The Phi Phenomenon was first discovered by Max Whertheimer in 1912. The basis of the theory is that since the human brain can perceive 10-12 individual images per second so the anything faster than this then are mind bends the images together giving us the illusion that the image is moving.
The significance is as this is the maximum frames a human eye can take in per second, so to achieve the illusion of movement then the images need to be faster than this, anything less than this then we would notice and the image will stop blending together. This was the fist frame rate, however with film there is still a flicker.
Undercrank Vs. Overcrank.
In the time before you had mechanical projectors which projected the film you had to do it by hand. The problem of this is because the frame rate was controlled by the person manually cranking the film, you had two issues with this they were undercranking and overcranking. They are:
- Undercrank: this is the process in the older days when you had to make the camera work by hand so this is when record a slower frame rate than the final projection.
- Overcrank: this is similar to under cranking but instead you are recording a faster frame rate than the final projection.
What impact did the introduction of sound have on frame rate?
The introduction of sound into film was one of the most drastic changes in cinema history, the sound was recorded separately and run beside the picture. This meant that the picture had to be very precise and accurate in order to run with sound, the frame rate to match this was 24 fps.
Why did 24 become the international frame rate standard?
With 48 projected frames being the goal, they settled for the next best thing, 24fps with a double bladed shutters which allowed the image to be seen 48 times a second. The reason why it was 24 was because it was easily divided by factors. For example, you know that 12 frames would be half a second, and a quarter would be six and a third would be 8. Another main factor was that the cost of film was expensive and 24 was the lowest number needed for sound to blend with picture.
The issue with bandwidth was that it was different to film but it still had the issue of flickering frames so the idea came from both German engineer Fritz Schroter and American engineer Richard Ballard in 1930 & 1932 respectively. There idea was to interlace the broadcast.
What is interlacing?
Interlacing is the idea of breaking up each frame into two sections, these were known as upper field and lower field. These were then broadcasted on TV in a comb like fashion which stop the flickering. However, the main issue was intermodulation. This is when there is almost like a wave going down your screen, this was caused by the electrical current generated from the TV. The US standard power rate was 60Hz, this equalled 60 fields per second, and this in frame rate equals 30fps.
How was the challenge of intermodulation tackled?
The problem with intermodulation was that a beat in the picture would create a wave which ran down the screen, to tackle this problem they set the refresh rate to that of the power of the TV. For example, in the United States this was 60Hz.
What was the significance of 60Hz and how does it relate to 30fps?
60Hz is the power generated from the average TV in the United States and this means you see 60 fields per second on your TV screen, this then results to the full 30fps and since this was over the standard of 24fps people were able to watch TV more clearly.
VHF Vs. UHF
In 1948 the Federal Communications Commission (FCC) put a temporary ban on TV licenses as it tried to figure out what to do with the new UHF spectrum. This represented the bandwidth to run colour into TV, this was anything from 300-3000MHz. this different from the VHF system, this was simply what TV had already been running on and it was from 30-300MHz.
How was the colour standard arrived at?
The National Television System Committee (NTSC) and the RCA combined together to try solve the issue of finding a colour standard that was compatible with the old TV sets. They used a theory first used by George Valensi in 1938. Valensi idea was to break down the colour picture into luminance and chrominance, this meant that broadcasters could embed the colour as a sub carrier in the television signal. What this meant was that new colour TVs could pick up this signal and broadcast them where the older black and white TVs would jest ignore it. This meant that colour television and black and white television were separated.
What challenge did bandwidth present to achieving colour standard and how was this problem overcome?
The only problem with new coloured television’s was that the bandwidth used by the new TVs standard could potentially interfere with the audio signal creating a mismatch between picture and audio and create intermodal beating. This problem was solved by reducing the framerate by 0.1%, so that the two signals would never fully match up.
What was the fields per second ratio that was eventually developed as the standard in colour and what was the resulting frames per second ratio?
The fields from second went from 60 to 59.94 frames per second for the standard in colour. The resulting frames per second went from 30 to 29.97fps. This helped created a two separate signals for colour television and black and white television.
What is PAL and why was it developed?
PAL stands for Phase Alternating Line and this is the European equivalent of the NTSC and they were created in 1967 by the German Broadcasting Authorities. PAL was created as they wanted to solve the colour gradient problem that NTSC was having by using a 50Hz power outlet rather than the Americans 60Hz.
What are fields per secod and frames per second of PAL & SECAM?
Both PAL and SECAM both run at 50 fields per second which then converts to 25fps.
How we get from 24fps to 60i video stream.
- Firstly, the 24fps is slowed down by 1%. This is done by timesing 24fps by 99.9% which equals to 23.976fps. Simple maths then makes us realise that you need 4 sets of 23.976fps to fit into 5 frames of 29.97.
- Secondly, you convert this fraction into a 3:2 Pulldown. Imagine the film strip is set as four letters: A, B, C and D. First of all A would be split into 3 with it being in the upper field, lower field then upper field again. Then B would be split into 2, one being on the lower field then the upper field. C would then follow what A did and fill up 3 fields but this time it would be lower, upper and lower. And then D would then fill the next two, this then replicates the 3:2 pulldown I mentioned. This is not perfect however as the screen would cause telecine judders every 3 frames.
- Finaly, to counteract this effect we would have to use the reverse telecine. This a technology that worked backwards to construct a true 23.976fps into a 60i video stream.
How do modern Digital Cameras avoid the telecine progress and with what affect?
Modern digital cameras avoid the telecine progress altogether by recording 23.976fps directly on the hard drives, this was effective as you did not have to go down the 3:2 pull down method.
How are 24fps films telecine’d onto SECAM or PAL 25fps?
This process is much simpler than the 3:2 pulldown method as all you have to do is use a 2:2 pulldown. This means that you have to do is times the 24fps by 4% this results in the frames being translated onto an upper and a lower field. However the increase raises the pitch of the audio by a noticeable 0.679 semitones, this can be counteracted by a pitch shifter.
Explain High Frame rates and temporal resolution?
What are the issues with higher frame rates in narrative filmmaking?
https://www.youtube.com/watch?v=mjYjFEp9Yx0
No comments:
Post a Comment