• Basic Concepts
Video is a collection of bit-mapped still images (called
frames) that are taken one after the other. When the file is
played these pictures are shown in quick succession to give
the impression of a moving image.
The number of frames is called the frame rate and this is
measured in FPS (Frames per Second). The standard
rate is 25 FPS.
MM Topic 4 - Video Data
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• Basic Concepts
The size of code used to represent the colour of each pixel
in a frame will also affect the file size and picture quality.
As with still images this is called the bit-depth or colour
depth.
Resolution is the number of pixels in the area that has
been captured. This is usually measured in megapixels.
A higher setting for resolution when a shot is being
captured will mean better quality but of course, a higher
file size.
The bit-rate is the number of bits that are sent in one
second when transmitting a video file. If the data cannot
be sent quickly enough then the video will not be able to
display in real-time.
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1. Digital video camera
2. Web cam
Digital Video camera
3. Video capture card
A digital video camera(camcorder) functions in the same way as
a still digital camera. The user points the camcorder at the
scene they wish to capture and a lens focuses the image onto an
array of CCDs (Charge Coupled Devices)
Each CCD corresponds to one pixel in the image being
recorded. The CCD is a sensor which changes the light
striking it into an electrical signal.
More expensive cameras have 3 CCDs for each pixel, one
each for red, green and blue. This improves quality.
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1. Digital video camera
Digital Video camera
This analogue signal is then changed
into digital using an analogue to
digital converter (ADC).
2. Web cam
3. Video capture card
Unlike a digital camera a camcorder must capture a
continuous stream of images. Therefore the ADC needs to
work much faster in a camcorder than in a camera.
Another consequence of capturing a constant image stream
is that file sizes are very large - indeed video is the largest
of any of the media types (text, images, sounds) you look at
in Higher, even after it has been compressed.
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1. Digital video camera
Digital Video camera
2. Web cam
3. Video capture card
Digital Signal processor
The camcorder has a piece of hardware in it called a
DSP(Digital Signal Processor)
The DSP automatically alters the contrast and brightness
of the video for maximum quality.
It also compresses and encodes the data stream into
MPEG format before saving it onto a storage medium—disk,
memory card or tape.
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1. Digital video camera
Webcam
2. Web cam
Webcams are used for real-time
3. Video capture card
chat, including instant messaging
(IM), telephone calls over the
internet, live broadcasts and video
conferencing.
Webcams are a lot less expensive than camcorders
because:
● there are fewer CCDs.
● the lower resolution means that the ADC need
not be as fast as that fitted on a camcorder.
● they have no backing storage facility
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3. Video Capture Card
1. Digital video camera
2. Web cam
If a recording was made with an
analogue video camcorder then the
conversion from analogue and
compression will not have taken
place.
3. Video capture card
An analogue camcorder can be connected to a device that
can be fitted to computers called a video capture card.
This receives the video data, converts it to digital and
compresses it.
A video capture card can also be used to receive data
from an analogue video recorder or analogue TV.
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1. Uncompressed AVI
2. MPEG
1. Uncompressed AVI
AVI stands for Audio Video Interleave and is a standard
developed by Microsoft. It is commonly used with Windows
Media Player (also developed by Microsoft).
AVI is so-called because it mixes in (interleaves) sound
and video so that they are stored efficiently, rather than as
separate entities where video and audio could be
separated and played out of sync.
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1. Uncompressed AVI
1. Uncompressed AVI
2. MPEG
AVI is uncompressed but it does have methods of
keeping file size compact:
● frame rate is set at a maximum of 30 fps
● final file size cannot exceed 2 Gb.
● resolution cannot exceed 320 * 240,
This makes AVI useful for some files but restricts it to
relatively small sections. Microsoft is currently developing
a successor to AVI called Windows Media Video which
will allow compression.
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1. Uncompressed AVI
2. MPEG
2. MPEG
MPEG(Moving Picture Expert Group) is perhaps the most
popular standard for video.
It comes in many different versions: MPEG 1, MPEG 2,
MPEG 3, MPEG 4 and MPEG 7with most DVDs using
MPEG 2.
MPEG is a compressed format that uses intraframe and
interframe techniques. Sound is stored as a separate
‘layer’ within the file.
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2. MPEG
1. Uncompressed AVI
2. MPEG
Intraframe compression
This involves removing data from each frame.
MPEG looks for and removes data that either cannot be
detected by the human eye or will not affect the overall
quality too much.
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2. MPEG
1. Uncompressed AVI
2. MPEG
Interframe compression
This look for savings between frames.
MPEG does not store all the frames but instead it stores
certain key frames (i-frames) as jpegs.
It then looks for the changes between frames and stores
these changes as delta frames. Thus the whole of every
frame is not stored—only those areas that might change.
If an area of a video clip such as a background does not
change over a long period then this does not have to be
stored for each frame.
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To calculate the size of a video file in bytes you need to
know the following:
● The resolution of a single frame
● The colour depth
● The frame rate (no of frames per second)
● The length of the video (time it runs for)
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Carry out the following steps
a. Calculate the number of pixels in a single frame by
multiplying height by width
b. Calculate the storage needed for a single frame by
multiplying the number of pixels by the bit depth.
c. Calculate the storage needed for 1 second by
multiplying the size of a frame by the frame rate.
d. Calculate total file size by multiplying size of 1
second by the length of the clip in seconds.
e. Convert the answer into a suitable unit.
MM Topic 4 - Video Data
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Calculate the file size of a one minute uncompressed video
clip recorded at 25 fps (frames per second. The resolution was
set at 640x480 and each frame is stored in 16 bit colour.
1. Number of pixels = 640 x 480 = 307,200 pixels.
2. Frame size = 307,200 x 16 = 4,915,200 bits.
3. 1 second of clip = 4,915,200 x 25 = 122,800,000 bits.
4. Total file size = 122,800,000 * 60 = 7,372,800,000 bits.
5. Convert into bytes = 7,372,800,00 /8 = 921,600,000 bytes.
6. Convert into Kilobytes = 921,600,00 / 1024 = 900,000 Kilobytes.
7. Convert into Megabytes = 900,000 / 1024 =878.91 Megabytes.
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Calculate the file size for the following:
(Note that the No of colours is given. You have to
work out the colour depth. (bit depth)
Resolution
640 x 420
800 x 600
1024 x 768
No of colours
16
65,536
256
fps Time(secs)
25
20
13
24
30
12
MM Topic 4 - Video Data
File Size
64.09 MB
285.64 MB
270 MB
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Once a video clip has been captured and transferred to a
computer it can be viewed and, if desired, edited.
To edit video clips you will need to have video editing software
such as Adobe Premiere Elements, Pinnacle Studio,
ULead Video Studio or Windows Movie Maker.
A user would typically be working with several video clips to
create a movie.
Typical activities would include copying clips, cutting frames
from clips, adding or removing audio tracks, adjusting audio
volumes, adding titles and credits and inserting transitions
and effects.
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Three features which are common to all
video editing applications are:
1. Timeline
2. Transition
3. Sequencing
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1. Timeline
1. Timeline
2. Transition
The timeline area of the screen
displays the sequence of video
clips in order of viewing.
3. Sequencing
This where you view the various media elements, video,
audio, still images, titles in an order and sequence which can
then be edited.
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1. Timeline
2. Transition
2. Transition
Transitions are added between
clips and to create an effect when
the change is made from one clip to
another.
3. Sequencing
Common transitions are wipe (a line wipes across the old
image, taking it away as it goes and replacing it with the new
picture), fade out (the old image gradually fades to black),
dissolve (the old image fades out while the new one fades
in without the screen going black), hard cut (the clip
suddenly jumps to a completely new image) and peel or
page turn (the old image peels away to reveal the next clip).
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1. Timeline
3. Sequencing
2. Transition
3. Sequencing
Sequencing simply refers to the order in which video and
audio clips are placed.
Clips can be easily put into any order and then
duplicated, inserted or deleted
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The final stage of the process is to display the video on
screen and a video card is used for this.
The DSP (Digital Signal Processor), also called a GPU
(Graphics Processing Unit) carries out all of the work
needed to display full quality video clips.
The DAC (digital to analogue converter) changes the
binary information into electrical signals that can be sent to
monitors.
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