In Part 1 of this series on timecode we looked at the rough history of SMPTE timecode use in professional audio including basic concepts of synchronisation and also some of the different formats such as LTC, VITC and BITC. In Part 2 we will now begin to look at the origins of standardised frame rates in film.
To recap, a frame rate is a number that corresponds to the amount of still images we see in a single second of motion picture. The human brain can itself perceive between 10 – 12 individual still images per second – once we go faster than that the images blend together in our brains and give the illusion of motion.
However in projected film if we just saw the uninterrupted rolling images changing before our eyes the movement of the film roll whilst it was changing between frames would create a blurry mess spoiling the intended effect.
Maintaining The Illusion
To maintain the illusion utilising the mechanics of rolling film and projectors meant that between each still frame a revolving shutter would block the projector lamp creating a period of ‘blackness’ whilst the film reel moved to the next frame*. This way each still image maintained its fidelity.
*The black frames created by the projector shutter between each frame of video are described as flicker. This effect can still be observed on very early projected films.
This charming instructional video from 1936 shows a revolving shutter in action.
Now I mentioned earlier that the threshold for our brains to get fooled into this motion picture illusion is around 12fps, however projecting at 12fps with its accompanying flicker is quite distracting and would be deemed intolerable to paying movie theatre audiences so a solution had to be found.
The Persistence Of Vision
Cue inventor extraordinaire and movie pioneer Thomas Edison who theorised that the human eye needed to see at least 46 still images a second for the black frames in between caused by the rotating shutter to become unnoticeable. The effect achieved is sometimes referred to as the ‘persistence of vision’.
Whilst running a camera and/or projector at 46fps would have been possible, it was prohibitively expensive due to the greatly increased costs of film stock required. At the turn of the century some clever projection engineers soon discovered that this increase in projector frame rate could be achieved by showing the same frame of film multiple times before switching to the next frame. The circular shutter was modified to have multiple holes instead of just one that when rotated would allow the frame to be quickly projected on and off 2 or 3 times depending on the hole or blade count of the shutter wheel before switching to the next frame of film.
Playing back a 16fps film with a triple bladed shutter gave an effective projected frame rate of 48fps which was just over Edison’s recommendations which is why 16fps became one of the first standards used in silent movie production.
I use the term ‘standard’ very loosely here due to the hand cranked nature of early cameras and projectors with early silent films coming in anywhere between 14 – 26 fps. This allowed for creative expression by the directors of the era but also exploitation by movie theatres who would often intentionally ‘over crank‘ or speed up the frame rate of the movie to squeeze in extra showings at the end of the day.
'The Talkies' Take Over
With the rise in popularity of sound in movies variable frame rates soon became a thing of the past with audiences not willing to tolerate the fluctuations in pitch associated with such practices. So, in 1929 for the first time in motion picture history a proper international standard frame rate of 24fps was adopted.
24fps was chosen as it allowed for compatibility with double bladed shutter systems giving an effective 48fps projected frame rate whilst being fast enough to maintain sufficient audio fidelity of the accompanying synchronous optical audio track which was printed on the film stock itself. 24fps is also divided easily by 2,3 and 4 meaning editors could more easily calculate frame and time relationships (with half a second = 12 frames and so on).
Faster frame rates such as 32fps would have been more expensive whilst slower rates such as the existing 16fps did not allow for good enough audio quality on the optical track.
Is High Frame Rate (HFR) The Future Of Film?
The look of 24fps as our format for film and its associated characteristic motion blur is now so ingrained into our psyche that there has been much debate and outcry surrounding a new breed of HFR cinema releases such as Peter Jackson's 'The Hobbit' trilogy (shot and theatrically released at 48fps) as to whether or not they can even be described as ‘film’. With popular video sharing services, such as YouTube, now allowing HFR content to be streamed natively it can only be a matter of time before it gains more popularity in movie theatres - but only time will tell.
Join us in part 3 where we’ll look at more modern timecode frame rates including broadcast television and explain why a decision not to annoy a small portion of black and white television owners in the 1950's has led to some complex frame rates and workflows.