ENGG.tv

Learning Objectives:

Describe how time division multiplexing (TDM) can be used to improve the capacity of a PCM communications link, using TDM.

Time-division multiplexing (TDM) is a method of putting multiple data streams in a single signal by separating the signal into many segments, each having a very short duration. Each individual data stream is reassembled at the receiving end based on the timing.
The circuit that combines signals at the source (transmitting) end of a communications link is known as a multiplexer. It accepts the input from each individual end user, breaks each signal into segments, and assigns the segments to the composite signal in a rotating, repeating sequence. The composite signal thus contains data from multiple senders. At the other end of the long-distance cable, the individual signals are separated out by means of a circuit called a demultiplexer, and routed to the proper end users. A two-way communications circuit requires a multiplexer/demultiplexer at each end of the long-distance, high-bandwidth cable.
If many signals must be sent along a single long-distance line, careful engineering is required to ensure that the system will perform properly. An asset of TDM is its flexibility. The scheme allows for variation in the number of signals being sent along the line, and constantly adjusts the time intervals to make optimum use of the available bandwidth. The Internet is a classic example of a communications network in which the volume of traffic can change drastically from hour to hour.

Axhind, CC BY-SA 3.0, via Wikimedia Commons

Learning Objectives:

Describe the operation of Frequency Division Multiplexing.

FDM is widely used in radio and television systems (e.g. broadcast radio and TV) and was widely used in multichannel telephony (now being superseded by digital techniques and TDM). The multichannel telephone system illustrates some important aspects and is considered below. For speech, a bandwidth of 3kHz is satisfactory.

FDM is an analog multiplexing technique that combines analog signals into one medium by sending signals in several distinct frequency ranges over a single medium.
The spectrum of each input signal is shifted to a distinct frequency range.
Total available bandwidth of a medium is divided intoa series of non-overlapping frequency bands, each of which is used to carry a separate signal.
Carrier frequencies separated so signals do not overlap (guard bands)
Channel allocated even if no data
e.g. radio and television broadcasting at different frequencies over the air at same time, cable television and analog telephone systems, etc.

Axhind, CC BY-SA 3.0, via Wikimedia Commons

Learning Objectives:

Understand the basics of AM, its importance in analog communication, and its role in superimposing information on a carrier signal.
Learn how to represent an AM waveform as the product of a carrier signal and a modulating signal.
Express AM mathematically using equations to represent the carrier signal, modulating signal, and the modulated signal.

Amplitude modulation (AM) is defined as a process in which the amplitude of the carrier wave c(t) is varied about a mean value linearly with message signal m(t) keeping frequency and phase constant.

Solved Example: 9197-01

An amplitude modulated signal has a carrier frequency of 10 kHz. The upper sideband is transmitted at 11 kHz. The bandwidth required for the AM signal to transmit is:

A. 10 kHz

B. 2 kHz

C. 11 kHz

D. 1 kHz

Correct Answer: B

Solved Example: 9996-01

The maximum power efficiency of an AM modulator is:

A. 25%

B. 50%

C. 33%

D. 100%

Efficiency of modulation can be given as \[\eta = \dfrac{P_S}{P_S + P_C} = \dfrac{\mu ^2}{2 + \mu ^2}\] $\mu$=1 is the optimum value \[\eta_{max} = \dfrac{1}{1 + 2} = 33\%\]

Correct Answer: C

Learning Objectives:

Understand the importance and operation of PCM.

With Pulse Code Modulation (PCM), the amplitude of sound wave is sampled at regular intervals and translated into a binary number.
The difference between the original analog signal and the translated digital signal is called quantizing error.
PCM Processes have four parts:
- Flitering:
- Sampling: Analog signal is sampled every T$_s$ seconds, which is referred as sampling interval. The three sampling methods are: Ideal, neutral and flattop.
- Quantization: The process of measuring the numerical values of the samples and giving them a table value in a suitable scale. In PCM, 8 bit code is used and so we have 256 intervals for quantizing (128 levels in the positive direction and 128 in the negative direction.)
- Encoding:

Advantages of PCM

It is robust against noise and interference.
Uniform transmission quality.
Efficient SNR and bandwidth trade-off.
It provides secure data transmission.
If offers efficient re-generation.
It is easier to add or drop channels.

Disadvantages of PCM

Large bandwidth is required for transmission.
More attenuation.
Quantizing error is present.
Encoding, decoding and quantizing circuit of PCM is complex.

Basic Modulation-Demodulation Concepts