Proposed definition of HD on the web, with examples
- Posted: Apr 01, 2009 at 5:08 PM
- 1,072 Views
I gave a quick response in the comments that I wanted to expand into a proposed definition.
The infamous Table 3 of the ATSC specification lists 18 different resolution and frame rate combinations for digital broadcasting, But they drew a pretty clear line on what’s HD and what’s not: 1280x720 is HD, and anything less isn’t.
A more pedantic definition could be that HD should be at least 1920x1080p24 or 1280x720p60. And we’ll get there on the web before too long. But for now, I want the full 720p experience as a minimum bar. I can see two flavors of that which we can define as the ragged low end of web HD; anything below these are something else.
For film source content, the widest aspect ratio in common use in 2.4:1. With 1280 wide, if we crop to the active image area and then round down to the next divisible-by-16 value for optimum compression, we’re left with 1280x528. For 1.85:1 movies, the equivalent is 1280x688
Lots of production codecs for HD aren’t square pixel. HDCAM is 1440x1080, and DVCPROHD is 1280x1080 or 960x720, even though both formats are always 16:9. So, I’m also inclined to allow 960x720 anamorphic for HD, but only in 30p. DVCPROHD’s 24p mode is 1280x1080p24.
If we calculate the pixels per second of the above, we get:
So the anamorphic is still more pixels/sec than square pixel 2.4:1. We could arguably define “Web HD” as “at least 16M pixels/second” as well.
Hassan Wharton-Ali brought up another good point on the thread - HD should actually be HD quality. It can’t be a lousy, over-quantized encode using a suboptimally high resolution just so it can be called HD.
A good test is the video should look worse (due to less detail), not better (due to less artifacts), if encoded at a lower resolution at the same data rate. If reducing your frame size makes the video look better when scaled to the same size, then the frame size is too high!
I know these are going to be too wide for my default blogging interface, but I need to stick an iframe somewhere in order to lock down the frame size exactly, due to how Silverlight Streaming works. Autoplay is turned off so your browser doesn’t automatically start two HD clips at once.
NOTE, yes I know in fact they still are doing Autoplay for some reason. I'm trying to fix that right now. Just make sure to pause the one you aren't watching. At least I got the size locked down (although still with too much padding...).
These are WMV files, but encoded with EEv2 SP1 using Smooth Streaming settings (so 1-pass CBR, 2 sec Closed GOP, and all that jazz). These are pretty basic encodes; I didn’t do anything tricky, as we WANT to see some artifacts as we’re defining these as the low bound of HD and below the low bound of HD. I do three versions of each clip
The SD is in there as an anchor to show that a lower resolution can actually look better at lower bitrates. Clearly the SD looks better most of the time, so 1 Mbps doesn’t count as HD. And those 16 Kbps of audio make a real difference with WMA 10 Pro, taking us from 32 to 44.1 KHz.
If you mouse the player, it’ll pop up controls (but without scaling the video – one of the nice features of the Black Glass template). To pick a particular clip, click on the icon that looks like poker chips (although upon reflection I note they’re actually film reels).
I haven’t used this clip in ages, but it’s always great to trot out as an edge case of hard encoding. It’s a Michael Bay joint, full of whip-pans, super-fast editing, frenetic motion, and film grain. Low bitrates can look fine for 80% of the frames in a clip, but fall apart for 20%. This clip has some good sequences where there’s multiple edits a second, so you get lots of chances to see the bandwidth stress. If only we had an encoder that could dynamically adjust frame size based on content complexity…
Just a short version of it this time, mainly because I’m tired of slow DSL uploads. The HD sizes are anamorphic, the SD is square pixel.