INTERNATIONAL JOURNAL OF ELECTRICAL, ELECTRONICS AND COMPUTER SYSTEMS, VOLUME 8, ISSUE 1, MAY 2012

MATLAB Implementation of Modulator for Video Signals SRIVIDYA.P #1, Dr. K.R NATARAJ *2, Dr.K.R.Rekha#3, M.G.Suresh*4 Research Scholar, VISVESVARAYA TECHNOLOGICAL UNIVERSITY Belgaum, Karnataka,India [email protected] Professor, Dept of ECE, SJBIT Kengeri, Bangalore, India. [email protected] Head of Department, Dept of ECE, SJBIT Kengeri, Bangalore, India. Research Scholar, VISVESVARAYA TECHNOLOGICAL UNIVERSITY Belgaum, Karnataka,India Abstract— The modulators are the basic requirement

of the communication systems. They are designed to reduce the channel distortion & for use in RF communication. Hence many types of carrier modulation techniques have already been proposed according to channel properties & data rate of the system. This paper deals with implementation of a QPSK modulator for video signals using Matlab. The Input video signals of varying frequency ranges from 50 KHz to 6MHZ are considered. The carrier frequency used is 30MHz. Finally, the results obtained are evaluated and discussed. Keywords— QPSK, Modulator, conversion, Video signals.

Matlab,

A/D

signals can be multiplexed together to form one signal. Storage and retrieval of voice, data or video at intermediate points (in the transmission path) is easy and is inexpensive in terms of storage space. Because of all these advantages, digital communications has grown quickly [1]. The objective of digital communication system is to transmit information at as fast a rate as possible, while maintain the probability of error within the admissible limits for the specific application for which the system has been designed. Further, it must do this by using minimum possible bandwidth and power by maintaining a high bandwidth efficiency and power efficiency. Fig 1 shows the block diagram of communication system.

I. INTRODUCTION With the advances in high speed, programmable digital signal processing (DSP) chips, modern communication links are using a combination of DSP techniques and digital communication methods to realize faster, reconfigurable, and modular systems. The proposed system will serve as a practical tool useful for simulating the transmission of video signals. II. DIGITAL COMMUNICATION Digital communications and signal processing refers to the study concerned with the transmission and processing of digital data. This is in contrast with analog communications. The analog communications use a continuously varying signal, a digital transmission can be broken down into discrete messages. Transmitting data in discrete messages allows for greater signal processing capability. Detecting and correcting errors are easier in Digital communication. Digital signals can also be sampled instead of continuously monitored and multiple

Fig. 1 Block diagram of Communication System.

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INTERNATIONAL JOURNAL OF ELECTRICAL, ELECTRONICS AND COMPUTER SYSTEMS, VOLUME 8, ISSUE 1, MAY 2012

III. ANALOG- TO-DIGITAL CONVERSION In digital communication, information is transmitted in the form of a continuous string of binary ones and zeroes. Thus it is necessary to convert the analog baseband signal, such as a sound or video recording, to a digital signal. Pulse code modulation (PCM) is a conventional technique that converts an analog waveform into a sequence of binary numbers. The first step is to establish a set of discrete times at which the input signal is sampled. According to a classic theorem of sampling theory stated by Harry Nyquist, the minimum sampling rate fs is twice the highest baseband frequency fm , or fs = 2 fm . The next step is to represent each analog sample value by a binary number. If there are n bits per sample, then there can be 2n – 1 possible level in each sample. Fig 2 shows the block diagram of Analog to Digital conversion.

frequency. Baseband signal is called modulating signal and band pass signal is called modulated signal. Baseband: Describes signals and systems whose range of frequencies is measured from 0 to a maximum bandwidth or highest signal frequency. Carrier: It is a waveform (usually sinusoidal) that is modulated to represent the information to be transmitted. This carrier wave is usually of much higher frequency than the modulating (baseband) signal. V. DIGITAL MODULATION Modulation can be done by varying the Amplitude, Phase, or Frequency of a high frequency carrier in accordance with the amplitude of the message signal. These techniques give rise to amplitude-shiftkeying (ASK), frequency-shift-keying (FSK) and phase-shift-keying (PSK), respectively.

VI. QUADRATURE PHASE SHIFT KEYING MODULATOR QPSK is a bandwidth efficient band pass digital modulation scheme that makes use of quadrature multiplexing. Just like in Binary Phase Shift Keying (BPSK), in QPSK too, the message information is carried in the phase of the transmitted signal. In QPSK modulation the carrier can assume one of four phase states, consisting of 0, π/2, π, and 3*π/2, which Fig. 2 ADC Block Diagram represent the symbols 00, 01, 11, and 10. A QPSK Sampling – Takes samples at time nT. Where T is the modulator is usually thought of as two Binary Phase time period and fs=1/T is the sampling period. Analog Shift Keying (BPSK) modulators that are out of phase signals must be sampled to be processed by MATLAB. by 90. For a given information bit rate Rb the power Infinite sampling rate is the ideal, but we settle for a required by BPSK and QPSK remains same. However, sampling rate that is about twice the highest since each QPSK symbol consists of two bits while bandwidth. Highest bandwidth of analog video signal each BPSK symbol consists of only one bit, the considered is 6MHz. Therefore sampling frequency bandwidth required for QPSK modulation is only half that for BPSK. Therefore, QPSK has been the considered is 12MHz. preferred form of digital modulation in satellite Quantization – Scalar quantization is a process that communications[2] ,[3]. maps all inputs within a specified range to a common QPSK involves the splitting of data stream bk(t) = value. This process maps inputs in a different range of values to a different common value. In effect, scalar bo,b1,b2,….., into an in phase stream bI(t) = b0,b2,b4,…., and a quadrature stream quantization digitizes an analog signal. bq(t)=b1,b3,….Both the streams have half the bit rate Binary Encoding – It converts each quantized value of the data stream bk(t), and modulate the cosine and sine functions of a carrier wave simultaneously. As a into a binary code word. result, phase changes across intervals of 2Tb, where Tb is the time interval of a single bit (the bk(t)s)[4]. IV. MODULATION In QPSK two sinusoids (sin and cos) are taken as Modulation is the process of varying a carrier basis functions for modulation. Modulation is signal in order to use that signal to convey achieved by varying the phase of the basis functions information. It includes encoding of information from a message source in a manner suitable for depending on the message symbols. In QPSK, transmission. It involves translating a baseband modulation is symbol based, where one symbol message signal to a band pass signal at frequencies contains 2 bits. Each such pair of bits is called a dibit. that are very high compared to the baseband The following equation (1) outlines QPSK modulation technique.

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INTERNATIONAL JOURNAL OF ELECTRICAL, ELECTRONICS AND COMPUTER SYSTEMS, VOLUME 8, ISSUE 1, MAY 2012

When n=1, the phase shift is 45 degrees. This is called pi/4 QPSK. The constellation diagram of QPSK will show the constellation points lying on both x and y axes. This means that the QPSK modulated signal will have an in-phase component (I) and also a quadrature component (Q). This is because it has only two basis functions. A QPSK modulator can be implemented as follows. A serial to parallel converter is used to separate odd and even bits from the generated information bits. Each of the odd bits (quadrature arm) and even bits (in-phase arm) are converted to NRZ format in a parallel manner. The signal on the in-phase arm is multiplied by cosine component and the signal on the quadrature arm is multiplied by sine component. QPSK modulated signal is obtained by adding the signal from both in-phase and quadrature arm. Fig 3 shows the block diagram of QPSK modulator.

A codebook tells the quantizer which common value to assign to inputs that fall into each range of the partition. A codebook is represented as a vector whose length is the same as the number of partition intervals. Quantization distorts a signal. Distortion can be reduced by choosing appropriate partition and codebook parameters. Following is the partition and code book selected. partition = [-7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7]; Codebook = [-7.5 -6.5 -5.5 -4.5 -3.5 -2.5 -1.5 -.5 .5 1.5 2.5 3.5 4.5 5.5 6.5 7.5]; The quantiz function accepts the input signal, partition and codebook and returns a vector that tells which interval each input is in. 5. 6. 7. 8.

9.

Encoding the quantized signal. For each sample 4 bits are used for encoding. The encoded signal is split into odd and even bits. I channel and Q channel bits and NRZ encoded. The signals obtained in I channel and q channel are then modulated using the carrier sine and cosine waves. The carrier selected is 30MHz. Finally, the I channel and Q channel signals are combined to obtain QPSK modulated signal. VIII. MATLAB SIMULATION RESULTS

Fig. 3 QPSK Modulator Block Diagram

VII. MATLAB IMPLEMENTATION STEPS Selecting the base band signal frequency. A time varying video signal varying in frequency from 50 KHz to 6MHz is chosen. 2. Selecting the sampling frequency. Sampling frequency selected is 12 MHz. 3. Sampling the input signal. 4. Quantizing the signal. Two parameters determine a quantization: a partition and a codebook. A quantization partition defines several contiguous, non overlapping ranges of values within the set of real numbers. To specify a partition in the Matlab environment, distinct endpoints of the different ranges in a vector is to be specified. 1.

Fig. 4 Quantized input signal

Fig 4 shows the quantized values for 3 MHz input signal. X axis shows the samples and Y axis shows the amplitude.

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INTERNATIONAL JOURNAL OF ELECTRICAL, ELECTRONICS AND COMPUTER SYSTEMS, VOLUME 8, ISSUE 1, MAY 2012

IX. CONCLUSION A simple direct QPSK digital modulator model is implemented in MATLAB simulation software. The modulator generates QPSK signal for time varying video signal. The carrier frequency is 30MHz and input frequency is varied from 50 KHz to 3MHZ. REFERENCES [1]

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[2] Fig. 5 Output of Q channel and I channel.

[3]

Fig 5 shows NRZ format for odd bits (quadrature arm) and even bits (in-phase arm), Carrier waves for the arms (Sine and cosine) and the multiplied component in each arm. X axis depicts time and Y axis depicts amplitude.

[4] [5]

[6]

[7]

[8]

J Proakis “Digital Communications”, 3rd Edition, International Edition, McGraw-Hill (1995). M.Kovac, J.Kolouch “BPSK, QPSK Modulator simulation Module”2004. Sanjit Krishnan Kaul, “QPSK, OQPSK,CPM Probability of error for AWGN and Flat Fading Channels” , Wireless Communication Techniques Spring 2005, Department of Electrical Engineering, Rutgers University, Piscataway, NJ. http://www.home.agilent.com/agilent/ product. (Digital modulator /Demodulator Agilent E8408A VXI) Prof. Jianfeng Feng, Dept of Computer Science and Centre for Scientific Computing, University of Warwick,UK,“Digital Communications and Signal Processing with MATLAB Examples”. Tomas Kratochvil, David Kriz, “Utilization of Matlab for the digital modulation methods simulation in DVB area”, Institute of Radio Electronics, Brno University of Technology. Teena Sakla , Divya Jain, Sandhya Gautham, “Implementation of Digital QPSK Modulator by using VHDL/Matlab”,International Journal of Engineering Science and Technology, Vol.2(9),2010,pp 4827-4831. Kriz D, “simulation of the digital modulation technique QPSK and QAM in DVB area”, Student project, FFEC BUT, Brno, 2004.

Fig. 6 Modulated output.

Fig 6 shows the QPSK modulated wave.

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