1950s).More efficient in the East Block when warding off "imperialist television" was the audio carrier frequency offset, which gave the option of either seeing a western broadcast without sound, or just hearing the audio.

author: Halcyon
author_rank: Unknown
timestamp: August 19, 2020, 11:25:25 pm
content: Quote from: janoc on August 19, 2020, 10:20:54 pmThere is a reason why NTSC was commonly referred to as "Never Twice the Same Colour". I always did appreciate the extra resolution of PAL.

author: tooki
author_rank: Super Contributor
timestamp: August 20, 2020, 07:16:43 pm
content: Quote from: Halcyon on August 19, 2020, 11:25:25 pmWhereas 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.

author: TimFox
author_rank: Super Contributor
timestamp: August 20, 2020, 10:11:09 pm
content: The reason why North America used approximately 60 Hz frame rate while Europe used approximately 50 Hz is the local mains frequency, unrelated to PAL/SECAM/NTSC standards. The North American b/w standard is actually 60 Hz. If there be line-related noise on the (color) video, a "hum bar" results, moving vertically with a repetition frequency equal to the beat between 60 Hz and 59.94 Hz frame rates.

author: schmitt trigger
author_rank: Super Contributor
timestamp: August 20, 2020, 10:35:42 pm
content: Interesting Israeli story.It shows how an absurd government regulation, even a well-meant one, will always provide incentives for people to break it.Speaking of the PAL system....people sometimes forget that there were several PAL "flavors". The most common was the PAL-B/G, and the oddest one was PAL-M, used exclusively on Brazil and which was a bizarre mix of PAL and NTSC.

author: NiHaoMike
author_rank: Super Contributor
timestamp: August 21, 2020, 02:38:03 am
content: 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.

author: Halcyon
author_rank: Unknown
timestamp: August 21, 2020, 02:41:22 am
content: Quote from: NiHaoMike on August 21, 2020, 02:38:03 amAlthough these days, that is all mostly irrelevant. Many countries, including Australia did away with analog TV transmissions years ago. Now all we have to do is get rid of interlacing and everything will be peachy.

author: vk6zgo
author_rank: Super Contributor
timestamp: August 21, 2020, 03:30:03 am
content: Quote from: NiHaoMike on August 21, 2020, 02:38:03 amThe 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.

author: NiHaoMike
author_rank: Super Contributor
timestamp: August 21, 2020, 03:47:06 am
content: Quote from: vk6zgo on August 21, 2020, 03:30:03 amIf 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?

author: vk6zgo
author_rank: Super Contributor
timestamp: August 21, 2020, 03:59:02 am
content: Quote from: tooki on August 20, 2020, 07:16:43 pmWas 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.

author: vk6zgo
author_rank: Super Contributor
timestamp: August 21, 2020, 04:25:03 am
content: Quote from: NiHaoMike on August 21, 2020, 03:47:06 amNo, 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!"

author: basinstreetdesign
author_rank: Frequent Contributor
timestamp: August 21, 2020, 04:44:07 am
content: Quote from: NiHaoMike on August 21, 2020, 03:47:06 amYes 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.

author: tooki
author_rank: Super Contributor
timestamp: August 21, 2020, 05:16:57 am
content: Quote from: Halcyon on August 21, 2020, 02:41:22 amHear, hear!!!I am actually irrationally angry that digital video standards (other than than 480i, which matched the analog SD signal, and which predates the HD standards) even include interlaced modes. It never made sense for 1080i to even exist, given that practically no TVs exist that can display it truly natively! (How many CRT HDTVs are there out there, really??)

author: Bud
author_rank: Super Contributor
timestamp: August 21, 2020, 05:57:52 am
content: With interlaced method of transmission you send half of the picture 50 or 60 times per second. With progressive method you send full picture at full resolution each time, therefore doubling the bandwidth. I can speculate that 1080i may have to do with lower bandwidth media including perhaps interconnecting cables. It would be less demanding to the system throughput than 1080p.

author: Halcyon
author_rank: Unknown
timestamp: August 21, 2020, 07:33:10 am
content: Quote from: Bud on August 21, 2020, 05:57:52 amIt looks shit, mainly on LCDs.

author: janoc
author_rank: Super Contributor
timestamp: August 21, 2020, 08:09:10 am
content: Quote from: tooki on August 20, 2020, 07:16:43 pmTooki, don't tell me that you can see the difference between 50Hz and 60Hz flicker with a naked eye and in the absence of flickering ambient lighting (i.e. no fluorescents or flickering LEDs around).

author: tooki
author_rank: Super Contributor
timestamp: August 21, 2020, 10:21:23 am
content: Quote from: janoc on August 21, 2020, 08:09:10 amWith regards to CRT TVs? Absolutely. It’s clearly visible, especially in peripheral vision. 60Hz really is the lower bound for being flicker free, 50Hz is just below it. There’s a reason 100Hz (50Hz x2) TVs were developed, but nobody ever had to make 120Hz (60Hz x2) TVs. It’s why cinemas used, at rock bottom minimum, frame rate doubling projectors (48fps) but preferred frame-tripling (72fps) to ensure no flicker.

author: tooki
author_rank: Super Contributor
timestamp: August 21, 2020, 10:25:48 am
content: Quote from: Bud on August 21, 2020, 05:57:52 amInterlacing was originally invented to reduce the demands on the circuitry of TV receivers, limited by 1930s technology.It (sorta) made sense to preserve that in digital video standards intended to be displayed on CRT SD TVs. In digital, they should have just made it 1080p30 instead of 1080i. (The things where you’d want the extra frame rate of p60 are also the ones where you want the clear movement of progressive, which is why sports, for example, typically got broadcast in 720p rather than 1080i.)

author: NiHaoMike
author_rank: Super Contributor
timestamp: August 21, 2020, 01:22:45 pm
content: Quote from: basinstreetdesign on August 21, 2020, 04:44:07 amSo then move the sound carrier that slight amount.Quote from: Bud on August 21, 2020, 05:57:52 am1080p at 30 FPS would be exactly the same pixel rate. Also, since the video is compressed, doubling the frame rate does not double the bandwidth.Instead of interlacing, they could have "checkerboarded" the half frames so that the artifacts would just appear as a blurred image instead of more visible lines.

author: janoc
author_rank: Super Contributor
timestamp: August 21, 2020, 01:33:31 pm
content: Quote from: tooki on August 21, 2020, 10:21:23 amI think the issue is more that if you watched a 60Hz TV you didn't have any 60Hz light sources around (everything being 50Hz in Switzerland), so you didn't get the beating effect that makes it much more visible. Or you have superhuman eyes.Quote from: tooki on August 21, 2020, 10:21:23 amThat's not true, for example 120Hz CRTmonitors were completely common (and expensive!). And pretty much all 100Hz TVs were multi-standard, they were able to double both 50Hz PAL/SECAM signals and 60Hz NTSC.

author: Bud
author_rank: Super Contributor
timestamp: August 21, 2020, 01:54:29 pm
content: Quote from: NiHaoMike on August 21, 2020, 01:22:45 pm..and you would get horrible flicker as a bonusQuoteAll that shenanigan just because someone wanted even 30Hz ?

author: tooki
author_rank: Super Contributor
timestamp: August 21, 2020, 02:57:42 pm
content: Quote from: janoc on August 21, 2020, 01:33:31 pmSorry, your assumptions are just all wrong.When I moved from USA to Switzerland (obviously not bringing any TVs with us, since they wouldn’t work), I immediately noticed how TVs here were flickery. Nothing to do with lighting artifacts, nor with a side by side comparison, but rather something that I was readily able to detect in isolation. This has nothing to do with “superhuman” vision: It’s widely documented that human peripheral vision is easily capable of much faster flicker detection than central vision.It’s my understanding that if a 100Hz TV supported NTSC input (which was FAR from universal!), that 60Hz was not frame-doubled. Maybe some could. Either way, that’s neither here nor there, since NTSC countries didn’t have 120Hz TVs. They never existed because 60Hz didn’t flicker the way 50Hz does. Nobody in an NTSC country would have had a 100Hz TV, even if it could do 120Hz.120Hz-capable computer displays were common among high end CRTs, but that’s got nothing to do with TVs.

author: newbrain
author_rank: Super Contributor
timestamp: August 21, 2020, 03:09:42 pm
content: Quote from: tooki on August 21, 2020, 10:21:23 amI can relate to this. No superpowers, but a kid I was nauseated by TV flicker (50 Hz), and I could not understand how people endured fluorescent lights.Now, it's been ~10 years since I looked at a CRT (wow, never thought of that!), but for fluorescent lights at least, I don't notice it any longer. I changed for sure, they might have too...

author: eti
author_rank: Super Contributor
timestamp: August 21, 2020, 11:54:35 pm
content: Was it designed in "Mehiko"?

author: NiHaoMike
author_rank: Super Contributor
timestamp: August 22, 2020, 01:27:10 am
content: Quote from: Bud on August 21, 2020, 01:54:29 pmThe display can be refreshed 2 or more times per frame. That's how classic 24 FPS movie projectors don't flicker.QuoteCheckboarding and frame rate are independent. The best solution is to just do 1080p at 60 FPS and compress more to stay within bandwidth limits, but if sending half frames at 60Hz has to be done, the artifacts introduced by a checkboard pattern is much less noticeable than interlacing. Actually, checkerboarding really is a slight modification to interlacing.Quote from: newbrain on August 21, 2020, 03:09:42 pmThey definitely have, pretty much every CFL has used an inverter since the early 2000s.

author: BrianHG
author_rank: Super Contributor
timestamp: August 24, 2020, 11:14:12 pm
content: Quote from: tooki on August 21, 2020, 05:16:57 amWhat the hell. My 37inch multiscan Mitsubishi back in 1993 did HDTV in RGB fine once the standard became available.Ok, well, brand new at the time, it was a 20K$ monitor.And yes, 5 years later when I could finally reliably play DVDs on a PC, I ran the screen at 72fps progressive making the motion in movies perfectly v-synced. Well, once in awhile, you could catch a dropped frame as the video card's 72Hz wasn't in perfect tune with the DVD player software of the time which I believe locked onto the sound card's 48KHz clock since it had to sync AC3 without disruption.My desktop screens were all running at either 96Hz or 120Hz. No flicker at all.

author: vk6zgo
author_rank: Super Contributor
timestamp: August 25, 2020, 02:17:51 am
content: With the invention, & subsequent miniaturisation of frame stores, system conversion has become a trivial task.A bit off topic, (but close to the sub-topic of things the eye can perceive), have you ever turned your head quickly whilst gazing at an analog 'scope screen from mid-distance?It looks as if the time/div setting has been reduced &, to me, is most obvious when looking at a sine wave-----the strobing effect of your motion makes you see less cycles spread out further on the screen.I don't think it would work with a DSO, though.

author: tooki
author_rank: Super Contributor
timestamp: August 25, 2020, 05:18:51 am
content: Quote from: BrianHG on August 24, 2020, 11:14:12 pmI wasn’t talking about computer displays, which is why I said “CRT HDTVs”, not just “CRTs”. I assume your 37” Mitsu was a computer display (or broadcast monitor), not a TV, right?Anyhow, I didn’t say there werenoCRT HDTVs. Just a vanishingly small number, compared to the number of flat-panel HDTVs sold over the years.

author: tooki
author_rank: Super Contributor
timestamp: August 25, 2020, 05:24:03 am
content: Quote from: vk6zgo on August 25, 2020, 02:17:51 amWell, I wouldn’t call it trivial. If the sole thing you’re converting is color system (e.g. PAL <-> SECAM), then it’s trivial. If you have to rescale a resolution, it’s a smidgen harder. But any time you have frame rate conversion or interlacing involved, it gets a LOT hairier. Good converters do similar things as the motion smoothing in our TVs: object tracking and interpolation. If you don’t do this, you get jerky movement and (on interlaced source material) combing.

author: igendel
author_rank: Frequent Contributor
timestamp: August 25, 2020, 08:01:48 am
content: Funny thing, I've heard a lot about the Mehikon (I'm just a couple of years too young to have any personal recollection of it) - but it always bugged me, how color can be restored to a B/W picture, and no one could explain so I stopped asking I guess. Now I finally understand... thanks!

author: Ultrapurple
author_rank: Super Contributor
timestamp: August 25, 2020, 10:08:31 am
content: Fascinating info from basinstreetdesign - thank you!The 50/60Hz thing lives on today. I was recently looking at the possibility of acquiring a '4k' TV. Reading the small print in the spec, it's amazing how many of them quote a 120Hz refresh rate for the panel. What's wrong with that? I live in a 50Hz country and if you watch 50Hz (100Hz) material on a 60Hz (120Hz) screen you get awful judder. Once you see it you'll hate it forever.I have yet to see a domestic TV that specifically says it adapts the panel refresh rate to suit the source material (though I admit I haven't done an exhaustive market survey).As an aside, the first couple of generations of Roku-based boxes used for the NowTV service were hard-wired 60Hz - which made as much sense as a chocolate teapot because all the native source material (basically, UK broadcast TV) would be 50Hz.I find it quite nausea-inducing to watch an American show that has been standards converted from 60Hz to 50Hz, converted back to 60Hz in the Roku box, and finally played on a 50Hz screen. Yet my partner doesn't even notice it.

author: tom66
author_rank: Super Contributor
timestamp: August 25, 2020, 11:18:15 am
content: Quote from: Ultrapurple on August 25, 2020, 10:08:31 amPerhaps it's different nowadays, but my 2012 Panasonic plasma has a 100Hz, 60Hz and 72Hz mode. The 100Hz mode is for 50Hz (frame doubling with low flicker), the 60Hz mode is of course for NTSC/60Hz content and the 72Hz mode is for frame-tripled 24fps content. You could see, on an oscilloscope, the drive waveforms changing between these modes, with the screen briefly blanking. The reset pulse, ramp timings etc all changed slightly and indeed the '100Hz' and '72Hz' modes had a slightly lower black level as they could refresh the panel more often without 'mal-discharge'.My older 2008 Panasonic plasma had 48Hz, 50Hz and 60Hz modes for a similar purpose, although the 48Hz mode was very close to the flicker-fusion point and so you often could see flickering in your peripheral vision.

author: NiHaoMike
author_rank: Super Contributor
timestamp: August 25, 2020, 12:51:40 pm
content: Quote from: Ultrapurple on August 25, 2020, 10:08:31 amThe app mpv can perform time domain interpolation.https://mpv.io/manual/stable/#gpu-renderer-optionsQuoteLook for "adaptive sync" or "G-Sync" in the specifications.

author: BrianHG
author_rank: Super Contributor
timestamp: August 25, 2020, 09:26:32 pm
content: Quote from: tooki on August 25, 2020, 05:18:51 amOk, the 37inch was a studio screen/monitor with NTSC/PAL Y/C. YUV, RGB TTL and Analog inputs.The most HDTV CRT TVs which were sold to consumers were pretty much all in Japan as they had OTA analog HDTV back then.

author: vk6zgo
author_rank: Super Contributor
timestamp: August 25, 2020, 11:36:05 pm
content: Quote from: tooki on August 25, 2020, 05:24:03 amTV Networks have been converting from PAL/SECAM 625 to NTSC 525 & vice versa since the mid 1970s.The earliest standards conversion units back in 1967 converted between 625 BW & 405 BW, but maybe did it "on the fly", by strategically dropping lines, rather than with frame stores.(Granted, this was only line rate conversion.)The last generations of telecine chains could convert from "sixteen & two thirds" fps to 25 fps, so that old film of the Kaiser's armies looked suitably ominous, instead of having them prancing around like Smurfs, as just running the old film at 25 fps did.Of course, telecine has the advantage of being a series of full frame stills, unlike a normal analog video frame, where the movement progresses line by line.The "frame grabbers" widely available in the 1990s had problems with this.A test card, or a frame sourced originally from cine film would "grab" perfectly, but video originated by a TV camera would blur (or perhaps "smear" is a better term) on rapid movements.

author: NiHaoMike
author_rank: Super Contributor
timestamp: August 26, 2020, 02:27:13 am
content: Quote from: vk6zgo on August 25, 2020, 11:36:05 pmSo basically equivalent time sampling?

author: vk6zgo
author_rank: Super Contributor
timestamp: August 26, 2020, 06:09:16 am
content: Quote from: NiHaoMike on August 26, 2020, 02:27:13 amMaybe it's just another way of looking at things, as scanning does "sample" the scene presented to the camera, & each tiny increment of movement is a change from line to line, so maybe it can be regarded as being higher in frequency than the "sample" rate.But, as an analog person, I tend to think in tems of it being to do with the fact that analog TV doesn't actually present 25 or whatever frames per second as individual still pictures, but presents a number of lines which take finite time, during which motion is happening in the live scene the original camera is seeing.As with many other things in analog TV, it depends upon human physiology.(Dogs, with their different vision characteristics, did not recognise their kind on analog TV--- they can with digital TV)"Persistence of vision" prevents us from seeing this happening as other than a smooth movement over multiple frames, but the "grabber" can see it, & in endeavouring to record it, tries to capture the xhange over multiple lines.I'm afraid I never went into how the cheaper "frame grabbers" work.Broadcast standard equipment seems to be able to do this without problems-- these were the cheaper ones we used mainly to grab & in turn source announcement "slides" as part of the emergency stuff at the transmitter.Scenery & such, without movement also "grabs" OK with these.