Lost in Translation: Going from TV to PC

It wasn’t too long ago that television broadcasters considered streaming programming over the public internet to PCs a curiosity. Poor video quality and lack of a derivative revenue stream were the major impediments. But once a few bold broadcasters ventured into cyberspace, networks and local stations realized that, 不管这对他们有没有好处, they needed to make some form of their programming available over the internet.

今天, every broadcaster plans for some amount of programming to be repurposed over the internet and regards that programming’s cyberspace dissemination as part of its brand and product promotion. 晚间新闻, 体育集锦, 现场转播, and complete episode rebroadcasts appear on local station and network websites as well as unauthorized video sites. Fifteen-second preroll commercial spots provide revenue streams and are sometimes offered as part of an integrated platform ad-sales strategy.

In the early days of this new TV programming consumption channel, 网络电视, viewers were satisfied with the novelty of simply being able to watch a show. Content availability was more important than image quality.

时代变了, and the quality of broadcast TV that has been repurposed for internet distribution to PCs is of critical concern. Even though internet bandwidth has increased by several orders of magnitude, 个人电脑已经变得更加强大, compression algorithms have gained greater efficiency, and displays have grown larger and higher in resolution, the video quality of TV programs delivered online is rarely acceptable on a computer display. Even when they are subjectively acceptable to most viewers, TV shows delivered online never quite attain the quality level they do when viewed on a DTV receiver.

Most discussions about the feasibility of HD video online focus on bandwidth and encoding, but those issues are only part of the story. 为本文的目的, we’ll assume that the available bandwidth for the transmission of TV packets over the internet is more than sufficient to deliver content at its encoded bitrate. We’ll also assume that the PC has more than enough computer power to handle the required video processing in real time.

相对完善
Beauty may be in the eye of the beholder, but image quality can be objectively measured. Television and imaging researchers have been tying to establish rigorous objective quality metrics and benchmarks for years. Their efforts have not been in vain; a number of TV vendors now offer image-quality assessment test equipment.

But why doesn’t TV programming ever look as good on a PC as it does on a TV? A number of factors and variables lead to diminished image quality. A generic image processing chain for conversion of a TV program for presentation on a computer (or any) display is shown in 图1.

图1. TV-for-internet repurposing workflow

Image processing includes color space conversion, 色阶转换, 原色调整, 伽马校正, 像素网格大小调整(缩放). Display technology figures into the equation since all the parameters in the conversion process must be optimized for the target display technology. Additional processes such as sharpness enhancement and display color temperature adjustment (white balancing) add to the complexity of delivering broadcast TV on a PC, but I won’t discuss those processes in this article.

If any of these parameters are out of broadcast "spec," image quality will not match the source. Even if these errors are so small that individually they may not produce noticeable artifacts, cumulatively they can wreck a pristine video image.

颜色空间
Computer graphics and TV systems don’t talk the same color language. Computers process image data in RGB (red, 绿色, blue) color space while TV broadcasts content using color represented in terms of luminance (Y) and chrominance (R-Y and B-Y), 色差信号.

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