HD Resolutions

By Geoff Boyle FBKS
EU-based Director of Photography
www.cinematography.net/geoff
Reprinted with permission

What is real HD?  The answer to that question should be obvious, unfortunately it isn't.  This is partly because of confusion caused by the FCC in the US and partly because of misinformation disseminated by manufacturers to make it easier to sell their products. Hype in other words.

To a purist it's very simple, HD is 1080 * 1920, is progressive, is 4:4:4 and is recorded uncompressed, I'll get back to what all that means later.  Unfortunately, according to the FCC, HD can have as few as 480 active lines as long as they are progressive scan.   Equally unfortunate is the contention of some manufacturers that systems that only store 109 pixels of the original 1920 of colour are HD.  I certainly wouldn't be happy if someone paid me a seventeenth of my daily rate but I'm supposed to be happy with a seventeenth of the colour in a picture!

Lets get to the definitions of the terms:

• 1080 * 1920 should be fairly clear; the picture is 1080 lines or pixels high and 1920 pixels wide. The Varicam works at 720 * 1280 but can run at twice the frame rate of other cameras as a result. Normal PAL TV, or SD, is 576 * 768; widescreen SD is 576 * 1024.
  
• Progressive, normal TV works in an interlaced way, the 25 pictures you see every second are actually shown in 2 parts, they are sliced into alternate lines and the odd lines are transmitted followed by the even lines. These are called fields and are transmitted 50 times per second to give you 25 complete frames per second.  A major problem with this is that there can be movement between the odd and even lines and this gives an odd feel to the pictures. I believe that this is one of the reasons that people prefer the look of film on TV.   When film is scanned the odd and even lines of a frame are identical so you get no inter line blurring.  This is also how progressive scan TV works, like film it creates 25 original complete frames every second and they are displayed/transmitted as 25 complete frames every second.  The different formats are identified with a p or an i to differentiate them, so to me a 1080p image is better than a 1080i one.
  
• 4:4:4 what the hell does that mean and why should you care?  It means simply that you have 4 quarters of the monochrome image, 4 quarters of colour picture A and 4 quarters of colour picture B.

Why its described this way is irrelevant?

Normal SD is generally recorded at 4:2:2 this is because research has shown that people are more tolerant to less detail in colour than they are in black & white, so TV systems were designed to save bandwidth by losing half the colour information.   Of course whilst your eye may be tolerant of that loss keying, colour correction and effects programs aren't. So for critical application you need to work in 4:4:4.

What do HD systems work in?

Well they vary from 3:1:1 via 4:2:2 to full 4:4:4. I'm going to deliberately avoid naming companies here but how anyone can expect you to make pictures with a full range of post production possibilities when they throw away a quarter of the monochrome picture and three quarters of the colour picture is totally beyond me!

Finally we come to compression, there are two different approaches to judging compression, mathematically lossless which has a maximum limit of three times and visually lossless which has whatever maximum the person designing it thinks is visually lossless.  Now bear in mind that the visually lossless decision is made by an engineer, and they are not famed for their visual sense, I mean, look at how they dress! They are also under pressure from an employer to get the data rate down.

Personally I'm only interested in uncompressed although the new Sony HDCam SR portable is only 2.5 times compressed at 4:4:4 so I guess I'd be happy with that! If I could get it that is, its not due for delivery for at least another 6 months.  A comparison chart :

Resolution Colour Bit Depth Compression

• Film 4,000+ 4:4:4 20+ None
• Viper 1080 4:4:4 10 None
• HDCam 1080 3:1:1 10 4.7
• Varicam 720 4:2:2 10 6.7
• DigiBeta 576 4:2:2 10 2
• MiniDV 576 4:2:0 8 5

Now some of the manufacturers may decide to show those numbers in a different way, using much larger number for the HD images but the chart is what they break down to. Anything else is designed to confuse you and obfuscate the issue.

To find out more about this and other cinematography issues, visit the Cinematography Mailing List website at http://cinematography.net/geoff.  CML is a place for professional cinematographers and their crew to talk and exchange ideas about cinematography.  This site and the associated discussion lists are run and maintained by professional Cinematographers in their spare time. We have people helping with this in the UK, US, Australia and Germany.

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