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To: .com who wrote (35143)	8/12/1998 4:43:00 PM
From: John Rieman	Respond to of 50808

More MPEG-2 quality testing needed................................ tvbroadcast.com Looking At Layers Ed Kiyoi, product marketing manager for Tektronix, thinks of DTV test and measurement in terms of "layers" of the signal. When comparing analog to digital, there are more levels and different requirements for digital signals. The digital signal itself is a digitized representation of an analog signal, and must be tested on several levels. Kiyoi said the digital baseband layer must be monitored to make sure the signal has been digitized correctly (in compliance with SMPTE standards). The transport layer shows the "analog characteristics of the digital signal," such as jitter. And the signal will still need to be verified through waveform analysis. With analog, engineers (and, for that matter, far less technically inclined staff members) could infer an acceptable level of picture quality with a waveform monitor and vectorscope. That's not necessarily sufficient in the digital world. Compression means adding more test equipment to insure signal integrity. "In order to get those digital baseband signals ready for that RF modulation, we go through this process of compression, and that adds a couple of very unique test requirements," Kiyoi explained. Digital broadcast signal compression requires compliance with the MPEG-2 protocol, so set-top boxes and DTV sets can properly decode the signal. As a result, Kiyoi said, protocol analysis of the compression is very important. "Because compression is a fairly new technology for the broadcast industry," he offered, "there's also a whole new set of impairments that could result in the compression process that we now need to be able to detect and measure." One of the most easily identified problems with compression are artifacts. Basically, the compression algorithms divide pictures into macroblocks (8 x 8 blocks of pixels). When video is subject to excessive compression (overly quantized), this macroblock structure becomes apparent--the video image looks like it's made out of blocks. "It's not something you can detect using just waveform analysis," Kiyoi explained. "You really need to get in and analyze the actual picture content to be able to detect some of these compression impairments." This direct (or objective) picture quality assessment is an area where broadcasters as a whole have little experience. In the past, human viewer trials were needed to assess the video quality, but new devices, including the Tektronix PQA200, are now able to provide a practical analysis of picture quality. Price And Practicality