/ lab / field notes on scan lines
The raster scan, interlaced vs progressive, and the letterbox math that decides which lines actually carry picture.
A frame is not a single image. It is a sequence of horizontal lines, drawn one after another, top to bottom — and the engineering decisions around which lines actually carry picture are older than HDTV and still alive in every codec you ship.
When you look at a screen right now, what is actually emitting light? A single racing dot? A glowing line moving down? Two million pixels all lit at once? The honest answer is: it depends on which screen — and once you know, half of broadcast engineering makes sense. Slow time way down and watch the two mechanisms diverge.
The CRT fires one electron beam (three for color — R, G, B guns aimed at a shadow mask). At 60 Hz the beam visits every one of 2 073 600 pixels in 16.67 ms. So at any single instant, ONE pixel out of two million is glowing. You see an image because (a) phosphor takes ~10 ms to fade and (b) your visual system integrates ~50 ms windows. For fifty years what you saw on TV was your brain stitching together a strobing dot.
1 / (1920 × 1080 × 60 Hz) ≈ 8 ns per pixelAn OLED panel has no beam. Every pixel is its own emitter, present and lit simultaneously. The HDMI cable still ships data line-by-line — that is the part that survives from broadcast — but the panel updates each row when its data arrives. So you see a stable image with a tiny rolling band (typically magenta in these diagrams) crossing top-to-bottom every 16.67 ms. Slow it down and the wave becomes visible.
1920 × 1080 = 2 073 600 emitters, always onwhen you set R = 255, G = 0, B = 0, only the red sub-light emits. when you set all three to 255, all three emit and your eye reads white. the panel does not have a "white light" — only red, green, and blue, blended by your visual system.
1× is real time at 60 Hz · drop below 0.05× to actually see a single line
At 60 Hz, the beam draws all 1080 lines in 16.67 ms. That is 64 800 lines per second — far faster than your eye can resolve, which is exactly why you do not see horizontal stripes when you look at a TV. Slow it down enough and the abstraction leaks.
1080 lines × 60 Hz = 64 800 lines/secBetween two lines the beam jumps back to the left edge (horizontal retrace). Between two frames it jumps back to the top (vertical retrace). The lines drawn during those jumps carry no picture — they are the blanking interval, and the broadcast industry has been hiding metadata in there for fifty years.
h-retrace ≈ 6 µs · v-retrace ≈ 1.3 msProgressive draws every line in order: 1, 2, 3, …, 1080. Interlaced draws odd lines first (1, 3, 5, …) as Field 1, then even lines (2, 4, 6, …) as Field 2 — two interleaved fields per frame. Same bandwidth, twice the temporal sampling, one ugly artifact when something moves.
60 fields/sec × 540 lines/field = 30 full frames/sec — the same bandwidth as 1080p30, but motion sampled twice as often.
The cost: anything that moves between F1 and F2 lands on different vertical positions. Stack the fields and you get combing — those zigzag horizontal teeth on a moving edge.
each row samples the underlying motion at its own rate. low fps means fewer samples per second — and the gap between them is what your eye reads as judder. the wagon-wheel rate (where the wheel reverses) is where temporal aliasing wins.
A panel today is progressive. To display 1080i it has to invent the missing lines of each field — copy from the neighbour, blend across time, or motion-adaptive guess. Cheap deinterlacers blur; good ones cost CPU and still smear on diagonals.
deinterlace = bob | weave | motion-adaptive | yadifField 1 carries odd-numbered lines (1, 3, 5, …, 539). Field 2 carries even lines (2, 4, 6, …, 540). For 1080i: 540 lines per field, 60 fields per second, captured 1/120 s apart.
F1 ∪ F2 = 1080 lines · 1/30 s apartA 1920×1080 frame is a container with a fixed aspect ratio of 1.78:1. Movies are not. To carry a 2.39:1 picture inside a 1.78:1 frame you center the picture vertically and pad the top and bottom with black. The active lines — those that carry picture — start at "first line" and end at "last line." Everything outside is blanking.
wider than the frame — letterbox
frame is 1920 wide · 1080 tall · 1.78:1
picture height = 1920 ÷ aspect — wider aspect means a shorter picture inside the 1080-tall frame · 1920 ÷ 2.39 = 803 lines
bar height = (1080 − picture height) ÷ 2 · first line = 1 + bar · last line = 1080 − bar · first = 139 · last = 942
SMPTE RP 219 and friends specify the active picture area for each aspect ratio inside the 1080-line container. A 1.85:1 master legitimately occupies lines 21–1059, with lines 1–20 and 1060–1080 carrying nothing but black. The metadata that says "this is 1.85" is what your player reads to crop or pillarbox correctly.
"HD" is a folk term. The shipping side of video has at least eight technical answers, and which one you choose depends on whether your viewer is in a theatre, on a couch, on a phone, or on a 60 Hz arcade cabinet.
| case | format | why |
|---|---|---|
| Theatrical master · 35 mm flat | 1080p24 or 2160p24 | Projection rate is 24 fps; anything else introduces frame-rate conversion and motion judder. Stay progressive. |
| Live sports for cable | 720p59.94 or 1080i59.94 | High field/frame rate matters more than spatial resolution. ABC/ESPN ship 720p60; NBC/CBS ship 1080i. Both are 60 Hz. |
| Streaming series · drama | 1080p23.976 / 2160p23.976 | Mirrors the cinema rate. Netflix and Apple TV+ default to 23.976 for cinematic series, with HDR metadata layered on top. |
| YouTube talking head | 1080p29.97 or 30 | Compression-friendly, friendly to upload bandwidth. 60 fps reserved for screen-recordings and gameplay. |
| Archive transfer · 90s VHS | 480i59.94 or 576i50 | Match the original line standard. Upscaling to 1080 introduces flicker artifacts unless deinterlaced first. |
| Carry a 2.39:1 picture | Any 1080 format · letterboxed | Active picture occupies lines 140–940. Lines 1–139 and 941–1080 are blanking. Player metadata announces the aspect. |
Every term on this page has a precise meaning. Here are six of them — what they actually are, where they show up in the demos above, and what the math looks like.
one horizontal row of pixels
A scan line is the smallest unit of picture in the temporal model. At 1920×1080 a single line is 1920 pixels wide and one pixel tall. The line is the unit of horizontal sync, of line-rate processing, and of every per-line trick that gets played in colour subcarriers, closed captions, and HDR metadata. Lines are numbered top-down, conventionally starting at 1.
Appears in: the beam animation, the F1/F2 labels, the "first line / last line" readouts.
half a frame, drawn alone
In interlaced video, a field is one pass through the odd-numbered lines or the even-numbered lines. F1 carries lines 1, 3, 5, … and F2 carries 2, 4, 6, …. Two fields make one frame. Field 1 is drawn first in time, then field 2, so anything moving between them lands at a slightly different vertical position — which the eye glues together into motion, until it does not, at which point the artifact is called combing.
Appears in: section 02 — the interlaced demo and the field-rate math.
the time the beam is not drawing
The horizontal blanking interval is the time the beam takes to fly back from the right edge to the left to start the next line. The vertical blanking interval is the longer pause at the end of a field before the next field begins. Neither carries picture — but both carry metadata: closed captions (line 21 NTSC), VITC timecode, teletext, and a long history of clever extras.
Appears in: the retrace overlay in the raster demo, the "lines 1–20" letterbox bar.
width ÷ height
The picture aspect ratio is the width of the picture divided by its height. 16:9 = 1.78. Theatrical "flat" = 1.85. Cinemascope = 2.39. The display aspect ratio is what the panel can render — usually 1.78 today. When picture and display do not match, you pad the difference: black bars on top/bottom (letterbox) or on the sides (pillarbox).
Appears in: section 03, every aspect ratio in the sweep slider.
black bars top and bottom
Letterboxing is what happens when a wide picture is delivered inside a narrower container. The picture is scaled to fill the width and centred vertically; the remaining height is filled with black. The black is not "wasted" — it has a precise line range, the player knows about it, and you can store metadata on those bars if you want. Pillarboxing is the same trick rotated 90°.
Appears in: section 03, the bar-height math, every "bars" readout.
24, 25, 30, 50, 60 — and the .94
Frame rate is full pictures per second; field rate is half-pictures per second. 1080p60 means 60 frames; 1080i60 means 60 fields = 30 frames. The decimal you see (23.976, 29.97, 59.94) is 1000/1001 of the integer rate — a 0.1% slowdown imposed when NTSC colour was bolted onto a 60 Hz monochrome system in 1953, and never undone.
Appears in: section 04, every format card with a fps suffix.
the reference image, since 1976
The seven 75%-saturated color bars are not a test pattern in the casual sense — they are an engineering reference. Every bar has a known Y, Cb, and Cr value; a calibrated monitor renders the white bar at 75 IRE, the black bar at 7.5 IRE, and the six colour bars at precise saturation. If the wrong bar looks wrong, your signal path is wrong, full stop.
Click any bar to read its name and chroma offsets. Same image you have seen on every late-night sign-off screen for half a century.