Digital video bit-rate reduction is impressive. It can squeeze an HDTV signal of about a billion bits per second down to a few million. But it’s not magic.
Consider a sleeping bag stuff sack. Down-filled bags can be greatly compressed. Something large enough for a big human to get into can be squeezed into a tiny pouch. But a pouch just big enough for a single sleeping bag won’t accommodate a double. The same is true of HDTV signals.
At the International Broadcasting Convention (IBC) in Amsterdam last month, the European Broadcasting Union (EBU) demonstrated the principle. Beginning with a Sony HDC-1500 camera with 1920 picture elements per line and 1080 picture-carrying lines per frame, it being Europe, they shot at 50 progressively scanned frames per second. The raw data rate was 1920 x 1080 x 50 x 10 bits per pixel x 2 for 4:2:2 color encoding or a little over two billion bits per second.
That’s twice as much as “normal” HDTV, either 1080 lines (by 1920 pixels) interlaced at 25 frames per second (in Europe) or 720 lines (by 1280 pixels) progressively scanned at 50 frames per second, 1080i or 720p. Either is about a billion bits per second; the full 1080p is about two billion.
The EBU also used some 65mm film shot material digitized to an even higher scanning format (2160 progressive scanning lines at 50 frames per second). In either case (film or video), the pictures were converted to three picture streams: 1080p, 1080i or 720p. The three streams were then compressed using MPEG-4 AVC compression to a range of bit rates. The compressed data streams were then decoded and presented on three identical Pioneer Ex5000 1920 x 1080 progressively scanned displays.
When uncompressed HDTV signals were fed to the displays, all three looked very good. Partisans of 1080i or 720p might have pointed to one or the other as looking slightly better. Those in the 1080i camp say 1920 x 1080 looks better than 1280 x 720. Those in the 720p camp say 50 progressive frames (or 60 in the U.S.) look better than 25 interlaced (or 30 here). But both arguments combined in the 1080p signal. It was 1920 x 1080 and 50-frame progressive. It looked best.
Unfortunately, uncompressed HDTV signals don’t fit into digital broadcast, cable or satellite channels. A complete U.S. digital TV broadcast channel carries a payload of a little over 19 million bits per second (European channels are somewhat larger). After audio and other non-video signals are subtracted, the maximum available for HDTV is about 18 Mbps.
It would take about 56 U.S. broadcast channels to carry a single uncompressed 1080i or 720p HDTV signal, which is more than exist in any one place; 1080p would take twice as many. That’s where compression comes in, and MPEG-4 AVC compression is about as good as it currently gets.
Ignoring the more-spatial-resolution vs. more-temporal-resolution arguments, progressively scanned images can be more efficiently compressed than interlaced ones. That difference in efficiency showed up on the screens at the EBU IBC demo. As expected, at compressed data rates, 720p looked better than 1080i.
Like 720p, however, 1080p is also progressively scanned. It should share the same compression-efficiency advantage over 1080i. Unfortunately, 1080p has twice as much data to begin with. It’s like the double-sized sleeping bag versus the single. The double is certainly roomier when uncompressed, but it won’t fit into the same stuff sack as the single. Similarly, although 1080p looked better than 720p uncompressed, at the EBU IBC demo it looked worse after compression.
It’s important to note that both the picture source and the displays were 1080p (or, in the case of the film, even better). Had the EBU started with a 1280 x 720 camera, the resulting contrast losses due to fewer pixels in the imager might have been traded for the increased compression artifacts of 1080p.
It’s also worth noting that compression systems continually improve. But broadcasters will likely use the improvements to cram more into their channels, leaving 720p’s advantage over 1080p.
The demonstration was just one of many instructive aspects of IBC2006. In a conference session, Aardman Animations executives explained why they were sending stop-motion experts to Dreamworks to give a retro look to a new computer-graphics movie.
Less is more, more or less.
Mark Schubin is an engineering consultant with a diverse range of clients, from the Metropolitan Opera to Sesame Workshop.
