1/21 Chapter 5 – Signal Encoding and Modulation Techniques

2/21 Analog Data, Digital Signal  Digitization is conversion of analog data into digital data which can then:  be transmitted using NRZ-L (digital signal)  be transmitted using code other than NRZ-L  be converted to analog signal by using modulation techniques (ASK, PSK, FSK) signal

3/21 Codec (Coder-decoder)  Analog to digital conversion done using a codec (coder-decoder). Two techniques: 1.Pulse Code Modulation (PCM) 2.Delta Modulation (DM)

4/21 Pulse Code Modulation (PCM)  Sampling Theorem:  “If a signal is sampled at regular intervals at a rate higher than twice the highest signal frequency, the samples contain all information in original signal”  Samples can be used to reconstruct the original signal  e.g., Hz voice data, requires 2*4000=8000 sample per sec  These are analog samples, called Pulse Amplitude Modulation (PAM) samples  To convert to digital, each of these analog samples must be assigned a binary code

5/21 Pulse Code Modulation (PCM) Example  The signal is assumed to be band-limited with bandwidth B  The PAM samples are taken at a rate of 2B, or once every Ts=1/(2B) seconds  Each PAM sample is quantized into one of 16 levels  Each sample is then represented by 4 bits.  8 bits→256 level →better quality  4000Hz voice→ (8000sample/s)* 8bits/sample= 64Kbps

6/21 Pulse Code Modulation (PCM) Block Diagram  By quantizing the PAM samples, the resulting signal is an approximation of the original one  This effect is known as quantization error or quantization noise  The Signal-to-Noise-Ratio (SNR) for quantizing noise:

Linear Versus Non-Linear Encoding 7/21  Linear Encoding (uniform quantization):  Equally spaced quantization steps  Lower amplitude values are relatively more distorted  Non-Linear Encoding (non-uniform quantization):  Non-equally spaced quantization steps  Large number of quantization steps for signals with low amplitude, and smaller number of quantizing steps for signals with large amplitude

8/21 Companding (Compressing-Expanding)  Instead of non-linear encoding, use companding+linear encoding  Companding gives more gain to weak signals than to strong signals on the input. At output, the reverse operation is performed CompressingExpanding X Y X

9/45 Delta Modulation (DM)  An analog input is approximated by a staircase function that moves up or down by one quantization level (  ) at each sampling interval (T s ).  A 1 is generated if the staircase function is to go up during the next interval; a 0 is generated otherwise.  The staircase function tracks the original waveform

10/21 Delta Modulation Operation  For transmission:  the analog input is compared to the most recent value of the approximating staircase function.  If the value of the analog input exceeds that of the staircase function, a 1 is generated; otherwise, a 0 is generated.  Thus, the staircase is always changed in the direction of the input signal.  For reception:  The output of the DM process is therefore a binary sequence that can be used at the receiver to reconstruct the staircase function. Staircase

11/21 Pulse Code Modulation (PCM) Versus Delta Modulation (DM)  DM has simplicity compared to PCM  DM has worse SNR compared to PCM  PCM requires more bandwidth  eg., for good voice reproduction with PCM  want 128 levels (7 bit) & voice bandwidth 4khz  need 8000 sample/s x 7bits/sample = 56kbps  PCM is more preferred than DM for analog signals

12/21 Analog Data, Analog Signal  Modulate carrier signal with analog data (voice)  Why modulate analog signals?  higher frequency can give more efficient transmission  permits frequency division multiplexing (chapter 8)  Types of modulation  Amplitude Modulation (AM)  Frequency Modulation (FM)  Phase Modulation (PM) DemodulatorModulator analog data carrier signal modulated signal

13/21 Amplitude Modulation (AM)  AM is the simplest form of analog modulation  Used in AM radio with carrier  Used also in analog TV broadcasting  Analog data modulates a carrier signal  Mathematically, the AM wave can be expresses as

11/45 Time Domain description of AM Signal  Derive an expression for the AM wave if the input signal:  Envelope of AM signal:

15/21 Frequency Domain description of AM Signal  The Double SideBand Transmitted Carrier (DSBTC): Upper Side Band (USB) Lower Side Band (LSB) Carrier

11/45 Frequency Domain description of AM Signal  Consider a voice signal m(t) with a bandwidth that extend from 300Hz to 3000Hz being modulated on a 60 KHz Carrier

17/21 Variations of AM signal  Double Side Band Transmitted carrier (DSBTC)  wast of power as the carrier is transmitted with the side bands  wast of bandwidth as both upper and lower side bands are transmitted (each side band contains the complete spectrum of the message signal m(t) ): Transmitted bandwidth=B T =2B  Double Side Band Suppressed Carrier (DSBSC) Less power is required as no carrier is transmitted  wast of bandwidth as both upper and lower side bands are transmitted: Transmitted bandwidth=B T =2B  Single Side Band (SSB) Less power is required as no carrier is transmitted Less bandwidth as one side band is transmitted: B T =B

18/21 Angle Modulation  Frequency Modulation (FM) and Phase Modulation (PM) are special cases of angle modulation  Used in FM radio with carrier  The angle modulated signal is expressed as:  Phase Modulation (PM): - Example: find s(t) if

19/21 Frequency Modulation (FM)  The angle modulated signal is expressed as:  FM when: - Example: find s(t) if

20/21 Transmitted Bandwidth for AM, PM and FM  Transmitted bandwidth for AM:  Transmitted bandwidth for PM and FM:  Thus, both PM and FM require greater bandwidth than AM

21/21 AM, PM, FM