Why is TV snow in black and white?

April 8, 2009 9:57 PM

Why is TV snow in black and white?

As best I can tell, the "snow" you see on your TV is random signals, from a dead channel or a blank VHS tape or whatever. My question is, if your have a color TV, why does the snow always appear in black and white? Why wouldn't you see a colorful display of random noise?

posted by abc123xyzinfinity to technology (6 answers total)

In NTSC the color is determined by the phase of a 3mhz wave mixed into the video signal. That's how color was done in a way that was backwards compatible with older TV signal standards. B&W sets ignored the 3mhz signal, and black and white broadcasts didn't have the signal.

My guess with the lack of color snow is that there isn't any coherent 3mhz signal in there. Just a bunch of random noise, so it looks like a black and white signal to the TV.

Interestingly, most modern TVs just show a blue field when they don't detect a signal. Kind of ruins the beginning of Neuromancer: "The sky was the color of television, tuned to a dead channel". Makes it sound like a bright, sunny, cloudless day. Rather then an oppressive Grey.
posted by delmoi at 10:11 PM on April 8, 2009 [12 favorites]

It's because a color TV signal is basically a black & white signal with some extra color information overlaid on top of it. But the color information has some extra handshakey type stuff so if it's not there the TV knows and won't try and show color.
posted by aubilenon at 10:11 PM on April 8, 2009 [1 favorite]

Here's more from Wikipedia:

For a TV to recover hue information from the I/Q phase, it must have a zero phase reference. It also needs a reference for amplitude to recover the saturation information. So, the NTSC signal includes a short sample of this reference signal, known as the color burst, located on the 'back porch' of each horizontal line (the time between the end of the horizontal synchronization pulse and the end of the blanking pulse.) The color burst consists of a minimum of eight cycles of the unmodulated (fixed phase and amplitude) color subcarrier. The TV receiver has a "local oscillator" which it synchronizes to the color bursts and then uses as a reference for decoding the chrominance. By comparing the reference signal derived from color burst to the chrominance signal's amplitude and phase at a particular point in the raster scan, the device determines what chrominance to display at that point. Combining that with the amplitude of the luminance signal, the receiver calculates exactly what color to make the point, i.e. the point at the instantaneous position of the continuously scanning beam.

So not only would there be no coherent color signal, there would also be no 'reference' color burst at the beginning of the scanline. So the TV would have no idea what color to make the dot.
posted by delmoi at 10:17 PM on April 8, 2009

Also see this cool post about restoring color in black-and-white PAL broadcasts.
posted by odinsdream at 6:50 AM on April 9, 2009

As an addendum, the heat generated by an ATSC digital to analog converter box melts all the snow before it gets to your TV.
posted by BrotherCaine at 5:29 PM on April 9, 2009

Seriously though, does anyone know if SECAM/SECAM-L/MESECAM analog transmissions have faintly colored snow?
posted by BrotherCaine at 5:35 PM on April 9, 2009