What’s the Difference between 1080p and 1080i?

Why is there an ‘i’ or a ‘p’ on my resolution? That letter at the end of the number refers to whether a television signal is ‘interlaced’ or ‘progressive’. To give a full explanation, I’m going to give you some history. If you’re not interested in the history lesson, all you need to know is that interlaced is lower quality, because it only displays half the screen at a time (odd lines, even lines, odd, even, etc). Progressive does a full screen every time.

 

Where’s My Ray Gun?

When TVs first came out, they used something called a Cathode Ray Gun to paint dots of light on a glass screen. If you think about an ink-jet printer, it worked something like that but with dots of light shot from inside the TV onto the back of the screen, not drops of ink on a page. That’s why we still call an old style television a ‘CRT’ which stands for Cathode Ray Tube.

There were a lot of challenges for engineers designing the first TV sets to overcome. One of those was that the guns they first built were a little slow. Painting a full screen of dots, the gun had to move really, really fast. It was hard to build guns that would hold up to that kind of speed over a long life span. So, to make it easier on the guns, instead of painting a full screen 60 times per second, they painted two half-screens thirty times each, alternating between the odd lines and the even lines. This meant that the gun didn’t have to move nearly as fast, and they could build more reliable sets with decent picture quality.

Imagine if your inkjet printer worked like that. What if it just painted the odd lines, and then rewound it back to the beginning and printed all the even lines the second time through? It wouldn’t be able to match all those lines up perfectly on the second pass, and you’d get spots where two dots that were supposed to be side by side would be on top of one another or too far apart, resulting in a jagged image. Same with TV. Interlaced images are often jagged. They’re aren’t as clear.

Lacking Upgrade Mobility

It worked, and it was good enough, but the problem is, once you standardize on something, it can be hard to change. Eventually they could build reliable screens that could handle progressive images, but if they switched all the broadcast, cable, and satellite systems to a progressive image, it would mean that all the older televisions would no longer work. If they changed the system, people who just bought a new TV the year before, expecting it to last for 15-20 years would be understandably upset.

The solution, of course, was to build a new televisions that could play both interlaced and progressive signals, and that’s what happened. The first time that people were able to actually get commercial content that was progressive was with DVD players. When DVD players first came out, there were interlaced and progressive models. Today, virtually every DVD player sold will display a progressive signal.

So, old Standard Definition television comes in two flavours, interlaced 480i and progressive 480p. The new higher definition standard 720p does not come in an interlaced variety. The highest level of resolution, 1080, does come in both 1080i and 1080p. This is somewhat confusing, I know.

The reason why 1080 comes in both flavours is that a true 1080p signal is a LOT of information. Broadcast systems and even the cables which run between your TV and other devices simply weren’t able to handle a true HD signal. That’s why all HD broadcasts are still at the time of this writing either 720p or 1080i, not 1080p. The infrastructure to deliver that much information isn’t in place yet and won’t be until TV broadcasters switch to an entirely new system which can handle the new requirements.

The DTV Transition

When that happens, old TVs built before the new standards were in place (March 2007) will stop functioning without a special device to convert the new type of signal into the old type of signal. As of March 2009, anyone in the US who wants to watch TV will need a converter box. In Canada, the transition to all digital isn’t scheduled to happen until 2011. Some countries have already made the transition, and others are on track. You can check your country’s progress on the Wikipedia Digital Television Transition page.

The US first tried to mandate a switch over to all digital years ago, but abandoned the attempt when it became clear what a mess it would be, and extended the deadline several times. The North American television system is complex and involves a lot of different companies working together. I don’t know how things will all work out, but I have two expectations:

1. It’s going to be messy. The system is too big and there are too many cooks in the kitchen for a smooth transition.
2. It’s going to get fixed before too long. There is just too much money at stake for the big companies that run TV to let chaos reign for an extended period of time. The US government has required the change. While big TV companies may have been too lazy to get all the work done in advance (let’s hope I’m being pessimistic), as soon as they start losing money, expeditious action will surely be taken.