The TDM system is based upon the sampling of the amplitude of the information signal atregular intervals, and the subsequent transmittal of one or more pulsations to stand for eachsample. In an parallel pulsation system, for the intelligence contained in the information signal to betransmitted, the features of the pulsation must be varied in conformity with the amplitude ofthe sample. This can be achieved by changing the amplitude, breadth, or the place of thepulses, to give either pulse amplitude, pulse continuance, or pulse place modulation.With PAM, pulsations of equal breadth and spacing have their amplitudes varied harmonizing to the features of the modulating signal.
Pulse transition ( PM ) consists of two signal, the bearer signal is in digital signifier which is used to convey the information signal which in bend modulate the bearer signal to bring forth 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 changing the amplitude of pulsations. The unmodulated signal consists of a uninterrupted train of pulsations of changeless frequence, continuance, and amplitude. During transition the pulsation amplitudes are changed to reflect the information being encoded. In PAM the consecutive sample values of the parallel signal are used to consequence the amplitudes of a corresponding sequence of pulsations of changeless continuance happening at the trying rate. No quantisation of the samples usually occurs.In principle the pulsations may busy the full clip between samples, but in most practical systems the pulse continuance, is limited to a fraction of the trying interval. Such a limitation creates the possibility of interleaving during one sample interval one or more pulsations derived from other PAM systems in a procedure known as time-division multiplexing ( TDM ) . This type of transition is so sensitive to resound as AM.
Pulse Code Modulation ( PCM ) :
Is a method of encoding information in a signal by changing the amplitude of pulsations. Many samples are quantized where there are merely predefined values of amplitude are selected to be coded to binary codification. This binary or digital signals need to be demodulated in the receiving system portion to the original signal utilizing a decipherer and so a low base on balls filter to counterbalance for any mistake or noise signal. This method of transition is preferred than the old method PAM because it ‘s good known by it ‘s unsusceptibility to resound because it ‘s coded in a digital signal. It besides provide high quality passage pure of deformation or extension.
Pulse Phase Modulation ( PPM ) :
Is about similar to Frequecy Modulation FM except that the stage of the bearer is altering where in FM the frequence of the bearer signal is changing. If the carrieng signal is as below
Then the modulated will be
The PPM can be modulated harmonizing to the size of the signal as in two theoretical accounts.
For little amplitude signals, PM is similar to amplitude transition ( AM ) and exhibits its features of sensitiveness to resound and it ‘s hapless efficiency.
For a individual big sinusoidal signal, PM is similar to FM, and its bandwidth is about
, and can exhibits it ‘s features of it ‘s imunity to resound and deformation.
Pulse Duration Modulation ( PDM ) :
Transition of a pulsation bearer wherein the value of each instantaneous sample of a modulating moving ridge produces a pulsation of relative continuance by changing the taking, draging, or both borders of a pulsation.
Pulse Frequency Modulation:
it is a method of transition where a modulated moving ridge is used to modulate a pulsation bring forthing signal. The amplitude and clip are kept changeless and the altering moving ridge in instantaneous same clip intervals.
-B43D1 Digital Communication Trainer-Digital Transmision of Analog Signals
( Figure B ) – Oscilloscope ( figure C ) – Connections
1-PAM TIME-DIVISION MULTIPLEX:
The trainer set up is as shown in figure 1 below.
Connect the power supply in proper mutual opposition to the CRO and
the AF channel input ( 1,4 ) were connected into the CRO.
The wave form is displayed on the screen of the CRO.
the TX channel PAM was connected into the CRO.
The wave form of the end product is displayed in the CRO screen.
the RX PAM was connected into the CRO channel to expose the PAM wave form.
the channel of AF end product ( impart 1,4 ) were connected into the CRO to reproduce the two wave forms.
The informations were recorded and the graphs were drawn.
figure 1. PAM TIME-DIVISION MULTIPLEX
Consequences and Discusions:
AF Channel Input ( 1,4 ) :
( Figure CH1 )
figure ( ch2 )
Time Division Multiplexing is really based on the peak rate of the signal where a samples merely are taken to be transmitted.
As we can see in the figure above a digitized samples are being transmitted non the original moving ridge.A tow input signals are being input to impart 1 and impart 4 to be transmitted utilizing PAM Time-Division Multiplex. By utilizing a high frequence bearer signal to convey the two signals through the clip division rule where the signal 1 is transmitted through channel-1 in a clip slot different than that used for channel 4.
Let ‘s see first conveying the first signal utilizing channel-1 where the clip division multiplexer set it to clip slot 1.
The signal as shown in figure ( ch1 ) above shows the input signal of channel 1 to be transmitted.the features of this signal are:
-Peak-to-peak electromotive force of 5.36 V.
-Frequency of 150 Hz
-time division of 2.5 ms/ div
Now for input signal in channel 4 has different features different of input singal to impart 1 to go possible for the TDM to convey the two signals through a individual line.
The features of signal are:
-Peak-to-peak electromotive force of 5.24 V.
-Frequecy of 600 Hz.
The procedure of PAM transition is done in few stairss:
Samples of the two signals of channel 1 and impart 4 are taken across clip so it will mensurate the amplitude of each signal sample. Those amplitude measurings of the signals are encoded in binary codifications that represent the samples.
The double star of digital information is sent to the receiving system portion as distinct binary codifications.
The sampled signals in the sender portion are as shown below with their electrical features:
2- signals at the TX PAM:
-Peak-to-peak electromotive force 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 receiving system the sampling station has to be synchronized with the entrance wave form so that the PAM samples matching to source-1 will look on the channel-1 end product, and so signal 2 in channel 4.
The end product signal in the standard channel is as below:
-Peak to top out of 5.20 V ( 2 v/div )
-frequency of 600 Hz ( impart 4 )
Finally after the signal reached the receiving system portion and being decoded back into the original signal we got those end products of channel 1 and impart 4.
AF Channel Output 1:
The features of the signal are:
-Peak-to-peak electromotive force ( 5.20 V )
-frequency ( 150 Hz ) .
AF Channel Output 4:
The features of the end product singal of chnnel 4 are:
– Peak-to-peak electromotive force ( 4.08 V )
-frequency ( 600 ) .
We realize that the end product signal is reproduced to corresponded to the input signal in channel 1 and impart 4 but there are decrease in the amplitude although the frequence is kept the same. This is due to add-on or minus of a noise signal.
PAM is susceptibleness to resound and the ground for such susceptibleness is that any intervention in the transmittal line will eighther add or deduct from signal electromotive force, therefore the amplitude will alter consequently. As we know that the amplitude of the electromotive force is stand foring the signal so any unwanted alteration in the signal will do a deformation ( unwanted pulsations ) 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 CRO to expose the signal at the sender.
-The Rx is connected to the CRO to expose the signal at the sender.
-The end product channel of the PCM is connected to the CRO and the demodulated signal is reproduced.
figure 2. BASIC PCM OPERATION
Consequences and Discusion:
Pulse Code Modulation ( PCM ) offers a method of get the better ofing some of the disadvantages of other types of pulse transition. In PCM, the instantaneous amplitude of the sampled signal is represented by a coded agreement of binary figures or spots ensuing in a series of pulsations. All pulsations are the same tallness and the same form. Therefore, it is merely necessary for the receiving equipment to observe the presence or absence of a pulsation.
The incoming message signal Figure ( PCM1 ) below is the signal to be transmitted utilizing the PCM which has the electrical features of the followers:
-Peak-to-peak electromotive force of 5.36 V.
-Frequency of 150 Hz
-time division of 2.5 ms/ div
Those features are the same for the first signal being modulated utilizing the PAM transition. PCM is sampled with a train of rectangular pulsations, narrow plenty to closely come close the instantaneous sampling procedure. The trying rate must be greater than twice the highest frequence.
Figure ( PCM1 ) AF input at PCM Encoder
Operation in the Sender:
The operation of the sender to bring forth a PCM goes through three phases, trying, quantisation and encryption.
– The sampling operation: it generates pulsed modulated singal which is sampled with a train of rectangular pulsations, narrow plenty to closely come close the instantaneous sampling procedure. The trying rate must be greater than twice the highest frequence.
F min & A ; gt ; 2 F soap
– The quantisation operation: is the operation of puting the sampled amplitudes into a discete values. Each sampled amplitude is approximated to the nearst degree selected from a set of quantisation degrees.
The degrees of quantal degrees can be found by this expression
Quantization degrees ( N ) = where N is the figure of spots used.
-The encoding operation: it converts the selected quantal degree into a binary represnation of 0 and 1.
The figure below shows the whole procedure that the input signal goes through until we got the transmited signal as quantal and coded signal contains the information of the original signal.
The basic operation of the P CM sender
The familial signal is drawn as below with the electrical features as followers:
-Peak-to-peak electromotive force ( amplitude ) of 5.36 V.
-Frequency of 22 kilohertzs.
– 10 µs/div clip.
As we can recognize that the amplitude of the coded signal in the familial channel keeps the same as the input signal to The AF channel but in rectangular signifier. But the frequence is really high so that it can go really far.
Figure ( PCM2 ) : TX PCM signal at PCM Encoder
Operation in the Receiver:
After conveying the coded signal utilizing the sender channel, this signal needed to be demodulated once more to acquire the original signal. This can be done by utilizing a receiving system channel to acquire back this original signal. The procedure of the receiving system operation goes through two stages. Decoding and Expanding, and Reconstruction or Reproduction.
– Decoding and Expanding ;
The first operation in the receiving system is to renew the standard pulsations. These clean pulsations are so regrouped into codification words and decoded into a quantal PAM signal. The sequence of decoded samples represent an estimation of the sequence of tight samples produced by the quantizer in the sender. In order to reconstruct the sequence of decoded samples to their right comparative degree, we use an expander with a characteristic complementary to the compressor, used in the sender. Ideally, the compaction and enlargement are precisely reverse, except for the consequence of quantisation. The combination of a compressor and an expander is referred as a compander.
The signal received by the sender channel is as shown below with following electrical features:
-peak-to-peak electromotive force of 5.28 V
– frequence of 21 kilohertzs.
-time of 10µs/ div.
We can recognize from the graph and electrical features that the signal in the receiving system channel is about the same except for little decrease of amplitude and frequence. This might because of some little mistakes which is comparatively unexpected.
This operation is achieved by go throughing the expander end product through a low-pass Reconstruction filter whose cutoff frequence is equal to the bandwidth of the message signal. The recovery of the message signal is merely the appraisal non exact Reconstruction.
Now the procedure of receiving is to the full achieved and the standard signal is reproduced.The features of the standard signal and graph are as follows:
-Peak-to-peak electromotive force of 4.88 V
– frequence of 150 Hz.
-time of 2.5 ms/ division
And the graph is as below.
From our observations we realized that the standard signal ( end product signal ) is non precisely as the input instead there are decrease in it ‘s amplitude but the frequence and clip are precisely the same. this is due to that the quantizer at the PCM encoder produces an mistake signal at the PCM decipherer end product.
By exposing both the input and recovered end product signals on a dual-trace CRO, a little hold can be observed. This is due to the Parallel-to-Serial transition in the PCM encoder ( 1 clip slot ) and the Serial-to-Parallel transition in the decipherer ( another clip slot ) .
The frequence of the input signal effects it ‘s end product features, if a higher frequence than 150 Hz it will produces a signal with less deformation.
3- PCM TIME DIVISION MULTIPLEX:
-The apparatus 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 CRO to expose the signal at the sender.
-The Rx channel was connected to the CRO to expose the signal at the sender.
-The end product channel of the PCM was connected to the CRO investigation to expose the end product PCM TDM.
Figure 3- PCM TIME DIVISION MULTIPLEX
Consequences and Discussion:
In a PCM TDM system there are several parametric quantities to see, including:
a ) Number of message channels ‘n ‘
B ) Bandwidth of each message channel Bm
degree Celsius ) Message trying rate
vitamin D ) Bit rate of the PCM TDM signal
First, the parallel signal at the AF input port ( figure AF ) is converted into a digital PCM signal at the TX end product port ( figure TX ) .
( figure AF ) – AF Input at PCM Encoder
Figure ( TX ) – Texas PCM At PCM Encoder
There is a timing signal of some kilohertz that transmit pulsation that occurs one clock
rhythm before the assigned clip slot. Each pulsation enables the Encoder to sample/encode the
message signal to be transmitted in the undermentioned clip slot.
At the receiving system Encoder, the PCM signal at the RX port ( figure RX ) is recovered into an linear signal at the the end product port ( figure OT ) .
Figure ( RX ) – RX PCM At PCM Encoder:
Figure ( OT ) – End product at PCM Decoder
Similar to the sender Encoder, the receiving system has an clock signal that
enables the operation of the encoder. In order for the receiving system to decrypt a familial PCM
codification in a given clip slot, the transmission and receiving pulsations must be synchronized.
It is easy to demodulate a pulse codification signal utilizing a decipherer at the having part.The digital pulsations have to transmitted in contrary mode where the first pulsation to be transmitted is the smallest pulsation in magnitude and the last 1 is the highest pulsation. In the reciver portion a current beginning is using the pulsation codes into An RC circuit as shown in the figure below
The current beginning will provide additive charges to the capactor which will increase every clip a pulsation is applied to C1 provided it does n’t bear down between pulsations. The resistance R1 is suited to let the capacitance to dispatch half of it ‘s value between pulses.when the PCM detector detects the presence and absence of pulsations it will reproduce the amplitude of the signal representd by the pulse codification group. For this ground, PCM has the advantage of unsusceptibility to resound and distortion regardless of the figure of times the signal is transmitted.
The chief advantages of PCM transmittal are: lower cost, easiness of multiplexing and shift, and better noise unsusceptibility. Its chief disadvantage is the rigorous timing and synchronism demands.
Pulse transition involves communicating utilizing a train of repeating pulsations. there are many sorts of pulse transition 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 chiefly involves merely the PAM and PCM. In pulse-amplitude transition ( PAM ) the amplitude of a train of constant-width pulsations is varied in proportion to the sample values of the modulating message signal. The pulsations are normally spaced at equal clip interval. PAM has advantage of being simple and it ‘s noise unsusceptibility more than linear transition. The procedure PAM involves taking Samples of the input signals across clip so mensurating the amplitude of each signal sample. Those amplitude measurings of the signals are encoded in binary codifications that represent the samples. The double star of digital information is sent to the receiving system portion as distinct binary codifications.
Pulse-code transition ( PCM ) is an extension of pulse amplitude transition ( PAM ) that incorporates quantisation of the samples to distinct degrees. The trying rate must be satisfied which means that the minimal frequence must transcend twice the highest frequence. The PAM involves three stages, trying, quantising, and encoding in the sender portion. At the receiving system portion it involves decrypting the signal and so reproducing the signal. PCM offers advantages over linear transition techniques such as its unsusceptibility to transmittal noise and its ability to be processed digitally. PCM can be processed in the digital sphere ( 0 and 1 ) , leting for a scope of signal changes that would otherwise be impossible in the parallel sphere.
TDM ( Time-division multiplexing ) is the clip interleaving of samples from several beginnings so that the information signifier these beginnings can be transmitted serially over a individual communicating channel. The TDM can be used to convey PAM and PCM signals through one signle line with different channels where the signal are transmitted utilizing different clip slots.
One advantage of utilizing pulse transition it permits the coincident transmittal ofSeveral signals on a time-sharing footing ( clip division multiplex ) . It has disadvantage of slow transmittal where signals have to be transmitted serially so a hold clip of spread is created which make the transmittal of informations slower.