What color would an RGB LED be if all its inputs were noise - Page 1
Forum section: electronics / beginners

author: MuhScopeBroke
author_rank: Contributor
timestamp: October 01, 2023, 03:03:14 am
content: Just a thought

author: vk6zgo
author_rank: Super Contributor
timestamp: October 01, 2023, 03:11:59 am
content: Quote from: MuhScopeBroke on October 01, 2023, 03:03:14 amIt would need to be a very high level of noise!

author: Zoli
author_rank: Frequent Contributor
timestamp: October 01, 2023, 04:24:03 am
content: Quote from: MuhScopeBroke on October 01, 2023, 03:03:14 amVoltage or current?

author: EEEnthusiast
author_rank: Frequent Contributor
timestamp: October 01, 2023, 04:55:01 am
content: It will produce white noise....

author: magic
author_rank: Super Contributor
timestamp: October 01, 2023, 07:36:16 am
content: At typical levels of noise seen in most electronics, it would be black

author: Microdoser
author_rank: Frequent Contributor
timestamp: October 01, 2023, 08:07:17 am
content: Quote from: EEEnthusiast on October 01, 2023, 04:55:01 amQuote from: magic on October 01, 2023, 07:36:16 amYeah, one of these depending on whether the voltage is over the threshold level

author: CaptDon
author_rank: Super Contributor
timestamp: October 01, 2023, 02:08:28 pm
content: I understand your question and we would hope the color would be whiteish, however due to the different voltage thresholds of each color of LED it would probably be very red heavy. It would also depend on the efficiency of each of the three LED's. With the assumption that all LED's were equally efficient and all had the same identical V/I curve then white would probably be the answer. The correction factor used with LED screens to produce white from a 'white' signal is called Gamma Correction if I recall correctly. Back in the N.T.S.C. (Never The Same Color) and C.R.T. days if you study the circuitry the gains of the individual color stages were not equal. This also had a bearing on how to get a correct black and white picture on a monochrome television during a color broadcast or a color television showing a black and white movie. The correction was so complex that a color television detecting a lack of color information would engage a circuit called 'Color Killer' which turned off the color demodulator stage and corrected the Gamma to reduce the slightly greenish hue that would exist without the color killer circuitry. Good question, deep thoughts. Cheers mates.

author: MuhScopeBroke
author_rank: Contributor
timestamp: October 01, 2023, 02:19:46 pm
content: Amazing response, thanks

author: wasedadoc
author_rank: Super Contributor
timestamp: October 01, 2023, 05:40:12 pm
content: Quote from: CaptDon on October 01, 2023, 02:08:28 pmSorry to tell you that your recollection is incorrect and the part of your post that I have quoted is a load of tosh.Gamma correction arose from the fact that with CRTs the light output is not linearly proprtional to the grid to cathode voltage. Gamma correction was only employed in TV cameras, even monochrome ones, to add an approximation to the inverse characteristic so that the end to end result of light hitting a spot on the camera tube to the phosphor on the TV CRT emitting light was closer to linear. With colour TV each of the R, G and B signals in the camera needed correction.This inbuilt non-linearity in video signals was and still is enshrined in the standards which means that a display device not using a CRT and not having a voltage to light curve similar to that of a CRT should incorporate non-linear circuitry to make it perform more like a CRT.The colour killer in TV receivers had nothing to do with gamma or green hue. The absence of colour burst could be used for two purposes:1. Disabling any colour subcarrier notch filter in the luminance channel.2. Disabling the colour demodulator prevented any noise in the chrominance frequency range appearing as coloured noise on monochrome transmissions.

author: CaptDon
author_rank: Super Contributor
timestamp: October 01, 2023, 06:35:46 pm
content: O.K., I guess I agree with your statements. The broadcast chain I worked for owned and operated a 1MW E.R.P. Ch43 UHF station and part of my rant is what was explained to me by the chief engineer. Also, even on a properly adjusted C.R.T. television if the movie or program being shown was black and white and the 3.58 color burst was present the color killer could still be adjusted to kill the demod process. If the color killer didn't squelch the demod the picture would have a slight greenish hue. If the color killer was active the picture was a more pure white. There were times on some t.v. sets where during a black and white program I could see the action of the color killer gating on and off by a shift in picture tint. Was the color killer completely and only triggered by the burst signal? If so then the burst signal was gating on and off at the transmitter site? Or was the color killer on the T.V. set not adjusted properly? Is the correction called Gamma? There was the R-Y and the B-Y and green was derived. Amazing that the color signal of N.T.S.C. had a narrow bandwidth and looked like shit when you took the luminance away. Amazing how easy it is to fool the eye.

author: wasedadoc
author_rank: Super Contributor
timestamp: October 01, 2023, 08:46:26 pm
content: The chief engineer at the transmitter site or the chief engineer of the entire TV company? Sadly it is not unkown for the former to understand little about the baseband signal that is coming into the transmitter site.Gamma is the power in the approximate formula relating CRT brightness and grid to cathode voltage.brightness proportional to voltage raised to the power gamma.Gamma is a number typically between 2.2 and 2.8.Pro and semi-pro cameras with 3 sensors have always used prism assemblies to separate the scene into red, green and blue components to be scanned by the tubes or solid state sensors. The gamma correction is applied to each of the RGB signals and then the luminance and two colour differences are derived.Despite many, many hours between 1971 and 1976 observing BBC1, BBC2 and ITV on 3 side by side Prowest professional monitors I never saw the on and off green tint on monochrome transmissions that you mention. PAL decoders could use the absence of burst swing to activate the colour killer. I don't recall there being any adjustment of the colour killer threshold on any of the professional or domestic decoders I encountered.Even if the colour killer was only acting intermittenly I see no reason why that would intentionally cause a green tint to come and go.

author: vk6zgo
author_rank: Super Contributor
timestamp: October 02, 2023, 12:11:52 am
content: In Australia, we all learnt about gamma correction, so the "Engineer" in charge of a a TV Transmitter Site (much more usually a Supervising Tech) would know about where it is used.As to the "colour killer", we were regaled with stories of how the burst would be turned off to present BW material, but I found that was not done in practice, with the burst left on at all times.As Oz came a bit later into colour TV, (1975) perhaps the circuitry was a bit more sophisticated, although it didn't seem overly so.In the lead up to the introduction of colour, I was involved in modifications to our 1959 vintage Marconi TV Tx to make them more suitable for the new signals.Initially, we had no colour test equipment, but we "made do" by injecting colour subcarrier into a sawtooth video generator, in lieu of the 1MHz usually used for "line time luminance non-linearity testing" so we could do "Differential Gain" as required by the new specs,To this end, we had a Wayne Kerr generator to produce the 4.433MHz, which was, for convenience mounted in an empty spot in the rack, just above the VDAs.It worked quite well for a rough lashup, & for stability, was left running.The ABC were diligent in the extreme about removing the burst until the official day, so were surprised to receive comments from early adopters that they were seeing swirls of colour on their BW pictures & had a "grizzle" to us.It turns out that there was a very small amount of leakage from the Wayne Kerr into the VDAs directly underneath it, which were, unfortunately, in circuit during normal transmission.It turns out that the "colour swirl" problem was peculiar to HMV TVs, with other sets, including the Philips K9 we had at the site, totally clear of this problem.The generator was removed from the rack & placed upon a mobile table instead, & the problem went away.Around this time a number of "clever people" managed to "jump the gun" on colour by devising "chroma lock" circuits that would allow the display of colour pix in the absence of burst.Somebody at work had "a bit of a fiddle" with the K9 & managed to bodge up a very simple mod to do this.This worked well, & produced quite good pictures, considering the Tx was not yet completely modified for the new service, but would sometimes lock on "out of phase", presenting such delights as "Pot Black" with a snooker game played by green players on a green table with a brown felt playing surface,

author: Microdoser
author_rank: Frequent Contributor
timestamp: October 02, 2023, 09:08:53 am
content: Quote from: CaptDon on October 01, 2023, 02:08:28 pmIt's not impossible the theoretical LEDs are controlled by a controller circuit that sorts all of that out (meaning any input would trigger the LED and the input is standardised across the 3 LEDs, say from 0V-1V) and if the noise is fed into that circuit, which then controls the LEDs, then the colour would be white, just not as bright as 100% on. Brown noise, on the other hand, would not produce brown. That would require rather more sophisticated circuitry, more akin to a spectrum analyser.

author: Terry Bites
author_rank: Super Contributor
timestamp: October 02, 2023, 09:07:15 pm
content: Dunno. Make a shift register or 'duino PRNG. Get some power mosfets and give it a whirl.My feeling is that it will be a warm white..............

author: themadhippy
author_rank: Super Contributor
timestamp: October 02, 2023, 10:27:43 pm
content: QuoteGood luck getting brown light out of a led,nearly as impossible as getting grey light out of ,well anything

author: sparkydog
author_rank: Regular Contributor
timestamp: October 02, 2023, 10:54:58 pm
content: Getting "brown" out of an LED is relatively easy; you just need a whitereference. (.) If it couldn't be done, you'd never see "brown" on an OLED display, and if you've ever seen an OLED display, you probably know they're quite capable of producing "brown".I'd expect brown or pink noise to also produce... "white". (Assuming a setup in which white noise produces "white".) Maybe¹. As long as theaveragevalue of the signal being fed to each channel is the same, the spectral distribution is irrelevant. In fact, as long as the average signal is the same, it shouldn't matter if you fed white noise to one channel, pink noise to another, and brown noise to the third.¹ Of course, what you'llactuallyget is all sorts of colors. If they vary quickly enough for persistence of vision to smooth them out, that will look "white" (or possibly gray, depending on what references are present). If it varies slowly, it... will look like random colors.