Chapter 2Digital data

Ola A. Younis

Elements of digital media

•Symbols: representation for something else.

Example: a group of letters often serve as symbols

of words.

•Data: the givens of experience-mesurments, facts,

observations and beliefs. Example: a list of numbers ( 12,

9, 20….)

• Data is either digital or analog.

•Information: data made useful, data interpreted

and applied to produce understanding. Example: the

previous list of numbers are hours!

Digital codesDigital information starts with digital data, and digital

data starts with digits (0-9).

•Bits : binary digit…. 0 or 1.

•Bytes: 8-bit code=256 distinct item.

•Effective code: is one that can represent each

desired data item with unique combination of

symbols.

•Efficient code: is one that does not waste

processing, storage or transmission storage.

FilesA computer file is a container for binary code, which

is the universal language of a computer.

• Size: measured in number of bytes, Kilobyte=2 , Megabyte=2 ,

Gigabyte=2 or Terabyte=2

• Format: a convention that specifies how instructions and data

are encoded in computer file.

• File extension..

• File Compatibility: assuring that a computer can process the

instructions or data that are encoded in a particular data format.

•Maintenance: Identification, categorization and

preservation.

10

30

20

40

GIF

Li & Drew5

GIF87• For the standard specification, the general file format

of a GIF87 file is as in Fig. 2.2.

Li & Drew

6

Fig. 2.2: GIF file format.

• Screen Descriptor comprises a set of attributes that belong to every image

in the file. According to the GIF87 standard, it is defined as in Fig. 2.3

Li & Drew

7

Fig. 2.3: GIF screen descriptor.

• Color Map is set up in a very simple fashion as in Fig. 2.4

However, the actual length of the table equals 2(pixel+1) as given in

the Screen Descriptor.

Li & Drew

8

Fig. 2.4: GIF color map.

• Each image in the file has its own Image Descriptor, defined as in Fig. 3.15.

Li & Drew

9

Fig. 2.5: GIF image descriptor.

Digitization

The process of converting analog data to digital format is called

digitization and it starts with sampling.

• Sampling: the process of analyzing small element of an image or

sound and representing that element in a digital code.

• Quantization: the process of rounding off the value of sample to

the closest available value in the digital code being used.

• Sample resolution: number of bits used to represent a digital

sample.

• Sample rate: the number of samples taken in a given unit of

time.

• Fig. 2.6: An analog signal: continuous measurement of pressure wave.

11

Li & Drew

• • The graph in Fig. 2.7 has to be made digital in both time and amplitude. To digitize, the signal must be sampled in each dimension: in time, and in amplitude.

▫ (a) Sampling means measuring the quantity we are interested in, usually at evenly-spaced intervals.

▫ (b) The first kind of sampling, using measurements only at evenly spaced time intervals, is simply called, sampling. The rate at which it is performed is called the sampling frequency (see Fig. 2.7(a)).

▫ (c) For audio, typical sampling rates are from 8 kHz (8,000 samples per second) to 48 kHz. This range is determined by the Nyquist theorem, discussed later.

▫ (d) Sampling in the amplitude or voltage dimension is called quantization. Fig. 2.7(b) shows this kind of sampling.

12

Li & Drew

•Fig. 2.7: Sampling and Quantization. (a): Sampling the analog signal in the time dimension. (b): Quantization is sampling the analog signal in the amplitude dimension.

13

Li & Drew

(a) (b)