2.
Pulse Amplitude Modulation (PAM) <ul><li>If a message waveform is adequately described by periodic sample values, it can be transmitted using analogue pulse modulation wherein the sample values modulate the amplitude of pulse train. </li></ul><ul><li>Therefore, the amplitudes of regularly spaced pulses are varied in proportion to the corresponding sample values of a continuous message signal x ( t ) . </li></ul><ul><li>This technique is termed </li></ul><ul><li>Pulse Amplitude </li></ul><ul><li>Modulation </li></ul>
3.
Generation of the PAM signal <ul><li>There are two operations involved in the generation of the PAM signal: </li></ul><ul><li>Instantaneous sampling of the message signal x ( t ) every Ts seconds, where the sampling rate f s = 1/ Ts is chosen in accordance with the sampling theorem </li></ul><ul><li>Lengthening the duration of each sample so obtained to some constant value τ (sample‐and‐hold) </li></ul>
7.
Block diagram of PAM generation System for recovering message signal m(t) from PAM signal s(t).
14.
Pulse Code Modulation <ul><li>PCM is the most commonly used technique in digital communications </li></ul><ul><li>Used in many applications: </li></ul><ul><ul><li>Telephone systems </li></ul></ul><ul><ul><li>Digital audio recording </li></ul></ul><ul><ul><li>CD laser disks </li></ul></ul><ul><ul><li>voice mail </li></ul></ul><ul><ul><li>digital video etc. </li></ul></ul><ul><li>They are a primary building block for advanced communication systems </li></ul>
15.
Pulse Code Modulation <ul><li>Based on the sampling theorem </li></ul><ul><li>Each analog sample is assigned a binary code </li></ul><ul><ul><li>Analog samples are referred to as pulse amplitude modulation (PAM) samples </li></ul></ul><ul><li>The digital signal consists of block of n bits, where each n -bit number is the amplitude of a PCM pulse </li></ul>
16.
Pulse Code Modulation (PCM) <ul><li>As in the case of other pulse modulation techniques, the rate at which samples are taken and encoded must conform to the Nyquist sampling rate. </li></ul><ul><li>The sampling rate must be greater than, twice the highest frequency in the analog signal, </li></ul><ul><li>f s > 2 f A (max) </li></ul><ul><li>Telegraph time-division multiplex (TDM) was conveyed as early as 1853, by the American inventor M.B. Farmer. The electrical engineer W.M. Miner, in 1903. </li></ul><ul><li>PCM was invented by the British engineer Alec Reeves in 1937 in France. </li></ul><ul><li>It was not until about the middle of 1943 that the Bell Labs people became aware of the use of PCM binary coding as already proposed by Alec Reeves. </li></ul>
17.
Figure The basic elements of a PCM system. Pulse Code Modulation
19.
<ul><li>Robustness to noise and interference </li></ul><ul><li>Efficient regeneration </li></ul><ul><li>Efficient SNR and bandwidth trade-off </li></ul><ul><li>Uniform format </li></ul><ul><li>Ease add and drop </li></ul><ul><li>Secure </li></ul>Advantages of PCM
21.
Quantizing <ul><li>The process of converting analog signals to PCM is called quantizing </li></ul><ul><li>Since the original signal can have an infinite number of signal levels, the quantizing process will produce errors called quantizing errors or quantizing noise </li></ul><ul><li>The dynamic range of a system is the ratio of the strongest possible signal that can be transmitted and the weakest discernible signal </li></ul><ul><li>In a linear PCM system, the maximum dynamic range is found by: </li></ul>DR = (1.76 + 6.02 m ) dB
23.
Two types of quantization: ( a ) midtread and ( b ) midrise.
24.
Illustration of the quantization process. (Adapted from Bennett, 1948, with permission of AT&T.)
25.
Companding <ul><li>Companding is used to improve dynamic range </li></ul><ul><li>Compression is used on the transmitting end and expanding is used on the receiving end, hence companding </li></ul>
26.
Intersymbol Interference <ul><li>If the system impulse response h(t) extends over more than 1 symbol period, symbols become smeared into adjacent symbol periods </li></ul><ul><li>Known as intersymbol interference (ISI) </li></ul>
27.
Intersymbol Interference <ul><li>Example </li></ul><ul><ul><li>Response h ( t ) is Resistor-Capacitor (R-C) first order arrangement- Bit duration is T </li></ul></ul>Time (bit periods) amplitude Time (bit periods) amplitude <ul><li>For this example we will assume that a binary ‘0’ is sent as 0V. </li></ul>Modulator input Slicer input Binary ‘1’ Binary ‘1’
28.
Intersymbol Interference <ul><li>The received pulse at the slicer now extends over 4 bit periods giving rise to ISI. </li></ul><ul><li>The actual received signal is the superposition of the individual pulses </li></ul>time (bit periods) amplitude ‘ 1’ ‘ 1’ ‘ 0’ ‘ 0’ ‘ 1’ ‘ 0’ ‘ 0’ ‘ 1’
29.
Intersymbol Interference <ul><li>For the assumed data the signal at the slicer input is, </li></ul><ul><li>Clearly the ease in making decisions is data dependant </li></ul>time (bit periods) amplitude Note non-zero values at ideal sample instants corresponding with the transmission of binary ‘0’s ‘ 1’ ‘ 1’ ‘ 0’ ‘ 0’ ‘ 1’ ‘ 0’ ‘ 0’ ‘ 1’ Decision threshold
30.
Delta Modulation <ul><li>In Delta Modulation, only one bit is transmitted per sample </li></ul><ul><li>That bit is a one if the current sample is more positive than the previous sample, and a zero if it is more negative </li></ul><ul><li>Since so little information is transmitted, delta modulation requires higher sampling rates than PCM for equal quality of reproduction </li></ul>
33.
The modulator consists of a comparator, a quantizer, and an accumulator. The output of the accumulator is Slope overload distortion and granular noise