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Nonlinear Encoding □ Quantization levels not evenly spaced □ Reduces overall signal distortion □ Can also be done by companding Companding • The process of compressing and then expanding. • The higher amplitude analog signals are compressed prior to transmission and then expanded in receiver. • Improving the DR of a communication system. Companding Functions Method of Companding □ For the compression, two laws are adopted: the -law in US and Japan and the A-law in Europe. □ -law □ Vout □ A-law Vout Vmax ln( 1 Vin Vmax ) ln( 1 ) A Vin Vmax Vmax 1 ln A Vin 1 ln( A Vmax ) 1 ln A Vin 1 0 Vout A 1 Vin 1 A Vout Vmax= Max uncompressed analog input voltage Vin= amplitude of the input signal at a particular of instant time Vout= compressed output amplitude A, = parameter define the amount of compression □ The typical values used in practice are: =255 and A=87.6. □ After quantization the different quantized levels have to be represented in a form suitable for transmission. This is done via an encoding process. Example 3 □ A companding system with µ = 255 used to compand from 0V to 15 V sinusoid signal. Draw the characteristic of the typical system. □ Draw an 8 level non-uniform quantizer characteristic that corresponds to the mentioned µ. Cont’d... μ-law A-law PCM Line Speed □ The data rate at which serial PCM bits are clocked out of the PCM encoder onto the transmission line. samples bits line speed X second sample □ Where □ Line speed = the transmission rate in bits per second □ Sample/second = sample rate, fs □ Bits/sample = no of bits in the compressed PCM code Example 4 □ For a single PCM system with a sample rate fs = 6000 samples per second and a 7 bits compressed PCM code, calculate the line speed. Virtues & Limitation of PCM The most important advantages of PCM are: □ Robustness to channel noise and interference. □ Efficient regeneration of the coded signal along the channel path. □ Efficient exchange between BT and SNR. □ Uniform format for different kind of baseband signals. □ Flexible TDM. Cont’d… □ Secure communication through the use of special modulation schemes of encryption. □ These advantages are obtained at the cost of more complexity and increased BT. □ With cost-effective implementations, the cost issue no longer a problem of concern. □ With the availability of wide-band communication channels and the use of sophisticated data compression techniques, the large bandwidth is not a serious problem. Time-Division Multiplexing □ This technique combines time-domain samples from different message signals (sampled at the same rate) and transmits them together across the same channel. □ The multiplexing is performed using a commutator (switch). At the receiver a decommutator (switch) is used in synchronism with the commutator to demultiplex the data. Cont’d… □ TDM system is very sensitive to symbol dispersion, that is, to variation of amplitude with frequency or lack of proportionality of phase with frequency. This problem may be solved through equalization of both magnitude and phase. □ One of the methods used to synchronize the operations of multiplexing and demultiplexing is to organize the multiplexed stream of data as frames with a special pattern. The pattern is known to the receiver and can be detected very easily. Block diagram of TDM-PCM communication system □ A single-bit PCM code to achieve digital transmission of analog. □ Logic ‘0’ is transmitted if current sample is smaller than the previous sample □ Logic ‘1’ is transmitted if current sample is larger than the previous sample Cont’d… Operation of Delta Modulation Cont’d... □ Analog input is approximated by a staircase function □ Move up or down one level () at each sample interval (by one quantization level at each sampling time) output of DM is a single bit. □ Binary behavior □ Function moves up or down at each sample interval □ In DM the quantization levels are represented by two symbols: 0 for - and 1 for +. In fact the coding process is performed on eq. □ The main advantage of DM is its simplicity. Cont’d... The transmitter of a DM System The receiver of a DM system Delta Modulation - Example DM circuit’s problem Cont’d… •Slope overload distortion is due to the fact that the staircase approximation mq(t) can't follow closely the actual curve of the message signal m(t ). In contrast to slope-overload distortion, granular noise occurs when is too large relative to the local slope characteristics of m(t). granular noise is similar to quantization noise in PCM. •It seems that a large is needed for rapid variations of m(t) to reduce the slope-overload distortion and a small is needed for slowly varying m(t) to reduce the granular noise. The optimum can only be a compromise between the two cases. •To satisfy both cases, an adaptive DM is needed, where the step size can be adjusted in accordance with the input signal m(t). Cont’d... □ In summary □ Slope overload □ Due to the input analog signal amplitude changes faster than the speed of the modulator □ to minimize : the product of the sampling step size and the sampling rate must be equal to or larger than the rate of change of the amplitude of the input analog signal. □ Granular noise □ Due to the difference between step size and sampled voltage. □ To minimize : increase the sampling rate, decrease the step size of modulator DM Performance □ Good voice reproduction □ PCM - 128 levels (7 bit) □ Voice bandwidth 4khz □ Should be 8000 x 7 = 56kbps for PCM □ Data compression can improve on this □ e.g. Interframe coding techniques for video Cont’d... □ Adaptive Delta Modulation (ADM) □ A Delta Modulation system where the step size of the DAC is automatically varied depending on the amplitude characteristics of the analog signal. □ A well designed ADM scheme can transmit voice at about half the bit rate of a PCM system with equivalent quality. □ Converting standard logic level to a form more suitable to telephone line transmission. □ The line codes properties: 1. Transmission BW should be small as possible 2. Efficiency should be as high as possible 3. Error detection & correction capability 4. Transparency (Encoded signal is received faithfully) Cont’d... □ Six factors must be considered when selecting a line encoding format; 1.transmission voltage & DC component 2.Duty cycle 3.Bandwidth consideration 4.Clock and framing bit recovery 5.Error detection 6.Ease of detection and decoding Why Digital Signaling? □ Low cost digital circuits □ The flexibility of the digital approach (because digital data from digital sources may be merged with digitized data derived from analog sources to provide general purpose communication system) Digital Modulation □ Using Digital Signals to Transmit Digital Data □ Bits must be changed to digital signal for transmission □ Unipolar encoding □ Positive or negative pulse used for zero or one □ Polar encoding □ Uses two voltage levels (+ and - ) for zero or one □ Bipolar encoding □ +, -, and zero voltage levels are used Non-Return to Zero-Level (NRZ-L) □ Two different voltages for 0 and 1 bits. □ Voltage constant during bit interval. □ no transition, no return to zero voltage □ More often, negative voltage for one value and positive for the other. Non-Return to Zero Inverted (NRZ-I) □ Nonreturn to zero inverted on ones □ Constant voltage pulse for duration of bit □ Data encoded as presence or absence of signal transition at beginning of bit time □ Transition (low to high or high to low) denotes a binary 1 □ No transition denotes binary 0 □ An example of differential encoding Multilevel Binary(Bipolar-AMI) • • • • zero represented by no line signal one represented by positive or negative pulse one pulses alternate in polarity No loss of sync if a long string of ones (zeros still a problem) • No net dc component • Lower bandwidth • Easy error detection 0 1 0 0 1 1 0 0 0 1 1 Pseudoternary □ One represented by absence of line signal □ Zero represented by alternating positive and negative □ No advantage or disadvantage over bipolar-AMI 0 1 0 0 1 1 0 0 0 1 1 Manchester □ There is always a mid-bit transition {which is used as a clocking mechanism}. □ The direction of the mid-bit transition represents the digital data. □ 1 low-to-high transition □ 0 high-to-low transition □ Consequently, there may be a second transition at the beginning of the bit interval. □ Used in 802.3 baseband coaxial cable and CSMA/CD twisted pair. Differential Manchester □ mid-bit transition is ONLY for clocking. □ 1 absence of transition at the beginning of the bit interval □ 0 presence of transition at the beginning of the bit interval □ Differential Manchester is both differential and biphase. [Note – the coding is the opposite convention from NRZI.] □ Used in 802.5 (token ring) with twisted pair. □ * Modulation rate for Manchester and Differential Manchester is twice the data rate inefficient encoding for long-distance applications. Example 5 □ Sketch the data wave form for a bit stream 11010 using □ NRZL □ Bipolar AMI □ Pseudoternary END OF PART 2