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Why do color TVs make black and white snow?
- deleted 7y ago[deleted]
- phkahler 7y agoForward and backward compatibility around the transition to color television was one of the most amazing innovations ever. Hiding the color subcarrier in-band but largely invisible on black and white sets was a work of collective engineering genious.
- pishpash 7y agoAnalog backward compatibility seems more complicated now.
- dylan604 7y agoAfter dealing with video, anything audio related seems way less complicated.
- FranOntanaya 7y agoAnd a curse long after frame modes lost any real world purpose. The workflows ended surviving much longer than the technology that justified them.
- jschwartzi 7y agoYeah I vividly recall encountering the same front/back porch and horizontal and vertical synchronization pulses in an LVDS output to a touchscreen as would be seen in analog television. Simply because you would need that data to work with monitors that have backward compatibility. This was on the TI AM335X so a fairly recent microcontroller.
- Taniwha 7y agoWell let's talk about gamma correction - originally created because B/W Tv tubes were not linear, rather than doing the linear->tube conversion in each TV (which typically had ~10 tubes in them - think 10 transistors) they did the conversion in the Tv studio for ALL the TV receivers and just standardized the non-linearity of the TV screen tube's response. We kept that same gamma when we switched to digital color spaces (and MPEG compression) - it's why noiry dark movies (think of Blade Runner with people in dark spaces smoking) - because of gamma correction there are few digital codes available at the dark corner of the digital color cubes and the result tends to be blocky simply because the color space simply can't represent those colors .... all because of a technical choice made in the late 30s to reduce the cost of Tvs
- buckminster 7y agoSome related engineering genius: the artefacts created in the black and white picture by the colour signal are hardly noticeable, but they are enough to recover the colour from a black and white recording! https://colour-recovery.fandom.com/wiki/The_Unofficial_Colour_Recovery_Wiki https://colour-recovery.fandom.com/wiki/The_Unofficial_Colou...
- jacobush 7y agoThat's amazing! I wonder if there are still photographs floating around which could be colour decoded too.
- amiga 7y agoMaybe a recorded broadcast of a photo?
- jacobush 7y agoOnly if the photo itself was in colour and broadcast in colour. It's the other way around I was thinking of - a still shot (in black and white) of a PAL or NTSC signal would still contain enough information to decode the colour now, in retrospect. For instance, a high resolution (still) film photo of a large high resolution TV from back in the day.
- fenwick67 7y agoThey even shimmed in closed caption data and widescreen signalling to that same signal they've been using since the 40s. It's honestly beautiful.
- dylan604 7y agoNot to mention timecode, Macrovision, and a slew of other proprietary information was stored in the vertical blanking part of the signal. I really miss the hackable nature of analog signals.
- hazeii 7y agoThough there was a time when presenters knew not to wear anything with narrow vertical or fine chequerboard patterns, because the resulting high-frequency luminance signal would bleed through into the colour signal causing phantom colour effects. Interestingly the effect occasionaly occurs today, here in the UK where people use old Sky boxes to provide an RF output to a TV in another room.
- deleted 7y ago[deleted]
- Taniwha 7y agoWasn't that long ago - it changed fashion - all those hounds-tooth jackets, paisley ties etc etc stopped being seen in public (on TV) and were replaced by solid colors.
- JdeBP 7y agoBack in the 1980s, D-MAC was touted as the system that was going to eliminate that problem. Separation of analogue chrominance and luminance was specifically called out as a feature in popular science explanations.
- YeGoblynQueenne 7y agoDo you mean like when a moire pattern may sometimes appear to have a ghostly coloured after-image? I've seen this in old black-and-white films shown on colour TV. I've been trying to describe that to people to learn more about what it is but I always fail to. I feared there might be something wrong with my eyes. Is there a name for this effect?
- hazeii 7y agoFor Tv's the colour is caused because the colour signal is hidden as a high frequecy component that older B&W Tv's would ignore. Conversely, a high frequency signal caused by closely spaced light and dark patterns fools colour TV's into thinking there's a colour signal present (as the filters in TV's improved the magnitude of the effect reduced, but it's there to this day in analogue TV). What you're seeing is probably Fechner Color [0]. I've seen the effect myself, as I recall there was a bit of a craze for demoing it some years back. [0] https://en.wikipedia.org/wiki/Fechner_color https://en.wikipedia.org/wiki/Fechner_color
- dbcurtis 7y agoYour comment made me think of a personal anecdote. File it under "Why I love living in Silicon Valley". Our neighborhood blocked off the street for a Fourth of July picnic a few years ago. Sitting in a lawn chair talking to a long-retired neighbor over a beer, we got to talking about old analog TV standards. (We are both hams, so conversation tended to turn to radio-ish things.) I remarked about how clever I thought it was mixing the video carrier and audio subcarrier to get the audio IF. And he says: "Oh, it wasn't always like that. When I was a kid, there was a TV station in Philadelphia that transmitted 2 1/2 hours two nights a week. They taped up the schematics for a receiver inside a big picture window at their station, and we would all go down with notebooks and copy down the latest changes so that we could update our receivers. The audio subcarrier went through a lot of changes. AM, FM, and they moved the frequency around." So... I realized that here was a guy who was watching television back when the way you got a TV receiver was to build it from scratch yourself. Wow number 1. I said: "I suppose when the war came along you were sent to radar school like almost everybody else that worked on television?" Answer: "No. Did you ever hear of Eckert and Mauchly?" Me: stunned.
- jacobush 7y agoSo, let me get this straight - he worked on the design or construction of the ENIAC? :)
- dbcurtis 7y agoI believe so. Or the war-time version they did for the Army. I don't remember all the history of those machines exactly.
- dylan604 7y agoWhile not in Silicon Valley, but the other Valley down in Burbank, I had a similar conversation. I was introduced to this person as we had a common interest in the proper methods of 24fps->29.97fps->23.976 video conversions. He told me of the summer he and his brother created a new method to replace the old school flying spot scanning technique that was common at the time. The new process included the ability of one of the first video noise removal capabilities. The coolest part of the story to me was that the first TV broadcast from the moon was sent through the bit of equipment he had built before being broadcast to the public.
- dylan604 7y agoI too hold this 30fps->29.97 solution to be quite ingenious. I wonder if they had any insight into the future of broadcast formats if they might have chosen a different path? I have no idea if anything else was possible, but looking back monday morning blah blah blah This backwards compatibility has been the bane of my career. The 29.97 or 30000/1001 frequency, the interlacing, and the effects it had on 24fps film transfer has given me a niche career path, but one so frustrating when dealing with media content butchered by people that had zero understanding of the history of why film/video content was formatted the way it was.
- jacquesm 7y agoThere are two other tricks like that: stereo radio transmission and stereo gramophone records.
- LocalH 7y agoThey didn't do it without slightly modifying the existing standard (luckily still within tolerance). Thus the source of that infernal 1000/1001 factor that pushed field rates from 60Hz to approximately 59.94Hz
- klodolph 7y agoThe other question is “why don’t you see that noise when the TV is tuned in?” The TV has automatic gain control. When the signal is weak, it will amplify it up to the right level. If the signal is just background noise, then it will amplify that background noise until it’s at the “right level” for a proper TV signal. So, the gain is lower for stronger signals, and very high when there is no signal at all. Fun fact: B&W TVs are more suitable as computer terminals because they have more bandwidth for the luma signal. Color TVs have to filter out the chroma signal, which reduces the bandwidth for the luma. If you are stuck with a color TV for your terminal, consider modifying it to accept a signal after the low-pass filter. This is not too hard for someone comfortable with a soldering iron, someone who isn’t scared of drilling a hole in their TV. Thank you for coming to my TED talk.
- jacobush 7y agoThis I have wondered many times even since a kid but even now I think I would have had a hard time asking the question right. Thanks!
- LaserDiscMan 7y agoFor those who are interested, automatic gain control on VHS players was exploited by Macrovision to prevent the copying of commercial tapes. Analog video signals contain synchronizing pulses, which are necessary for correct sweeps. These pulses are not part of the visible lines shown by the TV. A Macrovision tape contains spikes of varying amplitudes within the synchronization pulses. The spikes are placed far enough from the visible lines (usually the first 40ms of a frame), so that the TV's AGC is not affected, and the picture remains stable. When a VCR is receiving a Macrovision signal to record, the AGC (which unlike the TV does care about the first 40ms of a frame) amplifies the signal, causing the visible lines to be heavily altered in an erratic fashion. This sometimes appears as "snow" on the duplicated tape. Some modern "VHS-To-PC" device drivers/conversion software will detect a Macrovision signal and refuse to accept it. It was defeated in numerous ways, but the hassle free way to duplicate a Macrovision tape was simply to use an older VCR without an AGC circuit.
- thedance 7y ago
- beezle 7y agoIt wasn't entirely black and white snow. Also was a bit of pink and green too. Been awhile since I saw any though
- dylan604 7y agoI'm guessing it's been since 2009. https://en.wikipedia.org/wiki/Digital_television_in_the_United_States#Transition_from_analog_to_digital_terrestrial_broadcasts_in_2009 https://en.wikipedia.org/wiki/Digital_television_in_the_Unit...
- jimmaswell 7y agoI think I remember seeing colorful noise on top of a weak color signal. Makes sense given the color burst is there.
- russellbeattie 7y agoWhat a coincidence, just this morning I was reading a list of best opening sentences in novels, and smiled at this one: "The sky above the port was the color of television, tuned to a dead channel." - William Gibson, Neuromancer. But then I thought sadly, that more and more people won't understand that reference as time goes on. I can't remember the last time I saw real channel snow on TV. Probably the last time I owned a tube TV, which is easily a decade or so now... Digital TVs will automatically blank the bad channel out, so the experience of flipping to a blank channel and seeing/hearing snow is something few people will experience nowadays. Like hearing a record scratch, a real bell on a telephone or a fax screeching.
- badsectoracula 7y agoChances are you may have a different image than what Gibson had since the way a dead channel appears on a CRT had changed over the years and Gibson had mentioned that he was thinking of some very old TVs from his childhood. Here is a relevant thread on scifi stackexchange including a video of a TV from the era Gibson described: https://scifi.stackexchange.com/questions/163304/what-color-was-the-sky-above-the-port-in-neuromancer https://scifi.stackexchange.com/questions/163304/what-color-... But in practice i think everyone would understand even without seeing it the first time - it is like phonographs/gramophones, most people nowadays haven't used one in real life but they do know how they look like.
- the_af 7y agoI must admit I ctrl-F'ed this thread to see if my question from scifi.stackexchange was here. Thanks for making my day :)
- dylan604 7y agoThe static disappeared with the transition to DTV, not the switch away from CRTs. Analog signals can be "fuzzy", digital signals are either received or not.
- im3w1l 7y ago
- hazeii 7y agoI used to have a Philips 100Hz CRT TV, which used digital processing to 'double' the frame rate. One nice feature was it had a freeze-frame feature, back when such things were pretty rare. So the party trick was to leave it on showing frozen black and white 'snow', and when someone mentioned it just say 'oh, it's stopped again' and bang on top of it with my fist. The snow would then start moving again, in the usual manner. The trick was, of course, furtively pressing the 'unfreeze' button on the remote control in my other hand.
- _sbrk 7y agoNo color burst on the front porch, so the color killer circuit disables color.
- basementcat 7y agoJust want to take this opportunity to remind everyone that a portion of the noise power in "black and white snow" is the Cosmic Microwave Background from the Big Bang. http://www.astro.yale.edu/vdbosch/cmb_Osher.pdf http://www.astro.yale.edu/vdbosch/cmb_Osher.pdf
- justlexi93 7y agoThe short and academically correct answer: a random noise signal is random. The lack of organization precludes that there is no separate color carrier present, and the set goes, as is already answered, into b/w mode. For split seconds there might be randomly something that looks like a color carrier might be present, then there might be a quick colorish flicker. yes, you do not see wrong: while the generated signal is strictly b/w there are reasons that the academical requirement for b/w noise is not really happening: technical reasons and physiological reasons. A little ramble of why we still see sometimes color in real random noise: There are: physiological reasons: 'optical illusion' reasons that have to do with improper function of an analog TV reasons that have to do with improper function of analog to digital conversion that can make random noise appear tinted on a digital TV (a 'should not happen' that happens actually all the time on cheap TVs, there is a reason for 'cheap' and 'costly' digital TVs...) Let's go, or just stop here if you like... everything has basically been said, this is just for your enjoyment: pink artifacts: 'snow', when we see it: our eyes do add color tints , as they do add 'correlations'. Some see faces and stuff (TV people...maybe talk to someone about it... ;-D ) there are some optical illusions out there that move some b/w patterns and we (at least I do, don't have my bionic eyes yet..) do see color although there is none at all on the moving pattern google for 'optical illusions' Technical reasons why 'should be b/w noise' may have some hues and colored pixels: color in random noise on an analog color TV (or computer monitor): Reminding: noise from 'no signal' or from a blank analog magnetic medium on an analog TV cannot have a color signal which would require the organization of the noise into frames of certain durations ... There are instances where a color TV will render random noise (b/w) as color: The color convergence is out of alignment and the three (r/b/g) guns focus slightly off-center of the color triple on the screen (instead of 100% r/b/g it gives a little tint), either always, or per pixel and randomly. Stationary tints mean you need to demagnetize, random tints per pixel mean there is noise on the beam control. "I see color noise on a digital TV..." A digital TV has no noise, it works or not. A rendered analog signal (from a VCR empty tape or from an analog tuner / receiver) will be b/w at the origin. Then the analog signal is converted to a digital signal and here trouble can lurk in the detail: Before the conversion from analog to digital, the above rules for analog signals do apply: no blanking period means no color signal went in. Your analog inputs to a digital TV or monitor can come from dual tuners: even if there are no analog TV stations you might have an analog signal from some arcade games, from a camera abusing of an analog TV channel, or an old VCR you light into your digital TV via analog channel 2 or 3 (you need to know what a rotary dial phone is to understand this....). Again: academically these should be strictly b/w after conversion from analog to digital. Some reasons how conversion imprecision can add color to 'white' noise: Bad converters used to convert from analog to digital can include 'dust' in the rapidly generated digital pixel value. That is a software issue in the converter. A 0 is not exactly zero, a one is a little less than all bits on, and some bits are picked sometimes in the middle. When I say here and from here on '0' and '1' I am talking about 'all bits 0' and 'all bits 1' in the per pixel color depth value. Bad video a/d converter designs use FP units on fast and cheap general-purpose chips: again: 0 is not exactly zero ever and 1 is at most .999999. they do some mantissa tricking to process, e.g., a 24 bit pixel signal through an FP unit which is a general-purpose, cheaper (and faster, vectorized) chip than a real video converter chip. This creates a hue that is different per pixel and per instance (the same digital pixel lights up in slightly different hues each time it is used).
- ohiovr 7y agoFirst answer is right. I have some cave man era video experience. Color is a clever add on hack to the black and white transmission standard that allowed forwards and backwards compatibility with all sets.
- hyperpallium 7y agoWouldn't random colours average to greys to the HVS, pretty quickly, anyway?
- kmbfjr 7y agoThey originally did it all with vacuum tubes no less.