Why does 1080i exist?

[Glenn Chan]

Even though the compression schemes exist to manage the bandwidth of broadcast material and the processing power of the latest generation of multiprocessor computers can (given enough money) handle encoding true 1080p60 in real-time, affordable consumer camcorders cannot encode the analog information coming from the sensor(s).

I don't think that was a major factor since most broadcast material originates on professional camera equipment. And since HD is about quality, it's less likely that consumer equipment would be used for HD.

MPEG2 isn't really very well suited for compressing 1080p60.

MPEG2 is better suited at compressing 1080p60 than 1080i60.

The interlacing really messes with motion compensation and chroma subsampling and makes those techniques less efficient. Suppose you started with 1080p60 and wanted to compress that down. It would be preferable to use existing compression tricks (e.g. quantize the DCT coefficients more heavily) than to use interlacing.
Or to look at it slightly differently... suppose you're dealing with JPEG still images. Is it better to apply heavier JPEG compression, or to resize it down and then blow it back up? Only at very low bitrates does the latter make sense (and actually JPEG could be designed to handle very low bitrates better; they just don't do it because it isn't necessary for JPEG applications).

2- On the other hand... some of the arguments for 1080i are:
a- With 1080i, you can scan the sensor using row-pair summation to double sensitivity (this blurs the image vertically, but that is desirable for interlaced to avoid flickering lines on a CRT).
b- Tape transport mechanisms can only handle so much bandwidth. Going with 1080p60 would double the necessary bandwidth. Looking backwards, HDCAM and DVCPRO HD are formats where they really don't have enough bandwidth; 1080p60 would make that worse. *Neither tape format is really full HD.

3- IMO, I think 1080i is undesirable because it wastes bandwidth (whatever means you use to transmit it, e.g. OTA, satellite, etc.). Compressing interlaced images is less efficient than compressing progressive images.

Nowadays, bandwidth is becoming really valuable with all sorts of services fighting over it. e.g. more TV channels, wireless data, (wireless audio), etc. etc.

[Robert M Wright]

Progressive frames lend themselves to more efficient compression, but it's not realistic to use MPEG2 for 1080p60, at bitrates that would allow the stream to be recorded on MiniDV tape (in real time). It could be done reasonably with AVC though.

[Kevin Shaw]

So to sum up the answer to the original question, the reason we have 1080i is to capture and transmit 1080i/p resolution for 1080i/p displays in a cost-effective manner. If you have a 720p display you might as well feed it a 720p signal, but that's only about half the native resolution of modern 1080 HDTVs.

As far as frame rates are concerned, 30p and lower options are prone to a "juddery" look which can be distacting, and I see a lot of examples of this on TV lately. The most popular long-term format standards will likely be 1080/24p and 1080 or 720 res at 60p, but not 720 at 30p.

[John Miller]

Originally Posted by Kevin Shaw

So to sum up the answer to the original question, the reason we have 1080i is to capture and transmit 1080i/p resolution for 1080i/p displays in a cost-effective manner. If you have a 720p display you might as well feed it a 720p signal, but that's only about half the native resolution of modern 1080 HDTVs.

As far as frame rates are concerned, 30p and lower options are prone to a "juddery" look which can be distacting, and I see a lot of examples of this on TV lately. The most popular long-term format standards will likely be 1080/24p and 1080 or 720 res at 60p, but not 720 at 30p.

For many people, purchasing as 1080 native TV is, well, a waste of money. Unless you have a particularly large display and/or you sit close to it you won't be able to resolve the detail anyway....

[Robert M Wright]

Originally Posted by John Miller

For many people, purchasing as 1080 native TV is, well, a waste of money. Unless you have a particularly large display and/or you sit close to it you won't be able to resolve the detail anyway....

That's true. At a comfortable casual viewing distance, you have to have exceptional eyesight to be able to perceive any difference (between 1080p and 720p).

[Glenn Chan]

For many people, purchasing as 1080 native TV is, well, a waste of money. Unless you have a particularly large display and/or you sit close to it you won't be able to resolve the detail anyway....

Scaling/resampling will introduce an additional layer of artifacts in the form of less sharpness, more aliasing, or more ringing artifacts.

You don't just lose resolution... you lose resolution and some combination of:
- Losing even more resolution
- More aliasing
- More ringing

In practice, it's not quite as bad as it sounds. But you'll see this stuff as:
- Harder to read text or pick out fine details in the image.
- Crawling on edges.
- Halos around edges.

[Daniel Browning]

Originally Posted by John Miller

For many people, purchasing as 1080 native TV is, well, a waste of money. Unless you have a particularly large display and/or you sit close to it you won't be able to resolve the detail anyway....

Originally Posted by Robert M Wright

That's true. At a comfortable casual viewing distance, you have to have exceptional eyesight to be able to perceive any difference (between 1080p and 720p).

The human eye can resolve 1.25 lines per minute of arc, so one only needs to sit close enough that the display fills at least 10 degrees of the field of view to begin to exceed the resolution of 720p. By 20 degrees, 1080p is exceeded: that's a 34-inch screen at 7 feet for normal eyesight. That is a comfortable, casual viewing distance.

[Kevin Shaw]

I have a 42" 1080p LCD hooked up to my PS3, and the result is excellent for high-quality source material like digital photos. If people can't see a difference from 720p displays it may be because there isn't enough really good HD source material yet.