Digital Signal Formats. Syed A. Rizvi

Transcription

1 Digital Signal Formats Syed A. Rizvi 1

2 Why Digital Signaling? Low cost of digital circuitry Better resilience to noise Flexibility: digitized data derived from analog sources (voice, video signals etc.) can be combined with pure digital data (computer data) to form a general purpose communication system 2

3 Analog to Digital Conversion Pulse Amplitude Modulation Pulse Code Modulation Delta Modulation Differential Pulse Code Modulation 3

4 Pulse Amplitude Modulation Pulse amplitude modulation (PAM) is a simple technique to convert an analog signal to a pulse-type signal where the amplitude of the pulse denotes the analog information PAM can easily be generated using an analog switch, as shown in figure below 4

5 PAM (Continued) Baseband Analog Waveform Switching Waveform Resulting PAM Signal 5

6 Pulse Code Modulation In this technique, a digital word (series of bits) is generated representing an instantaneous sample of an analog waveform (signal) 6

7 PCM (Continued) Output Voltage Input Voltage Analog, PAM, and Quantized PAM Signals Error Signal Quantizer Output-Input Characteristics PCM Signal 7

8 PCM (Continued) Performance of a PCM System with Uniform Quantization (Noiseless Channel) Number of Quantizer Levels Used Length of the PCM Word Used Bandwidth of PCM Signal S/N (db) 2 1 2B* B B B B B B B , B 96.3 *B is the bandwidth of Analog Signal 8

9 PCM (Continued) Advantages: Relatively inexpensive digital circuitry PCM data can be merged with pure digital data and can be transmitted over common high-speed digital communication system Regenerative repeaters can be used to reconstruct a clean PCM signal from noisy/distorted PCM signal at appropriate intermediate points along the transmission path Superior S/N performance than that of an analog system Disadvantage: Higher bandwidth required than is required by an analog system 9

10 Delta Modulation (DM) Very simple technique Inexpensive to implement Poor S/N performance when compared to PCM 10

11 DM (Continued) Operation: 1. Generate a flat-topped PAM signal from the analog input signal 2. Compare the PAM signal with the accumulated output of the Integrator (AO/P) 3. If PAM > AO/P, Comparator s output is 1; otherwise it s 0 4. Comparator s output is fed to the Integrator to generate accumulated output 5. Output of the Comparator form the DM signal 11

12 DM (Continued) 12

13 DM (Continued) S/N performance of a DM system as a Function of step size A small step size causes slop-overload distortion A larger step size causes granular noise 13

14 Amplitude Modulation (AM) In amplitude modulation, the amplitude of a high frequency carrier is changed with the amplitude variations in the signal to be transmitted (baseband signal) % Positive Modulation = Amax Ac 100 A c Baseband Signal % Negative Modulation = Ac Amin 100 A c % Modulation = A A 2 max min A c 100 Resulting AM Signal 14

15 AM (Continued) AM Broadcast Station Technical Standards Item Assigned frequency Channel Bandwidth Carrier Frequency Stability FCC Technical Standard KHz (in 10 KHz increments) 10 KHz +/- 20 Hz of the assigned frequency % Modulation Maintain 85-95%; max: 100% negative, 125% positive Maximum power licensed 50 kw 15

16 Frequency Modulation (FM) In frequency (phase) modulation, the instantaneous frequency (phase) of the carrier is changed with the change in the baseband signal. Baseband Signal Instantaneous Frequency of the Corresponding FM Signal Corresponding FM Signal 16

17 FM (Continued) FCC FM Broadcasting Standard Item Assigned frequency Channel Bandwidth Carrier Frequency Stability Noncommercial Stations Commercial Stations FCC Technical Standard 88.1MHz to 107.9MHz (in 200 KHz increments) 200 KHz +/- 2 khz of the assigned frequency 88.1 MHz to 91.9 MHz 92.1 MHz to MHz 17

18 FM (Continued) FCC Two-way FM Mobile Radio Standard Item Assigned Frequency FCC Technical Standard MHz (low VHF band) MHz (2-m Amateur band) MHz (high VHF band) MHz (3/4 m Amateur band) MHz (UHF band) MHz (UHF, T band) MHz (900-MHz band) 18

19 Time Division Multiplexing (TDM) Time division multiplexing is a technique for transmitting information from several different sources serially over a communication channel by time interleaving of samples from these sources. 19

20 TDM (Continued) In an n-channel TDM system, the bandwidth of the TDM system is n times the highest bandwidth of the source. TDM receiver stores the multiplexed data and directs it to appropriate output channel using frame synchronization. Cross talk is referred to the phenomenon when PCM samples from one channel appear in another channel. This is caused by poor filtering of PCM samples. 20

21 Frequency Division Multiplexing (FDM) Frequency division multiplexing is a technique for transmitting information from different sources simultaneously over a wideband channel. 21

22 FDM (Continued) Each signal is modulated on a separate carrier. A composite baseband signal is formed by summing the modulated subcarriers. The composite signal is then modulated on the main carrier. 22

23 Amplitude Shift Keying (ASK) In this technique, a carrier sinusoid is switched (keyed) on and off with a binary signal. 23

24 Phase Shift Keying (PSK) In this technique, the phase of a sinusoid carrier is shifted 0 o or 180 o with a binary signal. 24

25 Frequency Shift Keying (FSK) In this technique, the frequency of a sinusoidal carrier is shifted from one frequency to another. In this way, one frequency of the carrier represents binary 1 and the other represents a binary 0. 25