
Why do color TVs make black and white snow? - bluedino
https://engineering.stackexchange.com/questions/33737/why-do-color-tvs-make-black-and-white-snow
======
klodolph
The 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.

~~~
segfaultbuserr
> _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._

Even better, if you want a color terminal, feed the original RGB signal (or
its approximation) directly to your TV, don't use RF modulators or a composite
input, so there's minimum degradation of video quality. It should be easy on a
relatively modern TV. Follow this flowchart.

1\. Is it an European TV with a SCART connector? If yes, it already has RGB
input. [0]

2\. Does the TV include a YPbPr component input? If yes, read Linear
Technology Application Note #57 [1], which shows you a circuit to convert RGB
to YPbPr using LT6550 chip (or find a similar commercial converter box).

3\. Does the TV have an S-video input? If yes, read Analog AD725 datasheet [2]
and use this chip to convert RGB to Chroma-Luma component signal for S-Video
(or find a similar commercial converter box).

4\. Does the TV have onscreen display (i.e. menus overlaid on top of the
image)? If yes, there already exists an RGB signal in the TV for feeding the
OSD. You can hack your TV and expose the RGB signal input for your own use
[3]. Caution: high voltage, discharge the tube, and never work on it with
power on.

5\. If everything else fails, don't forget the fact a TV's electron gun is
ultimately driven by an RGB signal. You can design a simple driving amplifier
to feed the RGB signal directly to the electron guns. Unfortunately, I cannot
find a reference design for the moment. But it's doable.

[0] [https://en.wikipedia.org/wiki/SCART](https://en.wikipedia.org/wiki/SCART)

[1] [https://www.analog.com/media/en/technical-
documentation/appl...](https://www.analog.com/media/en/technical-
documentation/application-notes/an57fa.pdf)

[2] [https://www.analog.com/media/en/technical-
documentation/data...](https://www.analog.com/media/en/technical-
documentation/data-sheets/AD725.pdf)

[3] [https://hackaday.com/2014/09/21/component-video-input-
hack-i...](https://hackaday.com/2014/09/21/component-video-input-hack-is-a-
master-hack-for-crt-televisions/)

~~~
klodolph
My advice was aimed at people living in 1980 who had a spare TV they could use
for the computer. I suppose your advice is good for people living in 1990 who
don’t have a computer monitor yet.

~~~
DonHopkins
Once the Hobart Phase kicks in and has a chance to to work its magic for a few
decades, your sagely advice will be relevant once again.

[https://en.wikipedia.org/wiki/Counter-
Clock_World#The_Hobart...](https://en.wikipedia.org/wiki/Counter-
Clock_World#The_Hobart_Phase)

~~~
r00fus
Wow so that episode of Red Dwarf was actually based on this? My respect for
PKD rises.

------
phkahler
Forward 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.

~~~
hazeii
Though 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.

~~~
YeGoblynQueenne
Do 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
For 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)

~~~
YeGoblynQueenne
Thanks. It could well be Fechner Colour.

------
russellbeattie
What 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
Chances 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-...](https://scifi.stackexchange.com/questions/163304/what-color-was-
the-sky-above-the-port-in-neuromancer)

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
I must admit I ctrl-F'ed this thread to see if _my_ question from
scifi.stackexchange was here. Thanks for making my day :)

------
hazeii
I 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.

------
basementcat
Just 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)

------
ohiovr
First 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.

------
beezle
It wasn't entirely black and white snow. Also was a bit of pink and green too.
Been awhile since I saw any though

~~~
dylan604
I'm guessing it's been since 2009.
[https://en.wikipedia.org/wiki/Digital_television_in_the_Unit...](https://en.wikipedia.org/wiki/Digital_television_in_the_United_States#Transition_from_analog_to_digital_terrestrial_broadcasts_in_2009)

------
hyperpallium
Wouldn't random colours average to greys to the HVS, pretty quickly, anyway?

------
_sbrk
No color burst on the front porch, so the color killer circuit disables color.

------
kmbfjr
They originally did it all with vacuum tubes no less.

------
justlexi93
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 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).

