CRT technology question - Page 1
Forum section: electronics / rf-microwave

author: cheater
author_rank: Regular Contributor
timestamp: September 17, 2019, 02:47:48 pm
content: Hi!First of all, not sure where this should go, but since this is about CRT TVs, the closest subforum I could find is radio, which is here...I was wondering if you could take an existing TV, one of the better quality tubes, and upgrade it to support higher resolution, or refresh rate. If you're willing to make your own raster scan driver, and possibly upgrade the rgb preamps, what is actually limiting a CRT TV from supporting increasingly higher resolutions? I have a large, high quality Sony CRT TV (32", 16:9, one of the last generations) and I wonder what resolution the tube could theoretically support. A raster scanner isn't an insurmountable amount of work, neither is a simple RF preamp.Thanks.

author: james_s
author_rank: Super Contributor
timestamp: September 17, 2019, 03:05:40 pm
content: A crt is not a fixed pixel device, actually it has no concept of pixels at all, though the dot/stripe pitch of a color tube is often the limiting factor. The smallest achievable spot size that can be focused will also be a limit but otherwise the resolution is determined by the scan circuitry and the bandwidth of the video amplifier driving the cathode(s). The deflection yoke is also an integral part of the scan circuitry design.There are also vector monitors that use a conventional CRT but do not scan a raster at all, they have X and Y inputs and draw lines between arbitrary points. These were used mostly in arcade games in the late 70s-early 80s.

author: james_s
author_rank: Super Contributor
timestamp: September 17, 2019, 03:11:53 pm
content: It's worth mentioning though that the deflection circuits in a late generation tv, especially a Trinitron, and even more especially a wide aspect ratio tube are very very specialized stuff and the horizontal scan is intimately linked with the EHT. Designing a replacement that is an improvement over what is already there would be the most astonishing DIY achievement I have ever seen. What you already have is the pinnacle of decades of engineering.

author: Audioguru again
author_rank: Regular Contributor
timestamp: September 17, 2019, 04:25:33 pm
content: Kiss the old CRT TV goodbye and bury it. Then buy a modern ultra high definition 4K LCD TV.Are 8K TVs available yet?

author: dave j
author_rank: Regular Contributor
timestamp: September 17, 2019, 04:32:42 pm
content: Find out your display's aperture grill size first. There's no point producing some super duper new raster scan driver only to find the tube can't benefit from it.

author: james_s
author_rank: Super Contributor
timestamp: September 17, 2019, 04:42:55 pm
content: I still think CRTs look better than LCD for SD content, but for HD there's not really much point. Not that there's anything on TV that's worth watching anymore.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: September 17, 2019, 04:56:11 pm
content: The real limits on resolution have been mentioned above and will prevent any major increase in the number of displayable pixels.In BW CRTs the limit is set purely by the spot size. Modest improvements could be achieved here by implementing a high resolution digital variable focus scheme to compensate for the changing geometry across the screen and brightness variation effects, but it is fairly hard to make all of those negatively charged particles come together in one spot. You would also have to adjust beam current to keep power density on the phosphor reasonable. I am not enough of a human factors guy to know whether the net result - four pixels written over time with similar power density but lower individual dwell than the single original pixel - would be satisfactory or would seem dim.In color CRTs you have all of above, plus the shadow mask/phosphor geometry. In Trinitrons there are vertical stripes of the three color phosphors which limit the horizontal resolution. In principle the vertical resolution could be improved without impact from the phosphor geometry. In a conventional color CRT there are triads of phosphor, each triad aligned with a single hole in the shadow mask which is where the electron beam is focused. The pitch of the holes limits both horizontal and vertical resolution. While some TVs and monitors may not have utilized the full shadow mask resolution that resolution is the ultimate limit on output resolution. No external electronics can change that.As to higher scan rates there is no fundamental reason you couldn't step that up, but it wouldn't be a simple tweak on the existing elx. Lots of energy involved in pushing that beam around and lots of resonant circuits. To double scan rate you have to roughly double the power and change the resonant frequencies of noted circuits by the same factor. Since dwell on pixel would be halved you would have to double beam current to maintain brightness.While this might be a fun project to take on, as james_s says it would be an extremely challenging one. There is a real message when an infant technology like LCDs goes from initial introduction to total dominance of the market in roughly five years. If CRTs had an easy path (easy by the standards of industrial giants like Sony, Zenith and Phillips) to higher resolution they would have taken it and staved off the demise of their product for a few years.

author: james_s
author_rank: Super Contributor
timestamp: September 17, 2019, 05:11:25 pm
content: I actually think that even if CRTs had clearly superior image quality to even the average person, LCD still would have taken over the market similarly. It's not image quality that won the war, it's flat and thin, at least up to a point. Since practically the dawn of TV, the holy grail has been a TV thin enough to be mounted to a wall and LCD and plasma displays made that happen. Lots of wives were happy to dispense with the big hulking box dominating the livingroom and replace it with something just a few inches thick.I observed many years ago when CRTs were king that most people don't care about picture quality. The best selling TVs were always the cheapest junk from places like Kmart and Walmart and only enthusiasts would spend the extra money to get the really good stuff like Sony. I'd see TVs all over that had the color cranked up way too high or the tint way off so people's faces looked saturated pink or orange. Even as recently as 10 years ago I was seeing a lot of HD LCD sets hooked up with composite video which of course only supports SD. Millions of people had HDTVs yet had no idea that they needed a HD source and HD capable interface to take advantage of it. Most people don't even notice if the picture looks like crap.

author: Gyro
author_rank: Super Contributor
timestamp: September 17, 2019, 09:25:59 pm
content: Quote from: james_s on September 17, 2019, 04:42:55 pmI used to own a Sharp LCD TV made specifically for the European PAL market, the LCD was native SD resolution. It was stunning on SD without any upsampling artifacts, very natural. It looked pretty darn good ondownsampledHD too.

author: Yansi
author_rank: Super Contributor
timestamp: September 17, 2019, 09:37:25 pm
content: Hey Gyro, those TFTs also had typically analog inputs for color signals. No digital RGB involved, no cheap rubbish video ADC sampling.

author: james_s
author_rank: Super Contributor
timestamp: September 18, 2019, 12:43:45 am
content: All LCD panels are inherently digital, so there has to be an ADC in there somewhere. The panel itself will always have digital RGB internally.

author: cheater
author_rank: Regular Contributor
timestamp: September 18, 2019, 01:19:11 am
content: Quote from: CatalinaWOW on September 17, 2019, 04:56:11 pmI understand convergence and noise both factor into this. We're able to design much lower noise circuitry now, especially when using a digital source with a DAC close to the output. Convergence might be interesting, I guess a LUT that works at the scan rate would be necessary.QuoteIs that exactly true? Sure, the color would change every e.g. 0.1mm, and you'd have gaps between them as well: 0.02mm of black, 0.1mm of green, 0.2mm of black, 0.1mm of blue, and so on. However, within each color, you could still have multiple different values display, if your brightness value changed quickly enough. So the pixel striping might be hard to notice, especially for moving objects. Essentially, it's as if you had a display where you had a pattern like this: 1 black pixel, 5 pixels that'll only display shades of green, then a black pixel, and so on.QuoteI thought the resonant circuits used in modern TVs were to ensure scan stability using existing integrated analog technology. TV driver electronics were an industry unto themselves, but it doesn't necessarily mean that you couldn't design something similar using e.g. digital control or even hook up an arbitrary function generator. I don't have an AFG, but I would love someone who has a dual AFG and a HV source and suitable amplifier try and display a high pitch raster on a modern (ish) CRT TV. Sadly here my lab is insufficient. However, I've tried similar things on my Tek 7k scope and it worked a treat.QuoteDo you? You'll scan twice as fast, but you'll also scan twice.QuoteAgreed, I'm not discounting the complexity of this - but I'm also not satisfied with just giving up on the idea because "what could be done has been done". Electronics have progressed a lot in the last 50 years and CRTs could easily enjoy an upgrade.LCD technology is 16 years older than vacuum tubes.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: September 18, 2019, 03:41:58 am
content: Quote from: cheater on September 18, 2019, 01:19:11 am

author: james_s
author_rank: Super Contributor
timestamp: September 18, 2019, 05:32:02 am
content: Quote from: cheater on September 18, 2019, 01:19:11 amMost modern CRTs use static convergence which is accomplished through careful design and adjustment of the deflection yoke and the adjustment rings on the neck of the tube. Trinitron tubes are somewhat unique in that they employ electrostatic dynamic convergence. The Trinitron aperture grill tube represents the pinnacle of CRT technology and the wide aspect ratio versions are a significantly greater challenge. There's an interesting book about the development of the Trinitron which took years to get right, I forget what the book was called though.QuoteThat's not quite how it works in practice, look at a color vector monitor and you'll see that it looks distinctly like pixels even though there are none, it is drawn as a continuous line. The beam can sort of illuminate only part of a group of phosphor strips but you still get a pronounced screendoor effect.QuoteThe horizontal scan circuit in a TV is a resonant tank that incorporates the HV transformer. There is quite a large amount of power bouncing back and forth which is required to deflect the beam at the horizontal scan rate. The vertical scan is normally not resonant but it runs at the field rate which is typically 60Hz compared to the horizontal line rate of 15.7kHz for NTSC. Now there are the vector monitors I mentioned earlier which are of course not resonant but they are incapable of displaying a normal raster, the deflection is not nearly fast enough and the deflection amplifier produces a tremendous amount of heat. I'm not going to say it would be impossible to design a non-resonant horizontal deflection circuit but it would be an extraordinary engineering feat and it would consume a very large amount of power to slam the required large current through the deflection yoke required to push the beam rapidly back over for the next sweep. Even the highest resolution CRT computer monitors still had resonant horizontal deflection, they incorporated all sorts of tricks to support multiple scan rates. They would switch capacitors in and out of the tank and vary the B+ to the scan circuit.QuoteIf the goal is to increase resolution then yes, you have more "pixels" to paint on the screen, assuming the same field rate this means you have to increase the line rate and spend less time in any given location meaning higher beam current. Ever play with an analog oscilloscope? What happens to the brightness when you turn up the horizontal sweep rate?QuoteIt has been done, haven't you had a high resolution CRT computer monitor? The fanciest one I owned could do 2048xsomething or other resolution and it was a very complex piece of equipment. HD studio monitors were available that could do 1080p, I think that's as high as they ever got. A computer monitor is optimized for a different use case than a TV though, it's possible to get very high resolution with a very small spot size by sacrificing brightness, a TV is capable of a far brighter picture than a monitor.Electronics have progressed yes, in some ways, but at the same time there are types of parts you can't get anymore. Try sourcing a really fast video amplifier for driving CRTs, they're hard to come by these days because nobody is developing CRT tech anymore. Look for a the specialized high voltage, high speed transistors used for horizontal outputs and you'll have trouble finding a lot of them, most are not being made anymore. CRT display design is a very, very specialized engineering discipline that virtually vanished 10+ years ago.Could it be done? Possibly, but how many decades of your life are you willing to devote to learning the ins and outs? How many thousands of dollars are you willing to spend prototyping? What do you know that escaped designers of high end CRT displays just 10-15 years ago? Frankly this is one of those cases where I'd assume if you had the expertise necessary to get anywhere at all with this you'd be well along the way already instead of asking here. If you want a starting point, study service manuals for high end SVGA computer monitors and the very last of the HD CRT displays that could do 1080p, I have a 15" Sony studio monitor that can do 1080i and it looks phenomenal, AFAIK they were over $10k in the early 2000s. They used special tubes that were the hand picked cream of the crop.If you do manage to succeed at this, you will have outdone every hobbyist engineer I've ever heard of, it would be the most amazing one-man project I've ever seen.

author: dzseki
author_rank: Frequent Contributor
timestamp: September 18, 2019, 06:06:12 am
content: FYI, I have a hobby of running and tweaking (back in the day) high-end CRT projectors.These are running 3 single colour CRTs (7"-9" size each) and can light up 2-2.5m wide screens. These machines were really pinnacle of the technology with everything you can imagine:dynamic electromagnetic beam focus and astigmatism (per colour), excessive convergence capability (digitally controlled of course), relatively high bandwidth, no shadow mask, these were multi sync devices with 150kHz (or more) horizontal scan limit and 160Hz (or more) refresh limit. Such a beast can even display a relaxing HD (1080P) picture with ultimate on/off contrast.The down side is they are really beasts with 60+ kg weight and a matching size, though while hanging from the ceiling size usualy does not really matters (providing you dare to sit under it)Some of the highest end models were: Barco 909, Sony G90 or Electrohome Marquee 9500 Ultra.The later one was in production up until 2012, mostly used in military simulators because of the excellent reliability of the tubes (compared to conventional projector lamps available back then) and the no-lag inputs. No surprise their price tag was in the 40k USD range...BTW such projectors can be found basically for free all over the world, type and condition varies of course...

author: Yansi
author_rank: Super Contributor
timestamp: September 18, 2019, 07:51:24 am
content: Quote from: james_s on September 18, 2019, 12:43:45 amWell, No, at all. Sorry. TFT panel is a pretty analog device. Those with with digital inputs utilise either R-DACs or switched capacitor DACs.I am pretty sure there is no ADC in the analog TFTs used in many SD video monitors not that long ago.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 18, 2019, 08:27:00 am
content: Quote from: james_s on September 18, 2019, 05:32:02 amIndeed, I estimate my(ex-?)Trinitronneeds almost 1kVAR of horizontal deflection power at 106kHz. That's a lot of magnetic field, and a lot of rate-of-change. If it were delivered by a class AB amplifier, it would burn all that as heat. Instead, the 1600V 30A output transistor, and related components, does it in probably under 100W.To handle different sweep rates, there's a buck converter, and different capacitors (S-correction) and inductors are switched in and out of circuit to tweak sweep linearity. Actually, the buck is not so much setting deflection supply voltage; I think it's both simpler and trickier than that sounds, as it's not a filtered DC output, and it's part of pincushion control.Regarding dynamic stuff: there's an entire class B amplifier (discrete BJTs) running from 100 or 200V, and delivering AC (cap coupled via RCs inside the FBT) to the focus and convergence electrodes. It's the second largest heatsink in the whole monitor.Did I mention HV is independent (they went with a MOSFET here, it's practically any old SMPS)? All those things (RC coupling networks) are integrated into the FBT.At least it's not the kind with the charged aperture grille, or whatever it was -- I forget why they needed that in some (earlier?) designs but I've seen that before, double FBTs chained to make something like 30 and 33kV.Tim

author: james_s
author_rank: Super Contributor
timestamp: September 18, 2019, 07:06:49 pm
content: Quote from: Yansi on September 18, 2019, 07:51:24 amWell I have never seen a TFT panel with an analog interface, but if you can find me a datasheet for one I'll be happy to eat my words.

author: james_s
author_rank: Super Contributor
timestamp: September 18, 2019, 07:12:48 pm
content: Quote from: T3sl4co1l on September 18, 2019, 08:27:00 amIIRC that was the predecessor to the Trinitron, it was a single-gun tube and used the charged aperture grill to steer the beam to the correct phosphor stripes. Clever design, but it was never a commercial success IIRC.BTW, I think the book I was thinking of earlier is called "The Sony Vision" and details the history of the Trinitron.

author: Gyro
author_rank: Super Contributor
timestamp: September 18, 2019, 07:24:52 pm
content: Quote from: james_s on September 18, 2019, 07:06:49 pmSorry to say that the Sharp one was LVDS interface (I have no isea about earlier ones).

author: Berni
author_rank: Super Contributor
timestamp: September 18, 2019, 07:46:52 pm
content: Quote from: james_s on September 18, 2019, 07:06:49 pmWell yes the pixels themselves are analog, but indeed you never see analog input directly into there. This is probably because digital is more convenient for the LCD controller. Unlike a CRT display that draws one pixel at a time, LCDs draw a whole line at a time so you can't just stream analog pixel data directly to the display panel via some giant MUX. In order to do that you would need some sort of bucket brigade CCD structure to convert the serial stream of analog pixels into a line of pixels and then at the right time trigger a sample and hold to grab the whole line and push it onto the screen matrix. It has taken a long time for proper full color LCDs to enter the mass market, so by then it was likely easier to just make a bunch of tiny slow DACs driving each line and get the serial pixels converted into a line using digital logic instead.I wouldn't be surprised if you can find a really early LCD panel from the old days that does use a analog bucket brigade to drive the matrix columns, but in general this is never done this way. If you want a fully digital display that does not involve analog you can take a look at DLP projector chips. The mirrors in those DLP chips only have two valid positions so PWM is used to get it to display grayscale.Quote from: cheater on September 17, 2019, 02:47:48 pmOh and there is nothing uprising about a CRT showing 1080p video. For computer monitor CRTs resolutions like 1600x1200 used to be pretty common and that's the same 2 million pixels as 1080p and the sort of video signal could be sent over VGA just fine (Tho you did need good quality cables to not have any visible image degradation). However some crappy computer monitors that could scan fast enough for 1600x1200 had shadow masks too course to make such a high resolution look as nice as you would expect it to look. If you had a fancy computer CRT you could get resolutions like 2560x1600 (This is 10% more pixels than 1440p) This is all in progressive scan at >= 60Hz.

author: james_s
author_rank: Super Contributor
timestamp: September 18, 2019, 09:23:24 pm
content: Quote from: Berni on September 18, 2019, 07:46:52 pmI would be really shocked to see one. The oldest graphical LCDs I have are from the late 80s-early 90s and they use driver ICs that are essentially shift registers. You clock the data out one line at a time, and the display is purely monochrome in the early ones, any sort of shading is accomplished by dithering.

author: NiHaoMike
author_rank: Super Contributor
timestamp: September 19, 2019, 04:57:03 am
content: Quote from: Audioguru again on September 17, 2019, 04:25:33 pmYes, if you're willing to pay for it.https://www.dell.com/en-us/shop/dell-ultrasharp-32-8k-monitor-up3218k/apd/210-alez/monitor-accessories

author: Berni
author_rank: Super Contributor
timestamp: September 19, 2019, 05:45:23 am
content: Quote from: james_s on September 18, 2019, 09:23:24 pmI don't mean that far back. I meant the early full color RGB LCD panels used to display PAL/NTSC video. In that case an analog bucket brigade could make sense to use since it was likely early days for digital video, but on the other hand CCD technology was widely used to make image sensors on silicon.The color displays pretty much all use TFT technology in the panel and that requires analog voltages to be put into the little memory cells in each pixel. So because of that the column driver for the matrix has to generate a analog voltage for each column line while the rows are just digital signals that tell the TFT cells when to latch in the new value. All sorts of weird positive and negative bias voltages are needed to make the whole analog dance work, but modern LCD controllers generate them all themselves using charge pumps.

author: james_s
author_rank: Super Contributor
timestamp: September 19, 2019, 06:11:11 am
content: Quote from: Berni on September 19, 2019, 05:45:23 amI have some early TFT color displays too, they all have digital RGB interfaces, usually 6-10 bits per pixel and then H and V sync and a pixel clock. The slightly newer ones use LVDS which is just serialized digital RGB. I'm not saying it's impossible that someone made a TFT with an analog interface but I've never seen or heard of one and don't think they exist. If someone can find me a datasheet I'll change my mind though.

author: Yansi
author_rank: Super Contributor
timestamp: September 19, 2019, 09:12:18 am
content: James_s, I really do not like wasting time on people who just set their mind and are willingless to learn stuff up. Jeez, go to google and learn how a TFT works. It is NOT a digital device.Column drivers has to output analog signals! There is absolutely no point for a (good old fashioned) video monitor to digitize the video signal and then right after puke it back via a DAC, degrading the quality twice in a row.For example this one from my collection of TFT panels, not that even old:https://www.beyondinfinite.com/lcd/Library/LG-Philips/LB070W02-TMJ2.pdfNow please eat your words and start learning harder

author: ebastler
author_rank: Super Contributor
timestamp: September 19, 2019, 10:32:59 am
content: Quote from: cheater on September 18, 2019, 01:19:11 amOh, come on. If you consider Friedrich Reinitzer's 1888 discovery "the origin of LCD technology", you might as well consider bottled vegetables "the origin of vacuum tube technology".Edit: Besides my slightly facetious comment above, it's also worth noting that cathode ray tubes are actually older than vacuum tubes used as "valves": The Crookes tube, using partial vacuum, was demonstrated in the 1870s, before the discovery of liquid crystals (https://en.wikipedia.org/wiki/Crookes_tube). Braun's cathode ray tube dates from 1897 -- nine years after liquid crystals, but still before vacuum valves for switching and amplification.

author: Berni
author_rank: Super Contributor
timestamp: September 19, 2019, 11:52:50 am
content: Quote from: james_s on September 19, 2019, 06:11:11 amI will admit that i never actually held a analog input LCD panel in my hands (Or had one but didn't know it had analog input) as they are indeed rare. But just because something is rare does not mean it does not exist, you just need to look for it a bit harder.The Parallel RGB bus is still plenty used today btw. Most low resolution screens (about under 640x480) still use it, or have a integrated LCD controller that talks via a parallel 24bit/16bit/8bit or SPI bus. There are also chips out there to convert Parallel RGB into most other serial forms and back again. The LVDS bus is actually starting to decline in popularity in favor of MIPI and eDP.Quote from: Yansi on September 19, 2019, 09:12:18 amThis is actually surprisingly new, good find.

author: StillTrying
author_rank: Super Contributor
timestamp: September 19, 2019, 12:29:40 pm
content: Quote from: Yansi on September 18, 2019, 07:51:24 amDid they make TFTs or even LCDs with a certain number of horizontal rows of pixels?How would the number of rows of pixels line up with the number of horizontal real time video lines? Displaying PAL's 576 visible lines on a 400 or 600 row LCD would be a mess.Quote from: james_s on September 18, 2019, 07:06:49 pmA very small display TFT panelcouldbe all analogue in the horizontal lines direction, but I don't think there'll be any all analogue SD video TFTs, they'll all beRGB -> ADC -> a stored frame -> DAC updating the individual pixels when it gets around to it.QuoteIf there's a SD video TFT all analogue RGB input to TFT pixels so will I, I'll let you choose the restaurant.

author: NiHaoMike
author_rank: Super Contributor
timestamp: September 19, 2019, 11:43:13 pm
content: Quote from: StillTrying on September 19, 2019, 12:29:40 pmYou can probably find one in one of the early portable color LCD TVs. Keyword is early, the somewhat newer ones use a common digital LCD and a video processor ASIC.

author: Berni
author_rank: Super Contributor
timestamp: September 20, 2019, 05:26:55 am
content: Quote from: StillTrying on September 19, 2019, 12:29:40 pmThis is what the LCD in the link is. Its s SD video displayQuote from: Yansi on September 19, 2019, 09:12:18 amBut its only 480x234! How in the world do you fit 576 scan lines onto such a display? Do we have an analog video resampling chip? Well no that would be silly of course.Problem is that LCDs don't like refreshing at 25 Hz because the pixel update process is a bit glitchy and the cells fade quickly so if you update a TFT panel too slowly you get strobing. One trick to get around it is to use this old interlaced crap to there advantage and simply draw both the even and odd fields to the same pixels to get 50Hz update rate. So we only really display 288 fields. But wait 288 is still too much to fit on a LCD with only 234 lines! Well... again this is a video signal designed for a CRT and the thing with old CRTs is that they are not quite square, but have a more roundish shape to them. So in order for TVs to fill all of the screen they scan slightly past the edge on all sides, this is called overscan. Only professional video production monitors are designed to show all of the overscan area. Since a LCD is perfectly square it can simply cut away the overscan area and just show the part that is supposed to show to the consumer, hence why it has less lines than PAL has fields.If you have a better solution on how to show SD video on a LCD then i would love to hear it.

author: StillTrying
author_rank: Super Contributor
timestamp: September 20, 2019, 11:59:28 am
content: It's a struggle to get TV video to fit that 480 X 234 pixel 16:9 LCD/TFT version.https://www.maximintegrated.com/en/design/technical-documents/tutorials/7/734.html

author: james_s
author_rank: Super Contributor
timestamp: September 20, 2019, 09:07:39 pm
content: Quote from: Yansi on September 19, 2019, 09:12:18 amNo need to be a dick about it, I know how a TFT works, however this is the first I've ever encountered that does indeed appear to have an analog RGB input, that's pretty fascinating.It would be interesting to see it side by side with a modern digital input one using one of the common LCD controller ICs. Modern panels tend to look much better, but I don't know if it would offset the double conversion or not.