The resolution ladder
| Name | Pixels | Also called |
|---|---|---|
| Standard definition | 720 × 576 | 576i, PAL |
| High definition | 1280 × 720 | 720p, "HD ready" |
| Full high definition | 1920 × 1080 | 1080i, 1080p, "Full HD" |
| Ultra high definition | 3840 × 2160 | 4K UHD, 2160p |
| 8K ultra high definition | 7680 × 4320 | 8K UHD, 4320p |
Each step up the ladder doubles both the width and the height, so it quadruples the total pixel count. A 4K UHD picture is four 1080p pictures arranged in a square, not twice as detailed as the name might suggest.
Interlaced and progressive
The letter after the number matters more than people expect. In interlaced video — the "i" in 1080i — each frame is split into two fields: one holding the odd-numbered lines, one holding the even. They are drawn in alternation, which halves the bandwidth needed. It was a sensible compromise for broadcasting and a legacy of cathode ray tube televisions.
In progressive video — the "p" in 1080p — every line of every frame is drawn in one pass. Motion is cleaner and there is no combing on fast movement. Flat panels are progressive by nature, so an interlaced signal has to be de-interlaced before it can be displayed, which is one of the things a television's picture processing is quietly doing.
This is why 720p can look better than 1080i on moving pictures despite having fewer pixels. Sport is the usual example: fewer lines, but all of them drawn at once.
4K, UHD, and why they are not the same thing
The two terms get used interchangeably in shops, but they describe different standards. UHD is 3840 × 2160 — the broadcast and consumer format, and what a television sold as "4K" almost always is. DCI 4K is 4096 × 2160, the digital cinema standard, and it is slightly wider.
The difference rarely matters when buying a television. It matters if you are working with video, because the two are not the same shape and something has to be cropped or padded to move between them.
Australian television
Australia broadcast in analogue PAL at 576i for decades. The switch to digital was staged region by region and completed at the end of 2013, when the last analogue transmitters were shut off. Free-to-air broadcasting has been entirely digital since.
Australian digital television uses the DVB-T standard, and broadcasters have gradually moved their main channels to high definition as capacity allowed. Anything received over the air is compressed to fit the available spectrum, which is why the same programme can look noticeably better streamed than broadcast even at the same nominal resolution.
Resolution is only one part of picture quality. Bitrate, compression, colour depth, frame rate and how well a panel handles contrast all matter, and a well-encoded 1080p stream will usually beat a heavily compressed 4K one.
What happened to the word
"HDTV" belonged to a specific moment: roughly 2005 to 2015, when the industry needed a word to distinguish new digital widescreen sets from the analogue ones they replaced. Once every television sold was high definition, the label stopped doing any work.
Marketing moved on to 4K, then UHD, then to the panel technology itself — OLED, QLED, mini-LED — because that is where the visible differences now sit. The formats HDTV named are still in daily use. The name isn't.