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4.2 Digital Transmission

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  1. 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)

  2. 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

  3. Cont’d…

  4. Operation of Delta Modulation

  5. 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.

  6. Cont’d... The transmitter of a DM System

  7. The receiver of a DM system

  8. Delta Modulation - Example

  9. DM circuit’s problem

  10. 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).

  11. 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

  12. 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

  13. 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.

  14. 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)

  15. 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

  16. 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)

  17. 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

  18. 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.

  19. 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

  20. 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

  21. 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

  22. 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.

  23. 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.

  24. Example 5 • Sketch the data wave form for a bit stream 11010 using • NRZL • Bipolar AMI • Pseudoternary

  25. END OF PART 2