Popular Mechanics recently published an article talking about one of the things that frustrates knowledgeable HDTV owners and merely confuses not-so-knowledgeable ones. There is no standard for the amount of acceptable compression on an HDTV signal.
As it turns out, television bandwidth (in the electromagnetic spectrum sense) is very closely related to television bandwidth (in the throughput sense.)
Compression is used to deliver multiple HD channels – the higher the compression, the more channels you can fit either into your slice of the EM spectrum, your satellite bandwidth, or your cable service, you know, whatever multi-billion dollar communications infrastructure you have lying around.
The problem is that in order to deliver more HD channels, some companies apply a level of compression that is destructive to image quality, reducing fine detail and introducing compression artifacts – those magic rectangular blobs you would get from, say, an overcompressed XviD.
In fact, there’s no real regulation over high-definition picture quality at all—”none whatsoever,” one industry consultant told me. … After all, 1080 lines of poor-quality pixels may technically be “high-definition,” but that doesn’t mean it looks very good.
One of the most important factors in determining picture quality is bit rate, or how much video and audio data is being sent down the pipe for each program. The technology behind digital television relies heavily on digital compression, and the ATSC specifies that digital TV use the MPEG-2 compression standard, which is also utilized by DVDs, although some satellite broadcasters use the more efficient MPEG-4 advanced video coding (AVC) standard. These compression technologies are necessary in order to deliver a large number of channels to consumers. Without these codecs, an uncompressed HD video stream could require as much as 1 gigabit per second of data capacity—that’s 52 times the capacity of the average broadcast channel. With compression, the same stream can be shrunk almost infinitely. But compression is often used overzealously, and picture quality suffers as a result.
In fact, the end picture quality can be so poor that it defeats the very purpose of broadcasting in HD to begin with, as poster “Bfdtv” showed through a series of screenshots on the AVS Forum.
The problem with determining the point at which compression significantly degrades the “quality of television experience” is a highly subjective measure, and differs not only from person to person, but from content to content – high compression is probably fine for talking-head news shows with little movement, but not very good for sports, for example. Crime drama shows might fall somewhere in the middle. Some measure of compression is obviously acceptable, otherwise YouTube, iTunes shows, and yes, pirated movies and TV shows wouldn’t be popular.
I asked Robert Zitter, HBO’s chief technical officer, what his company’s requirements were. “We do a contractual relationship with all of our distributors, and one of the items that’s addressed in there is what they can and cannot do with our signal,” he says. “But much of it cannot be quantified. There’s not a device that’s made that can look at a picture and say pass or fail. We all wish we had it—it would make the negotiations a lot easier. Ultimately, it winds up being more subjective than quantifiable.”
I think Zitter may be wrong, however. While this is a difficult problem, it is not necessarily an unsolvable one. We already have a system in place for measuring another “subjective” thing in a “quantifiable way” – voice call quality.
Voice call quality, which is also a “subjective” measurement, is now something that is calculated in VoIP deployments as a Mean Opinion Score [MOS]. Today we can derive the MOS and the subjective end-user experience by tracking all the packet drops, latency, jitter, and general network quality of a VoIP call. MOS for picture quality determined from the amount of macroblocked pixels, the compression rate, and the effective level of detail isn’t far off into the future.
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