HD – 720p, 1080i and 1080p27 Sep 2006
After a conversation with Ine on HD formats (1080i vs. 1080p), I researched the topic a bit further. Let me resume some of the things I have learned up till now:
Real HD and HD-ready
HD or ‘high definition’ as defined for screens, projectors and TV, defines 2 resolutions. The smaller one has 720 lines of each 1280 pixels, the bigger one 1080 lines of each 1920 pixels. They can be used with different frame rates: refreshed at 24 fps (a common movie standard) up to 50/60fps (often used for TV). To limit the necessary bandwidth in some cases ‘interlaced scanning’ is used: 1 frame contains all the odd lines, the next only the even lines. This effectively halves the throughput, at the cost of image quality (rapid moving lines appear jagged).
The two most common formats are:
- 720p60: 1280×720, 60 fps progressive scanning, used e.g. in USA-based HDTV broadcasts
- 1080i50 or 1080i60: 1920×1080, 50 or 60 fps interlaced scanning. The higher resolution makes it better for larger screens and movies, but the interlacing has a bad influence on fast moving images (like e.g. sports).
What kind of resolution do we have now? Regular digital TV (SD or ‘Standard Definition’) consists of 480 lines of 720 pixels each. DVD, for instance, allows for 480i and 480p. So, HD delivers at least 3x that resolution.
“HD Ready“, a label that a lot of TVs/screens carry now, just indicates that:
- The minimum native resolution of the display (e.g. LCD, PDP) or display engine (e.g. DLP) is 720 physical lines in wide aspect ratio.
- The display device accepts HD input via Analogue YPbPr1, DVI or HDMI
- HD capable inputs accept the following HD video formats: 1280×720 @ 50 and 60Hz progressive (“720p”), and 1920×1080 @ 50 and 60Hz interlaced (“1080i”)
- The DVI or HDMI input supports content protection (HDCP)
from eicta.org (PDF)
Even if the display can only show 720p, and so must ‘downsample’ an incoming 1080i signal to that lower resolution, it can be called “HD Ready”.
Bandwidth for HD
You might know my interest in bandwidth, so what do these HD standards translate in? I’m not taking any compression into account:
- 720p60: 1280×720 gives 0,92 megapixels per frame. If we take a 24-bit color value per pixel (RGB), this means 2,76 MB data per frame, which multiplied with the 60fps gives 1,3 Gbps or 165 MB/s.
- 1080i60: 1920×1080 means a 2 megapixel frame which translates into 6,2MB per frame. The interlacing cuts the bitrate in half: 6,2MB x 60 / 2 = 1,49 Gbps or 186 MB/s.
- 1080p60 is obviously double the throughput of 1080i: 2,98 Gbps or 373 MB/s
HD-DVD and Blu-ray?
For those hoping for 1080p from HD DVD, don’t hold your breath—Toshiba confirmed that the data is recorded on HD DVD in 1080i, and there are no plans to change that. The players have already been designed for 1080i discs, and it would take a redesign to enable them to handle 1080p discs, even if there were plans to produce them.
Blu-Ray already has a Samsung player capable of playing 1080p discs. But at $999 that’s a pricy gamble of the fact that 1) Blu-Ray will not be the Betamax of HD and 2) movies will actually be mastered as 1080p Blu-Ray.
1080p is the best
Sure, 1080p is the best resolution home devices could have these days. There are some issues with that, however:
- almost no sources are capable of delivering 1080p input. HDTV can do 1080i at the best, DVDs give you 480p, Blu-ray doesn’t do it yet, HD-DVD will never have it.
- bandwidth requirements are painful. 1080p generates image data at 3Gbps. We need better compression algorithms than MPEG2 to get that down to manageable bitrates. MPEG4 or JPEG2000 might help.
- would you see the difference between 1080i and 1080p? According CNET only on really big screens (more than 60″) when sitting very close to the screen. Not your everyday situation.
If you want to buy a real HDTV -and you have the budget – go for a 1080i one. Don’t settle for 720p and don’t hold your breath for 1080p.