Notes on Digital Video

1. AS Level Computing
Harrow International School Bangkok
Digital Video
Video Frame Rates
All video you have ever seen, whether film or digital, is just a collection of still shots played rapidly to
give the illusion of motion. Just like a flipbook.
The frequency which these pictures move is called the frame rate. The faster the frame rate, the
smoother the motion.
The frame rate used in most movies is 24fps (frames per second) as this is the lowest frame rate at
which our eyes perceive as seamless fluid motion. TV is often broadcast at 30fps. High speed
cameras shoot at 60fps as this is useful if you wish to slow the footage down. There are some crazy,
expensive cameras which shoot at over 1,000fps and some film directors are now choosing to
experiment with 48fps – like Peter Jackson’s The Hobbit (although it was generally not seen as a
success – this article discusses why: http://gizmodo.com/5969817/the-hobbit-an-unexpected-masterclass-
in-why-48-fps-fails).
30fps is generally the basic standard for computer games, but many people aim to achieve 60fps.
60fps is generally the maximum due to many LCD monitors only have a refresh rate of 60Hz. You can
see the difference between 30fps and 60fps using these websites here:
http://www.30vs60fps.com/
http://frames-per-second.appspot.com/

2. AS Level Computing
Harrow International School Bangkok
Progressive and Interlaced Encoding
Progressive footage is when the footage is shown at a full frame each time. So 30fps would mean
there are 30 full images loaded per second to show fluid motion.
The problem is that it is not fast enough to give the appearance of fluid motion, especially for events
like sport or any fast moving footage. To get round this, interlacing was created.
This is recorded at 60fps, but each field only contains half the image – the odd lines or the upper
field, and the even lines or the lower field. These alternate 60 times per second, but to the human
eye it gives a more fluid motion than progressive, but it can be sent using the same amount of data.
This is still regarded as 30fps or 60i.
The problem with interlaced footage comes when you want to do more with it than just view it. It is
very difficult to change the scale of the video, play it in slow motion or take a still image.

3. AS Level Computing
Harrow International School Bangkok
Spatial and Temporal Redundancy
Video compression is very important, especially when it comes to wanting to store large amounts of
videos on hard drives, or streaming videos over the internet. With efficient techniques, video files
sizes can be reduced without any noticeable effects on visual quality.
A video codec is made up of two algorithms. An encoder is an algorithm used to compress the file,
and a decoder, which is an inverse-algorithm, to restore the file so it can be viewed.
MPEG-4 and H.264 are two types of video codecs. Video compression looks at what has changed in
each frame and only sends the data to update the parts of the image that have changed. There are
two types of techniques, spatial and temporal redundancy.
Spatial looks at a single frame and compares it to its neighbours. If the data is the same (ie. the
background in a Skype conversation) then it will only update the new data.
Temporal redundancy looks a series of frames and compares them. It then compresses the data by
not encoding the pixels that have not changed. This is known as inter frame compression.
http://www.axis.com/products/video/about_networkvideo/compression.htm

4. AS Level Computing
Harrow International School Bangkok
Inter Frame Compression Algorithms
Inter frame prediction compression has three different types of frame; I -frame, P-frame and B-frame.
An I-frame (inter frame) is an independent frame that can be decoded without referring to anything
else. They are always used at the start of a video, or starting points for new viewers on a live stream
for example. The downside to them is that they are the largest files because they contain a lot of
data.
P-frames (predictive inter frame) refer back to previous I-frames or P-frames to code the image.
They contain less data but are more likely to contain transmission errors due to their reliance on the
previous data received.
B-frames (bi-predictive inter frame) refer back to previous I-frames or P-frames and also future
frames. They are not used by all codecs.
For a more detailed write-up:
http://www.axis.com/products/video/about_networkvideo/compression.htm
Streaming Video Protocols
The HTTP (Hyper Text Transfer Protocol) used to be the
protocol used to transfer a video from an internet server to your
computer. However this was not great as you had to download the
whole video before you could start watching it.
Now, the RTSP (Real Time Streaming Protocol) is used on most
websites like YouTube where you can stream content to your computer so you can start watching
before it has downloaded all of the content, and you can jump to any
place in the video. Definition: RTSP is used to ESTABLISH and
CONTROL media sessions between client and server.
Multimedia Container Formats

5. AS Level Computing
Harrow International School Bangkok
A container format is a file format that can contain various types of data, compressed in a manner of
standardized codecs. The container file is simply used to identify the different data types inside it.
Some containers, like MKV can support audio, video, subtitles, chapters, and meta-data.
The most popular multimedia container formats are:
● AVI (standard Microsoft Windows container).
● MOV (standard QuickTime container).
● MP4 (standard container for the MPEG-4 multimedia portfolio).
● Ogg (standard container for Xiph.org codecs).
● Matroska (not standard for any codec or system, but it is an open standard).
● ASF (standard container for Microsoft WMA and WMV).
● RealMedia (standard container for RealVideo and RealAudio).
To read about multimedia container formats in more detail:
http://www.divxland.org/en/article/15/multimedia_container_formats#.VFXnFfmUfTk
To read more about how internet video streaming works:
http://www.techradar.com/us/news/internet/how-internet-video-streaming-works-1095211