How a television draws a picture

Analog video from the wire up: one voltage carrying brightness, color, and timing, and the electron beam that develops it into light. The foundation under everything else on this shelf.

Composite video is one voltage, changing over time, on one wire. About a volt from bottom to top, and inside that volt: the brightness of the picture, the color of the picture, and the timing that says where every part of it belongs. Nothing else. No packets, no checksums, no addresses in any digital sense. A frame is never sent as a picture — it is sent as a reading, taken left to right, line by line, top to bottom, the way you would read a page aloud to someone who is transcribing it.

The raster

An NTSC frame is 525 lines. They arrive interlaced: the odd-numbered lines are drawn as one pass, then the even-numbered lines as a second pass filling the gaps — about 60 of these fields per second, about 30 complete frames. Each line takes roughly 63.5 microseconds. Within a line, brightness is simply the instantaneous voltage: higher voltage, brighter point. A bright vertical stripe in the picture is a brief bump in voltage that recurs at the same moment in every line.

one scanline, ~63.5 us
      _______________________________________________
     |                                               |
     |        ~~ active video (the picture) ~~       |
_____|_                                              |____
  |    \  _/\_                                            |
  |     \/    \  <- color burst (3.58 MHz reference)      |
  |sync |                                                 |sync
  |tip  |back porch                                       |tip

Between every line the voltage drops to a level blacker than black — the sync pulse — and just after it rides a short burst of reference tone. That is the addressing system, and it has a page of its own.

Color is a trick

Color television had to work on black-and-white sets already in living rooms, so color was added on top rather than alongside: a 3.58 MHz subcarrier rides on the brightness signal, its amplitude carrying saturation and its phase carrying hue. Phase is timing — which is why a timing error of a few nanoseconds, invisible as geometry, shows up as a shifted color. The sync explainer covers what that fragility costs and how to work around it.

The tube develops the signal

A CRT is not a screen in the modern sense. It is a glass vacuum vessel with an electron gun at the back and a phosphor coating on the inside of the face. The gun fires a beam at the phosphor; magnetic coils around the tube’s neck steer that beam left to right and top to bottom in step with the sync pulses; the incoming voltage drives the beam’s intensity moment to moment. Where the beam lands, the phosphor glows — and then begins to fade.

So at any instant, most of what you think you are watching does not exist. There is one moving point of light, a short comet-tail of still-glowing phosphor behind it, and darkness everywhere else. Persistence of vision — your eye’s own refusal to let go of what it just saw — assembles those into a stable picture. The tube never holds a frame. There is no framebuffer, no stored image, nowhere to point and say “the picture is kept here.” The picture is a performance, redrawn sixty times a second, completed only in the viewer.

This is why I describe pointing a camera at a CRT as photographing a process rather than a bitmap. The camera catches the phosphor mid-development: the glow, the decay, the scan structure. Those are not artifacts on top of the picture. On a tube, they are the picture.

Why this matters for bending

Because the whole signal is one continuous voltage with no error correction and no fence between picture, color, and timing, anything done to it lands somewhere visible. Load the line and the picture smears. Nudge the timing and the hues rotate. Starve the sync and the geometry folds. There is no illegal value — a display doesn’t validate a composite signal; it obeys it. That is the difference between analog glitch and digital glitch: corrupt a digital file and you usually get either nothing changed or nothing at all, because everything between the data and the screen is built to reject damage. Corrupt an analog signal and the display faithfully draws the damage, which turns out to have structure worth looking at.

From here: the addressing system that holds a bent picture on screen, what controlled damage looks like when you step it like a datasheet, and the instruments built to do it on purpose.

Numbers above are NTSC; PAL differs (625 lines, 50 fields, a 4.43 MHz subcarrier) but the architecture is the same.