24fps film & sound-on-film

[JaredSMark]

Way back when, on top of the obvious reasons they settled on 24fps as the standard (cost of film), did they settle on the 24fps frame rate since it was pretty much the least speed they could run tape when recording sound-on-film to retain high frequencies? At least, is that one of the reasons? I also heard it may be due to the noise floor.

[j1clark@ucsd.edu]

Way back when, on top of the obvious reasons they settled on 24fps as the standard (cost of film), did they settle on the 24fps frame rate since it was pretty much the least speed they could run tape when recording sound-on-film to retain high frequencies? At least, is that one of the reasons? I also heard it may be due to the noise floor.

At the time that sound was introduced, there was no 'tape' recording. Well, in general. 'tape' recording was a post WWII development. At the time of the first 'talkies', the two methods that have been somewhat 'proven' were recording mechanically to disk, similar to recording phonograph records, and optical recording on film. 'recording on film' won out for a number of reasons.

Hence, 24 fps, was 'found' to be the lowest speed to provide the 'best quality audio recording', while recording the film image.

There may have been other reasons why 24... as opposed to 25... but it was sort of a cost/benefit type choice.

[Richard Crowley]

Sound was not recorded ON film (the same film or different stock) at the time that 24 FPS was established as the standard speed. As Mr Clark says, sound was recorded on transcription disks (phonograph records) which had their speed synchronized to the film speed.

24 FPS was selected for visual reasons, the slowest (=least expensive) speed that would work with the POV (persistance of vision) of the average viewer to produce the illusion of a moving picture. Of course technology has allowed us to raise the frame rate for electronic media and higher rates are used for premium production and exhibition.

24 frames per second was chosen because it was the slowest (and thus cheapest) speed which allowed for sufficient sound quality.
http://en.wikipedia.org/wiki/Film#Technology

The first half of this sentence is correct, but the last part is completely fanciful and bogus. Note that WIkipedia has flagged this article for multiple problems, including sources and atrribution.

This article has a better explanation of frame rates and appears more factual: http://en.wikipedia.org/wiki/Frame_rate

[Noiz2]

As mentioned above "tape" was first developed during WWII by the Germans. After the war a soldier shipped back two of the machines (prototypes?) and formed AMPEX. Sound was recorded to optical tracks on 35mm film. I believe this was a second "sound camera" that recorded the optical track and then this was optically printed back to the release print. Originally sound was on records that played back in sync to the film, kind of like SDDS does today. But sync was hit and miss and optical tracks printed on the film became standard.

I also remember something about 24 being the slowest speed with acceptable wow and flutter. I think that is correct since silent films were shot at around 19-20 FPS. Generally hand cranked so speed was a bit flexible. One of the reasons most people think of silents as jerky films is because most have only seen silents played back incorrectly at 24fps so everything becomes a bit herky jerky.

With sound you needed a stable speed and picture and sound needed to run at the same speed to stay in sync so film sped up to meet the needs of sound. Probably the last time picture changed to accommodate sound ;~).

[JaredSMark]

https://vimeo.com/3388256 - at 1:14 he mentions, "audio engineers determined the absolute minimum speed they needed to optically record synchronized sound was 24fps". So it didn't have much to do with sound quality, but more about the synchronization issues? I'm wondering exactly how this synchronization works and why 24p was the lowest they could use.

I was under the impression that recording 'faster' on tape gave you a better frequency response (kind of like digital sample rate), so that's why I guessed that when the article said "minimum speed that allowed for sufficient sound quality", they were talking about frequency response (quality).

[BRH]

24 FPS was selected for visual reasons, the slowest (=least expensive) speed that would work with the POV (persistance of vision) of the average viewer to produce the illusion of a moving picture.

I believe this is correct. 24FPS with a standard 2 bladed shutter would be 48 interruptions per second... just at the point where you eye/brain starts to notice the flicker. Has nothing to do with sound.

[j1clark@ucsd.edu]

I believe this is correct. 24FPS with a standard 2 bladed shutter would be 48 interruptions per second... just at the point where you eye/brain starts to notice the flicker. Has nothing to do with sound.

The silent films ranged from 14-24 fps. This was set in stone with the introduction of sound, since the sound recording/playback was 'better' than when 20-24, 'common enough' silent speeds, were used. The actual film transport rate was set at 18 in per second.

I'm pretty sure that if the 'studios' at the time could have gotten away with 16 fps 'sound', even with 6 bladed shutters... they would have done it... that would have been 66% less film to buy at any stage of the film making process.

There are two basic ways that sound was recorded optically. 1) varying the density of the optical track, 2) varying the width of the optical track. The variable density option was the first used. This consisted of a light controlled by the 'audio' signal, and a slit opening that exposed the negative film media to the light.

If an 'audio signal' varies faster than the transport moves the film, the 'slit' acts as an integrator, and as such is a 'low pass' filter. Therefore the lower the speed, the lower the cutoff frequency of this low pass filter. For 'reasonable' sound reproduction, one would want to have 5-10 KHz worth of 'bandwidth', and so, the 'fastest' film transport that would deliver that, would be the 'better' choice, than a slower speed.

At the time in the US 24 fps was the standard 'top end'. There were demonstrations of higher speed, but at that rate, given, cost of film, exposure time required for the visual image(*), exposure time for the 'audio' optical track, 24 seems to have been the 'best' solution.

I still don't know 'why 24' as opposed to 25... but it may be that the perception of motion starts about 12 fps, so 24 is 'double' that. But still some number of silent films where not shot at 24 fps. However blades in the projectors did chop the image to smooth it out.

*What seems to escape some people is that for a given ISO value, doubling the film transport speed, halves the exposure, and therefore requires double the light for the same resultant density... then in film film there's 'reciprocity' failure when this 'simple' law fails, and even more light is needed than a simple doubling...

In the case of 'optical audio recording' this would correspond to faster transport, would require a doubling of the light until reciprocity failure took over, and requiring even more light...

[Noiz2]

Well as you can see this is a little like asking what ADR stands for.

I'm not sure about the 48 frame thing. In science classes we were taught that it takes 1/4 of a second for the eye to register an image. Certainly they were getting smooth motion with 16 fps so?

The sound quality issue has to do with speed but also the way a projector works. There is a loop that goes through the plate and is projected. The film jumps frame to frame so the "audio head" is located elsewhere where the transport is smoothed down. This is where the speed comes into play. Even then sound wasn't at a frame rate but at a speed.

As Richard mentioned, originally the recordings were on platters. But once things standardized recording was on optical recorders. Sync is simple. All the machines move sprocket for sprocket together. If you have a start mark you are mechanically locked and always in sync. For release all standard films were released with an optical track that was part of the film so sync was never a playback issue.

Yes in theory faster speed in analog tape gets you better fidelity. But a lot also depends on things like bias and noise reduction. This is kind of moot though when your talking about optical tracks, especially early mono ones. No noise reduction and the Academy curve eliminates pretty much anything over 10K and below 100hz. So speed doesn't get you much.

[Rick R]

I believe this is correct. 24FPS with a standard 2 bladed shutter would be 48 interruptions per second... just at the point where you eye/brain starts to notice the flicker. Has nothing to do with sound.

Yes, that and because the US electricity is at 60Hz. In some other parts of the world (the UK for instance) is @ 50Hz. and the FPS is 25.
FYI, the confiscated WWII era German tape machines were know as "Magnetophone".

[Noiz2]

Yes, that and because the US electricity is at 60Hz. In some other parts of the world (the UK for instance) is @ 50Hz. and the FPS is 25.
FYI, the confiscated WWII era German tape machines were know as "Magnetophone".

Well kind of, but your mixing video and film. 35mm film is 24 everywhere PAL video is at 25fps (because of 50Hz power) and NTSC was 30 fps (for the same reason). The 30 got bastardized into 29.97 to accommodate color (and because some political maneuvers, we were going to have something very similar to PAL).

Film was never synced to the AC freq.