What's in your pocket protector?
By Paul McGoldrick
There is magic in a child's face when you talk about how things were in the “old days.” Although vinyl record sales are in the ascendant today — dwarfing cassette and minidisk sales, for example — most children have never seen a phonograph in any of its many forms.
It is the same within our industry. A technician or engineer trained in the last ten years probably cannot imagine a tubed oscilloscope on which — even with magic delay triggering from Tektronix — one could not look at an individual line of video; or the early days of UHF transmission, when the pathetic VHF panoramic receiver (we didn't yet call them spectrum analyzers) we had was made even worse by the connection of a downconverter at the front end. Yes, there were displays there somewhere, but which mirror image was the real spectrum we were supposed to be tuning? On a good Sunday (such work was always done on a Sunday, when there was no programming until the evening) you might see eight completely separate spectrum displays. We had no idea whether the downconversion and the doubtful filtering in the panoramic receiver were really making the whole thing just a complete lie.
In those days, too, the most significant question on a promotions-review panel would be to ask a candidate to define quadrature distortion. But we were proud of our newly found knowledge of that and ICPM, even though we couldn't really measure either.
There was one high-power VHF transmitter that would always fail its annual performance tests. It was reliable, but once a year we had to retube the aural side and the video modulator. Then some bright spark decided to question the validity of the test equipment…
But time and again the boys in Beaverton came to the rescue with boxes that defined what we could measure and, even if we were not too sure the results were real, at least the commonality of product made it as if we were all on the same playing field. By and large, measurements became extremely stable, and you were able to accept both program circuits and equipment as being to specification.
It is always rather amusing, then, to see subcarrier phase and gain numbers in semiconductor products like operational amplifiers quoted to three decimal places. I don't know of anybody in our industry who would claim that this could be done. When you talk to the manufacturers of these products the answer is usually in the form of, “Oh, we connected twenty amplifiers in series and then divided the numbers we measured by twenty.” Right. Even if there was a consistent relationship between parts, there has been experiment after experiment showing that you cannot just add differential phase and gain numbers.
I was recently pleased with the performance of a product I thought was going to turn out to be a toy — a tone generator and cable tracer from B+K. The generator puts out a warbling tone at +3dBm (into 600Ω), and the tracer (a very high gain amplifier) can find the cable that tone is on when you are within about nine inches of it. It really works! And it saved having to replace a long multi-core cable. I just had to make a simple splice.
That product is delightfully simple. But so are many of the test products available to us today. It hasn't been that many years since you would have had to go to driver's ed to be qualified to drive a spectrum analyzer; now most of them do a lot of the thinking for you, and the limitations are greatly reduced. The front ends of digital oscilloscopes now have impressive bandwidths, and the displays have a truly reassuring analog look!
We also have waveform monitors/vectorscopes that allow for accurate measurements rather than just acting as operational guides (although I still see a lot of rackwide test equipment even in relatively modern engineering areas). And the computer is fast becoming the mother ship for a lot of test and measurement activities, and may just take over as a picture monitor for test displays.
But whether you are measuring differential phase in your still-analog system, or you are monitoring multiformat SDI material, there are things that don't change. The tools have advanced dramatically — because they have had to keep up with the technology, and real competition came along — but the purpose of the tools remains the same: to measure and promote quality. Today's buyers of vinyl disks would no doubt be difficult people to argue with about audio quality, but we have come a great deal further than that.
Paul McGoldrick is an industry consultant based on the West Coast.
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