Lost in Translation: Going from TV to PC
Consider that a scenario where each processing stage degrades the image quality by 1%. Five processing steps will result roughly in a 5% quality loss; 2% quality loss per step degrades the image 10%—certainly noticeable, if not annoying.
Internet Delivery Issues
Up to this point, we’ve assumed that the internet will deliver a pristine flow of TV data. But can sustained bitrates of 1–2Mbps for SD and 6–10Mbps for HD be maintained over the internet? Even though ISPs boast of up to 10Mbps download capabilities, in reality, bitrates can vary greatly over time. Buffering must be able to store a sufficient amount of data to ensure a continuous feed of data to the decoder.
One "showstopper" that occurs when delivering TV over the internet is packet jitter. Packets arrive at varying, indeterminate times at the PC due to the fact that they may travel through different paths over the internet.
Lost packets can also wreck the party; any amount of lost data will cause an artifact, will freeze the display, or will lock up the PC altogether. DTV transmission includes forward error correction (FEC) data to protect and recover data that is lost over a noisy transmission channel. Internet delivery of broadcast TV can’t afford the additional bandwidth for FEC data.
Only as Good as the Source
All compression codecs are not created equal. Encoders have a toolkit of features and techniques to choose from. Video quality is not checked during the compression process. Compliance to compression-codec standards is judged on the production of a syntactically correct bitstream, not the selection of compression tools. Compressing video streams at equal bitrates using different encoders can produce video of widely varying quality.
Transrating content—that is, reducing the bitrate for internet distribution—will always degrade image quality. Cross-conversion or transcoding content originally compressed in MPEG-2 to AVC/MPEG-4 Part 10 or VC-1/Windows Media, Flash, or QuickTime is a complex processing challenge with inherent quality loss. It is best to repurpose baseband video and perform a single encode.
On With the Quest
Obviously, there are numerous imaging parameters that can be set incorrectly when preparing broadcast TV for internet delivery; even minor variances can accumulate to the point where image quality is unacceptable. The irony is that most viewers like a distorted image to begin with. Marketing research has determined that consumers actually prefer displays with saturated colors and images enhanced with artificial detail. So guess how DTV displays are preset at the factory!
But broadcast DTV is not perfect. HDTV produced by the ATSC prototype, even with generation-zero MPEG-2 encoders and decoders, produced perceptually perfect images. However, many terrestrial broadcasters are providing a multiplex of HD and SD programs. Unfortunately, all the images suffer. Internet data pipes leading to the home and long-haul ISP networks are evolving to higher bandwidths; it won’t be too long before 100Gbps backbones to the home are technically and commercially viable. When that happens, internet TV may, in fact, be capable of delivering video that’s better than broadcast-quality TV.
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