The Litchfield, Connecticut Cable Advisory Council recently asked the Connecticut state franchise authority to develop "measurable and definitive" picture quality standards for digital television. Among other things, the Council wanted the government to prohibit pixelation in MPEG video; prohibit audio/video delay beyond 10 milliseconds; prohibit frame freezes; and develop a picture quality standard for MPEG compression. That particular franchise authority has not yet decided what to do, but you can be sure that other consumer advisory councils will be asking their franchise authorities to impose similar regulations.
If you look in the FCC Rules, you will see a variety of analog technical rules in Section 76.605, but no specific digital technical rules. For that, you have to go Section 76.640. It doesn't list individual specifications, but instead requires cable systems to comply with ANSI/SCTE 40 2004, Digital Cable Network Interface Standard. ANSI/SCTE 40 2004 specifies numerous requirements and limits for digital cable systems, including: RF channel spacing and frequency range; transit delay from headend to most distant customer; carrier-to-noise-plus-interference ratio; composite triple beat; AM hum modulation; group delay variation; chroma/luma delay; phase noise; amplitude variation; micro-reflections; burst noise; adjacent channel rejection; and carrier level at the terminal input.
If a cable system meets these requirements, digital video signals will reach subscribers unimpaired. All the relevant technical rules are already in place. And that means that local franchise authorities are preempted from regulating digital cable standards.
Pixelation. So let's look at the Council's specific demands. Pixelation means that individual pixels are visible to the eye. In a digital television program that has been compressed using MPEG-2 video coding, whether individual pixels are visible to the eye depends on a combination of factors, including the amount of action in the scene, the data rate of the compressed signal, the size of the TV display, the resolution of the TV display, and the distance between the viewer and the display. So for example, if the TV screen is very large or if the viewer is very close to the screen, individual pixels are likely to be visible. This is outside the control of the cable operator.
Audio/video synchronization. Lack of synchronization between audio and video ("lip synch") can be annoying to viewers. Digital TV stations are the primary culprits, not cable systems or programmers. The chain from program production to reception consists of many links, and each link could introduce a loss of synchronization. At the production and post-production stages, audio-video synch errors can occur in the capture stage, in film-to-video transfer, and in editing. When TV networks distribute the signals to affiliate TV stations, signals are first decompressed and then recompressed, which can affect synchronization. Because cable operators typically pass through cable and broadcast programming without doing any compression and decompression, the control of lip synch errors is outside of their control.
Frame freezes. A frame freeze occurs when the transmission quality of the signal is degraded so much that the receiver is unable to decode the video, and the last picture in the receiver's buffer memory continues to be displayed. This degradation might potentially occur on many of the links in the distribution chain before it gets to the home, including satellite and broadcast links. Consequently, if freeze frames do occur on cable system delivery of MPEG digital signals, it may be due to degradations outside the control of the cable operator.
Picture quality. The process of evaluating perceived picture quality of digital video signals is complicated. Objective measurement techniques are not used, since compression-related impairments may be dependent on the amount of action in the scene. The standard approach uses subjective picture quality tests in which subjects judge selected video images. Subjective tests are expensive and time-consuming, because they require groups of viewers to watch carefully-selected clips of program material under controlled viewing conditions.
Both academic researchers and test equipment manufacturers have been trying to develop less-expensive objective picture quality measurement techniques for digital video. But there is no agreement on any single approach. In any event, even if MPEG picture quality assessment were feasible, the picture quality would be beyond the cable operator's control because compression is done by the cable programmer or the TV station.
There you have it—another clueless consumer council trying to meddle in technology.
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