Frames, Lines and Changing Times

Somewhere in your camera menu sits a number that makes no sense at all. 29.97 frames per second. Not 30, not 29, but a slightly apologetic 29.97 that looks more like the total on a lunch receipt than a technical standard. The story behind it stretches back nearly a century and explains most of the odd settings in your editing software.

Television did not arrive fully formed. In the 1930s John Logie Baird's largely mechanical system went head to head with Marconi-EMI's all electronic 405 line system, and in 1936 the BBC ran both on alternate weeks from Alexandra Palace. EMI won, Baird was sent home, and Britain stayed with 405 lines right up until 1985. The full saga is over in who invented TV.

In 1941 the US settled on 525 lines, and after the war most of Europe went with 625. More lines meant more detail but also more bandwidth, which was always in short supply. Only around 480 of those American lines and 576 of the European ones actually carried picture, with the rest tucked away for timing signals. Those two numbers still haunt us today as the standard definition resolutions of DVDs and old tape formats.

The frame rate came from somewhere far less glamorous - the electricity coming out of the wall. American power runs at 60Hz and European at 50Hz, and early TV sets were so leaky that any mismatch showed up as hum bars rolling slowly up the picture. Studio lights flickered in step with the mains too, so locking the picture to the power supply kept everything steady. That is why North America ended up with 30 frames per second and Europe with 25, a split we are still living with.

Even those rates were too much for the technology of the day, so engineers cheated. Each frame was split into two fields, first the odd lines and then the even lines, giving 60 or 50 pictures a second of smooth motion for half the bandwidth. Your eyes blended them together and nobody complained, much like a perfectly hidden edit in a podcast. That little trick is the i in 1080i, and it will come back to bite us later.

Then came color, and a very awkward problem. Millions of people already owned black and white sets, and in 1950 the US briefly approved a CBS color system that those sets could not display at all. It flopped, and by 1953 a new NTSC standard had cracked it by keeping the black and white picture intact and hiding the color on a subcarrier. Old sets simply ignored the extra signal, while new sets used it to paint the picture in glorious color.

Here is where our mystery number arrives. That color subcarrier clashed with the sound carrier and created an ugly visible pattern, and nobody wanted to move the sound and break every TV in the country. So engineers nudged the frame rate down by one tenth of one percent instead, turning 30 into 29.97. It fixed the picture but left editors with drop frame timecode, which skips the odd frame number so your clock does not drift by 3.6 seconds every hour.

Europe watched all this and decided to do it better. German engineer Walter Bruch at Telefunken came up with PAL, which flips the color signal on every other line so that errors cancel each other out. American sets were famous for drifting skin tones, earning NTSC the nickname Never The Same Color, while PAL viewers rarely had to reach for the tint knob. France, never one to follow the crowd, went its own way with SECAM, which also spread across much of Eastern Europe.

For half a century the screen itself barely changed shape. TV was 4:3, almost square, while cinema had gone wide in the 1950s to lure audiences away from the box in the corner of the living room. In the 1980s SMPTE engineer Kerns Powers proposed 16:9 as a clever compromise, wide enough to hold most movie formats with only modest black bars. Getting broadcasters to actually make the switch, though, took the best part of twenty years.

I watched that switch happen from the inside. Around 2003 I was hosting on a live UK shopping channel from Studio 3 at Thames Television, and a brand new control room had been built from scratch. It was designed not just for shopping TV but to house a wall of shiny widescreen monitors ready for the 16:9 future. The old control room was far too small for all that, so it was quietly retired and turned into a dressing room, which tells you everything about how big the jump to widescreen really was.

Of course viewers at home were not all ready at once. Widescreen shows on old square sets got letterboxed with black bars top and bottom, square shows on new widescreen sets got pillarboxed with bars at the sides, and sometimes the picture was simply cropped and panned around to fill the frame. Europe went widescreen while still in standard definition, while much of North America made the jump at the same time as HD. And if you think 16:9 is wide, take a look at a 57 inch 32:9 monitor that makes 16:9 look positively timid.

Widescreen and high definition really took off together once broadcasting went digital. The US switched off its full power analog transmitters in June 2009, and the UK finished its digital switchover in 2012. Digital gave us 720p, 1080i and 1080p, and suddenly the interlacing trick from the 1930s was back on the spec sheet. Broadcasters loved 1080i because it squeezed into less bandwidth, but it was never the gold standard.

That title belongs to 1080p, which is 1920 pixels across by 1080 lines down, drawn all in one go rather than in two halves. That is just over two million pixels per frame, and it is the default on most cameras and phones in home studios today. Even so, look closely at your frame rate options and the old ghosts are still there - 25 and 50 for European heritage, 29.97 and 59.94 for American. The plug socket is still quietly running your timeline.

Which brings us to converting between them, and the headache that is film. Movies are shot at 24 frames per second, which does not divide neatly into 60 fields, so engineers invented 3:2 pulldown. Every four film frames get spread across ten video fields by holding one frame for three fields, the next for two, and so on. It works, but it adds a slight judder to slow camera pans that eagle eyed viewers still grumble about.

Europe took the lazy route. Rather than mess around with pulldown, broadcasters simply played 24 frame films at 25 frames per second, making every movie run about four percent faster. A two hour film finished nearly five minutes early, and unless the audio was corrected, every voice and music cue was pitched up slightly. So if a favorite film score always sounded a touch brighter on British TV, now you know why.

Converting live TV between American and European standards was even messier. Early standards converters were little more than a camera pointed at a TV screen, with predictably blurry results. By the 1980s and '90s, motion compensated converters were tracking movement across frames to build convincing new ones. They cost a fortune and lived in racks at broadcast centers, well out of reach of anyone with a spare bedroom studio.

Today that same job happens in software on a laptop. Editing tools use optical flow and AI frame interpolation to invent in between frames and convert 25 to 29.97 in minutes. Modern TVs with 120Hz panels can even show 24 frame film without any pulldown at all, since 24 divides into 120 perfectly. The same trick powers the motion smoothing setting most people switch off, which I covered along with every other confusing menu option in The Big Picture.

Streaming has finally cut the cord between your picture and the power socket, yet those old frame rates refuse to retire. Every time you choose 25 or 29.97 in your camera, you are making a decision that traces back to hum bars, flickering studio lights and a sound carrier nobody dared to move. So next time a client asks for 29.97 drop frame, you can nod wisely and tell them it is all the plug socket's fault. Now that is what I call a format with history.