technical aspects of digital video
TRANSCRIPT
Technical Aspects of Digital Video
What is Digital Video?● Digital video is very similar to film. It is simply a series of
still images.● The still images are played back quickly enough to
make objects on screen appear to be moving just like they do in the real world.
● The only difference is that the images are captured on fields of digital pixels rather than a frame of film.
Pixels● Pixels are tiny rectangles that contain three lights--Red,
Blue, and Green--and can produce about 16.8 million colors using different intensities of the three.
● They capture light, and save it as a single color. ● Each image is made up of thousands of pixels.● 1920x1080 is a form of HD, also known as 1080p.
○ This means each frame of video contains 1080 horizontal rows, each containing 1920 pixels.
○ So when you watch HD video, you are seeing 2,073,600 pixels in every frame.
Pixels
12x16
UHD
Aspect Ratios● 4:3 → Standard SD, most TVs used to
be this.● 16:9 → Standard HD, most TVs are
now like this.● 1.85:1 → Most Theatrical showings
since the 1960s.● 2.35:1, 2.39:1, 2.40:1, etc. →
Widescreen.● Everything Else → Weird, rare stuff.
4:3
16:9
CSI: Season 1 (‘00) & Season 15 (‘15)The SImpsons: Season 3 (‘91) & Season 23 (‘12)
Down Converting● When stations receive HD video, they have to convert it
to a format that can be displayed on SD screens (640x480 and 4:3).
● The most common ways that stations convert are by letterboxing, cropping, or squeezing.
Letterboxing● Letterboxing is where the
HD video is fit to the horizontal dimensions of the 4:3 SD screen. The problem with this technique is that you sacrifice even more resolution than you have to by not using all of the rows of pixels.
Cropping● Chopping the edges off to make the image fit.
Cropping (Cont’d)● Cropping can also be used to “up convert” old 4:3
video in order to make it fit a full 16:9 screen. ● Netflix and FX have both been caught doing this.● The problem is that you lose part of the picture, and
also lose some of the already inferior quality of the SD video.
The Simpsons on FXX
Original On FXNow.com and FXX
Examples of Netflix Cropping
Squeezing● Literally squeezing the image together.● This causes the original image to be distorted.
Analogy● Down Converting is like buying a couch, getting home, and realizing it is
too big to fit in your living room. You have three options:1. Return the couch and buy a love seat (Letterboxing)2. Saw off six inches of the couch from either end (Cropping)3. Hit the couch with a sledgehammer until it fits (Squeezing)
● No matter what option you choose, you will not have the same couch you wanted originally because your living room is too small. With an SD video feed, you can watch things that were shot in HD, but it will never be the same as having an HD TV.
Title Safe
Title Safe (Cont’d)● Different types of TVs display different amounts of the
information sent through them from the signal.● Everything inside the innermost box will definitely be on
viewers’ TVs and is considered “title safe.”● Everything inside the next box will show up on most
viewers’ HD TVs and is considered ‘action safe.”● Outside of both boxes is called “overscan,” and
probably won’t show up on most consumer televisions.
Frame Rates● A frame is a single still image.● The frame rate describes how quickly still images are flashed
on the screen to create a video.● Frame rates are referred to in frames/second, or fps.● Our brain can perceive ~10-12 fps as individual images, but
anything faster than that looks like motion.● Film cameras and projectors both originally required the
operator to hand crank the film through them, so early film frame rates were often uneven and varied from one showing to the next.
Frame Rates (Cont’d)● With the addition of
sound, frame rates had to be standardized to make sure sounds and actions always matched.
● 1929 → 24 fps becomes standard, and still is today.
Digital Frame Rates● When the US adopted the
NTSC broadcasting standards in 1941, it was not technologically feasible to play video on TV at 24 fps.
● The way films are projected, each frame is projected twice to avoid flicker, but the TVs of the time were not capable of that.
Digital Frame Rates (Cont’d)● The solution was to create two fields of alternating pixel rows
that -- when combined -- would make up one full frame of digital video.
● This is called interlaced footage.● The two different fields are flashed on the screen intermittently
at 60 fields/second. This is because the hum caused by AC electrical current in the US is 60Hz. The hum could cause horizontal intermodulation lines otherwise.
● The visible scan lines and motion blur associated with interlaced footage is called combing.
Wait, WTF does that mean?
Digital Frame Rates (Cont’d)● To make digital color video available through the same
broadcasts as black and white, the frame rate for color video had to be slightly offset so it didn’t interfere with the black and white sound frequencies.
● The result was that the standard for digital video was slowed down by 0.1%. This was possible because timing circuits were developed to regulate the oscillation of TVs’ electrical current to avoid intermodulation.
● In reality, video we see on TV is being played at 29.97 fps, not 30.
Digital Frame Rates (Cont’d)● The next problem facing TV producers was how to take
movies that were shot on film at 24 fps, and convert them to video that could be played at 29.97 fps on TV.
● If editors were to simply convert each film frame to a digital frame, movies would appear to be playing in fast-motion on TV
29.97 ÷ 24 = 124.9% of original speed.
Digital Frame Rates (Cont’d)● Solution → 3:2 Pulldown● 3:2 Pulldown takes four full frames, and turns them into
five interlaced frames.
Digital Frame Rates (Cont’d)● TVs today can process information much faster than the
original tube sets, so most display video in progressive frames.
● This means every frame is singular, and the whole image is displayed at once.
● When a TV’s resolution is described as “1080p,” it stands for the resolution as well as the way in which the TV refreshes images, which is progressive in this case.
Digital Frame Rates (Cont’d)● The new fad is for TV companies to market super high
“refresh rates,” which are the exact same thing as frame rates.
Digital Frame Rates (Cont’d)● Super high frame rates reduce motion blur, so they can
be valuable for watching sports and playing video games, but anything over 120Hz doesn't really do anything.
● Since most cameras don’t record at anything over 60fps, the high-frame rate settings actually play back more frames than were originally recorded. They do this by creating new frames in between the recorded frames that are basically guesses at what the image should look like. Video
Digital Frame Rates (Cont’d)Some Standard Digital Frame Rates
● 1080i60 → 1080 horizontal rows of pixels, interlaced footage, “60” fields/second, 29.97 frames/second
● 720p30 → 720 horizontal rows of pixels, progressive footage, 29.97 frames/second.
Codecs● A Codec is a term used to describe the way in which a
digital video file is compressed.● It stands for Coder/Decoder.● After video is captured, it is compressed into a smaller
file that is easier to move around.● The Codec is the file type that describes the
compressed file.
Codecs (Cont’d)● Lots of other file types also have codecs. .mp3 →
Audio, .jpeg → image, etc.● The most common video codecs are Cinepak, MPEG-2,
H.264, and VP8.
Codecs (Cont’d)● All codecs are either lossless or lossy. ● Lossless codecs reproduce an exact replica of the
original file. With lossy codecs, some image quality is lost when the file is compressed.
● There are lossless video codecs (PKZIP, PNG), but all streaming is only possible with lossy codecs. If lossless codecs were used for streaming, nobody’s internet would be able to stream the videos fast enough. To put in perspective how much data is contained in a film file..
Codecs (Cont’d)● Godzilla (2014)● It took four special effects
artists six months to fully animate Godzilla’s scales.
● 762 visual effects crew members worked on the film.
● With one computer, it would have taken 445 years to render the final project.
Codecs (Cont’d)● Shockingly, if you stream Godzilla
online, some detail will be lost from the film’s original format.
● Your TV, computer, etc. do not have the audio and video capabilities to even display the original movie, let alone download it fast enough to stream anyway.
Codecs (Cont’d)● There are two types of compression: Intra-frame and
Inter-frame.● Intra-frame compression compresses every single
image as an individual still image.
Codecs (Cont’d)● Inter-frame compression looks at adjacent frames to
only store the minimum amount of information to recreate the original image.
Codecs (Cont’d)
Inter-Frame Compression● The first frame of a new
shot is fully encoded. This is called the Key Frame.
● For the next frame, only the changes from the previous frame are encoded, saving file space. These frames are called Delta Frames.
Inter-Frame Compression● Think of it as if you
were drawing a cartoon and you could copy pictures, erase parts, and draw more. Would you draw a whole new picture every time?
Inter-Frame Compression
Inter-Frame Compression● Other popular file
formats like MPEG are even more lossy, because they save far fewer frames than the original video format.
Inter-Frame Compression● I-Frames are actual frames
from the original footage.● P-Frames show just what
changes from the I-Frames.● I-Frames are also called
“Anchor Frames,” and are used to predict P-Frames and construct B-Frames from scratch.
Inter-Frame Compression
Color Correction● While some examples are more obvious than others,
every major film now goes through some sort of digital color correction process during post production.
Color Correction● One of the major critiques of contemporary films is that
most of them use the same teal and orange coloring.
THE END