Why 720p looked terrible on LCD

If you bought a flat-panel TV in the mid-2000s, there's a good chance your first look at HD on it was a letdown. Sports looked smeary, game text was fuzzy, and a PC desktop was barely readable. The usual explanations are that 720p wasn't very sharp, or that early LCDs were just bad. Neither one holds up.

720p was a perfectly good format. The trouble was that it almost never reached a screen whose pixels matched it, and it often went through two or three different scalers on the way there.

CRTs hid the problem

A CRT draws its picture as continuous scan lines with an electron beam. There's no pixel grid to line the image up against, so a lower-resolution signal just looks a little soft. That softness was forgiving, and it hid a lot of compression artifacts too.

An LCD has an exact grid of pixels, and any image that doesn't match the grid has to be resampled to fit. When the scale factor is a whole number, each source pixel can map to a neat 2x2 or 3x3 block. When it isn't, nearly every screen pixel becomes a blend of its neighbors, calculated by whatever scaling chip the manufacturer picked.

Most "720p" TVs weren't 720p

Most LCD TVs sold as "720p" in the US, or "HD Ready" in Europe, didn't have 1280x720 panels. They had 1366x768 panels.

The 768 came from the computer world. 1024x768 (XGA) was the standard resolution for laptop and monitor panels, and manufacturers kept the same line count when they built widescreen TV panels. At 16:9, 768 lines works out to 1365.33 pixels across, which rounds up to 1366. Europe's HD Ready logo only required 720 lines, so these panels qualified. In the US they were simply sold as 720p sets. The same resolution went on to become one of the most common laptop screens of the 2010s.

Then there's 1360x768. Graphics hardware and VESA's timing formulas work in 8-pixel steps, and 1366 isn't divisible by 8, so PCs, game consoles, and many TVs' own advertised PC modes used 1360x768 instead. Spec sheets and retailers used both numbers, sometimes for the same model. When a 1360-wide signal hits a 1366-wide panel, the TV either shows it pixel for pixel with a 3-pixel black bar on each side, or stretches it by 0.44 percent. That's a tiny stretch, but it still means every pixel on the screen gets resampled.

True 1280x720 panels did exist. You mostly found them in projectors and DLP rear-projection TVs, not in the LCD sets lining store shelves.

What a 7 percent stretch does

Going from 1280x720 to 1366x768 enlarges the image by about 1.067x in each direction. Going to 1360x768 is 1.0625x horizontally and 1.067x vertically, so the two axes don't even scale by the same amount.

A small stretch sounds harmless, but it's a bad deal. Every pixel still gets resampled, so you pay the full cost of scaling, and the picture barely gets bigger in return. Anything that was exactly one pixel wide, like a thin line or the stem of a letter, now straddles two screen pixels and turns into a smear. Because the ratio is uneven, the source and screen pixels drift out of alignment and come back into line roughly every 15 or 16 pixels. Some columns stay fairly sharp and others go soft, in a repeating pattern across the screen. On a still image that reads as general fuzziness. On a camera pan, fine textures shimmer as detail slides in and out of alignment.

TV processing made it worse. Many sets shipped with edge enhancement turned on, adding light and dark outlines along edges to make them look crisper. On an image that scaling had just softened, that mostly added halos.

Overscan was the other problem. CRTs always hid the outer edges of the picture, and HDTVs kept the habit. By default, most sets cropped a few percent off every edge and enlarged what was left to fill the screen. That pushed the scale factor higher and meant that even a TV whose panel matched the signal wasn't showing it pixel for pixel. Turning it off meant finding a setting called something like Just Scan, Dot by Dot, Full Pixel, or Screen Fit, depending on the brand, assuming the TV had one at all.

Broadcast TV took the long way around

US networks split on HD formats. CBS and NBC went with 1080i. ABC, Fox, and ESPN chose 720p, largely because 60 full progressive frames per second handled fast motion well.

A lot of 1080i content got converted to 720p before the TV ever saw it. Cable and satellite boxes were a common culprit. People with "720p" TVs, and the installers who set them up, often locked the box's output to 720p to match, so the box converted every 1080i channel itself. The 720p networks also aired plenty of shows that were produced at 1080 and converted down for broadcast.

Follow one 1080i program through a box set to 720p. First the box deinterlaces it, turning the two interlaced fields into full frames. Quality varied a lot here, and a cheap "bob" deinterlacer that handles each field on its own leaves you with roughly 540 lines of real vertical detail. Then the box shrinks 1080 lines to 720. Finally, the TV stretches that 720p signal to 768 lines with its own scaler, its own filters, and its own sharpening.

The frustrating part is that the panel had 768 lines, which is more than 720. The round trip threw away vertical detail the screen could have shown, then had the TV invent replacement detail by interpolation, passing through two or three different chips along the way.

Compression piled on top. US broadcast HD used MPEG-2, and stations often squeezed the main channel's bitrate to make room for subchannels. Cable and satellite providers frequently compressed things further. Every scaling and sharpening step enlarged the blockiness and the "mosquito noise" around edges, which a CRT's softness used to blur over.

The short path usually looked better. With the box passing 1080i through untouched, the TV could do one deinterlace and one downscale straight to 768 lines, although the result still depended on how good the TV's deinterlacer was.

Games got scaled twice

The Xbox 360 and PS3 era was full of games that rendered below 720p to hold their frame rates. Halo 3 rendered at 1152x640, and Call of Duty 4 at 1024x600.

Take Halo 3 on a typical 768-line TV with the console set to 720p. The console's hardware scaler stretches 1152x640 up to 1280x720. Since 1152x640 is a 1.8:1 frame rather than 16:9, the two axes get resampled by different amounts, 1.111x horizontally and 1.125x vertically. Then the TV stretches the 720p signal again to 1366x768, with a different scaling algorithm and usually its own sharpening.

Two non-integer passes with two different filters are worse than one. The blur from the second pass stacks on top of the blur from the first, and the TV's sharpening amplifies whatever ringing the console's scaler already introduced. Most games of that era also had little or no anti-aliasing, so jagged edges and shimmer went through both passes as well.

The console's output setting made a real difference. Choosing 1080i or 1080p on a 768-line TV meant scaling up past the panel's resolution and then back down. The Xbox 360, though, could output 1360x768 directly, most famously over its VGA cable. That turned the whole chain into a single pass on the console and a pixel-for-pixel image on the panel. The PS3 only offered standard TV resolutions, so its games always went through the TV's scaler.

The same problem everywhere else

Connecting a PC was where it hurt most. Many of these TVs wouldn't accept 1360x768 or 1366x768 over HDMI, so people ran the desktop at 720p or 1080i and let the TV scale it, which turns small text to mush. TVs also treated HDMI inputs as video, and video processing often reduced color resolution (chroma subsampling), which smeared colored text even more. On a lot of sets, the VGA input was the only clean way in.

1080p panels didn't make the problem go away. They just changed the numbers. 720p on a 1080p screen has to be scaled by 1.5x, which still isn't a whole number, and that's part of why 720p video and 720p console games looked soft on 1080p monitors years later.

Plasma TVs had the same issue. Common plasma panels were 1024x768 at 42 inches, with non-square pixels to fill a 16:9 screen, and 1366x768 at 50 inches. That's a good sign the real story is about fixed-pixel displays in general, not LCD in particular.

Early LCDs did have weaknesses of their own that made everything look worse. An LCD holds each frame on screen for the whole refresh period (this is called sample-and-hold), which blurs anything your eyes are tracking, and slow pixel response added ghosting on top. That undercut the main reason ABC, Fox, and ESPN had picked 720p in the first place. Grayish blacks from CCFL backlights and showroom picture modes with brightness and sharpness maxed out didn't help either.

Proof the pipeline was the problem

When 720p reached a display that matched it, it looked clean. Native 1280x720 projectors and DLP rear-projection sets, with overscan turned off, showed 720p pixel for pixel, and the picture held up well. And the same "terrible" LCD TVs looked sharp when a PC fed them their exact native resolution, like a PC game running at 1360x768 through the VGA input.

How to make a 768-line TV look good today

That last point is also the fix, and it still works. If you have an old 1366x768 or 1360x768 TV, the goal is to send it exactly its native resolution so the TV's scaler never touches the image.

Start by finding the real panel resolution. The spec sheet or service manual is the most reliable source. The TV's EDID data, which a PC can read, shows which resolutions it will accept, and that isn't always the same thing.

Then use a source that can output that resolution. A PC or a Raspberry Pi can, sometimes with a custom resolution setting. Streaming sticks and modern consoles generally can't, since they only offer standard TV resolutions like 720p and 1080p, so they still leave the job to the TV.

Next, get the TV to map the signal pixel for pixel. That means a PC mode or the brand's version of Just Scan or Dot by Dot. On some sets only the VGA input, or an HDMI input labeled "PC," gives true 1:1 mapping, often with full color resolution as a bonus. Set sharpness to its neutral point, which isn't always zero.

Now let the source handle 1080p. Scaling 1080p down to 768 lines (0.711x) is effectively supersampling, with each screen pixel built from about two source pixels' worth of real detail. That's why 1080p downscaled to 768 looks noticeably sharper than 720p upscaled to 768, even though 720 is the closer number. A good media player or GPU does it in one pass with a high-quality filter, and the TV just displays the result.

Fitting 1080p onto a 1360x768 screen takes a tiny compromise, and none of the options are visible. You can scale to 1360x765 and leave a 3-line letterbox, fill all 768 lines and crop about 5 pixels off the sides, or accept a 0.4 percent change in aspect ratio. On a true 1366x768 panel, 1080p fits almost exactly, since 768 lines at 16:9 is 1365.33 pixels wide.

For over-the-air broadcasts, a network tuner feeding a PC skips the 720p detour completely. The PC deinterlaces 1080i once with a good deinterlacer and scales straight to 768 lines.

Finally, check your work with a 1-pixel checkerboard test pattern. If it looks like an even gray from your seat, the TV is showing pixels 1:1. If you see bands, moiré, or blur, something in the chain is still scaling. A pattern with a 1-pixel border will also show whether overscan is cropping the edges.

Set up this way, a secondhand 768-line TV still makes a nice screen for a kitchen, garage, or spare room.

The numbers in one place

720p finally fits

The lessons from all this still apply to every screen sold today. Match the panel's native resolution when you can. Keep scaling passes to a minimum. When scaling can't be avoided, scaling down from more detail beats scaling up from less. And turn off overscan.

There's a bit of irony at the end, too. 720p scales by exactly 2x to 1440p and exactly 3x to 4K, the resolutions that came after the panels it actually lived on, and modern GPUs can integer-scale it so each pixel becomes a perfect block. 720p finally divides evenly into the screens people buy, long after it had its chance to make a first impression.