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## 4.2 Digital Transmission

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**4.2 Digital Transmission**Outlines Pulse Modulation (Part 2.1) Pulse Code Modulation (Part 2.2) Delta Modulation (Part 2.3) Line Codes (Part 2.4)**DELTA MODULATION (DM)**• 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...**• 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**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.**LINE CODES**• Converting standard logic level to a form more suitable to telephone line transmission. • The line codes properties: • Transmission BW should be small as possible • Efficiency should be as high as possible • Error detection & correction capability • Transparency (Encoded signal is received faithfully)**Cont’d...**• Six factors must be considered when selecting a line encoding format; • transmission voltage & DC component • Duty cycle • Bandwidth consideration • Clock and framing bit recovery • Error detection • 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 bi-phase. [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