Published: TV Technology magazine Issue: 26 January 98 Solving the HDTV Switching Delay by Joe Fedele MIAMI Imagine your “on-air” signal going to black for a period of up to two seconds every time you made a switch to your outgoing feed in master control. That is exactly what would happen if broadcasters tried to execute a live insert, fade or cut commonly used in master control operations using today’s MPEG standards. And it is one of the many challenges facing broadcasters that are currently testing and implementing DTV. “Switching an MPEG serial digital bitstream, is proving to be a real problem for us,” said Joe Bal- kan, manager of Digital Technology at WHD (the model HDTV station in Washington, D.C.). We’ve found that a simple task, such as making a switch from your network feed to a local spot, will cause a loss of 26 frames or more of video.” ADDRESS THE ISSUE Problems such as this could turn routine operational functions into a nightmare if they are not properly addressed by MPEG software and equipment developers. This switching, or “splicing” problem, as it is called in MPEG, is due to the inherent architecture of a bitstream that uses motion estimation to increase the efficiency of its compression capabilities. MPEG is composed of I-, P- and B-frames. I-frames can be thought of as a reference frame that the P- (predictive) frames and B- (bidirectional) frames use to estimate motion and increase the level of compression in the signal. The challenges associated with splicing an MPEG bitstream can be loosely compared to cutting from one video source to another in NTSC. In analog video, a seamless switch can only be accom- plished if it is made during the vertical interval. In MPEG this switch must occur at an I-frame. But, while vertical intervals occur at a constant rate, I-frames “are not evenly spaced,” Balkan said. And the problems associated with splicing go far beyond a simple switch. “How do you do a weather crawl, a fade, wipe, squeeze or other video effect?,” asks Jim Kutzner, director of engi- neering for Comark Digital Services. “It’s a very big challenge for system designers.” One possible solution to the problem is to decode the MPEG stream back to video, make your switch or effect and then re-encode it back. But the process of encoding and decoding takes time. “It could be seconds,” Kutzner warns, “before the feed is re-established.” And even a few seconds, says Debra J. Grivois, director of engineering and operations at NBC O&O WCMH-TV (Columbus, Ohio), “is too much time.” “We are used to being an instantaneous group,” she explained. “A master control operator’s heart will generally stop if there is more than 10 frames of black between air cuts.” Then there is the question of quality. Encoding and decoding can add noise and artifacts under certain circumstances. Individual broadcasters will have to access just how much is too much. “It’s not going to be as pretty as you are capable of if the coding process is not handled properly,” Grivois said. Nor [will it be] as good as you’d like it to be.” NDS, based in the U.K. (with offices in Newport Beach, Calif.), demonstrated prototype non- seamless MPEG splicing technology in a hotel suite during the recent SMPTE conference in New York. Basically, the last frame of the network feed is frozen to fill the black frames before commer- cial insertion refills the encoder’s buffer, thereby providing a continuous picture for the viewer (see sidebar). Kutzner added that broadcasters designing new facilities must first decide how their facility will be operated. “Are we going to operate in a fully compressed mode or something in between?” he asks. EVERYTHING IN BETWEEN That “something in between” Kutzner refers to is an MPEG signal that uses a much milder com- pression ratio. “The restricted bandwidth capabilities of a 6 MHz TV channel limits a deliverable MPEG bitstream of only 19.39 Mbps to the home,” Balkan stated. “This translates into roughly an 80:1 compression ratio if you were to encode the SMPTE 292M standard for an uncompressed HDTV bitstream of 1.56 Gbps.” Although broadcasters are limited by the capabilities of a 6 MHz channel for home delivery, they are not restricted in distributing a much higher bit-rate within a facility or throughout the country. A 45 Mbps bitstream, for instance, would yield a compression ratio of only 35:1 for the same HDTV signal. “The big questions,” said Kutzner, “are what will be the cost of MPEG splicing and what is the quality going to be like if you can deliver the MPEG stream at 45 Mbps? At 45 Mbps your quality [of the video] will probably be OK through even a second decompression cycle. Broadcasters may have to rethink their implementation plans to accommodate the special problems imposed by MPEG.” And those demands are now being addressed in a SMPTE standard that is expected to be ready for publication early this year. As of this writing, the 25-page document, titled “Splice Points for MPEG-2 Transport Streams,” was in its third draft. The standard’s main focus will deal with speci- fying the different types of splice points and how they will be used in editing program material in an MPEG digital bitstream.