4.1 digital multimedia elements graphics audio video

4.1 Digital Multimedia Elements Graphics

Audio

Video

Graphics Bitmaps

A bitmap image is a collection of colour dots called pixels arranged in rows and columns.

A pixel is the smallest picture element that can be shown on a computer screen.

The number of colours can be displayed in a pixel is determined by the number of bits used to represent the pixel (known as colour depth).

The larger the colour depth, the larger the number of colours can be displayed in the image.

4.1 Digital Multimedia Elements


Bitmaps

Comparison of images in different colour depths

Colour depth (bits per pixel) = 1Number of colours that can be displayed = 21 = 2File size = 1.3 MB


Bitmaps


Colour depth (bits per pixel) = 8Number of colours that can be displayed = 28 = 256File size = 10.3 MB


Bitmaps


Colour depth (bits per pixel) = 24Number of colours that can be displayed = 224 = 16,777,216File size = 30.8 MB

Graphics Bitmaps

Resolution Is the measurement of the number of pixels per unit

length in a bitmap image.

Is measured by pixels per inch (ppi). The higher the resolution, the more details of the image

can be shown. But the file size of an image will increase dramatically.



Bitmaps Resolution

Comparison of images with different resolutions

72 ppi, 99 KB

150 ppi, 351 KB

300 ppi, 1,333 KB

Graphics Bitmaps

Digital images for screen display are usually stored at a resolution ranging from 72 to 95 ppi.

For photograph printing, the images can be stored at a resolution ranging from 200 ppi to 400 ppi.

Common file formats of bitmap BMP JPEG GIF


Graphics Vector graphics

They are composed of a number of objects. A vector object can be a shape, a line or a string of text. The shape and location of vector objects are defined by

mathematical formulae. Can rescale and modify every detail of each vector object

easily without affecting the quality of vector graphics.




Bitmap

Vector graphics

Magnification of an image in vector graphic and bitmap format


The file size of vector graphics does not grow rapidly even when more and more vector objects are drawn in the image.

The file size of vector graphics is usually much smaller than that of a bitmap image.

They suit for representing simple charts, line arts and illustrations.

They are not suitable for representing images with complicated colour patterns.

Examples of popular vector graphic formats Windows Metafile (WMF) Shockwave Flash (SWF) Postscript (EPS)




Details of the original bitmap image (left) are lost when it is represented in vector graphic format (right).

Audio Audio files in a computer are the digitized data of music,

speech and any kind of audio signals. The quality of a digital audio file is determined by its bit

resolution and sampling rate. Bit resolution

Is the number of bits used to represent the amplitude of an audio signal.

The higher the number of bits used, the more precise the amplitude of the audio signal will be.


Audio Sampling rate

Is the number of samples taken each second during the conversion process

Is measured in Hz. The higher the sampling rate, the better the quality of the

digital audio will be.


Audio type Bit resolution Sampling rate (Hz)

Voice over telephone

8-bit 11,025

FM broadcast 16-bit 22,050

Compact disc 16-bit 44,100

DVD 24-bit 96,000Characteristics of different types of audio

Audio


The smooth 16-bit waveform indicates that it has a better quality than the jagged 8-bit waveform.

Audio file with 16-bit resolution

and 44 kHz sampling rate



Audio


The 44 kHz audio file contains more samples to represent the original audio signal and therefore it is more precise than the 11 kHz audio file.



Audio file with 8-bit resolution and 11 kHz

sampling rate

Audio Stereo audio contains two streams of audio signals,

known as left sound track and right sound track. Mono audio contains only one sound track. Audio data can be stored either in uncompressed or

compressed format. WAV is the most popular type of uncompressed audio file

format. WMA and MP3 are examples of compressed audio widely

used in portable music players and web pages.


Video Is a sequence of full-motion images together with sound

tracks. The file size of it can be very large.


Videos can be played by using Windows Media Center.

Video


Factor Description Example

Frame size

Number of horizontal and vertical pixels on each video picture (frame)

Frame size for VCD-PAL is 352 × 288 pixels. Frame size for DVD-PAL is 720 × 576 pixels.

Frame rate Number of frames displayed per second

Frame rate for both VCD-PAL and DVD-PAL is 25 fps. Frame rate of a MPEG-4 video played in PlayStation Portable is 30 fps.

Duration Play time of a video clip -

Compression

Reduction of quantity of data of video clips by changing the content or the way of representation of video frames

VCD: MPEG-1DVD: MPEG-2iPod and PlayStation Portable: MPEG-4Streaming video on the Internet: WMV and RealMedia

Colour depth

Number of bits used to represent a pixel

VCD: 24 or 32 bitsDVD: 24 or 32 bits

Factors affecting the size of a video file

Video The higher the frame size and frame rate, the better the

video quality and the larger the file size. A particular compression method has its own

specification of compression ratios, frame sizes and frame rates.

Different video file formats require different media players and plug-ins.

AVI is the most popular standard of uncompressed video.

MPEG-1, MPEG-2 and WMV are examples of compressed video file formats.


Video


WMVFile size = 40 MBFrame size = 320 × 240 pixelsFrame rate = 25 fps

Video files can be compressed into different video file formats for different playback media.

MPEG-1 (VCD)File size = 52 MBFrame size = 352 × 288 pixelsFrame rate = 25 fps

Uncompressed AVI video clipFile size = 300 MBFrame size = 720 × 576 pixelsFrame rate = 30 fps

Video


Video files can be compressed into different video file formats for different playback media.

Uncompressed AVI video clipFile size = 300 MBFrame size = 720 × 576 pixelsFrame rate = 30 fps

MPEG-2 (DVD)File size = 180 MBFrame size = 720 × 576 pixelsFrame rate = 25 fps

MPEG-4 video clipFile size = 45 MBFrame size = 320 × 240 pixelsFrame rate = 30 fps

Characteristics of analogue data and digital data

4.2 Analogue Data and Digital Data

Characteristic Analogue data Digital data

Way of measurement

Continuous signals of amplitude and frequency of the source are measured.

Information is represented in discrete form. In other words, a set of finite numbers of symbols (usually ‘0’ and ‘1’) is used to represent a piece of information

Efficiency on data storage

Low: The physical size of storage media such as cassette tape and video tape are much larger than the corresponding digital storage media. Their life-time is much shorter.

High: Data can be stored in compact media suchas CD-ROM and flash memory for long term storage.

Comparison of the characteristics of analogue data and digital data





Efficiency ontransmission

High rate of transmission error

Low rate of transmission error

Accuracy ofinformation

Accuracy depends on the sampling technique used.

Lower accuracy: Not all information is converted and recorded in the process of converting analogue signals to digital signals.

Replication

Absolute replication is nearly impossible. Data is lost in the replication process.

Absolute replication and error-free transmission of data are possible and easy.




Mode of data access

Mostly sequential access

Direct access is possible, depending on the media used for storage.

Efficiency on data analysis

Low: Analogue content is difficult to search for, analyze and reconstruct to produce other information.

High: Digital contents can be efficiently reused or processed by computers to generate information.



Applications of digital data in IT All electronic devices including computers and modern

data transmission channels handle data in binary forms.

Digital data and computer programs can be represented in sequences of binary numbers.

This makes operations of electronic devices on digital data such as storage, transmission, manipulation and duplication highly precise and efficient.

Store data Transfer processed data


Applications of digital data in ITWorkstation A Workstation B

CD-RDigital camera Hard disk Printer

Hard copy

Output copy

Duplicate Transfer data

Computernetwork

Daily-life applications of digital data

Transfer data


Conversion between analogue data and digital data

Digitization is the process of converting analogue data into

digital data. Digitization involves two procedures

Sampling Quantization

Sampling an analogue signal

Sampling The process of obtaining discrete sample data from the

continuous analogue signal for every fixed time interval


Quantizing an analogue signal

Quantization The process of converting the attribute of each sample

data into a digital code using a predefined scale The digital code is usually represented by a binary number.


Digitizing an analogue photograph

Conversion between analogue data and digital data


Sample data obtained from the analogue photograph

Original analogue photograph Digital code representation of sample

data

Conversion of analogue multimedia elements

Audio Sound is inherently analogue. A sound card is an electronic circuit board that is

responsible for conversion. Digital audio files can be stored in a large variety of file

formats, for example, WAV, MP3, WMA and RA. The quality of digital audio is determined by

Sampling rate Bit resolution (number of bits per sample) Number of channels



Audio


Standard configuration of a computer system for audio recording and playback

Mouse

Microphone

Monitor

System unit

Speakers

Keyboard


Audio Since the digital audio is an approximation of the

analogue source, the audio playback cannot be the same as the original one.

However, higher sampling rate and bit resolution can record and reproduce audio output closer to the original one.



Image Analogue images such as photographs or printed

materials can be converted into digital images using a scanner.

The scanned images are stored as bitmap graphics. The quality of a bitmap graphic is determined by

Resolution Colour depth Compression ratio



Image


Image typeSetting

Plain text and art

line

Black & white photo (for screen

display)

Colour photo (for screen

display)

Colour photo (for high quality

printing)

Resolution 300 ppi 72 ppi 72 ppi 300 ppi

Colour depth

1 bit / pixel (21 = 2 levels: black and white)

8 bits / pixel (28 = 256 levels of grey-scale).

24 bits / pixel (8 bits for Red, Green and Blue Respectively) (224 ≈ 16.8 million colours)

24 bits / pixel (8 bits for Red, Green and Blue respectively) (224 ≈ 16.8 million colours)

Colour mode Halftone Grey-scale RGB RGB

File formatrecommended

BMP JPEG JPEG JPEG

Compression No Yes Yes YesTypical settings for image scanning


Image No matter what the scanning settings are, the scanned

digital image cannot be the same as the original analogue source.

There is always a loss in information during the digitization process.


Conversion of an image from an analogue to a digital format


Image Printing a digital image on paper and projecting it on a

screen are examples of conversion of digital images into analogue images.

The colour ranges of different presentation media are different,so the output analogue images deviate from the digital images.

The manufacturing quality of the output devices is another factor affecting the appearance of the output images.


Conversion of analogue multimedia elements Video

To capture television programmes into a computer, we need a TV tuner card to receive TV signals from the antenna.

A TV tuner card converts analogue TV signals into a sequence of digital images and digital audio.

A video capture card works in a similar fashion except that it receives video sources instead of radio frequencies.

Example of video sources Video cassette recorders (VCR) Digital video camera Web cam Digital camera




TV tuner box

A TV tuner box (left) and a TV tuner card (right)

TV tuner card


Video The quality of a digital video is determined by the frame

size, frame rate and the video compression algorithm. Example of digital video file formats

AVI MPEG-1 MPEG-2 WMV MOV RM




Typical video setting for a web camFile format: compressed AVIFrame size: 320 × 240 pixelsFrame rate: 12 fps

Typical video setting for a digital video cameraFile format: MPEG-1Frame size: 352 × 288 pixelsFrame rate: 25 fps

Examples of different digital video file formats


Modern digital video cameras and web cams contain charge-coupled devices (CCD) used to convert analogue video signals into digital video data in real time.

The captured digital video data can be transferred directly to computers through USB or FireWire (IEEE 1394) ports.




Digital video camera

Charge-coupled device (CCD)

A digital video camera (left) and a charge-coupled device (CCD) (right)

4.1 digital multimedia elements graphics audio video

Documents

graphics vector graphics

quality of vector graphics

file size of vector

mb slide

bitmap format slide

digital aud

number of colours

location of vector objects