c3_Digital Video Primer.ppt

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Prepared by: AQ First Prepared on:1st September 2007 Last Modified on: -n-

Quality checked by: -n-

Copyright 2007 Asia Pacific University College of Technology and Innovation

Chapter 2:

Digital Video Primer

Digital Audio and Video(CT018-3-2)


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Digital Audio and Video (CT018-3-2)Digital Video Primer

Topic & Structure of the lesson


Color Sampling

Compression

CODEC

Data Rate

Video File Size and Data Rate

Types of Digital Video Files How video works

Optimising Video File for delivery


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Learning Outcomes

At the end of this lecture YOU will be able to:

Demonstrate an awareness of fundamental

knowledge of digital video.

Identify the types of digital video CODEC and its

container.


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Colour Sampling

Chroma Sampling

The CCDs that capture our image inside our videocamera, capture colour by the use of Red, Green, andBlue filters.

The phosphors in our CRT monitor, and filters in ourLCD monitor and DLP projector also use Red, Greenand Blue filters.

Why is it then, that digital video is stored andmanipulated as separate luma and chroma

components? Separate luma and chroma are used so that the

resolution of the chroma can be reduced with respect tothe resolution of the luma so that large savings can bemade in the amount of data that needs to be transmitted(it is a form of compression. )


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If you use digital video, file size is an important concernbecause digital video files tend to take up a lot of storagespace on your hard drive.

Compressing video is essentially the process of throwing

away data for things we can't perceive. Standard digital video cameras compress video at a ratioof 5 to 1, and there are formats that allow you tocompress video by as much as 100 to 1.

The more you compress, the more data you throw away.

Throw away too much, and the changes becomenoticeable.

With heavy compression you can get video that's nearlyunrecognisable.

Compression


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Compression strategies

Psychovisual video compression

Psychovisual video compression is similar to its audio counterpart.

Instead of discarding audio that we can't hear, psychovisual models

discard data that are eyes don't need.

An uncompressed clip that shows a chair in the same location for 60

seconds repeats the same data for that chair for each frame.

With psychovisual video compression, the data for that chair from a

single frame is stored and reused in subsequent frames. This type of compressioncalled statistical data redundancyis

one of the mathematical tricks that WMV, MPEG, and other video

formats use to compress video while retaining good quality.

Compression


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Lossless compression

The term losslessmeans no loss of data.

When a file is compressed in a lossless fashion, 100percent of the data is still there, much like when you zip

a document (the document file gets smaller, but all thewords are still there when you unzip it.)

You can save lossless video over and over without anyloss of data.

Compression simply squeezes that data into a smaller

space. Lossless compression saves less space because you

can compress data only so much before you have tostart discarding information.

Compression



Lossy compression

Lossy compression discards data in order to achieve alower bit rate.

Psychoacoustic compression and psychovisual

compression are lossy technologies that result insmaller files that contain less of the original source data.

And every time you save your file in a lossy file format,it discards more of the dataeven if you're saving it inthe same format.

A good rule of thumb is to move to a lossy format onlyas the very final step in your project.

Compression



Intraframe Compression Techniques

Intraframe compression is compression applied to still

images, such as photographs and diagrams, and

exploits the redundancy within the image, known asspatial redundancy.

Intraframe compression techniques can be applied to

individual frames of a video sequence.

Compression



Interframe Compression Techniques

Interframe compression is compression applied to asequence of video frames, rather than a single image.

In general, relatively little changes from one video frame

to the next. Interframe compression exploits the similarities betweensuccessive frames, known as temporal redundancy, toreduce the volume of data required to describe thesequence.

There are several interframe compression techniques, ofvarious degrees of complexity, most of which attempt tomore efficiently describe the sequence by reusing partsof frames the receiver already has, in order to constructnew frames.

Compression


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Sub-sampling

Sub-sampling can also be applied to video as aninterframe compression technique, by transmitting onlysome of the frames.

Sub-sampled digital video might, for example, containonly every second frame.

Either the viewers brain or the decoder would berequired to interpolate the missing frames at the

receiving end.

Compression




Codecs are the means by which video is compressed(for delivery purposes) or decompressed (for viewingpurposes).

Some other popular codecs are MPEG-1, RealVideo,

Windows Media Video, DV and MPEG-4. MPEG-1 is probably the most universal codec and is a

great delivery codec. Most PCs have some kind of MPEG-1 codec already installed

on their system.

Windows Media Video/Audio is a great codec in termsof quality and compatibility.

Since most PCs run the Windows OS, this is a safe codec touse for most situations.

CODEC




The RealVideo/Audio codec has a bit lessquality than WMV/WMA but it is a great webvideo format (i.e., streaming video).

DV codec is the codec used in digital videocamcorders and analog/digital converters.

MPEG-4 is the codec used in most portabledevices such as cell phones, PDAs and

portable media players. MPEG-4 has a good balance of quality and file size.

DIVX is one of the better known codecs in theMPEG-4 category.

CODEC




The file size is one of the considerations for video

filesa larger file requires more storage space.

But an important factor related to the smoothness of the

video playback is its data rate. The data rate refers to the amount of video data to be

processed per second.

The average data rate of a video is calculated by

dividing the file size by the total length of the video inseconds.

In the 1-second video example above, the data rate is

133 MB/s.

Data Rate




To play a video smoothly would require the computer

equipment capable of handling its data rate.

For instance, the data rate of a 48x-speed CD-ROM

drive is about 7 MB/s which is not high enough tohandle the data rate of this video.

In other words, the playback of this video on a 48x-

speed CD-ROM drive will be very jerky.

Data Rate





Video file size and data rate are closely related but theydo not have the same implication.

File size pertains to the totalamount of the data. Large

file size requires more disk space and longer transfertime.

Hence, a very long video, even with low data rate, canhave a large file size.

Data rate affects the smoothness of the video playback.

If the data rate is too high for a computer to handle, thedata will not be processed fast enough for continuousplayback.

The playback will be choppy.



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Types of Digital Video Files

*.avi (Audio-Video Interleaved)

*.mov (Quicktime Movie)

*.realVideo

*.mpeg (Motion Picture Experts Group) open standard version 2 is industry video standard

MPEG 1(for 352 pixels x 240 pixels x 30 frames per second (normally scaled up to 640x480)

MPEG 2 (Already in use for DVD, satellite TV, HDTV, it is for full-screen video at 1024 x 768 x 30 fps)

MPEG 4 (Much greater compression than existing standards, Optimized for streaming media. It isintended for low-bandwidth multimedia applications)

.dv (Digital Video) Used in digital video (DV) cameras

Technically almost the same as MPEG-2


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How video works


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How video works


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How video works


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Optimising Video File for delivery

Considerations while optimising video files fordelivery Native aspect ratio

Full screen or Wide Screen

Codec Selection of codec; MPEG, WMV

File format Selection of container; AVI, Quicktime

Data Rate Settings of data rate in Mbps

Audio quality Stereo vs Mono


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