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Measuring SDI

To the editor,

While reading your magazine, I was impressed with the high level of knowledge people such as Michael Robin have in regards to new technologies. I remember that EIA/TIA 250-C was clear on how to measure the video and audio signals after a long-haul or short-haul transmission path. Now that video compression techniques such as MPEG-2 are currently used to transmit the digital audio-visual signal from point A to point Z, the use of EIA/TIA 250-C is no longer valid. This is mainly because the interfaces are now bit-serial digital video and audio as per SMPTE 259M-C and SMPTE 272M. Does it make sense to measure the SDI signal at the output of the MPEG-2 decoder? Would the MPEG-2 decoder ‘create’ a brand-new SDI stream? How do we measure transmission-path problems at video level (SDI) over MPEG-2 networks? Is it only possible to check TR 101 290 at the DVB-ASI level? These questions have been around our facility for some time now, and nobody seems to know the answer.

Eduardo Gudis of Videotek responds:

It makes sense to measure SDI signals on the output of an MPEG decoder, but you should look at MPEG-2 encoding/decoding errors, such as macroblocking, frame dropping and audio-video lip-sync. The MPEG-2 decoder should in theory replicate what was fed into the MPEG-2 encoder at the source. But, because MPEG is a compression algorithm, that isn't always the case. Transmission-path problems can cause timing errors when MPEG-2 packets arrive and that causes PCR jitter. If too much PCR jitter is present at the decoder end, it may lose its ability to lock to the video/audio signals of a specific program present on the MPEG stream. At the DVB-ASI level, the TR 101 290 defines the important measurements, including PCR jitter.

Economic patriotism

Mr. McGoldrick,

Thank you for the editorial “Cutting Corners” in the February issue. I agree with you, and your article reminds me of an old saying, the gist of which is “penny-wise — pound-foolish.” I also coined the rhetorical question, “Do you know how many people die each day because someone assumed something?”

Ironically, in this same issue is a quote from Glen Sakata, director of sales for Harmonic, which says in essence, “Why employ 20 people when automation can replace 18 of them?” While I do not advocate returning to the days of one tech for each piece of equipment the station owns, I like what one former presidential candidate said: “There needs to be an economic patriotism in this country. If you can still make a profit, keep your shareholders happy and save some jobs, more American companies ought to consider doing that.” My prediction is that one day technology will bring us to the point where master control and ingest operations will be farmed out to a company overseas that can hire techs for far less than the kingly sum of $7 that most U.S. non-union stations pay.
Charlie Farr

Paul McGoldrick responds:

I love the idea of “economic patriotism”! Watching the export of technical jobs just to make a marginal difference to the bottom line is depressing. Although I'm sure you're right that master control could be run from somewhere like India, one would hope that owners will realize that the security of their operations is as important as continuity switching. One can imagine the FCC's reaction to hackers hijacking ABC.

I read an interview with Marc Andressen in which he said that the history of U.S. business is to replace jobs with new industries and opportunities. That has always happened, he said. He couldn't say what those new opportunities would be or how people could be trained for them. But it would be nice to think that he was right.

February Freezeframe:

Q. Name the nonlinear AV workstation introduced by Panasonic at the 1995 NAB.

A. The Panasonic WJ-MX1000 Postbox


Tim Costley
Guy St-Arnauld
Bobby Saggu