Multimedia information representationCharacters represented as Code word Images as lines, lines as coordinates, coordinate as a pair of digital value. Microphones , video cameras electrical signals of varying amplitude Amplitude is magnitude or image intensity Analog signal Conversion into digital form-signal encoder. Digitization Principles Analog signal Varying amplitude Fourier analysis Signal bandwidth Band limiting channel . The bandlimiting filter removes selected higher frequency components from source signal Sample and hold samples the amplitude of the filtered signal at regular intervals of time .Encoder Design Conversion of time varying analog signal by signal encoder(electronic circuit) The principles of an encoder Band limiting filter Analog to digital converter(ADC) consisting of sample and hold . quantizer circuit. Holds the sample amplitude constant between the samples Quantizer converts each amplitude into a binary value known as code word. The most significant bit of each code word indicates the polarity sign of the sample Positive value by 0 Negative value by 1 Both are relative to zero level . Sampling rate Nyquist sampling theorm To obtain an accurate representation of a time varying analog signal its amplitude must be sampled at a minimum rate that is greater or equal to twice the highest frequency component present in the signal. This is known as Nyquist rate represented as either Hz or samples per second (sps) Sampling at a rate lower than Nyquist rate results in additional frequency component being generated which distorts the original signal . In practice the transmission channel used has lower bandwidth than the source signal. To avoid distortion the bandwidth and hence the frequency range of the transmission channel determines the sampling rate rather than the bandwidth of the original signal In such cases the source signal may have higher frequency components than the nyquist rate It becomes essential to pass the source signal through band limiting filter . .Quantization Intervals To represent the amplitudes in a digital format it would require an infinite number of binary digits. In the example three bits are representing each sample including the sign bit. The result is four positive and four negative quantization intervals. When a finite number of digits are used each sample is represented by a corresponding number of discrete levels. If Vmax is the maximum positive and negative signal amplitude and n is the number of binary bits used then the magnitude of each quantization interval q is q=2Vmax/2n The signal anywhere in the interval will be represented by same binary codeword Each code word corresponds to a nominal amplitude level which is at the center of the interval The actual signal level may differ by q/2(+ or -) . Another factor influencing the choice of number of quantization intervals is smallest amplitude relative to peak amplitude. The error value varies randomly from sample to sample hence is also known as quantization noise.scale as decibels or db. The ratio of smallest amplitude to peak is called as dynamic range of the signal D .The difference between the actual and corresponding amplitude is called quantization error. . .D=20 log10(Vmax/Vmin)db It is essential to ensure that the level of quantization noise relative to the smallest signal amplitude is acceptable. The principles for it are Digital to analog convertor (DAC) Low pass filter (recovery or reconstruction filter Most multimedia applications involving audio and viedo the communication channel is a two way .Decoder Design The analog signal signals are stored .processed and transmitted in a digital form Prior to their output they need to be converted back.(loud speakers) The electronic circuit that performs this conversion operation is known as a signal decoder. Hence audio/video encoder and decoder are combined into a signal unit called audio/video codec or encoder-decoder. TEXT There are 3 types of text used to produce documents Unformatted text Formatted text hypertext .