Digital Pulse Modulation techniques.pptx

Digital Pulse Modulation PPT includes all digital pulse modulation techniques

1. Overview of Digital Pulse Modulation Techniques All wireless, fiber, and networked digital systems use modulation to encode information onto a carrier signal. As a carrier signal travels through some physical medium, it transports information between endpoints. Digital pulse modulation techniques are a subset of modulation methods for sending digital information over an analog channel. Digital modulation: These schemes use digital data to vary some quality (amplitude, phase, or frequency) of an analog carrier signal.  Digital pulse modulation: These techniques do not use digital signals with constant level for modulation. Instead, pulse modulation involves using quantized pulses to modulate a carrier signal or using a pulse train as the carrier signal for digital data (e.g.,  PAM4 signaling for high-speed networking ). 

2. The primary difference between these two sets of digital modulation techniques is the use of a truly digital signal vs. the use of pulses in modulation and transmission. Digital pulses do not have constant signal level, in contrast to true digital signals. As a result, digital modulation methods force carrier signal quantities to take specific signal levels. 

3. Transmission Bandwidth

4. DELTA MODULATION If the sampling interval ‘Ts’ in DPCM is reduced considerably, i.e. if we sample a band limited signal at a rate much faster than the Nyquist sampling rate, the adjacent samples should have higher correlation. The sample-to-sample amplitude difference will usually be very small. So, one may even think of only 1-bit quantization of the difference signal. The principle of Delta Modulation (DM) is based on this premise. Delta modulation is also viewed as a 1-bit DPCM scheme. The 1-bit quantizer is equivalent to a two-level comparator (also called as a hard limiter)

5. F eatures of Delta Modulation An over-sampled input is taken to make full use of the signal correlation. The quantization design is simple. The input sequence is much higher than the Nyquist rate. The quality is moderate. The design of the modulator and the demodulator is simple. The stair-case approximation of output waveform. The step-size is very small, i.e., Δ delta. The bit rate can be decided by the user. This involves simpler implementation. Delta Modulation is a simplified form of DPCM technique, also viewed as 1-bit DPCM scheme. As the sampling interval is reduced, the signal correlation will be higher.

6. Block diagram of a delta modulator

7. Working Principle Delta modulation transmits only one bit per sample. Here, the present sample value is compared with the previous sample value and this result whether the amplitude is increased or decreased is transmitted. Input signal x(t) is approximated to step signal by the delta modulator. This step size is kept fixed. The difference between the input signal x(t) and staircase approximated signal is confined to two levels, i.e., +Δ and -Δ. Now, if the difference is positive, then approximated signal is increased by one step, i.e., ‘Δ’. If the difference is negative, then approximated signal is reduced by ‘Δ’ . When the step is reduced, ‘0’ is transmitted and if the step is increased, ‘1’ is transmitted. Hence, for each sample, only one binary bit is transmitted.

8. D elta demodulator The delta demodulator comprises of a low pass filter, a summer, and a delay circuit. The predictor circuit is eliminated here and hence no assumed input is given to the demodulator. A binary sequence will be given as an input to the demodulator. The stair-case approximated output is given to the LPF.

9. Low pass filter is used for many reasons, but the prominent reason is noise elimination for out-of-band signals. The step-size error that may occur at the transmitter is called granular noise , which is eliminated here. If there is no noise present, then the modulator output equals the demodulator input.

10. From fig it is observed that the rate of rise of input signal x(t) is so high that the staircase signal can not approximate it, the step size ‗Δ‘ becomes too small for staircase signal u(t) to follow the step segment of x(t). Hence, there is a large error between the staircase approximated signal and the original input signal x(t). This error or noise is known as slope overload distortion . To reduce this error, the step size must be increased when slope of signal x(t) is high. Since the step size of delta modulator remains fixed, its maximum or minimum slopes occur along straight lines. Therefore, this modulator is known as Linear Delta Modulator (LDM).

11. Granular noise Granular noise occurs when step size is too large compared to small variations in the input signal. This means that for very small variations in the input signal, the staircase signal is changed by large amount because of large step size. The error between the input and approximated signal is called granular noise. The solution to this problem is to make step size small. Adaptive Delta Modulation To overcome the quantization error due to slope overload distortion and granular noise, the step size (Δ) is made adaptive to variations in input signal x(t). Particularly in the step segment of the x(t) , the step size is increased. Also, if the input is varying slowly, the step size is reduced. Then this method is known as Adaptive Delta Modulation (ADM). The adaptive delta modulators can take continuous changes in the step size or discrete changes in the step size

12. Introduction Up until now, we’ve always considered one transmitter sending one signal over one channel (or a medium) to one receiver. But in real life, to efficiently utilize the bandwidth of the channel, it is needed to send multiple signals over one channel. The techniques used to combine multiple signals for transmission over one channel are known as multiplexing techniques . Multiplexing Techniques Multiplexing is the process of sharing a single communication channel (or medium) for transmission of multiple signals. Why is this important? Often in communication (e.g. telephone systems) it is necessary or desirable to transmit more than one voice or data signal simultaneously. Telemetry: Many physical systems have multiple sensors which generate data that must be transmitted back to a monitor or control system. (For example, chemical plants, space shuttle, etc.) Oftentimes it is not practical or cost-effective to have a separate communication channel for each sensor.

13. An overview of signal multiplexing is shown in Figure 1. Note that the channel (or medium or link) shown in the figure can be a wire or free space (wireless communication) and A multiplexer (MUX) is a component that combines multipler signal into a single data stream, and A de-multiplexer ( DeMux )is a component that sepearate a single data stream into multiple signals.

14. Types of Multiplexing Multiplexing techniques can be categorized into the following three types: Frequency-division multiplexing (FDM): It is most popular and is used extensively in radio and TV transmission. Here the frequency spectrum is divided into several logical channels, giving each user exclusive possession of a particular frequency band. Time-division Multiplexing (TDM): It is also called synchronous TDM, which is commonly used for multiplexing digitized voice stream. The users take turns using the entire channel for short burst of time. Statistical TDM: This is also called asynchronous TDM, which simply improves on the efficiency of synchronous TDM.

15. Frequency Division Multiplexing Frequency Division Multiplexing (FDM) Frequency-division multiplexing is a form of signal multiplexing which involves assigning non-overlapping frequency ranges to different signals or to each "user of a medium. FDM achieves the combining of several signals into one medium by sending signals in several distinct frequency ranges over a single medium. Frequency division multiplexing involves translation of the speech signal from the frequency band 300-3400 Hz to a higher frequency band. Each channel is translated to a different b and and then all the channels are combined to form a frequency division multiplexed signal. In FDM, the speech channels are stacked at intervals of 4 kHz to provide a guard band between adjacent channels. Some of the commutation systems that use FDM include cable TV (each signal gets a 6MHz channel), FM stereo broadcasting (for Left and Right, and for Radio Data System (RDS)).

16. Use of guard bands in FDM If the channels are very close to one other, it leads to inter-channel cross talk. Channels must be separated by strips of unused bandwidth to prevent inter-channel cross talk. These unused channels between each successive channel are known as guard bands.

17. FDM are commonly used in radio broadcasts and TV networks. Since, the frequency band used for voice transmission in a telephone network is 4000 Hz, for a particular cable of 48 KHz bandwidth, in the 70 to 108 KHz range, twelve separate 4 KHz sub channels could be used for transmitting twelve different messages simultaneously. Each radio and TV station, in a certain broadcast area, is allotted a specific broadcast frequency, so that independent channels can be sent simultaneously in different broadcast area. For example, the AM radio uses 540 to 1600 KHz frequency bands while the FM radio uses 88 to 108 MHz frequency bands.

18. Time-division multiplexing (TDM) is a digital process that allows several connections to share the high bandwidth of a line. Instead of sharing a portion of the bandwidth as in FDM, time is shared. Time Division Multiplexing is the process of dividing up one communication time slot into smaller time slots.

19. Time Division Multiplexing (TDM) technique is used, where multiple signals share the same channel by taking turn transmitting. Data is broken up into frames and assigned to time slots. (Like cars merging in a lane). This technique is primarily used for digital data. Each signal uses the entire bandwidth of the channel when transmitting. On the receiving end, the demultiplexing process requires synchronization of the frames. This is often accomplished through a sync pulse. To help detecting transmission errors, additional error detection code (i.e. parity bits) may be added to each frame. In frequency division multiplexing, all signals operate at the same time with different frequencies, but in Time-division multiplexing all signals operate with same frequency at different times.

20. This is a base band transmission system, where an electronic commutator sequentially samples all data source and combines them to form a composite base band signal, which travels through the media and is being demultiplexed into appropriate independent message signals by the corresponding commutator at the receiving end. The incoming data from each source are briefly buffered. Each buffer is typically one bit or one character in length. The buffers are scanned sequentially to form a composite data stream. The scan operation is sufficiently rapid so that each buffer is emptied before more data can arrive. Composite data rate must be at least equal to the sum of the individual data rates. The composite signal can be transmitted directly or through a modem.

21. The composite signal has some dead space between the successive sampled pulses, which is essential to prevent interchannel cross talks. Along with the sampled pulses, one synchronizing pulse is sent in each cycle. These data pulses along with the control information form a frame. Each of these frames contain a cycle of time slots and in each frame, one or more slots are dedicated to each data source. Synchronous TDM is called synchronous mainly because each time slot is preassigned to a fixed source. The time slots are transmitted irrespective of whether the sources have any data to send or not. Hence, for the sake of simplicity of implementation, channel capacity is wasted. Although fixed assignment is used TDM, devices can handle sources of different data rates. This is done by assigning fewer slots per cycle to the slower input devices than the faster devices.

22. The maximum bandwidth (data rate) of a TDM system should be at least equal to the same data rate of the sources.

23. Advantages of TDM The user gets full bandwidth of the channel in a particular time slot. For burst signals such as voice or speech TDMA gives maximum utilization of the channel. Most suitable technique for digital transmission. It does not require precise carrier matching at both end of the links. Can expand the number of users on a system at a low cost. Simple circuit design. It uses entire channel bandwidth for the transmission of the signal. The problem of Intermodulation distortion is not present in TDM. Pulse overlapping can sometimes cause crosstalk but it can be reduced by utilizing guard time. Thus, is not much serious.

24. Disadvantages of TDM The transmitting and receiving section must be properly synchronized in order to have proper signal transmission and reception. Slow narrowband fading can wipe out all the TDM channels. It is not much suitable for continuous signals. Initial cost is high. The noise problem for analog communication has greater effect. Extra guard times are necessary. Applications of Time division multiplexing TDM finds its application mainly in a digital communication system, in cellular radio and in satellite communication system.