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					Multimedia Systems and

 Lecture 1 - Introduction to

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To provide an overview to the subject.
 To describe a number of fundamental concepts.

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What is multimedia?
Global Structure
Digital Technology

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What is multimedia?
 Multi- means many; much; multiple
 Medium means:
  An intervening substance through which something is
   transmitted or carried on.
  A means of mass communication such as newspaper,
   magazine, or television.
 Multimedia is woven combinations of text,
 graphic art, sound, animation, video and other
 kinds of elements.
Sample projects include: Newspapers,
 advertisements, libraries, websites, e-learning,
 user-interface design…etc.
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 What is multimedia?
When a viewer of a multimedia presentation is
 allowed to control what elements are delivered and
 when, it is interactive multimedia.
 Multimedia is an inter-disciplinary subject because
 it involves a variety of different theories and skills:
   These include computer technology, hardware and

    Arts and design, literature, presentation skills.

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Global Structure
 Application domain — provides functions to the user to
  develop and present multimedia projects. This includes
  Software tools, and multimedia projects development
 System domain — including all supports for using the
  functions of the device domain, e.g., operating systems,
  communication systems (networking) and database
 Device domain — basic concepts and skill for processing
  various multimedia elements and for handling physical

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  Course Overview
We can roughly divide the lectures into the following parts:
 Multimedia Elements which deals with the various properties of each
  kinds of elements:
    Sound, audio, voice and music
    Graphics, photographs and images
    Text, and layout
    Full-motion video and animation
 Application domain which deals how to develop multimedia
    Multimedia application development method
    Interface design and Software tools— element processing tools, authoring
 Supporting technology which deals the technology that are needed to
  support multimedia applications, such as:
    Data Compression
    Data and file format
    Multimedia input, output and storage
 Multimedia and the Internet
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Digital Technology
Everyday, we encounter many values that
 change continuously, for example, the voltage of
 the electricity that lights up our room varies
 continuously over time. These are also known as
 analogue signals.
However, modern computers are built to deal
 with entities in completely different way. These
 are known as digital computers because they
 work with digits.

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Digital Technology
 Because of this, when using
  a computer to process
  continuous signals, we first
  need to find a way to
  represent them so that the
  computer is able to handle
  them. Usually, this is a
  digital representation, i.e.,
  we use a series of numbers
  to denote the continuous

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Digital Technology
Then, we have to convert the continuous signal
 into the digital representation. This process is
 known as digitization.
The first step in the digitization process is
 sampling which takes samples of the continuous
 signal. The number of samples taken during a
 time period is known as sampling rate.
The second step is known as quantization where
 we restrict the value of the samples to a fixed
 set of levels.

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Digital Technology

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Ahmed Hosny

  Sound (Wave) Representation

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 Frequency is expressed in cycles per second - 1 cycle = 1 hertz (Hz)
  the higher the frequency, the higher the pitch of the sound.
 For example, a violin = 2000 Hz, and a brass band = 100 Hz. A high
  quality system will produce sounds in the range 20Hz to 20,000Hz
  (Human range).

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 The amplitude of an analogue audio signal is measured in
  decibels (dB).

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A Low Frequency Sound Signal

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A High Frequency Sound Signal

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Ahmed hosny

          Digitizing Sound
   (Analogue to Digital Conversion)

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 Readings of the analogue voltage are taken at uniformly
  spaced time intervals - this is called the sampling rate:
  the number of samples taken per second.
 In theory, the sampling rate must be at least twice the
  highest frequency in the range of analogue voltage
 Sampling rate = 2*f max; f : Frequency. Nyquist Rule
 For Human sound : 20Hz to 20,000Hz
  We take 2* 20,000 = 40,000 sample/sec

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Sampling the Low Frequency

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 Sampling the High Frequency

Clearly, the sampling rate for the high frequency signal is
inadequate - it does not 'catch' every peak and trough - the rate
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After sampling has been completed, it must be
 remembered that the sampled data is still
 analogue in form. It must therefore now be
 turned into digital data. This is achieved using a
 process called Quantization.
Quantization is the process of converting
 analogue values to digital values.

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Aliasing and Quantization Error
If we under sample, i.e., taking less samples
 than as required by Nyquist sampling theorem,
 some of the frequency components will be
 mistakenly converted into other frequencies.
 This is known as aliasing.
On the other hand, if we use too few levels to
 represent each sample value, there will be large
 amount of error for each sample.
This is known as quantization error. These errors
 can be thought of as noise on the signal.

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Quantization Interval
When 8 bits are used, there are 28 = 256 bands,
 and also, therefore, 256 quantization intervals.

The fewer the number of analogue bands, the
 wider each band must be, and thus the greater
 the range of analogue values which are
 translated to a single digital value.

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Quantization Interval
 It is better to use a larger number of bits for each
  quantization interval. For instance, when 16 bits are
  used, the number of bands/quantization intervals is 216 =
  65,536. Thus less coarse (and therefore more accurate)
  than 8-bit samples. 16-bit sampling is in fact the norm
  for sound digitization.

 The data rate of a stream of digitized data is calculated
sampling frequency x bits per sample / 8, expressed
                   in bytes/sec.
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Sampling and Sampling Rate
To record everything the human ear can possibly
 hear, we need to be sampling at a bit over
 44kHz, or something like 44,100Hz, or 44.1kHz,
 which is what audio compact disks (CDs) are.

By sampling at this rate, we effectively remove
 higher frequencies. For human listeners that's
 not a problem (since we cannot hear higher
 beyond 22kHz anyway).

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Sampling and Sampling Rate
For example, telephone samples at 8kHz,
 which effectively means that any frequency
 higher than 4kHz is wiped out; which is precisely
 why on the phone we sometimes cannot tell the
 difference between certain letters (like "f" and
 "s"), and why we need to spell our names (s-as-
 in-Solid, and f-as-in-fast), and the listener still
 manages to make mistakes when writing it

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Sample Size
 Another issue we have to care about when sampling
  sound is how big to make each sample. Basically, how
  many distinct values can a wave can take:
  * We want to record 0 dB – 60 dB (Normal
             60/8 = 7.5  8 bits needed
  * To record from 60 dB – 140 dB (Machines and
            140/8 =17.5  16 or 24 bits
 Most sound is encoded at 8-bit or 16-bit. Some (very
  rare) high end sound cards have 32-bit samples. Audio
  CD samples are 16-bit.

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Sample Size
For example, a 22kHz stereo sound, with 16-
  bit sampling will need to be sampled at 44.1kHz
  or 44,100 times per second. Each sample will be
  16-bits, and we'll have 2 channels (one left, and
  one right---for stereo). Thus, we'll have
44,100 sample/sec * 2 bytes/sample * 2
  channels = 1,411,200 bits/s
              = 176,400 bytes/s
             =173 kbyte/sec.

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High Data Volume of
Multimedia Information

Speech       8000 samples/s                8Kbytes/s

CD Audio     44,100 samples/s, 2 176Kbytes/s

Satellite    180X180 km^2                  600MB/image
Imagery      30m^2 resolution              (60MB
NTSC Video   30fps, 640X480                30Mbytes/s
             pixels, 3bytes/pixel          (2-8 Mbits/s

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