24fps because of high frequencies?

[invalidusername]

Originally Posted by HSLand

Way back when, 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 think i've read somewhere that 24 fps was first chosen because it was cheaper than 25 fps, and produced a good enough frame-rate. film is very expensive... it's as simple as that

we got used to it i guess.

[HSLand]

A lower frame rate was chosen because of the price, that's correct. But I've also heard that audio engineers decided 24fps was the least they could have for optimal sound quality when recording sound-on-film. My question is WHY 24fps was the least audio engineers would settle for. My guess is getting those higher frequencies.

[Branko]

Running at 24 fps allows better movement reproduction. Less flicker. Has nothing to do with sound.
And, btw, it is faster than most of the tape machines were at 15 ips.

[georgia]

What Branko said...

It had nothing to do with sound. It's all about the least amount of footage and still make a bunch of still frames look like fluid movement.


A nice simple explanation from Wikipedia...

The human eye and its brain interface, the human visual system, can process 10 to 12 separate images per second, perceiving them individually. The visual cortex holds onto one image for about one-fifteenth of a second, so if another image is received during that period an illusion of continuity is created, allowing a sequence of still images to give the impression of motion. Early silent films had a frame rate from 14 to 24 FPS which was enough for the sense of motion, but it was perceived as jerky motion. By using projectors with dual- and triple-blade shutters the rate was multiplied two or three times as seen by the audience. Thomas Edison said that 46 frames per second was the minimum: "anything less will strain the eye." In the mid- to late-1920s, the frame rate for silent films increased to about 20 to 26 FPS. From 1927 to 1930, the rate of 24 FPS became standardized for 35 mm sound film; a speed of 456 millimetres (18.0 in) per second. This allowed for simple two-blade shutters to give a projected series of images at 48 per second. Many modern 35 mm film projectors use three-blade shutters to give 72 images per second—each frame flashed on screen three times.


cheers
geo

[Mundox]

Museum of moving image in Queens NY has cool things that physically demonstrate how frame rates' effect our perception of images.
That museum is an under-appreciated gem.

[Etch-A-Sketch]

I was always taught it was because of the sound as well. 18 and 20 fps look just fine and aren't distinguishable from 24fps. But the sound/noise of the tape between 18fps and 24fps is noticeable.

The speed of magnetic tape dictates its signal to noise ratio (among other things). The slower the tape, the noisier it gets. Each channel adds more noise. So a 4 track mag would be twice as noisy as a 2 track mag and 4 times as noisy as a mono sound track. To get the SNR of a 2track at 7.5ips, you'd need to run 4 tracks at 15ips. Which 24fps (18ips) close.

there is a reason why you run a 2in 24 track deck at 30ips (if you don't have dolby A or SR) and why you can get away with running a stereo 2track tape machine at 7.5ips. When you sum the tracks the noise from each track sums as well.

Reel-to-reel audio tape recording - Wikipedia, the free encyclopedia

Also with tape, frequency response is effected with its speed. I don't think this was a major factor for films since even 7.5ips can reproduce up to 20KHz, but maybe?

media_2: analog tape characteristics

[Rick Sanchez]

24 frames had nothing to do with sound quality other than finding a standard frame rate that looked good.

There were a number of frame rates being used in silent pictures, but when the added sound, if it played at the wrong frame rate, the audio pitch change was very noticeable, so they decided to pick one that worked well, set a standard and go with it.

Or so I've been told.

[minister]

Just to add, even though Branko provided the answer 700 posts ago:

High Frequencies....

Motion Picture Sound Engineers of yesteryear wrote about anything above 8kHz as superfluous. Furthermore, The Academy Curve -- which is also known as the Normal Curve -- is defined as flat 100 Hz - 1.6 kHz, down 7 dB at 40 Hz, down 10 dB at 5 kHz, and down 18 dB at 8 kHz!

[ggegan]

Originally Posted by minister

Just to add, even though Branko provided the answer 700 posts ago:

High Frequencies....

Motion Picture Sound Engineers of yesteryear wrote about anything above 8kHz as superfluous. Furthermore, The Academy Curve -- which is also known as the Normal Curve -- is defined as flat 100 Hz - 1.6 kHz, down 7 dB at 40 Hz, down 10 dB at 5 kHz, and down 18 dB at 8 kHz!

The Academy Curve was noise reduction in the form of a monitor rolloff for minimizing optical noise. The idea was that mixers were supposed to push the frequencies that were rolled off to compensate. That's when you had to EQ the hell out of everything, no matter how well it was recorded. I mixed my share of Academy Curve mono deliveries.

[Tom Fleischman]

Originally Posted by ggegan

The Academy Curve was noise reduction in the form of a monitor rolloff for minimizing optical noise. The idea was that mixers were supposed to push the frequencies that were rolled off to compensate. That's when you had to EQ the hell out of everything, no matter how well it was recorded. I mixed my share of Academy Curve mono deliveries.

For mono theatrical releases, an Academy filter was applied to the monitor in the mixing stage. We mixed through the filter, pre-emphasizing the track with EQ.

Cross modulation distortion and poor developer application was what always gave me the biggest headaches with optical soundtracks. The piss-poor dynamic range limitations were a real bummer at times as well.