Why we can't drop drop-frame
by PAUL D. LEHRMAN
Drop or Non-Drop? Pull-Up or Pull-Down? Thirty or 29? And what the heck is 23.976? All questions that strike terror into the hearts of post-production audio engineers, sound designers and even composers, all over this great land.
Anyone who works with sound for television has a nightmare story about dealing with this nonsense. I've been collecting them for years. One of my favorites, from the days when analog multitrack ruled the post-production world, involved a studio that striped 29.97 SMPTE on one track and 60Hz Nagra resolve tone on another, and then told the synchronizer to lock to both. The hapless 2-inch machine bucked like a clothes dryer with a bowling ball inside, until the tape snapped.
Even now, in the age of digital, we still can't escape this insanity. I just went through yet another journey through sync hell with a score for a 15-minute film on Beta SP video that was being posted by an expensive transfer house in New York. I told them, I can give you the music on audio CD, on CD-R as a Pro Tools session, on CD-R as .AIFF or .WAV, on ADAT or DA-88, or on timecode DAT. Which one would you prefer? The response: Ordinary DAT will be fine just tell us at what frame of the film to start the audio. I thought this was great; among other things, it saved me the cost to rent a timecode DAT machine (and can someone explain why they are still three times the price of a digital 8-track?).
When I got the videotape back, the audio indeed started right on the money, but it ended about 40 frames after the picture did. For the next three days, the transfer house and I argued about mismatched frame rates. We used up my monthly allotment of cellphone air time, and then some, arguing about how to make the audio and the video come together. They tried different DAT machines, different video machines and different operators. They tried transferring it from DAT to another videotape, and pulled it up and down, and probably in and out as well. Nothing worked. Finally I asked, Is the DAT machine locked to house sync? How do you do that? came the response. I knew all was lost.
I persuaded them to send back the tapes, and I brought them over to a friend with lots of experience in this sort of thing, as well as access to a Beta deck. Everything locked up on our first try. All of the fretting about frame rates was totally unnecessary, and as long as everything in the house was running from the same master clock, there was no problem.
Our equipment certainly has gotten smarter, and for many of us, sync issues are routinely solved by having our computers do the required complex math on-the-fly, thus taking the worry out of it. But there's always that nasty little factor, pilot error: The equipment only works right if it's set up and operated correctly. At one point in the process of scoring this particular film, my synchronizer unbeknownst to me decided to change its default clock value to 30 fps instead of 29.97 non-drop, and suddenly all of my hits were in the wrong place and the MIDI tracks weren't agreeing with the audio. That one took a couple of hours to find, and a couple of years off my life. (And when we couldn't get the first transfer to work, I wondered whether or not it was my fault that they were having problems.)
In Europe, of course, they laugh at us. Their video frame rates are nice whole numbers, and they don't understand how we could be so stupid as to make our video run 0.1% off, or why we spend so much time and energy trying to deal with it. (And, in point of fact, the real frame rate is 29.9700267/sec, but who's counting?)
Frankly, I don't fully understand it either, although it's been explained to me numerous times. I'm not enough of a television engineer (despite my '70s-vintage First Class FCC license, which mostly proves I am good at taking multiple-choice tests, not that I know anything) to really know what it's all about, but I think I get the gist of it. Most people, however, who think they know why the 29.97 frame rate was adopted have it wrong. And it's quite likely that those who do understand it correctly, but believe it was actually necessary, are also wrong. I was first alerted to this by a friend, who at the time was president of a synchronizer company, who pulled me aside at a demonstration of his wares at an NAB show and whispered, You know, all of this B.S. really wasn't necessary. When I asked him to elaborate, he only said, enigmatically, Take a look at the documents.
The version, it seems, that most people know is this: In the early days of television, when everything was shades of gray, the standard frame rate in the United States (and Canada) was 30 frames per second. Because each television frame consists of two fields of alternating lines, the field rate was 60 Hz. When the color standard developed by RCA was adopted in 1953 by the NTSC (that stands for the National Television Standard Committee not, as some of our European friends insist, Never The Same Colour), it was discovered that the new color signals were susceptible to interference from 60Hz AC fields, and if there was any difference between the line frequency and the scan rate of the received signal, then it would show up as a visual beating. But if you altered the field rate enough, then the beating would speed up to the point that it was no longer visible.
Unfortunately, this version is simply not true. Line-frequency beating never was a problem. And if it were, the cure is worse than the illness: At a field rate of 59.94 Hz, if there really was beating caused by AC-line leakage, then you'd see a bar roll across the screen about every 17 seconds. Not pretty.
The real reason, and the one those in the know profess, is this: The lower frame/field rate is designed to prevent potential visual beating between the color (chrominance) subcarrier in the broadcast signal and the audio subcarrier. Why is this a problem, and how does changing the frame rate help? Well, for various esoteric reasons, the color subcarrier frequency in the television signal needs to be modulated onto the picture carrier at 455/2 times the horizontal line frequency. At 30 frames per second, and with 525 horizontal scan lines per frame, this frequency is 15.750 kHz, which means that the color subcarrier would have to be at 3.583125 MHz. The sound subcarrier (as established on the first monochrome TV systems) is at 4.5 MHz. If the two subcarriers were to interfere and beat against each other, then the 916.875kHz difference might be visible; and, in fact, according to one report presented to the NTSC, it was visible in some monochrome sets at the time, under some conditions. Because backward compatibility was a major consideration for the NTSC (and the reason the CBS-developed system was abandoned in favor of the more finicky RCA system), this was a problem.
This report, written by an obscure General Electric engineer, went on to say that if the difference signal happened to be an odd multiple of one-half the scan rate, then this beating would be reduced. If the frame rate was dropped 0.10001%, then the scanning frequency would be 15.734264 kHz, the chrominance subcarrier would be 3.579545 MHz, and the beat product (if there was one) would be 920.455 kHz, which is very, very close to the 117th multiple of half the scan rate. Did you get all that?
But a close look at the technical documents and the committee proceedings around this point seem to show that the problem never really existed. According to Mark Schubin, longtime technical editor and columnist for Videography magazine (to whom I am indebted for leading me to some excellent primary sources in dealing with this issue), there should not have been any cause for concern. Remember, the sound carrier is FM, he told me recently. The frequency swings and is never in its nominal position anyway, and so any beating wouldn't be steady, and therefore not visible.
Another video engineering expert, Tim Stoffel, says that a higher chrominance subcarrier frequency could have been used (495/2 fs), and the audio subcarrier also increased slightly to make the difference signal fall on the right multiple of the scan rate, and despite the change, most [black and white] sets would have tolerated it at the time. However, the TV set manufacturers' association screamed bloody murder, and so the decision was made to leave the carriers where they were (or pretty close) and change the horizontal scanning frequency and, consequently, the frame rate instead. This didn't make the transmitter manufacturers or the stations very happy, says Stoffel, because, It meant much expensive alterations to transmission equipment, as the AC line could no longer be used as a frequency reference for sync!
An engineer who was there at the beginning, Rollie Zavada of Eastman Kodak, diplomatically calls the decision to change the frame rate debatable. Other sources say that the very first generation of color sets, and also the black-and-white sets that were made by the time the color standard was adopted, had good enough filters on the audio section so that leakage between the subcarriers was simply not an issue.
The decision to lower the frame rate was probably, according to Schubin, a political decision more than anything else: I've talked to people who were there at the time who also think it wasn't necessary, but it was several people's entrenched position [to change the frame rate], so others went along with it, because, otherwise, the standard might be blocked forever. RCA was apparently among those not in favor of 59.94.
Thus, as he wrote in his April 1993 magazine column, all of the problems of NTSC's 29.97 frame-per-second rate may have been caused because some 1952 TV sets may have had a problem sometimes, and changing the carrier frequencies was said to reduce (not eliminate) the problem. In the same column, he points to another potential problem interference between stations located some geographical distance apart but on the same channel, which was also thought of at the time to be serious, and which might have been solved by a similar interleaving of frequencies. It turned out not to be an issue at all.
While it's depressing and frustrating to realize that changing the frame rate to an irrational number probably wasn't necessary, what's sadder still is to realize that apparently we're never going to get away from it, although we recently had the opportunity to do so. We are, after all, on the cusp of a new age of television: Digital TV and HDTV (which are not to be confused with each other, although the second is a subset of the first) are already on the air. The original HDTV standards (and there are many) all specified a frame rate of 30 fps.
Progressive, interleaved, 1080, 720, whatever variation you looked at, there was no mention of 29.97 anywhere in the proposals. But the HDTV programs now in production and going out over the air are running at you guessed it 29.97 frames. The FCC mandate for HDTV is incredibly vague and has over the years been increasingly dictated by the broadcasters themselves, which means networks and stations have been free to do just about anything they want with it. And dropping the frame rate is something that came easily.
Mark Schubin puts it this way: The colossal broadcast infrastructure is 59.94 Hz in all NTSC countries. Using 60 Hz would have meant dropping a field [from broadcast programs] every 16.67 seconds. Although devices were built that could deal with that, they were considered too cumbersome. So in the U.S., HD, like NTSC, is now 59.94 Hz-based. And it is likely, no, certain to remain so. What's that line about the sins of the fathers?
As I write this, I am very saddened to hear of the death of Douglas Adams, author of Hitchhiker's Guide to the Galaxy and other brilliant books, essays, radio and television scripts, computer games, etc., not to mention the first human to realize that the answer to the meaning of life, the universe and everything else was 42 and now all we had to do was figure out what the question was.
I knew Adams slightly, having met him at a couple of conferences and book signings, and having communicated with him in the early days of computer music on the PAN network, where his was an active (and very funny) voice. He was an avid amateur musician, and his use of MIDI sequencers and samplers found its way in to his Dirk Gently, Holistic Detective books. He owned some 30 guitars (all left-handed), and on his 42nd birthday joined his good friend David Gilmour and the rest of Pink Floyd onstage to play Dark Side of the Moon. He was a Macintosh fanatic from day one, and I was very proud that, for a while, I was listed on the masthead of one of the Mac magazines under the same contributing editors heading as he. But his true contribution to our industry and indeed the world was as a visionary and an augur, although he would never allow such serious labels to be applied to himself.
He envisioned how the technologies that we develop will soon be so ingrained into the culture that we won't even notice them, and he also saw how silly many of them might end up being. He wrote about computers trying to be helpful that end up putting their owners into life-threatening situations, and a brilliant, super-strong robot with chronic depression (something Adams himself struggled with). Despite all of our advances, he wrote, human behavior is never going to change very much: There will still be plenty of greed, arrogance, selfishness and just plain stupidity to go around. In both his fiction and his nonfiction, he provided a perspective that showed how destructively shortsighted so many of us are when it comes to the long-range survival of the planet and its life-forms (including us). And he inserted into that often deadly serious genre science fiction some of the wittiest and most memorable satirical writing since Jonathan Swift.
As the writings of Jack Kerouac limned the parameters of the Beat generation, and the writings of Abbie Hoffman and Paul Krassner codified the language of the Freak generation, so Adams' writings gave philosophy, vocabulary and attitude to the high-tech generation of the '80s. If you haven't read the five books of the Hitchhiker's trilogy (yes, five) in a while, do go back and take a look at them. They're just as funny, and just as prescient, as ever. (I love the band that's so loud that the sound crew has to be 400 miles away in orbit, and the primitive planet that is being colonized by marketing directors who can't invent the wheel until someone does research to find out what color it should be.) And you'll find that a lot of the terms we use to talk about today's technology were in fact invented although never without satirical purpose by him.
Adams died the way one of his characters would, totally unexpectedly and with more than a little irony: He suffered a fatal heart attack while lifting weights at a gym in Santa Barbara. He was 49. So long, Douglas, and thanks for allof everything.