There Is No HD Lie

Back in January George Ou on ZDNet posted a ridiculous article outlining the “HD lie”. Normally I would just shrug this off as just another idiot with a keyboard. Unfortunately this article has gained some attention after the recent update to the AppleTV and the availability of HD rentals from iTunes. I expect this sort of drivel on some random Digg comment but not on a web site with some amount of editorial control.

Video Compression Basics

The process of compressing video is pretty complicated but is a well understood art. Uncompressed video is essentially a stream of images played in a sequence. When played quickly enough this stream of images can fool us into thinking the images we’re seeing are moving. Uncompressed digital video takes up an enormous amount of space.

bits per pixel * pixels per frame * frames per second

Let’s plug some numbers in and see how this works:
Bits per pixel: 24 (that’s one byte for each color of the RGB color space) but to make things simple lets make it 3 bytes
Pixels per frame: 307,200 (that’s 640 * 480 or a standard VGA frame)
Frames per second: 30 (in order to perceive motion the frames need to be displayed at a rate above about 20 per second)

Which brings us to: 3 * 307200 * 30 = 27648000 bytes of data needed every second. That is 26.36 megabytes of data every second for video to make no mention of audio. A one hour video would take up nearly 95 gigabytes!

This is where video compression comes in. In images and video there tends to be not only a lot of redundant information but also information that can be thrown away without people noticing. For instance a JPEG compressed image generates a mathematical representation of a source image throwing away a lot of information in the process but without looking closely you’d probably never realize it. I will let Wikipedia’s article explain the science and process of video compression. With modern compression schemes that example video stream that needed 26 megabytes per second can be reproduced with a tiny fraction of that data rate. This data rate for video is referred to as the bitrate.

Not All Compression is Equal

This is a major sticking point for George in his article. He is basing his entire argument on differing bitrates between different services without taking into account what compression systems are being used. The first he rails about is MPEG-2 which is used in digital video broadcast, DVDs, and Blu-ray and HD-DVD discs. It is fairly extensible and scales upwards well. It performs best at a bitrate above 3Mbps and can scale up to around 38Mbps. The bitrate needed by MPEG-2 to produce good quality video is determined by the type of footage being encoded. For Standard Definition video a simple scene without a lot of motion between frames looks good at 3Mbps while an action scene with a lot of movement or complexity will need around 9Mbps. For HD video the required bitrates are a bit higher, the ATSC broadcast spec in the US offers 19.4Mbps for MPEG-2 video which allows for good quality 1080i video. All this being said, MPEG-2 doesn’t hold a candle to MPEG-4 AVC in terms of video encoding quality. AVC has a whole slew of features designed to make up for deficiencies that cropped up in the MPEG-2 system. Instead of simply throwing more bits at the problem AVC does more work during the compression and decompression phases allowing high quality at far lower bitrates than MPEG-2. This was a specific design goal of AVC. Typically AVC can give the same visual quality as half to a quarter of the bitrate of MPEG-2 which means it can easily deliver good quality SD video at 1-2Mbps and HD video at 4-6Mbps (for 720p). AVC also scales up to around 40Mbps for 1080p Blu-ray content. More bits don’t hurt but they’re not necessary to have good looking video if your codec is smart.

Is it really HD?

The HD standard covers a lot of different options. For broadcast it covers two major resolutions, 1280×720 and 1920×1080. The first is the most common resolution for HD televisions and is the lowest hanging HD fruit. It’s a noticeable improvement over SD resolution. More pixels on screen allows for more visual detail and usually a better movie watching experience. This idea extends to the 1920×1080 resolution which is an even bigger improvement over SD. This resolution is less commonly found in HD televisions but does provide a nice quality increase. Because an engineering specification can never be simple the HD spec allows for a lot of variation around these resolutions. It allows for different framerates for video based on the framerate of the source video. For instance 24fps and 30fps roughly equivalent to film and NTSC broadcast framerates.

There’s nothing about a particular bitrate or video codec that defines HD, it’s all about the resolution. The HD content from iTunes and Xbox 360 Marketplace is little different from over the air or cable HD broadcasts in terms of resolution. Both of these services offer 720p content because the files are far smaller than 1080p files. Even with AVC or VC-1 encoding a 1080p video requires a pretty high bitrate and thus results in large files which in turn take a long time to download. The average speed for broadband connections in the US is only 1.9Mbps which makes for long waits for even low bitrate SD video to say nothing of HD.

HD Content

HD content from Apple and Microsoft is not really on par with high quality Blu-ray discs in terms of absolute visual fidelity. Blu-ray video is 1080p and because of the disc medium offers obscenely high bitrates, even for AVC and VC-1. Even for advanced codecs like these having a higher bitrate overhead helps with the visual quality. At the same time they aren’t somehow inferior to their high bitrate MPEG-2 counterparts, these advanced codecs were designed to provide the same visual quality at lower bitrates specifically to be able to transmit HD video over common broadband connections.

Since upgrading my AppleTV I’ve rented and watched a couple of HD movies. These movies looked great on my TV, even in high action scenes. Blu-ray versions of these might have looked a bit better but they’re on par with HD content from places like Apple’s HD Gallery. I was really impressed with the quality considering I only needed to buffer the video for a few minutes before it was available to watch. The previews don’t come close to the quality of the actual movies but you’re getting to watch those for free so I honestly don’t expect phenomenal quality from them.

I think there’s a bit of HD hype burnout on the interwebs. People have been let down by badly mastered Blu-ray and HD-DVD movies and cable and satellite providers cutting corners when it comes to encoding their content. When a content provider with a fixed sized pipe like a cable or satellite company says they’re giving you “more channels!” it means they’re clamping down the bitrate of their content to fit more into the pipe without caring about the quality. Unlike AppleTVs and Xbox 360s most cable and satellite providers don’t buffer the video locally so if they only have a 5Mbps pipe they have to squeeze the video into that 5Mbps even if to get the best quality that particular video ought to be encoded at 6Mbps. The ITU didn’t convene the VCEG to work on AVC because they had nothing better to do, they wanted a more advanced codec than existed in the form of MPEG-2.