1. Chapter 2 Digital data Ola A. Younis

2. 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!

3. Digital codes Digital 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.

4. Files A 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

5. GIF Li & Drew 5

6. 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.

7. 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.

8. 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.

9. 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.

10. 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.

11. Fig. 2.6: An analog signal: continuous measurement of pressure wave. 11 Li & Drew

12. 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

13. 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)