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The TDM system is based upon the sampling of the amplitude of the information signal atregular intervals, and the subsequent transmission of one or more pulses to represent eachsample. In an analog pulse system, for the intelligence contained in the information signal to betransmitted, the characteristics of the pulse must be varied in accordance with the amplitude ofthe sample. This can be achieved by varying the amplitude, width, or the position of thepulses, to give either pulse amplitude, pulse duration, or pulse position modulation.With PAM, pulses of equal width and spacing have their amplitudes varied according to the characteristics of the modulating signal.
Pulse modulation (PM) consists of two signal, the carrier signal is in digital form which is used to transmit the information signal which in turn modulate the carrier signal to generate a modulated digital signal.is in the digital signal. Figure A below shows the modulating signal which is larger than the information signal figure B
Types of PM signals:
Pulse Amplitude Modulation (PAM) :
Is a method of encoding information in a signal by varying the amplitude of pulses. The unmodulated signal consists of a continuous train of pulses of constant frequency, duration, and amplitude. During modulation the pulse amplitudes are changed to reflect the information being encoded. In PAM the successive sample values of the analog signal are used to effect the amplitudes of a corresponding sequence of pulses of constant duration occurring at the sampling rate. No quantization of the samples normally occurs .In principle the pulses may occupy the entire time between samples, but in most practical systems the pulse duration, is limited to a fraction of the sampling interval. Such a restriction creates the possibility of interleaving during one sample interval one or more pulses derived from other PAM systems in a process known as time-division multiplexing (TDM). This type of modulation is so sensitive to noise as AM.
Pulse Code Modulation (PCM) :
Is a method of encoding information in a signal by varying the amplitude of pulses. Many samples are quantized where there are only predefined values of amplitude are selected to be coded to binary code. This binary or digital signals need to be demodulated in the receiver part to the original signal using a decoder and then a low pass filter to compensate for any error or noise signal. This method of modulation is preferred than the previous method PAM because it's well known by it's immunity to noise because it's coded in a digital signal. It also provide high quality transition pure of distortion or propagation.
Pulse Phase Modulation (PPM):
Is almost similar to Frequecy Modulation FM except that the phase of the carrier is changing where in FM the frequency of the carrier signal is varying. If the carrieng signal is as below
Then the modulated will be
The PPM can be modulated according to the size of the signal as in two models.
For small amplitude signals, PM is similar to amplitude modulation (AM) and exhibits its characteristics of sensitivity to noise and it's poor efficiency.
For a single large sinusoidal signal, PM is similar to FM, and its bandwidth is approximately
, and can exhibits it's characteristics of it's imunity to noise and distortion.
Pulse Duration Modulation (PDM):
Modulation of a pulse carrier wherein the value of each instantaneous sample of a modulating wave produces a pulse of proportional duration by varying the leading, trailing, or both edges of a pulse.
Pulse Frequency Modulation:
it is a method of modulation where a modulated wave is used to modulate a pulse generating signal. The amplitude and time are kept constant and the changing wave in instantaneous same time intervals.
-B43D1 Digital Communication Trainer-Digital Transmision of Analog Signals
(Figure B)- Oscilloscope ( figure C)- Connectors
1-PAM TIME-DIVISION MULTIPLEX:
The trainer set up is as shown in figure 1 below.
Connect the power supply in proper polarity to the oscilloscope and
the AF channel input (1,4) were connected into the oscilloscope .
The waveform is displayed on the screen of the oscilloscope.
the TX channel PAM was connected into the oscilloscope.
The waveform of the output is displayed in the oscilloscope screen.
the RX PAM was connected into the oscilloscope channel to display the PAM waveform.
the channel of AF output (channel 1,4) were connected into the oscilloscope to reproduce the two waveforms.
The data were recorded and the graphs were drawn.
figure 1. PAM TIME-DIVISION MULTIPLEX
Results and Discusions:
AF Channel Input(1,4):
Time Division Multiplexing is actually based on the peak rate of the signal where a samples only are taken to be transmitted.
As we can see in the figure above a digitized samples are being transmitted not the original wave .A tow input signals are being input to channel 1 and channel 4 to be transmitted using PAM Time-Division Multiplex. By using a high frequency carrier signal to transmit the two signals through the time division principle where the signal 1 is transmitted through channel-1 in a time slot different than that used for channel 4.
Let's consider first transmiting the first signal using channel-1 where the time division multiplexer set it to time slot 1.
The signal as shown in figure (ch1) above shows the input signal of channel 1 to be transmitted .the characteristics of this signal are:
-Peak-to-peak voltage of 5.36 V.
-Frequency of 150 Hz
-time division of 2.5 ms/ div
Now for input signal in channel 4 has different characteristics different of input singal to channel 1 to become possible for the TDM to transmit the two signals through a single line.
The characteristics of signal are:
-Peak-to-peak voltage of 5.24 V.
-Frequecy of 600 Hz.
The process of PAM modulation is done in few steps:
Samples of the two signals of channel 1 and channel 4 are taken across time then it will measure the amplitude of each signal sample. Those amplitude measurements of the signals are encoded in binary codes that represent the samples.
The binary of digital data is sent to the receiver part as discrete binary codes.
The sampled signals in the transmitter part are as shown below with their electrical characteristics:
2- signals at the TX PAM :
Electrical characteristcs :
-Peak-to-peak voltage of 5.20 V
-Frequecy of 150 Hz
-time division of 2.5 ms/div
This signal is the sampled signal of channel 1 input signal.
At the receiver the sampler has to be synchronized with the incoming waveform so that the PAM samples corresponding to source-1 will appear on the channel-1 output, and so signal 2 in channel 4.
The output signal in the received channel is as below:
-Peak to peak of 5.20 V (2 v/div)
-frequency of 600 Hz (channel 4)
Finally after the signal reached the receiver part and being decoded back into the original signal we got those outputs of channel 1 and channel 4.
AF Channel Output 1:
The characteristics of the signal are:
-Peak-to-peak voltage (5.20 V)
-frequency (150 Hz).
AF Channel Output 4 :
The characteristics of the output singal of chnnel 4 are:
- Peak-to-peak voltage (4.08 V)
We realize that the output signal is reproduced to corresponded to the input signal in channel 1 and channel 4 but there are reduction in the amplitude although the frequency is kept the same. This is due to addition or subtraction of a noise signal.
PAM is susceptibility to noise and the reason for such susceptibility is that any interference in the transmission line will eighther add or subtract from signal voltage, therefore the amplitude will change accordingly. As we know that the amplitude of the voltage is representing the signal so any unwanted change in the signal will cause a distortion ( unwanted pulses ) that affect the signal.
2-BASIC PCM OPERATION:
-The set-up is shown as in (figure 2) below.
-A Test Tone of 150 Hz is applied into input of the PCM encoder.
-The Tx is connected to the oscilloscope to display the signal at the transmitter.
-The Rx is connected to the oscilloscope to display the signal at the transmitter.
-The output channel of the PCM is connected to the oscilloscope and the demodulated signal is reproduced.
figure 2. BASIC PCM OPERATION
Results and Discusion:
Pulse Code Modulation (PCM) offers a method of overcoming some of the disadvantages of other types of pulse modulation. In PCM, the instantaneous amplitude of the sampled signal is represented by a coded arrangement of binary digits or bits resulting in a series of pulses. All pulses are the same height and the same shape. Therefore, it is only necessary for the receiving equipment to detect the presence or absence of a pulse.
The incoming message signal Figure (PCM1) below is the signal to be transmitted using the PCM which has the electrical characteristics of the following:
-Peak-to-peak voltage of 5.36 V.
-Frequency of 150 Hz
-time division of 2.5 ms/ div
Those characteristics are the same for the first signal being modulated using the PAM modulation . PCM is sampled with a train of rectangular pulses, narrow enough to closely approximate the instantaneous sampling process. The sampling rate must be greater than twice the highest frequency.
Figure (PCM1) AF input at PCM Encoder
Operation in the Transmitter:
The operation of the transmitter to generate a PCM goes through three stages, sampling, quantization and encoding.
- The sampling operation: it generates pulsed modulated singal which is sampled with a train of rectangular pulses, narrow enough to closely approximate the instantaneous sampling process. The sampling rate must be greater than twice the highest frequency.
F min > 2 F max
- The quantization operation: is the operation of setting the sampled amplitudes into a discete values. Each sampled amplitude is approximated to the nearst level selected from a set of quantization levels .
The levels of quantized levels can be found by this formula
Quantization levels(N) = where n is the number of bits used.
-The encoding operation: it converts the selected quantized level into a binary represnation of 0 and 1.
The figure below shows the whole process that the input signal goes through until we got the transmited signal as quantized and coded signal contains the information of the original signal.
The basic operation of the P CM transmitter
The transmitted signal is drawn as below with the electrical characteristics as following:
-Peak-to-peak voltage (amplitude) of 5.36 V.
-Frequency of 22 kHz.
- 10 µs/div time.
As we can realize that the amplitude of the coded signal in the transmitted channel keeps the same as the input signal to The AF channel but in rectangular form. But the frequency is very high so that it can travels very far.
Figure (PCM2) : TX PCM signal at PCM Encoder
Operation in the Receiver:
After transmitting the coded signal using the transmitter channel, this signal needed to be demodulated again to get the original signal. This can be done by using a receiver channel to get back this original signal. The process of the receiver operation goes through two phases. Decoding and Expanding, and Reconstruction or Reproduction.
- Decoding and Expanding;
The first operation in the receiver is to regenerate the received pulses. These clean pulses are then regrouped into code words and decoded into a quantized PAM signal. The sequence of decoded samples represent an estimate of the sequence of compressed samples produced by the quantizer in the transmitter. In order to restore the sequence of decoded samples to their correct relative level, we use an expander with a characteristic complementary to the compressor, used in the transmitter. Ideally, the compression and expansion are exactly inverse, except for the effect of quantization. The combination of a compressor and an expander is referred as a compander.
The signal received by the transmitter channel is as shown below with following electrical characteristics:
-peak-to-peak voltage of 5.28 V
- frequency of 21 kHz.
-time of 10µs/ div.
We can realize from the graph and electrical characteristics that the signal in the receiver channel is almost the same except for small reduction of amplitude and frequency. This might because of some small errors which is relatively unexpected .
This operation is achieved by passing the expander output through a low-pass reconstruction filter whose cutoff frequency is equal to the bandwidth of the message signal. The recovery of the message signal is only the estimation not exact reconstruction.
Now the process of receiving is fully achieved and the received signal is reproduced .The characteristics of the received signal and graph are as follows:
-Peak-to-peak voltage of 4.88 V
- frequency of 150 Hz.
-time of 2.5 ms/ division
And the graph is as below.
From our observations we realized that the received signal (output signal) is not exactly as the input rather there are reduction in it's amplitude but the frequency and time are exactly the same . this is due to that the quantizer at the PCM encoder produces an error signal at the PCM decoder output.
By displaying both the input and recovered output signals on a dual-trace oscilloscope, a slight delay can be observed. This is due to the Parallel-to-Serial conversion in the PCM encoder (1 time slot) and the Serial-to-Parallel conversion in the decoder (another time slot).
The frequency of the input signal effects it's output characteristics, if a higher frequency than 150 Hz it will produces a signal with less distortion.
3- PCM TIME DIVISION MULTIPLEX:
-The setup of the experiments is implemented as shown in ( figure3) below.
- An input signal of 600 Hz was input into the AF input of PCM.
-The Tx channel was connected to the oscilloscope to display the signal at the transmitter.
-The Rx channel was connected to the oscilloscope to display the signal at the transmitter.
-The output channel of the PCM was connected to the oscilloscope probe to display the output PCM TDM.
Figure 3- PCM TIME DIVISION MULTIPLEX
Results and Discussion:
In a PCM TDM system there are several parameters to consider, including:
a) Number of message channels 'n'
b) Bandwidth of each message channel Bm
c) Message sampling rate
d) Bit rate of the PCM TDM signal
First, the analog signal at the AF input port (figure AF) is converted into a digital PCM signal at the TX output port(figure TX).
(figure AF)- AF Input at PCM Encoder
Figure (TX)- TX PCM At PCM Encoder
There is a timing signal of some kHz that transmit pulse that occurs one clock
cycle before the assigned time slot. Each pulse enables the Encoder to sample/encode the
message signal to be transmitted in the following time slot.
At the receiver Encoder, the PCM signal at the RX port (figure RX)is recovered into an analog signal at the the output port(figure OT).
Figure (RX)- RX PCM At PCM Encoder:
Figure (OT)- Output at PCM Decoder
Similar to the transmitter Encoder , the receiver has an clock signal that
enables the operation of the encoder. In order for the receiver to decode a transmitted PCM
code in a given time slot, the transmitting and receiving pulses must be synchronized.
It is easy to demodulate a pulse code signal using a decoder at the receiving part.The digital pulses have to transmitted in reverse manner where the first pulse to be transmitted is the smallest pulse in magnitude and the last one is the highest pulse. In the reciver part a current source is applying the pulse codes into An RC circuit as shown in the figure below
The current source will supply linear charges to the capactor which will increase every time a pulse is applied to C1 provided it doesn't charge between pulses. The resistor R1 is suited to allow the capacitor to discharge half of it's value between pulses.when the PCM demodulator detects the presence and absence of pulses it will reproduce the amplitude of the signal representd by the pulse code group. For this reason, PCM has the advantage of immunity to noise and distortion regardless of the number of times the signal is transmitted.
The main advantages of PCM transmission are: lower cost, ease of multiplexing and switching, and better noise immunity. Its main disadvantage is the stringent timing and synchronization requirements.
Pulse modulation involves communication using a train of recurring pulses. there are many kinds of pulse modulation but the most popular types are Pulse Amplitude Modulation (PAM), Pulse Phase Modulation (PPM) , Pulse Duration Modulation (PDM) and Pulse Frequency Modulation (PFM).The experiment mainly involves only the PAM and PCM. In pulse-amplitude modulation (PAM) the amplitude of a train of constant-width pulses is varied in proportion to the sample values of the modulating message signal. The pulses are usually spaced at equal time interval. PAM has advantage of being simple and it's noise immunity more than analog modulation. The process PAM involves taking Samples of the input signals across time then measuring the amplitude of each signal sample. Those amplitude measurements of the signals are encoded in binary codes that represent the samples. The binary of digital data is sent to the receiver part as discrete binary codes.
Pulse-code modulation (PCM) is an extension of pulse amplitude modulation (PAM) that incorporates quantization of the samples to discrete levels . The sampling rate must be satisfied which means that the minimum frequency must exceed twice the highest frequency. The PAM involves three phases, sampling, quantizing , and encoding in the transmitter part. At the receiver part it involves decoding the signal and then reproducing the signal. PCM offers advantages over analog modulation techniques such as its immunity to transmission noise and its ability to be processed digitally. PCM can be processed in the digital domain (0 and 1), allowing for a range of signal alterations that would otherwise be impossible in the analog domain.
TDM (Time-division multiplexing) is the time interleaving of samples from several sources so that the information form these sources can be transmitted serially over a single communication channel. The TDM can be used to transmit PAM and PCM signals through one signle line with different channels where the signal are transmitted using different time slots.
One advantage of using pulse modulation it permits the simultaneous transmission ofSeveral signals on a time-sharing basis(time division multiplex). It has disadvantage of slow transmission where signals have to be transmitted serially so a delay time of gap is created which make the transmission of data slower.