LINE CODING
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1. There is a return to zero for the second half of each bit period RZ AMI Return to zero alternate mark inversion 3 level there is a half width output pulse if the input is a 1 no output if the input is a 0 This would be the same as UNI RZ But in addition there is a polarity inversion of every alternate output pulse Bij L Biphase level Manchester bipolar V volts For each input 1 there is a transition from V to V in the middle of the bit period For each input 0 there is a transition from V to V in the middle of the bit period DICODE NRZ Di code non return to zero 3 level for each transition of the input there is an output pulse of opposite polarity from the preceding pulse For no transition between input pulses there is no output The codes offered by the line code encoder are illustrated in Figure 2 below These have been copied from the Advanced Module Users Manual where more detail is provided Line coding D 45 input DATA 0 1 0 0 0 0 1 1 1 0 O 1 0 5V i TTL oV NRZ L 0 NRZ M 0 UNI RZ 0 BIP RZ 0 RZ AMI 0 DICODE o _ El Ep Figure 2 TIMS line codes The output waveforms apart from being encoded have all had their amplitudes adjusted to suit a TIMS analog channel not explicitly shown in Figure 2 When connected to the input of the LINE CODE DECODER these waveforms are de coded back to the orig2. re timed bit clock 8 333 kHz from 2 083 kHz bit clock MASTER SIGNALS Figure 3 simplified model of Figure 1 When a particular code has been set up and the message successfully decoded without error the BUFFER should be included in the transmission path By patching it in or out it will introduce a polarity change in the channel If there is no change to the message output then the code in use is insensitive to polarity reversals Note that the LINE CODE DECODER requires for successful decoding an input signal of amplitude near the TIMS ANALOG REFERENCE LEVEL 2 volt pp In normal applications this is assured since it will obtain its input from the DECISION MAKER procedure There are no step by step Tasks for you to perform Instead it is left to you to ensure that in the approximate order indicated 1 you read the TIMS Advanced Modules User Manual for more details of the LINE CODE ENCODER and LINE CODE DECODER modules than is included here 2 you select a short sequence from the transmitter message source Line coding DI 47 at least initially you synchronize the oscilloscope to show a snapshot of the transmitter sequence Later you may be interested in eye patterns examine each code in turn from the encoder confirming the transformation from TTL is as expected On the other hand and far more challenging is to determine what the law of each transformation is without help from a Textboo
3. LINE CODING PREPARAN e a aaas 42 why line coding cnra eriak aa E ana iaae 42 O E oes 43 teETMUNOLO aA E A gee tees Reheat 44 available line codes a e Giese ola 44 NR ata 44 INR Za Min dde 44 UNI RZ Sic Seine oes dans cosh cae ii dorado 45 Jei S A E NN 45 A NAO 45 O TAE 45 DICODEENRZ a Set ota ae 45 band AMA vivo cad iii 46 duobinary encoding ineen iie e E E i Ea 46 EXPERIMENT nt A E E wea vars 47 proced re A r A E E sua EN 47 TUTORIAL QUESTIONS nisinsin ei 48 Line coding Vol D1 ch 5 rev 1 0 41 ACHIEVEMENTS familiarity with the properties of the LINE CODE ENCODER and LINE CODE DECODER modules and the codes they generate PREREQUISITES an appreciation of the purpose behind line coding EXTRA MODULES LINE CODE ENCODER and LINE CODE DECODER PREPARATION This experiment is tutorial in nature and serves to introduce two new modules In your course work you should have covered the topic of line coding at what ever level is appropriate for you TIMS has a pair of modules one of which can perform a number of line code transformations on a binary TTL sequence The other performs decoding why line coding 42 D1 There are many reasons for using line coding Each of the line codes you will be examining offers one or more of the following advantages spectrum shaping and relocation without modulation or filtering This is important in telephone line applications for example where the transfer character
4. e particular waveform will return to zero for a finite part of each data 1 typically half the interval The term NRZ is an abbreviation for non return to zero and this waveform will not return to zero during the bit interval representing a data 1 e the use of L and M would seem to be somewhat illogical or inconsistent with each other For example see how your text book justifies the use of the L and the M in NRZ L and NRZ M e two sinusoids are said to be antipodal if they are 180 out of phase available line codes For a TTL input signal the following output formats are available from the LINE CODE ENCODER NRZ L Non return to zero level bipolar this is a simple scale and level shift of the input TTL waveform NRZ M Non return to zero mark bipolar there is a transition at the beginning of each 1 and no change for a 0 The M refers to inversion on mark This is a 44 D1 Line coding differential code The decoder will give the correct output independently of the polarity of the input UNI RZ Uni polar return to zero uni polar there is a half width output pulse if the input is a l no output if the input is a 0 This waveform has a significant DC component BIP RZ Bipolar return to zero 3 level there is a half width ve output pulse if the input is a 1 ora half width ve output pulse if the input is a 0
5. ignal at the TIMS ANALOG REFERENCE LEVEL or less It could be corrupted by noise Here it is re generated by a detector The TIMS detector is the DECISION MAKER module already examined in the experiment entitled Detection with the DECISION MAKER in this Volume Finally the TIMS LINE CODE DECODER module accepts the output from the DECISION MAKER and decodes it back to the binary TTL format Preceding the line code encoder may be a source encoder with a matching decoder at the receiver These are included in the block diagram of Figure 1 which is of a typical baseband digital transmission system It shows the disposition of the LINE CODE ENCODER and LINE CODE DECODER All bandlimiting is shown concentrated in the channel itself but could be distributed between the transmitter channel and receiver l 1 gt TTL LINE BANDLIMITED LINE SOURCE MESSAGE p SOURCE cope PH ANALOG p DETECTOR CODE DECODER TTL OUT SOURCE ENCODER ENCODER CHANNEL DECODER 1 1 TRANSMITTER 1 CHANNEL RECEIVER Figure 1 baseband transmission system The LINE CODE ENCODER serves as a source of the system bit clock It is driven by a master clock at 8 333 kHz from the TIMS MASTER SIGNALS module It divides this by a factor of four in order to derive some necessary internal timing signals at a rate of 2 083 kHz This then becomes a convenient source of a 2 083 kHz TTL signal for use as the sy
6. inal TTL sequence band limiting No matter what the line code in use it is not uncommon to bandlimit these waveforms before they are sent to line or used to modulate a carrier As soon as bandlimiting is invoked individual pulses will spread out in the time domain and interfere with adjacent pulses This raises the issue if inter symbol interference ISI A study of ISI is outside the intended scope of this text but it cannot be ignored in practice Bandlimiting by pulse shaping can be effected and ISI controlled by appropriate filter design An alternative approach duobinary encoding was invented by Lender duobinary encoding A duobinary encoder and decoder is included in the line code modules Duobinary encoding is also called correlative coding or partial response signalling The precoded duobinary encoding model implemented in the LINE CODE ENCODER module is described in the TIMS Advanced Modules User Manual 46 D1 l Lender A The Duobinary Technique for High Speed Data Transmission IEEE Trans Comm Electron vol 82 pp 214 218 May 1963 Line coding Figure 3 shows a simplified model of Figure 1 There is no source encoding or decoding no baseband channel and no detection For the purpose of the experiment this is sufficient to confirm the operation of the line code modules change polarity O ext trig SEQUEHCE LIME CODE BUFFER LINE CODE ENCODER AMPLIFIERS DECODER TTL out
7. istic has heavy attenuation below 300 Hz bit clock recovery can be simplified DC component can be eliminated this allows AC capacitor or transformer coupling between stages as in telephone lines Can control baseline wander baseline wander shifts the position of the signal waveform relative to the detector threshold and leads to severe erosion of noise margin error detection capabilities bandwidth usage the possibility of transmitting at a higher rate than other schemes over the same bandwidth At the very least the LINE CODE ENCODER serves as an interface between the TTL level signals of the transmitter and those of the analog channel Likewise the Line coding LINE CODE DECODER serves as an interface between the analog signals of the channel and the TTL level signals required by the digital receiver the modules The two new modules to be introduced are the LINE CODE ENCODER and the LINE CODE DECODER You will not be concerned with how the coding and decoding is performed You should examine the waveforms using the original TTL sequence as a reference In a digital transmission system line encoding is the final digital processing performed on the signal before it is connected to the analog channel although there may be simultaneous bandlimiting and wave shaping Thus in TIMS the LINE CODE ENCODER accepts a TTL input and the output is suitable for transmission via an analog channel At the channel output is a s
8. k or other reference of significant interest would be an examination of the power spectra of each of the coded signals For this you would need data capturing facilities and software to perform spectral analysis and so on sv resetting Resetting of the LINE CODE ENCODER and the LINE CODE DECODER after the master clock is connected or after any clock interruption is strictly not necessary for all codes But it is easier to do it for all codes rather than remember for which codes it is essential For more details refer to the TIMS Advanced Modules User Manual TUTORIAL QUESTIONS Q1 why introduce the complications of line encoding in a digital transmission system 02 apart from the inevitable delay introduced by the analog filter did you notice any other delays in the system You may need this information when debugging later experiments Q3 an important function of many line encoders is the elimination of the DC 48 D1 component When is this desirable Line coding
9. stem bit clock Because the LINE CODE DECODER has some processing to do it introduces a time delay To allow for this it provides a re timed clock if required by any further digital processing circuits eg for decoding or error counting modules Line coding DI 43 terminology e the word mark and its converse space often appear in a description of a binary waveform This is an historical reference to the mark and space of the telegraphist In modern day digital terminology these have become HI and LO or 1 and 0 as appropriate e unipolar signalling where a 1 is represented with a finite voltage V volts and a 0 with zero voltage This seems to be a generally agreed to definition e those who treat polar and bipolar as identical define these as signalling where a 1 is sent as V and 0 as V They append AMI when referring to three level signals which use V and V alternately for a 1 and zero for 0 an alternative name is pseudoternary You will see the above usage in the TIMS Advanced Modules User Manual as well as in this text However others make a distinction Thus e polar signalling where a 1 is represented with a finite voltage V volts and a 0 with V volts e bipolar signalling where a 1 is represented alternately by V and V and a 0 by zero voltage e the term RZ is an abbreviation of return to zero This implies that th
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