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If you let a color TV display something from an unplugged display port, it displays snow, right? Randomly varying black or white pixels on the whole screen.

My guess was that the television was decoding the noise from the unplugged wires as a video signal.

Assuming this, it makes sense that B&W TVs make B&W snow (they would decode the noise as being a B&W video signal). But then color TVs should make colored snow, right? Shouldn't they decode the noise as being a color video signal?

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    $\begingroup$ No ... I bet 9 out of 10 colour TV's - especially all flat-screen TV of various technologies will not display snow .. instead display commonly a moderate blue to indicate "signal missing" $\endgroup$ – eagle275 Mar 23 at 11:02
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    $\begingroup$ I'm curious; is this true for non-NTSC TV's (since a major aspect of NTSC was that it was backwards compatible with black and white TV, which I didn't think PAL had to deal with) $\endgroup$ – Foon Mar 23 at 13:10
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    $\begingroup$ @Foon, yes they did but in a slightly different way. $\endgroup$ – Prof. Falken Mar 23 at 19:24
  • $\begingroup$ @eagle275 Modern ones certainly do, but I recall at least ~three~ four earlier ones (each non-'flat-screen', which may or may not be relevant -- ~-time edit: actually all of the CRTs I remember were this way, and none of the flat-screens) that displayed 'snow'. $\endgroup$ – OJFord Mar 23 at 22:20
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    $\begingroup$ the television was decoding the noise from the unplugged wires as a video signal you are actually decoding the CBR + other noises of course $\endgroup$ – Ordiel Mar 24 at 2:51
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If the receiver does not detect the sub-carrier for the "colour burst" signal which is transmitted during the horizontal blanking period the receiver switches on the "colour-killer" circuit so the set reverts to black and white mode. The colour-burst signal - 8 to 10 cycles of 3.85 MHz - is unlikely to be generated by random noise.

enter image description here

Figure 1. The colorburst signal is transmitted on the "back porch" between the horizontal blanking pulse and the start of that line's luminance signal. The colorburst signal is used to synchronise the QAM (quadrature amplitude modulation) oscillator which can hold its frequency accurately enough for the remainder of the cycle and decode the I and Q information. Source: Maxim tutorials.

There's plenty of information on the web.

Links:

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    $\begingroup$ Thank you very much ! I started to freak out when I imagined what colored snow could look like ^^ $\endgroup$ – Axel B Mar 21 at 23:40
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    $\begingroup$ Graham Cox's answer on this Quora thread says colored static is possible on PAL TV's, but he did not know the cause and no one else expanded on it: quora.com/… $\endgroup$ – Ryan_L Mar 22 at 0:09
  • $\begingroup$ I'm not sure this is entirely correct because I've seen NTSC color sets sometimes display colored snow; it may depend on the way the color subcarrier detection works. $\endgroup$ – Anthony X Mar 22 at 13:51
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    $\begingroup$ @AxelB: Basically it'd look pretty much like this (image from this answer on SO). $\endgroup$ – Ilmari Karonen Mar 22 at 16:10
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    $\begingroup$ Technology Connections has a playlist about the history of analogue TV, including colour transmission. This video is the start of the ones about colour, but the whole playlist and channel have some really good history-of-tech videos. $\endgroup$ – Richard Ward Mar 22 at 20:41
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The 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 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).

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The question was well answered by "Transistor" on Mar 21 at 14:31. The only part missing was that the other colour tv systems of the past, known as PAL (Phase Alternate Line) and SECAM (Séquentiel Couleur à Mémoire) worked in a very similar fashion, both had a colorburst signal at 4.43 MHz, superimposed on a blanking portion of a horizontal synchro pulse. PAL with phase alternating between 135° and 225° from line to line. SECAM did not care about the phase since it used FM modulation for colour information. The important part is that all the analog tv receivers had a circuit designed to detect the presence of color signal, with noise immunity at the top of the requirements list. So when the signal is very week or noisy (or missing) the analog color tv set reverts back into a black and white mode.

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I disagree. Every color TV I got (and I repaired a lot of them in the '70's and '80's) displayed colored snow when tuned to blank air. In fact this was a much touted "feature" mentioned in consumer magazines when there was nothing interesting to watch.

The trouble with newer sets is instead of "working" and displaying what they receive, they are "too smart" and "refuse to work". This makes them useless for tasks for which they are not specifically designed, such as displaying stereo video (with shutter glasses). I must resort to heavy, cumbersome "boob tubes" for this.

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    $\begingroup$ That's interesting. (1) PAL or NTSC? (2) What does "tuned to blank air" mean? If they were tuned then what were they tuned to? (Word it for an international audience.) (3) Were you operating with other TVs on your bench that were displaying TV pictures? Note that the second paragraph is tending towards a rant. There's an edit link below your post if you wish to clarify it. $\endgroup$ – Transistor Mar 25 at 22:40

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