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Basic Elements of PCM
Click to allow Flash After you enable Flash, refresh this page and the presentation should play. View by Category Toggle navigation. Products Sold on our sister site CrystalGraphics. Title: Chapter 2: Modulation. Description: 'Modulation is defined as the process of modifying a carrier wave radio wave Tags: chapter excursion modulation narrowband.
Latest Highest Rated. Modulation is defined as the process of modifying a carrier wave radio wave systematically by the modulating signal audio This process makes the signal suitable for the transmission and compatible with the channel. This process is known as Demodulation.
It is simply a measurement for the degree of modulation and bears the relationship of Vm to Vc Therefore the full AM signal may be written as 22 Amplitude Modulation DSBFC Full AM Using Upper sideband component Carrier component Lower sideband component So, with the modulating process, the original modulating signal is transferred to a different frequency spectrum with a higher value frequency 23 Amplitude Modulation DSBFC Full AM The frequency spectrum of AM waveform contains 3 parts A component at the carrier frequency fc An upper sideband USB , whose highest frequency component is at fcfm A lower sideband LSB , whose highest frequency component is at fc-fm The bandwidth of the modulated waveform is twice the information signal bandwidth.
What is the degree of modulation required to establish a desirable AM communication link?
Answer is to maintain mlt1. This is important for successful retrieval of the original transmitted information at the receiver end. This will cause severe distortion to the modulated signal.
This will give rise to the generation of the maximum message signal output at the receiver without distortion. The multiplication is achieved by using a network with a nonlinear characteristic. Nonlinear networks are not true multipliers because other components are produced and need to be filtered out. Since it is more simple and highly effective device produces a waveform at its output that is proportional to the real envelope of its input i. Following this, the capacitor C charges to the peak positive voltage of the carrier.
It capacitor then holds this peak voltage, results the diode stop conducting. Suppose now that the input-carrier amplitude is made to increase. Again, the diode resumes conduction, and the capacitor charges to a new higher carrier peak.
To ensure that the capacitor voltage vc to follow the carrier peaks when the carrier amplitude is decreasing, it is required to include the resistor R, so the capacitor C may discharge.
PCM (Pulse Code Modulation)2.ppt
In this case the capacitor voltage vc has the form shown in AM waveform i. An additional LPF might be needed to effectively smoothen out the saw tooth distortion of the envelope waveform shown in figure AM waveform after the envelope detector. If it is transmitted using the conventional AM system, the required bandwidth is 9. But according to the standardization, TV signal is limited to 6MHz only.
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So, to reduce to 6Mhz bandwidth, a part of the LSB is not transmitted. In this case SSB transmission is not applied as it is very difficult to suppress a sideband accurately at high frequency.
The amplitude of the carrier is constant while its frequency and rate of changes varied by the modulating signal Frequency modulated signal 72 Frequency Modulation FM Introduction The FM modulator receives two signals, the information signal from an external source and the carrier signal from a built in oscillator.
The modulator circuit combines the two signals producing a FM signal which is passed on to the transmission medium.
Point B is where the information signal is at the max. During the time from point A to B, the FM signal increases in freq. From point B to C, the FM signal freq. From point D to E, the FM signal increases until reaching the centre frequency. A fundamental characteristic of an FM signal is that the frequency deviation is proportional to the amplitude of the modulating signal, Vm and independent of the modulating frequency, fm or 82 Frequency Deviation The highest frequency for FM wave is The minimum frequency for FM wave is The total change of the frequency from minimum frequency to the maximum frequency is called frequency carrier swing, fcs 83 FM Frequency Spectrum As obtained, the FM signal is 84 FM Frequency Spectrum By using mathematical expressions Where Jn is a Bessel Function from first type, nth order J0 - will give the amplitude of the carrier Jn will give the amplitude of the sidebands, with frequency 85 FM frequency spectrum From above equation, the FM waveform has a component at the carrier frequency and an unlimited series of frequency, above and below the carrier frequency as below figure.
The remaining columns indicate the amplitudes of the carrier and the various pairs of sidebands.
Chapter 2: Modulation - PowerPoint PPT Presentation
Sidebands with relative magnitude of less than 0. This means that the signal represented by that amplitude is simply shifted in phase ? As you can see, the spectrum of a FM signal varies considerably in bandwidth depending upon the value of the modulation index.
The higher the modulation index, the wider the bandwidth of the FM signal. With some values of modulation index, the carrier can disappear completely. However, since the values of Jn?
Example Assuming a maximum frequency deviation of 5 kHz and a maximum modulating frequency of 2. This power is contained at the various frequency Spectrum components, in amounts determined by the mf and the corresponding Bessel Function 96 Power in FM The FM average power is where Pc carrier power n number of pairs of significant sidebands The average power of the modulated carrier PT must be equal to the average power of the un-modulated carrier 97 Narrow Band FM NBFM Modulation index approximates to 1 The frequency modulation is between 5 kHz to 10khz Bandwidth 10 30kHz The maximum modulating frequency 3 kHz NBFM is used for communication, in competition with SSB, having its main applications in various form of mobile communication eg.
All the transmitted power in FM is useful, whereas in AM most of it in the transmitted carrier, which contains no useful information 2. FM has the advantages over the AM, of providing greater protection from noise for the lowest modulating frequency 3.
In FM, the transmitted amplitude is constant. This characteristic has the advantages of significantly improving immunity to noise and interference Disadvantages of FM compared to AM 1. Since the reception is limited to line of sight, the area of reception for FM is much smaller than AM 2. Equipments for the transmitter and receiver are more expensive and complex 3. A much wider bandwidth is required by FM, up to 10 times larger than needed by AM. This is the most significant disadvantage of AM Frequency Modulation Amplitude modulation has two drawbacks that is serious deficiencies in dynamic range and in noise immunity For these reason, Frequency Modulation FM is introduced.
This is due FM is offering a wide dynamic range which is suitable for high fidelity system such as in FM stereo and can reduce the effect of noise However, it require a wide bandwidth and a complex system transceiver FM Waveform PM Communication Chart Phase Modulation PM Phase modulation is a system in which the phase of the carrier signal is varied by the information signal.
The amplitude of the carrier is kept constant. That is, the is proportional to the slope or first derivative of the modulating signal. In PM, the phase deviation is proportionally to the amplitude of the modulating signal and is independent of its frequency 3.
In FM, the frequency deviation is proportionally to the amplitude of the modulating signal Vm as well as its frequency, fm 4. The main difference between PM and FM, is how the information signal will change the carrier signal. Pulse modulation Analog Pulse Modulation APM Digital Pulse Modulation Pulse modulation can be used to transmit analogue information, it is first converted into pulses by the process of sampling.
PULSE CODE MODULATION - PowerPoint PPT Presentation
The sampling theorem Nyquist theorem is used to determined minimum sampling rate for any signal so that the signal will be correctly restored at the receiver.
Nyquists Sampling theorem Where fs sampling frequency fm max maximum frequency of the modulating signal Sampling Three basic condition of sampling process Sampling at fs2fm max Sampling Sampling at fsgt2fm max This sampling rate creates a guard band between fm max and the lowest frequency component fs-fm max of the sampling harmonics.
PWM gives a better signal to noise performance than PAM Pulse Position Modulation PPM PPM is when the position of a constant width and constant amplitude pulse within prescribed time slot is varied according to the amplitude of the modulating signal. There are Immune to channel noise and interference Signals and messages can be coded for error detection and correction Can carry a combination of traffics It is easier and more efficient to multiplex several digital signal More economical Disadvantages Requires significantly more bandwidth Requires precise time synchronization between the clocks in the transmitter and receivers Pulse Code Modulation PCM PCM is a form of digital modulation where groups of coded pulses are used to represent the analog signal.
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The analog signal is sampled and converted to a fixed-length, serial binary number for transmission. It consists of prescribed numbers of discrete amplitude levels Principles of PCM Three main process in PCM transmission are sampling, quantization and coding.
Only samples are being transmitted. If sufficient samples are sent and sampling theorem are met, the original signal can be constructed at the receiver Quantization Quantization is a process of assigning the analog signal samples to a pre-determined discrete levels. The number of quantization levels, L depends on the number of bits per sample, n, used to code the signal where Principles of PCM The magnitude of the minimum stepsize of the quantization levels is called resolution, The resolution depends on the maximum voltage, Vmax and the minimum voltage, Vmin of the information signal, where Principles of PCM Minimum stepsize resolution Principles of PCM Illustration of the quantization process Principles of PCM Quantization error or quantization noise is the distortion introduced during the quantization process when the modulating signal is not an exact value of the quantization level.
The maximum quantization error, Quantization error can be reduced by increasing the number of quantization levels, but this will increase the bandwidth required. It depends on the sampling frequency and the number of bit per sample used to encode the signal. Modem is an interface device consists of modulator and demodulator used in point-to-point data communication systems, through the public switching telephone networks PSTN.
Also it gives voltage and current appropriate for interfacing with the telephone line At the receiver It converts analog signal back to digital data signals.
Simple ASK is no longer used in digital communication systems due to noise problems. Relatively easy for FSK generation The constant amplitude property for the carrier signal does not waste power and does produce some immunity to noise.
Advantages Many signals can share an existing channel and make better use of the channel capacity allow several different signal to be clustered into a single group, for easy handling and maintenance Four simultaneous transmissions on a single circuit Multiplexing Multiplexing Three common techniques of multiplexing- Frequency Division Multiplexing FDM Time Division Multiplexing TDM Wavelength Division Multiplexing WDM Frequency division multiplexing FDM In FDM, multiple sources that originally occupied the same frequency spectrum are each converted to a different frequency band and transmitted simultaneously over a single wideband transmission system.
TDM is compatible with digital signals and makes good use of digital circuitry for these signal Simplistically, TDM physically switches from originator to originator to share the time available, and the receiving unit does the same in synchronism. FDM the individual channels are assigned to different frequency slots but jumbled together in the time domain TDM offers simpler instrumentation.
The interference in FDM is normally due to imperfect bandpass filtering and non-linear cross modulation In FDM, the bandwidth is used effectively The transmission medium of TDM is subjected to fading Wavelength Division Multiplexing WDM WDM is a technology that enables many optical signals to be transmitted simultaneously by a single fiber cable The basic principle behind WDM involves the transmission of multiples signals using several wavelengths without their interfering with one another.
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