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Transmission Of A Tv Signal English Language Essay

Paper Type: Free Essay Subject: English Language
Wordcount: 3456 words Published: 1st Jan 2015

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In electronics, many different types of carrier signals exists and due to the fact that this signals simply carry data intended for a different audience, which most of the times, are farther than the area of production of such signals. This gave rise to various modulation techniques to ensure effective transfer of these signals bearing information without loss of quality or degradation. This paper deals on how to modulate a Television (TV) signal using 16-QAM (Quadrature Amplitude Modulation)

Keywords include: 16-QAM, TV Signals, Amplitude Modulation, Data, and Attenuation, baseband.

1.0 Introduction

Everyday different types of signals transverse the air around us, most of which we can’t feel or see, but we see their impact or influence everyday through different electronic gadgets we have or own personally.

Most of these electronic gadgets are able to work as a result of them being able to receive a signal and they able to correctly interpret that signal and we in turn appreciate the devices.

For the devices to correctly interpret these signals a kind of technique was used and this is known as Demodulation, this being that the devices are designed to receive this type of signals. The processes that led to being able to demodulate is what my focus is on; the process is known as modulation.

Why modulation? This is due to the fact that not all intended users of a particular device or service reside close to the production station and also production signals called baseband signals can’t travel much and would need a lot of power to transmit them over the air medium and this adds to the operating cost of the station and this is not acceptable.

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2.0 Transmission medium for TV signal

As with any kind of signal, the most important of that signal is to be received and decoded correctly, so that information encoded can be seen or viewed. The medium of passage of such signals is of consequence, because these helps to know the appropriate modulation to use and what, this information is tied to the channel coding, the advantages of the channel and the noise rate of that channel in consideration.

For example, the transmission medium for sound received by the ears is usually air, sound can also travel through solids, liquids and plasma Electromagnetic waves have no need of any material substance for it to propagate but can be affected by the transmission media they pass through at the boundary between the media by absorption, reflection or refraction.

Therefore, TV signals are like communicating data sent from one location to another which requires a pathway or medium. These pathways, called communication channels; two types of preferred media are: – (a) Cable (twisted-pair wire, cable, and fibre-optic cable) and (b) broadcast (microwave, satellite, radio, and infrared). {Note:- Cable media use physical wires of cables to transmit data and information, while Twisted-pair wire and coaxial cables are made of copper, and fibre-optic cable is made of glass.}

2.1 BASEBAND SIGNAL

The definition of a baseband signal is a signal that contains information, which may vary from audio signals, video signals, speech signal, image signal and many different other types depending on application and use. These signals can’t move more than a few metres after production; for example, a speech signal; two persons can hear themselves clearly in close proximity to each other, as the distance between them increase, they start hearing faint messages or signals from each other and at a point, they can’t hear each other again. Therefore for them to be heard no matter the distance between them there is need to modulate the speech signal generated by each person, so that each person can hear what the other person has said or is saying.

A signal can be said to be an analogue data stream with which it varies with respect to space and time; with this property, each signal can be sampled, quantized, and transformed into digital data streams or packets. These processes it goes from being analogue to digital data streams or packet is called modulation. There are different modulation techniques used to carry different signals depending on the transmission medium and the condition inherent in that medium.

2.2 Modulation

What is modulation? According to Margaret Rouse, modulation is the combination of signal to an electronic signal carrier, modulation is useful in direct current by turning it on and off to alternating current.

Literally, modulation can be said to be a way of quicken the transfer of data or information through a medium. Transmission of sound through air has restricted degree of power generated by the lungs, to give the extent at which the voice can reach there is need to transmit it through a medium such as phone line, radio etc. therefore, the successful conversion of information in this case voice, to pass through a medium such as wire or radio waves is known as Modulation (www.complextoreal.com ).

2.3 TYPES OF MODULATION

Modulation helps in many ways and it can be referred to as the process of transposing a signal wave onto a carrier wave in order to carry information. There are 3 three key properties of this carrier wave, which is a sinusoid wave with an amplitude, a phase and a frequency, these properties can be exploited to generate another kind of waveform to carry the intended information which is known referred to as a modulated signal. (www.encyclopediapro.com/mw/modulation)

In most part of the world, frequency spectrum is expensive to waste and often it’s a national property, which means, to use it, the intending user has to be licensed and must follow a protocol process to have that frequency band assigned to him solely and given the channels parameters he can use. This is because, there are many different users competing for the same frequency band, due to the fact that the lower the frequency given, the lower the operating cost of using that frequency band.

There are 2 broad class of modulation; they are the analogue modulation and the digital modulation.

In analogue modulation, there is a continuous change in response to the signal to be modulated; this can be seen in Phase Modulation, Frequency Modulation, Amplitude Modulation, Single-Sideband Modulation, and Vestigial-sideband Modulation.

In digital Modulation, there are changes in the signal position with respect to a starting position, while each position corresponds to a piece of information or symbol and can be represented on a constellation diagram. Examples include: Phase-Shift Key (PSK), Frequency-shift Key (FSK), Amplitude-shift Key (ASK), Minimum-shift Key (MSK), Gaussian minimum-shift Key (GMSK), Very minimum-shift Key (VMSK), Quadrature Amplitude Modulation (QAM), this a combination of PSK and ASK. Continuous phase modulation (CPM) and Trellis coded modulation (TCM). (www.encyclopediapro.com/mw/modulation)

For each of these types of modulation techniques, their phases, frequencies and amplitude are assigned a unique sequence of bits which corresponds to an equal number of bits and make up a symbol that represent that particular phase or code.

3.0 QAM

The modulation of interest for the purpose of his paper is the QAM, or Quadrature Amplitude Modulation, from research; it was noticed that QAM has both an analogue and digital modulation properties depending on how it’s to be applied.

QAM is made up of two modulation scheme, namely the Amplitude-shift Keying which is achieved by modulating or changing the state of the amplitude of two carrier waves to carry two analogue message signal or two digital data streams. The other is the Phase-shift Keying, in which the sinusoidal waves are usually out of phase to each other and apart by 90o. When these two waveforms are combined, it results to Quadrature Amplitude Modulation (two different amplitudes & two out of phase signals).

QAM, this modulation technique is used to encode and transmit digital cable channels, while the simplest of the QAM techniques is the 16-QAM i.e. 24 phase points, this is able to carry 6MHz bandwidth regulated by the authority’s policy. The type of signal interested in is the signal generated from a video source, this type of signal consumes more than the set frequency of 6MHz, a typical video source signal is huge, so using 16- QAM techniques, it is possible to encode the signal onto the QAM carrier wave to carry it and pass through the 6MHz bandwidth limit and still be able to carry additional signals such as the voice signal component of the video signal. However there are different video formats such as the NTSC, PAL, MPEG; this video formats also help in reducing the size of video signal, hereby assisting the 16-QAM to effectively encode the signal without loss of quality and making it noise proof. Other variants of QAM includes 64, 256 phase variants, but as this phase’s size increase, so do they incorporate noise and mismatching often affect the performance of the receiver to be correctly tuned in to receive the sequence of bits sent over the medium.

One of the uses of QAM is in the modulation of digital television systems, due to its high spectral efficiencies and the requirement of a cleaner path, and such hybrid fibre coaxial cables are used to deliver digital television signals to homes. (http://en.wikipedia.org/wiki/QAM_tuner, 2012) In the United Kingdom, 2 variants of the QAM are being used, this are the 16 and 64 variants and they are used for digital terrestrial TV like Freeview by British Telecoms & Top-Up TV, while for the 256-QAM, is being considered for the HD version of freeview which is delivered over high speed fibre optics links to homes across the country.

Therefore, the makeup of QAM is shown via the figure below and the different modulation techniques it uses to modulate and encode its own signal and how it lead to the development of QAM.

3.0.1 Amplitude Shift Keying (Ask)

According to N.Vlajic (2010), Ask is the modulation of digital data with changes in the amplitude of a carrier while both frequencies an d phase remain constant; carrier signal is varied by representing the binary with 0 and 1.

Also in ASK, the mode of operation is to vary the amplitude of the carrier wave to carry a baseband signal, while the frequency and phase remain constant.

Ask has two levels called binary bits of implementing its modulation process and it is often referred to as Binary Amplitude shift keying or on-off keying (B-ASK or OOK). Each binary bit corresponds to a symbol that represents an amplitude level.

According to (Glover and Grant, 2004, chapter 11, page 391), they said that in B-ASK, the two digital levels can be one and zero, to represent the pulse of a sinusoidal carrier wave, although any one of the changing amplitude can be inferred to be one and the other zero and from this result, it describes why it also referred to as on-off keying sequence. This method of digital sequence of one and zero can be referred to a switch and was used to transmit Morse codes (Wikipedia) in the early 20th century.

In recent times, additional amplitude levels has been developed for enhanced encoding schemes such as a 4 four level encoding scheme to take on two bits or 22with corresponding shift in amplitude, also there is an eight level scheme to represent 3 three bits or 23, and so on. This gives a reduced power required to transmit and hence advantageous to use. (Wikipedia-http://en.wikipedia.org/wiki/Amplitude-shift_keying)

A binary amplitude-shift keying (BASK) signal can be defined by

S (t) = A m (t) cos 2pfct. For: (0 < t < T). Where A is a constant, m (t) = 1 or 0, fc is the carrier frequency, and T is the bit duration. (http://engineering.mq.edu.au/~cl/files_pdf/elec321/lect_mask.pdf )

Implementation of ASK is relatively cheap and simple but its amplitude is usually affected by noise, distortion s and the propagation conditions on different route which makes the process more difficult. In ASK digital data is transmitted over an optical fiber.

Figure 1. ASK signal representation.

3.0.2 Phase shift keying (PSK)

Phase-shift keying (PSK) refers to a modulation scheme that is used to carry information by changing, or modulating, the phase of the carrier wave.

PSK can still be defined as the modulation of digital data with amplitude at peak, frequency remain constant and phase with carrier signal varied to depict binary 0 or 1, with binary 0 = 180 degree phase and binary 1 = 0 degree.PSK is not sensitive to error and productively make use of bandwidth, so high data rate is possible but has more complex signal detection (N. Vlajic, 2010).

This modulation scheme like the Amplitude shift Keying (or ASK) also uses a finite number of distinct signals to represent digital data. Each of the finite phases, is assigned a unique pattern of binary digits that forms a symbol. Phase shift keying comes in two forms, which are binary PSK (this uses two phases which are 180o apart) and Quadrature PSK (this uses four phases and the phases are 45o, -45o 135o and -135o). (Forouzan, 2013 pg 144)

3.0.2.1 Binary PSK

In Binary PSK, it has only two signal element said suggested by Forouzan, 2013, pg 142; with each phase set at 0o and the other at 180o. The general form for BPSK follows this equation Sn (t). (wikipedia- H. Stern & S. Mahmoud, Communications Systems, Pearson Prentice Hall, 2004, p283)

s_n(t) = sqrt{frac{2E_b}{T_b}} cos(2 pi f_c t + pi(1-n )), n = 0,1.

This yields two phases, 0 and π. In the specific form, binary data is often conveyed with the following signals:

s_0(t) = sqrt{frac{2E_b}{T_b}} cos(2 pi f_c t + pi )

= – sqrt{frac{2E_b}{T_b}} cos(2 pi f_c t)For binary “0”

s_1(t) = sqrt{frac{2E_b}{T_b}} cos(2 pi f_c t)

For binary “1”

[where fc is frequency of the carrier-wave.]

Also, the BER of a two signal BPSK in AWGN can be calculated as follows

P_b = Qleft(sqrt{frac{2E_b}{N_0}}right)

P_b = frac{1}{2} operatorname{erfc} left( sqrt{frac{E_b}{N_0}}right)

3.0.2.2 Quadrature PSK

This sub part of psk is such that it uses two separate BPSK modulations to generate a QPSK modulation scheme; this is able to work due to each BPSK band as an in-phase carrier and the other out of phase carrier. The phases are different and the first point is located at 45o, then 135o, then -135o and lastly -45o, the amplitude stays constant at 2½. The bit stream is then split into two bit streams I (in-phase) and Q (Quadrature) .The bit stream in-phase (I) is called the “even” stream and quadrature(Q) is called “Odd” stream. (http://turboblogsite.com/quadrature-phase-shift-keying-qpsk-modulation.html)

QPSK is used mostly to transfer digital data by varying the phases of the carrier signal, the 4 phase points tally with 4 points on a circle and can be so represented. The figure below shows the mapping of the 4 phase points.

QPSK diagram showing how four different binary codes can be transmitted

Fig: QPSK diagram showing how four different binary codes can be transmitted

The list below gives some of the other commonly used forms of phase shift keying, PSK, they are O-QPSK – Offset Quadrature Phase Shift Keying, 8 PSK – 8 Point Phase Shift Keying, 16 PSK – 16 Point Phase Shift Keying, QAM – Quadrature Amplitude Modulation, 16 QAM – 16 Point Quadrature Amplitude Modulation and 64 QAM – 64 Point Quadrature Amplitude Modulation (http://www.radio-electronics.com/info/rf-technology-design/pm-phase-modulation/what-is-psk-phase-shift-keying-tutorial.php)

Figure 2. PSK signal diagram representation

3.1 QAM bits per symbol

The advantage of using QAM is that it is a higher order form of modulation and as a result it is able to carry more bits of information per symbol. By selecting a higher order format of QAM, the data rate of a link can be increased. (http://www.radio-electronics.com/info/rf-technology-design/pm-phase-modulation/8qam-16qam-32qam-64qam-128qam-256qam.php)

The table below gives a summary of the bit rates of different forms of QAM and PSK.

Modulation

Bits per symbol

Symbol Rate

BPSK

1

1 x bit rate

QPSK

2

1/2 bit rate

8PSK

3

1/3 bit rate

16QAM

4

1/4 bit rate

32QAM

5

1/5 bit rate

64QAM

6

1/6 bit rate

3.1.1 Quantized QAM using constellation diagram

In QAM, the constellation diagram plots are usually arranged in square formation with different spacing and this spacing point represent binary bits per symbol, this spacing point is in the power of 2’s. The more the bits per symbol, the more it is to transfer more information over a given channel or medium. But as the bits increase, the points move closer and make the system susceptible to noise corruption, and this gives rise to a high BER ratio, making useless the information sent due to its high noise component.

There are different bit positions for the constellation diagram, this diagrams show the different position of each symbol and the states of QAM, and the constellation diagrams for BSPK, 16 QAM. 32 QAM, 64 QAM. Therefore, the figures below show various constellation diagrams:

http://www.radio-electronics.com/info/rf-technology-design/pm-phase-modulation/modulation-constellation-bpsk.gif

http://www.radio-electronics.com/info/rf-technology-design/pm-phase-modulation/modulation-constellation-16qam.gif

http://www.radio-electronics.com/info/rf-technology-design/pm-phase-modulation/modulation-constellation-32qam.gif

http://www.radio-electronics.com/info/rf-technology-design/pm-phase-modulation/modulation-constellation-64qam.gif

3.1.2 QAM applications

QAM is in many radio communications and data delivery applications. However some specific variants of QAM are used in some specific applications and standards.

For domestic broadcast applications for example, 64 QAM and 256 QAM are often used in digital cable television and cable modem applications. In the UK, 16 QAM and 64 QAM are currently used for digital terrestrial television using DVB – Digital Video Broadcasting. In the US, 64 QAM and 256 QAM are the mandated modulation schemes for digital cable as standardised by the SCTE in the standard ANSI/SCTE 07 2000.

In addition to this, variants of QAM are also used for many wireless and cellular technology applications.

4.0 Conclusion

In this review, it is clearly shown that Amplitude Shift Keying (ASK) and Phase Shift Keying (PSK) modulation techniques that form Quadrature Amplitude Modulation (QAM) ensures effective transfer of TV signals In QAM different combination of amplitude and phase are used to achieve higher digital data rate, the number of bit transmitted per time T (sec) interval can be further increased by increasing the number of levels used

References

Behrouz A. Forouzan , Data communications and Networking

H. Stern & S. Mahmoud, Communications Systems, Pearson Prentice Hall, 2004, p283)

http://engineering.mq.edu.au/~cl/files_pdf/elec321/lect_mask.pdf

http://en.wikipedia.org/wiki/Amplitude-shift_keying (Accessed 06 December 2012)

http://en.wikipedia.org/wiki/QAM_tuner, 2012 (Accessed: 28 November 2012)

http://turboblogsite.com/quadrature-phase-shift-keying-qpsk-modulation.html

http://www.radio-electronics.com/info/rf-technology-design/pm-phase-modulation/8qam-16qam-32qam-64qam-128qam-256qam.php (Accessed 06 December 2012)

http://www.radio-electronics.com/info/rf-technology-design/pm-phase-modulation/what-is-psk-phase-shift-keying-tutorial.php (Accessed 06 December 2012)

Ian A. Glover and Peter M. Grant, Digital communications, second edition

Intuitive Guide to Principle of Communication, www.complextoreal.com

Margaret Rouse, July 2005

Merriam Webster

N.Vlajic, Instructor, CSC 3213, fall 2010. Analog transmission of digital data (ASK, FSK, PSK, QAM)

Wikipedia, the free encyclopaedia (Accessed: 28 December 2012)

www.encyclopediapro.com/mw/modulation (Accessed: 28 November 2012)

 

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